Value of Etest penicillin V and penicillin G strips for penicillin susceptibility testing of Neisseria meningitidis

The NCCLS agar dilution method and Etest are currently accepted methods for susceptibility testing of Neisseria meningitidis to penicillin. We determined the MIC of penicillin V and penicillin G by both the agar dilution method and Etest using 43 strains of N. meningitidis. Although results for reference strains were within the accepted quality control range of penicillin MICs for both drugs, differences of two to three dilutions were seen between the two antibiotics with both methods. Penicillin V results cannot correctly predict the susceptibility to penicillin G for N. meningitidis if penicillin G breakpoints are used for penicillin V. However, adjusting the penicillin V breakpoints two dilutions higher (i.e., S < or = 0.25 and R > or = 8 microg/ml), concordance could be achieved for susceptibility categorization by the two compounds. An agreement of 98% +/- 1 dilution was obtained between Etest and the reference method when using penicillin G strips. We conclude that Etest with penicillin G strips is a convenient and reliable alternative method for determining the MICs of penicillin for N. meningitidis.     

Validation of a modified Kirby-Bauer disk diffusion method for metronidazole susceptibility testing of Helicobacter pylori

Triple antimicrobial therapy that includes metronidazole has been recommended as a first-line therapy for Helicobacter pylori because it has the highest eradication rates. However, resistance in H. pylori to metronidazole has been reported worldwide and its presence may reduce the efficacy of triple therapy. Various methods for testing H. pylori against metronidazole have been used including agar dilution, disk diffusion and the Etest but there has been little standardization of methods. One hundred isolates of H. pylori from different patients were tested for susceptibility to metronidazole by agar dilution, Etest and disk diffusion (5 μg disk). The agar dilution results confirmed the MIC susceptibility breakpoint to be ⩽8 μg/ml. Using this breakpoint there was close agreement (98%) between Etest and agar dilution results. For susceptible strains, MICs by E-test were generally one twofold dilution lower. Using the error-rate bounded method, agreement between disk diffusion zome diameter and MIC was 98% for agar dilution with breakpoints of ⩾12 mm and ⩽8 μg/ml and 100% for Etest with breakpointsof ⩾12 mm and ⩽8 μg/ml. The Etest discriminated better than agar dilution between susceptible and resistant strains and was simple to perform. The disk diffusion test is a reliable and cheaper alternative to the Etest with susceptibility being a zone diameter ⩾12 mm with a 5 μg disk. The prevalence of metronidazole resistance in this study was 40% by Etest.     

Emergence of quinolone resistance and cephalosporin MIC creep in Neisseria gonorrhoeae isolates from a cohort of young men in Kisumu, Kenya, 2002 to 2009

We evaluated antimicrobial resistance in Neisseria gonorrhoeae isolated from men enrolled in a randomized trial of male circumcision to prevent HIV. Urethral specimens from men with discharge were cultured for N. gonorrhoeae. MICs were determined by agar dilution. Clinical and Laboratory Standards Institute (CLSI) criteria defined resistance: penicillin, tetracycline, and azithromycin MICs of ≥2.0 μg/ml; a ciprofloxacin MIC of ≥1.0 μg/ml; and a spectinomycin MIC of ≥128.0 μg/ml. Susceptibility to ceftriaxone and cefixime was shown by an MIC of ≤0.25 μg/ml. Additionally, PCR amplification identified mutations in parC and gyrA genes in selected isolates. From 2002 to 2009, 168 N. gonorrhoeae isolates were obtained from 142 men. Plasmid-mediated penicillin resistance was found in 65%, plasmid-mediated tetracycline resistance in 97%, and 11% were ciprofloxacin resistant (quinolone-resistant N. gonorrhoeae [QRNG]). QRNG appeared in November 2007, increasing from 9.5% in 2007 to 50% in 2009. Resistance was not detected for spectinomycin, cefixime, ceftriaxone, or azithromycin, but MICs of cefixime (P = 0.018), ceftriaxone (P < 0.001), and azithromycin (P = 0.097) increased over time. In a random sample of 51 men, gentamicin MICs were as follows: 4 μg/ml (n = 1), 8 μg/ml (n = 49), and 16 μg/ml (n = 1). QRNG increased rapidly and alternative regimens are required for N. gonorrhoeae treatment in this area. Amid emerging multidrug-resistant N. gonorrhoeae, antimicrobial resistance surveillance is essential for effective drug choice. High levels of plasmid-mediated resistance and increasing MICs for cephalosporins suggest that selective pressure from antibiotic use is a strong driver of resistance emergence.     

Antibiotic susceptibility pattern of Mycobacterium marinum

In vitro activities of 17 antibiotics against 53 clinical strains of Mycobacterium marinum, an atypical mycobacterium responsible for cutaneous infections, were determined using the reference agar dilution method. Rifampin and rifabutin were the most active drugs (MICs at which 90% of the isolates tested were inhibited [MIC(90)s], 0.5 and 0.6 microgram/ml, respectively). MICs of minocycline (MIC(90), 4 microgram/ml), doxycycline (MIC(90), 16 microgram/ml), clarithromycin (MIC(90), 4 microgram/ml), sparfloxacin (MIC(90), 2 microgram/ml), moxifloxacin (MIC(90), 1 microgram/ml), imipenem (MIC(90), 8 microgram/ml), sulfamethoxazole (MIC(90), 8 microgram/ml) and amikacin (MIC(90), 4 microgram/ml) were close to the susceptibility breakpoints. MICs of isoniazid, ethambutol, trimethoprim, azithromycin, ciprofloxacin, ofloxacin, and levofloxacin were above the concentrations usually obtained in vivo. For each drug, the MIC(50), geometric mean MIC, and modal MIC were very close, showing that all the strains had a similar susceptibility pattern. Percent agreement (within +/-1 log(2) dilution) between MICs yielded by the Etest method and by the agar dilution method used as reference were 83, 59, 43, and 24% for minocycline, rifampin, clarithromycin, and sparfloxacin, respectively. Reproducibility with the Etest was low, in contrast to that with the agar dilution method. In conclusion, M. marinum is a naturally multidrug-resistant species for which the agar dilution method is more accurate than the Etest for antibiotic susceptibility testing.     

In Vitro Activity of Ertapenem versus Ceftriaxone against Neisseria gonorrhoeae Isolates with Highly Diverse Ceftriaxone MIC Values and Effects of Ceftriaxone Resistance Determinants: Ertapenem for Treatment of Gonorrhea?

Clinical resistance to the currently recommended extended-spectrum cephalosporins (ESCs), the last remaining treatment options for gonorrhea, is being reported. Gonorrhea may become untreatable, and new treatment options are crucial. We investigated the in vitro activity of ertapenem, relative to ceftriaxone, against N. gonorrhoeae isolates and the effects of ESC resistance determinants on ertapenem. MICs were determined using agar dilution technique or Etest for international reference strains (n = 17) and clinical N. gonorrhoeae isolates (n = 257), which included the two extensively drug-resistant (XDR) strains H041 and F89 and additional isolates with high ESC MICs, clinical ESC resistance, and other types of clinical high-level and multidrug resistance (MDR). Genetic resistance determinants for ESCs (penA, mtrR, and penB) were sequenced. In general, the MICs of ertapenem (MIC(50) = 0.032 μg/ml; MIC(90) = 0.064 μg/ml) paralleled those of ceftriaxone (MIC(50) = 0.032 μg/ml; MIC(90) = 0.125 μg/ml). The ESC resistance determinants mainly increased the ertapenem MIC and ceftriaxone MIC at similar levels. However, the MIC ranges for ertapenem (0.002 to 0.125 μg/ml) and ceftriaxone (<0.002 to 4 μg/ml) differed, and the four (1.5%) ceftriaxone-resistant isolates (MIC = 0.5 to 4 μg/ml) had ertapenem MICs of 0.016 to 0.064 μg/ml. Accordingly, ertapenem had in vitro advantages over ceftriaxone for isolates with ceftriaxone resistance. These in vitro results suggest that ertapenem might be an effective treatment option for gonorrhea, particularly for the currently identified ESC-resistant cases and possibly in a dual antimicrobial therapy regimen. However, further knowledge regarding the genetic determinants (and their evolution) conferring resistance to both antimicrobials, and clear correlates between genetic and phenotypic laboratory parameters and clinical treatment outcomes, is essential.     

Reevaluation of the Disk Diffusion Method for Sulfonamide Susceptibility Testing of Neisseria meningitidis

Lack of correlation between quantitative minimal inhibitory concentration (MIC) determinations and disk diffusion susceptibility tests in our laboratory prompted a study to reevaluate the use of the disk diffusion test for sulfonamide susceptibility testing of Neisseria meningitidis. One hundred and sixty-three recent clinical isolates of N. meningitidis were examined for sulfonamide susceptibility by the agar dilution and disk diffusion methods. Optimal inocula for each of the tests were determined, and thereafter all disk diffusion tests were compared with quantitative MICs as determined by the agar dilution method using sulfadiazine and an inoculum of 10⁶ colony-forming units (CFU)/ml. The clearest and most reproducible zone diameters were obtained with a 10⁷-CFU/ml inoculum in the disk diffusion test. There was complete correlation between the disk zone diameters for 300-μg disks of sulfadiazine and sulfathiazole and the agar dilution test MICs. All isolates with a zone diameter of <20 mm were resistant to sulfadiazine, whereas those with zone diameters of ≥30 mm were susceptible. False susceptible and false resistant readings were obtained with 300-μg sulfisoxazole disks. These data suggest that inocula and type of sulfonamide are critical factors in the disk diffusion test for meningococcal susceptibility testing. Sulfonamide disks are not interchangeable for susceptibility testing of meningococci.     

Comparative antimicrobial activity of gatifloxacin tested against Streptococcus spp. including quality control guidelines and etest method validation. Quality Control Study Group.

Gatifloxacin (formerly AM-1155 or CG 5501) is a new 8-methoxy fluoroquinolone with enhanced activity against Gram-positive cocci, especially Streptococcus pneumoniae and other streptococci. Recent clinical strains (599 isolates) were tested against gatifloxacin, three comparison fluoroquinolones, and penicillin by the reference broth microdilution, Etest (AB BIODISK, Solna, Sweden) and standardized disk diffusion methods (5 micrograms gatifloxacin disk). Gatifloxacin (MIC90, 0.5 microgram/ml) activity was generally comparable to that of trovafloxacin (MIC90, 0.25 microgram/ml), or sparfloxacin (MIC90, 0.5 microgram/ ml) and markedly superior to ofloxacin (MIC90, 2-4 micrograms/ml) against the streptococci. Rates of penicillin non-susceptibility were 41.9, 38.0, and 16.2% for S. pneumoniae (301 strains), viridans group streptococci (150 strains), and beta-haemolytic streptococci (148 strains). Etest results correlated well (95.7-100.0% +/- one log2 dilution) with the reference MIC results, but Etest tended to have elevated gatifloxacin MIC results compared to the broth microdilution method for the highly resistant isolates (MICs, > 2 micrograms/ml). Gatifloxacin disk zone diameters correlate well to reference MICs for all streptococci and proposed interpretive criteria (susceptible at < or = 1 microgram/ml or > or = 18 mm, and resistant at > or = 4 micrograms/ml or < or = 14 mm) did not produce discords between method results (absolute agreement). A nine laboratory quality control (QC) study conforming to the National Committee for Clinical Laboratory Standards (NCCLS) Guideline M23-T3 studied S. pneumoniae ATCC 49619 and gatifloxacin. Proposed ranges for QC of NCCLS tests were 0.12-0.5 microgram/ml for the broth microdilution test and 24-31 mm for the disk diffusion method. These reported results indicate that gatifloxacin was a potent fluoroquinolone with extensive activity against streptococcal isolates. In vitro test methods to measure this activity appear accurate and comparable; and QC guidelines have been established for routine clinical laboratory use pending approval by the NCCLS and the Food and Drug Administration (FDA).     

Nine-Hospital Study Comparing Broth Microdilution and Etest Method Results for Vancomycin and Daptomycin against Methicillin-Resistant Staphylococcus aureus

Vancomycin and daptomycin MIC results for 1,800 randomly selected oxacillin (methicillin [meticillin])-resistant Staphylococcus aureus (MRSA) bloodstream isolates from nine U.S. hospitals (collected from 2002 to 2006) were determined by a reference broth microdilution (BMD) method using frozen-form panels with precise incremental dilutions and by the Etest technique. The Etest provided vancomycin and daptomycin MIC results that were consistently higher (0.5 to 1.5 log₂ dilution steps) than those provided by the reference BMD method. The dominant MRSA population (91.2% of MRSA isolates) would be categorized as vancomycin nonsusceptible by the MIC results from the Etest method if the susceptibility breakpoint was adjusted downward to ≤1 μg/ml, as suggested by clinical outcome studies.Vancomycin is still used extensively for the treatment of oxacillin (methicillin [meticillin])-resistant Staphylococcus aureus (MRSA) bacteremia, as well as other less serious MRSA infections. However, vancomycin treatment failure is not uncommon, even when the MRSA strains are fully susceptible to vancomycin according to the criterion (breakpoint MIC, ≤2 μg/ml) used by the Clinical and Laboratory Standards Institute (CLSI). A reduction in the efficacy of vancomycin against MRSA strains for which vancomycin MICs are elevated (to 1 to 2 μg/ml) has been widely reported, suggesting that modest elevations in MICs may explain some suboptimal clinical outcomes (10, 13).The vast majority of clinical laboratories use automated systems and a distinct minority use the disk diffusion method as the routine susceptibility testing technique. However, the disk diffusion method and some automated systems do not accurately detect vancomycin-intermediate S. aureus (7). In addition, there have been a growing number of reports showing discrepancies between in vitro susceptibility test results for vancomycin and clinical outcomes of MRSA infections treated with this glycopeptide (6, 11). Thus, many laboratories are being requested to assess exact MICs by reference or alternative methods, such as that of Etest (AB Biodisk, Solna, Sweden).Although daptomycin remains very active against S. aureus, an association between reduced susceptibility to vancomycin and reduced susceptibility to daptomycin has been reported by some investigators (1). S. aureus isolates classified as daptomycin nonsusceptible according to MICs (≥2 μg/ml) are still rare, and no definitive resistance mechanism has been identified. However, genetic mutations and increases in daptomycin MICs after prolonged vancomycin and/or daptomycin treatment of infections associated with septic arthritis, osteomyelitis, septic thrombophlebitis, and endocarditis, especially in the presence of intravenous catheters and other prosthetic devices, have been reported previously (1). Thus, microbiology laboratories may be required to perform proper daptomycin susceptibility testing. Since a daptomycin disk diffusion test has not been developed, daptomycin susceptibility has to be evaluated by reference methods (2, 3) or the Etest method. In the present study, we evaluated the correlation between vancomycin and daptomycin MICs obtained by the Etest technique and the CLSI reference broth microdilution (BMD) method.(This study was presented in part at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy-46th Infectious Diseases Society of America meeting, Washington, DC, 25 to 28 October 2008.)A total of 1,800 MRSA strains from nine hospitals, one in each of the U.S. census regions, were tested. Each medical center contributed 200 MRSA strains collected from patients with bloodstream infections from 2002 through 2006 (target, 40 strains per year per center). The participant centers were as follows: Tufts Medical Center, Boston, MA; New York Hospital Queens, New York, NY; Ochsner Clinic Foundation, New Orleans, LA; University of Colorado Denver, Aurora, CO; University of Nebraska Medical Center, Omaha, NE; University of Washington, Seattle, WA; University of Alabama at Birmingham, Birmingham, AL; Wayne State University, Detroit, MI; and the Medical University of South Carolina, Charleston, SC.MICs of daptomycin and vancomycin were determined by the reference BMD method (using frozen-form panels), with the appropriate medium variation (the addition of 50 mg/liter of calcium) for testing daptomycin (2). Thirty-six precise incremental dilutions of vancomycin between 64 and 0.06 μg/ml (64, 32, 16, 8, 4, 3, 2.5, 2, 1.875, 1.75, 1.625, 1.5, 1.375, 1.25, 1.125, 1, 0.938, 0.875, 0.813, 0.75, 0.688, 0.625, 0.563, 0.5, 0.469, 0.438, 0.406, 0.375, 0.344, 0.313, 0.281, 0.25, 0.188, 0.12, 0.094, and 0.06 μg/ml) were tested, and 20 dilutions of daptomycin between 16 and 0.06 μg/ml (16, 8, 4, 2, 1.5, 1, 0.875, 0.75, 0.625, 0.5, 0.438, 0.375, 0.313, 0.25, 0.219, 0.188, 0.156, 0.12, 0.094, and 0.06 μg/ml) were tested. Isolates were also evaluated by Etest for susceptibilities to daptomycin and vancomycin according to the recommendations of the Etest manufacturer (AB Biodisk). To compare the results obtained with the BMD method and those obtained with the Etest technique, BMD results were rounded up to the next dilution provided for the Etest method. Resistance to oxacillin was confirmed by the reference BMD method (dilution range tested, 0.5 to 4 μg/ml) and cefoxitin susceptibilities tested by disk diffusion (2, 3). Quality control strains S. aureus ATCC 25923 and Enterococcus faecalis ATCC 29212 were evaluated concurrently with every set of tests.MICs of vancomycin obtained by the Etest method were consistently higher (+0.5 to 1.5 log₂ dilutions) than those obtained by the BMD method (Fig. ​(Fig.1a).1a). For strains from all centers combined, the overall vancomycin MIC mode determined by the BMD method was 0.75 μg/ml (corresponding to 75.3% of values, including those rounded up from measured 0.563-, 0.625-, and 0.688-μg/ml MICs) and 96.9% of strains exhibited vancomycin MIC results of ≤1 μg/ml. In contrast, when tested by the Etest, MICs for 58.3 and 32.1% of MRSA strains were 1.5 and 2 μg/ml, respectively (Fig. ​(Fig.1a1a).Open in a separate windowFIG. 1.Scattergrams showing the correlations between vancomycin (a) and daptomycin (b) MICs obtained by the BMD method and the Etest. Solid lines indicate CLSI susceptibility breakpoints, and broken lines indicate complete agreement between results from the two methods.For all strains except one, the Etest yielded higher vancomycin MICs than the reference method. Among 1,628 strains for which the Etest MIC was 1.5 μg/ml (1,050 strains) or 2 μg/ml (578 strains), only 44 (2.7%) had BMD MIC results of >1 μg/ml. Furthermore, among 13 strains considered to be nonsusceptible (intermediate) to vancomycin (MIC, 3 or 4 μg/ml) according to the Etest results, only 1 had a vancomycin MIC result of >2 μg/ml by the reference method, demonstrating a positive predictive value of only 7.7%.Etest MICs of daptomycin were also slightly elevated (+0.5 to 1 log₂ dilution) compared to BMD MICs (Fig. ​(Fig.1b).1b). When tested by the BMD method, the daptomycin MIC mode was 0.19 μg/ml (corresponding to 55.4% of values, including MICs of 0.156 and 0.188 μg/ml) and 92.9% of strains exhibited daptomycin MIC results of ≤0.25 μg/ml. In contrast, when tested by the Etest, the MIC mode was 0.38 μg/ml (corresponding to 37.2% of values), which is 1 doubling dilution higher than that obtained by the BMD method. Among nine strains considered to be daptomycin nonsusceptible according to the Etest results, only two had a MIC result of >1 μg/ml when tested by the reference BMD method, demonstrating a positive predictive value of 22.2% (Fig. ​(Fig.1b1b).Although the emergence of vancomycin-intermediate S. aureus strains and that of vancomycin-resistant S. aureus strains are reasons for concern, these organisms still are extremely rare (12). Nevertheless, a number of studies have demonstrated increased clinical failure with MRSA isolates for which vancomycin MICs are increased (>1 μg/ml) but still within the CLSI-defined susceptibility range (≤2 μg/ml) (3, 11, 13). In addition, there are reports of the increase of vancomycin MICs over time (designated “MIC creep”) at individual institutions, although this trend has not been validated by large, multi-institutional studies (14). Thus, it is becoming clear to physicians that an “S” result for vancomycin is not sufficient to appropriately guide therapy and that the clinical laboratory should provide accurate and reliable MICs.Since most clinical laboratories use automated systems to perform susceptibility testing and these systems do not provide a precise vancomycin MIC, many laboratories are using alternative methods for testing vancomycin in selected cases (6). The Etest method is an attractive option for alternative vancomycin testing since it is easy to perform and cost-effective for testing only one drug-bug combination. However, the results of the present study clearly show that the Etest provides vancomycin and daptomycin MIC results consistently higher (by 0.5 to 1.5 log₂ dilutions) than those provided by precisely performed reference BMD tests. Furthermore, the fact that the higher vancomycin MIC results provided by the Etest appear to be more reliable in predicting vancomycin treatment responses (6) indicates that the vancomycin susceptibility breakpoint should be reevaluated.The susceptibility testing method used may have a large impact on physician decisions for vancomycin and daptomycin treatment of MRSA infections. According to the information presented here, the dominant MRSA population (91.2% of strains) would be categorized as vancomycin nonsusceptible by the Etest method if the susceptibility breakpoint was adjusted to ≤1 μg/ml, as suggested by some published clinical outcome studies on recognizing isolates for which vancomycin MICs are 2 μg/ml (4, 5, 8-10, 14). Daptomycin MICs were also affected by the method used, but to a lesser degree (MICs were found to be 0.5 to 1 dilution step higher by the Etest method than by the reference method, with 0.4% false-nonsusceptible results), and may also suffer from the perception of declining daptomycin potency.     

Antimicrobial susceptibility testing of Helicobacter pylori comparison of E-test,broth microdilution,and disk diffusion for ampicillin,clarithromycin, and metronidazole

The optimal method for the determination of the minimum inhibitory concentration (MIC) of antimicrobials against Helicobacter pylori has not been established. The epsilometer agar diffusion gradient test (E-Test; AB Biodisk, Solna, Sweden) was compared with broth microdilution, the reference method, and disk diffusion for the antimicrobial susceptibility testing of 122 clinical isolates of H. pylori to ampicillin, clarithromycin, and metronidazole. Isolates were considered to be resistant when the MIC value was >8 μg/ml for either ampicillin or metronidazole and >2 μg/ml for clarithromycin. For an individual isolate, the MICs for ampicillin and clarithromycin determined by broth microdilution and the E-test were highly reproducible, with replicate results being within ±1 log₂ dilution. The correlation between the MICs determined by E-test and broth microdilution was excellent for both ampicillin and clarithromycin (90.1% and 88.5% were within ±log₂ dilution, and 98.3% and 96.7% of the values were within ±2 log₂ dilution, respectively). In no instance did the interpretation of “sensitive” or “resistant” differ. Conversely, only 70.5% of the E-test results for metronidazole were within ±1 log₂ dilution of the broth microdilution results. In addition, 15 (12.3%) of the H. pylori isolates interpreted as resistant by the E-test were sensitive by the broth microdilution method. All discrepancies occurred when the E-test MIC values fell between 8 and 32 μg/ml. The results of the ampicillin and clarithromycin disk diffusion assay correlated 100% with the results of the broth microdilution. However, these data suggest that when the E-test MIC results for metronidazale yield values between 8 and 32 μg/ml, the MIC should be reevaluated by another method.     

Comparative Evaluation of Colistin Susceptibility Testing Methods among Carbapenem-Nonsusceptible Klebsiella pneumoniae and Acinetobacter baumannii Clinical Isolates

We compared six colistin susceptibility testing (ST) methods on 61 carbapenem-nonsusceptible Klebsiella pneumoniae (n = 41) and Acinetobacter baumannii (n = 20) clinical isolates with provisionally elevated colistin MICs by routine ST. Colistin MICs were determined by broth microdilution (BMD), BMD with 0.002% polysorbate 80 (P80) (BMD-P80), agar dilution (AD), Etest, Vitek2, and MIC test strip (MTS). BMD was used as the reference method for comparison. The EUCAST-recommended susceptible and resistant breakpoints of ≤2 and >2 μg/ml, respectively, were applied for both K. pneumoniae and A. baumannii. The proportions of colistin-resistant strains were 95.1, 77, 96.7, 57.4, 65.6, and 98.4% by BMD, BMD-P80, AD, Etest, MTS, and Vitek2, respectively. The Etest and MTS methods produced excessive rates of very major errors (VMEs) (39.3 and 31.1%, respectively), while BMD-P80 produced 18% VMEs, AD produced 3.3% VMEs, and Vitek2 produced no VMEs. Major errors (MEs) were rather limited by all tested methods. These data show that gradient diffusion methods may lead to inappropriate colistin therapy. Clinical laboratories should consider the use of automated systems, such as Vitek2, or dilution methods for colistin ST.     

Development of gemifloxacin in vitro susceptibility test methods for gonococci including quality control guidelines. The Quality Control Study Group

Gemifloxacin (formerly SB-265805 or LB20304a) is a new fluoronapthyridone with documented activity against Gram-positive and -negative organisms. The activity of gemifloxacin was tested against 150 Neisseria gonorrhoeae strains, using reference agar dilution, standardized disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods. Gemifloxacin was very potent against ciprofloxacin (CIPRO)-susceptible strains (MIC(90,) 0.008 microg/ml) but was significantly less active against the CIPRO-resistant gonococci (MIC(90,) 0.12 microg/ml). Etest and reference agar dilution MIC results showed excellent correlation (r = 0.96), and 98.7% MICs were within +/- one log(2) dilution. Agar dilution MICs were also compared to zone diameters obtained using gemifloxacin 5-microg disks; and complete intermethod categorical agreement (100%) was achieved applying breakpoints proposed as follows: < or =0.25 microg/ml (zone, > or =25 mm) for susceptible and > or =1 microg/ml (zone, < or =21 mm) for resistant. Gemifloxacin MIC and disk diffusion te quality control (QC) ranges were established for N. gonorrhoeae ATCC 49226. Data were collected from > or = seven laboratories, three GC agar medium lots for both agar MICs and disk methods, and two lots each of the 5- and 10-microg disks. The proposed MIC QC range was 0.002 to 0.016 microg/ml and the calculated mm zone ranges (median +/- 0.5x average mm range) for both disks were similar, but contained only 88.1 to 91.9% of participant results. To achieve the acceptable > or = 95% of all study results within range, a 43 to 54 mm limits (5-microg disks) were necessary. The excellent broad-spectrum activity and a low reported adverse effects profile of gemifloxacin shows a potential for treatment of fluoroquinolone-resistant gonorrhea.     

Antimicrobial activity of gatifloxacin tested against Neisseria gonorrhoeae using three methods and a collection of fluoroquinolone-resistant strains

Gatifloxacin, a new 8-methoxy fluoroquinolone, was tested against 131 strains of Neisseria gonorrhoeae by reference agar dilution, disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods on supplemented GC agar. Gatifloxacin activity was equal to ciprofloxacin (MIC₅₀, 0.008 μg/mL) against strains fully susceptible to fluoroquinolones, but was generally four-fold more active (MIC₉₀, 0.064–0.094 μg/mL) against strains with par C or gyr A mutations and resistance to ciprofloxacin. Etest results were comparable to those generated by the agar dilution test [correlation coefficient (r) = 0.97]. Gatifloxacin zone diameters using 5-μg disks also correlated well (r = 0.86–0.87) with the agar dilution and Etest MIC results. Breakpoints for laboratory testing of Neisseria gonorrhoeae strains await clinical trial outcome correlations, but susceptibility at ≤0.125 or ≤0.25 μg/mL (≥34 mm) seems appropriate. All three tests used in this study seem applicable for laboratory testing of isolates from patients with uncomplicated gonorrhoeae receiving therapy with gatifloxacin.     

Comparison of NCCLS microdilution method and Etest in antifungal susceptibility testing of clinical Trichosporon asahii isolates

We investigated the in vitro activity of amphotericin B, fluconazole, and itraconazole against clinical Trichosporon asahii isolates (n = 43) by NCCLS M27A reference microdilution method and explored the correlation between Etest and NCCLS reference method. Microdilution MIC ranges following 48 h of incubation were 1-8, 0.25-16, and 0.06-4 microg/ml for amphotericin B, fluconazole, and itraconazole, respectively. The corresponding Etest MIC ranges were determined as 0.125- > 8, 0.25- > 64, and 0.03-8 microg/ml. Of interest, Etest tended to produce lower amphotericin B MICs and widen the MIC range compared to microdilution. The influence of Etest on fluconazole and itraconazole MICs was in contrary with that observed for amphotericin B. Etest MICs of fluconazole and itraconazole tended to be higher than microdilution MICs. The wider range of amphotericin B MICs obtained by using Etest methodology may facilitate discrimination of isolates with reduced susceptibility to amphotericin B. However, clinical significance of these findings remain yet unknown and determination of MIC breakpoint values is required.     

In vitro activities of daptomycin, vancomycin, and penicillin against Clostridium difficile, C. perfringens, Finegoldia magna, and Propionibacterium acnes

Daptomycin has in vitro activity against gram-positive anaerobic bacteria, although limited numbers of species have been tested. We studied the in vitro activities of daptomycin, vancomycin, and penicillin against more than 100 strains each of Clostridium difficile, C. perfringens, Finegoldia magna, and Propionibacterium acnes. Daptomycin Etest MICs and results from time-kill studies were determined for selected strains. For 392 of 421 strains (93%), daptomycin was inhibitory at < or =1 microg/ml, including 15 of 16 strains of C. difficile with elevated linezolid MICs of 8 and 16 microg/ml, all 32 strains with moxifloxacin MICs of > or =4 microg/ml, and all 16 strains resistant to clindamycin. Daptomycin MICs were also < or =1 microg/ml for all 16 F. magna strains resistant to clindamycin and all 32 strains resistant to tetracycline. Only one strain, a C. perfringens strain, had a MIC of >2 microg/ml to daptomycin. Eighty-five and 92.5% of the Etest MICs were within 1 dilution of the agar dilution method for all drugs at 24 and 48 h, respectively. In time-kill studies, a C. difficile strain was inhibited by both daptomycin and vancomycin at 1, 2, 4, 8, and 24 h; colony counts were decreased by 2.3 to 2.9 log at 24 h. Vancomycin was not bactericidal for C. perfringens; however, daptomycin showed bactericidal activity as early as 1 h at four and eight times the MIC and at 2 and 4 h at two and four times the MIC.     

Molecular epidemiology of β-lactamase production in penicillin-susceptible Staphylococcus aureus under high-susceptibility conditions

Little is known about the prevalence of β-lactamase production in penicillin-susceptible Staphylococcus aureus isolates under high-susceptibility conditions. We analyzed S. aureus isolates with penicillin G minimum inhibitory concentration (MIC) ≤ 0.12 μg/ml that were recovered from in-/outpatients (n = 108) between 2016 and 2017 in Japan. β-Lactamase production was detected by nitrocefin-based and Clinical and Laboratory Standards Institute penicillin zone edge testing and blaZ PCR. All isolates were categorized as having penicillin G MIC ≤0.03 μg/ml using an automated system; MICs determined based on the microdilution method were 0.016 μg/ml (2%), 0.03 μg/ml (44%), and 0.06 μg/ml (54%). Notably, no isolates harbored the blaZ gene. The results from the nitrocefin-based and zone edge tests were consistent with those obtained by PCR. S. aureus isolates with penicillin G MIC ≤0.03 μg/ml exhibited a low frequency of β-lactamase production. Thus, screening for β-lactamase production may be unnecessary for isolates showing such high susceptibility.     

Correlation between mutations in liaFSR of Enterococcus faecium and MIC of daptomycin: revisiting daptomycin breakpoints

Mutations in liaFSR, a three-component regulatory system controlling cell-envelope stress response, were recently linked with the emergence of daptomycin (DAP) resistance in enterococci. Our previous work showed that a liaF mutation increased the DAP MIC of a vancomycin-resistant Enterococcus faecalis strain from 1 to 3 μg/ml (the DAP breakpoint is 4 μg/ml), suggesting that mutations in the liaFSR system could be a pivotal initial event in the development of DAP resistance. With the hypothesis that clinical enterococcal isolates with DAP MICs between 3 and 4 μg/ml might harbor mutations in liaFSR, we studied 38 Enterococcus faecium bloodstream isolates, of which 8 had DAP MICs between 3 and 4 μg/ml by Etest in Mueller-Hinton agar. Interestingly, 6 of these 8 isolates had predicted amino acid changes in the LiaFSR system. Moreover, we previously showed that among 6 DAP-resistant E. faecium isolates (MICs of >4 μg/ml), 5 had mutations in liaFSR. In contrast, none of 16 E. faecium isolates with a DAP MIC of ≤2 μg/ml harbored mutations in this system (P < 0.0001). All but one isolate with liaFSR changes exhibited DAP MICs of ≥16 μg/ml by Etest using brain heart infusion agar (BHIA), a medium that better supports enterococcal growth. Our findings provide a strong association between DAP MICs within the upper susceptibility range and mutations in the liaFSR system. Concomitant susceptibility testing on BHIA may be useful for identifying these E. faecium first-step mutants. Our results also suggest that the current DAP breakpoint for E. faecium may need to be reevaluated.     

Antimicrobial activity of daptomycin tested against Staphylococcus aureus with vancomycin MIC of 2 μg/mL isolated in the United States and European hospitals (2006–2008)

We evaluated the activity of daptomycin (minimum inhibitory [MIC] and bactericidal [MBC] concentration) against Staphylococcus aureus strains with elevated (2 μg/mL) vancomycin MIC values. A total of 410 contemporary clinical S. aureus isolates (282 from the United States and 128 from Europe) with vancomycin MIC values of 2 μg/mL were tested by reference broth microdilution method. Vancomycin MBC and the presence of vancomycin-heteroresistant population (heterogeneous vancomycin-intermediate S. aureus [hVISA]) were evaluated in 31 randomly selected strains. Overall, 97.3% of isolates were susceptible to daptomycin (MIC₉₀, 0.5 μg/mL). Daptomycin exhibited potent bactericidal activity with MBC values at the MIC concentration (74.2%) or 1 log₂ dilution above the MIC (25.8%). In contrast, vancomycin MBC was ≥32 μg/mL in 12.9% of strains tolerance, and 25.8% of strains tested positive for hVISA (AB BIODISK GRD Etest, Solna, Sweden). In conclusion, S. aureus strains with vancomycin MIC of 2 μg/mL showed high rates of hVISA and vancomycin tolerance. Daptomycin retained potent bactericidal activity against S. aureus with decreased susceptibility to vancomycin.     

Comparative antimicrobial activity of gatifloxacin tested against Campylobacter jejuni including fluoroquinolone-resistant clinical isolates

Campylobacter jejuni is an important pathogen that causes gastroenteritis, as well as other disease states such as meningitis and septic arthritis. In this study, the Etest (AB BIODISK, Solna, Sweden) results were compared to a reference agar dilution method using gatifloxacin, a new 8-methoxyfluoroquinolone. A total of 53 strains of C. jejuni initially isolated from patients in California and Mexico were tested. Results demonstrated a high correlation (r = 0.88) between the two utilized in vitro dilution methods. In addition, gatifloxacin activity was compared to that of ciprofloxacin, metronidazole, amoxicillin, erythromycin, chloramphenicol, gentamicin, tetracycline, and trimethoprim/sulfamethoxazole using the Etest. Gatifloxacin (MIC₉₀, 4μg/ml) was approximately eight- to 16-fold more potent than ciprofloxacin (Mic₉₀, > 32 μg/ml), a commonly used fluoroquinolone for Campylobacter infections. Eight strains highly resistant to ciprofloxacin (MIC₉₀, > 32 μg/ml) were tested for cross resistance against the newer fluoroquinolones (gatifloxacin, levofloxacin, trovafloxacin) and the rank order of potency was: gatifloxacin (MIC₅₀, 16 μg/ml) > trovafloxacin = levofloxacin (MIC₅₀, > 32 μg/mL). However, only 25% ciprofloxacin-resistant strains were inhibited by ≤ 1 μg/mL of gatifloxacin or trovafloxacin. These results for gatifloxacin against C. jejuni strains must be further assessed in the context of in vivo trials before the clinical role of this new fluoroquinolone can be determined. The Etest appears to be a simple and precise susceptibility test method for testing C. jejuni isolates against fluoroquinolones and other alternative therapeutic agents.     

In Vitro Selection of Resistance to Four β-Lactams and Azithromycin in Streptococcus pneumoniae

Selection of resistance to amoxicillin (with or without clavulanate), cefaclor, cefuroxime, and azithromycin among six penicillin G- and azithromycin-susceptible pneumococcal strains and among four strains with intermediate penicillin sensitivities (azithromycin MICs, 0.125 to 4 μg/ml) was studied by performing 50 sequential subcultures in medium with sub-MICs of these antimicrobial agents. For only one of the six penicillin-susceptible strains did subculturing in medium with amoxicillin (with or without clavulanate) lead to an increased MIC, with the MIC rising from 0.008 to 0.125 μg/ml. Five of the six penicillin-susceptible strains showed increased azithromycin MICs (0.5 to >256.0 μg/ml) after 17 to 45 subcultures. Subculturing in medium with cefaclor did not affect the cefaclor MICs of three strains but and led to increased cefaclor MICs (from 0.5 to 2.0 to 4.0 μg/ml) for three of the six strains, with MICs of other β-lactams rising 1 to 3 twofold dilutions. Subculturing in cefuroxime led to increased cefuroxime MICs (from 0.03 to 0.06 μg/ml to 0.125 to 0.5 μg/ml) for all six strains without significantly altering the MICs of other β-lactams, except for one strain, which developed an increased cefaclor MIC. Subculturing in azithromycin did not affect β-lactam MICs. Subculturing of the four strains with decreased penicillin susceptibility in amoxicillin (with or without clavulanate) or cefuroxime did not select for β-lactam resistance. Subculturing of one strain in cefaclor led to an increase in MIC from 0.5 to 2.0 μg/ml after 19 passages. In contrast to strains that were initially azithromycin susceptible, which required >10 subcultures for resistance selection, three of four strains with azithromycin MICs of 0.125 to 4.0 μg/ml showed increased MICs after 7 to 13 passages, with the MICs increasing to 16 to 32 μg/ml. All azithromycin-resistant strains were clarithromycin resistant. With the exception of strains that contained mefE at the onset, no strains that developed resistance to azithromycin contained ermB or mefE, genes that have been found in macrolide-resistant pneumococci obtained from clinic patients.     

Tentative criteria for determining the in vitro susceptibilities of Haemophilus influenzae,including quality control parameters,to two fluoroquinolones (grepafloxacin and PD 131628)

Grepafloxacin (OPC 17116) and PD 131628 were evaluated against 150 Haemophilus influenzae isolates to propose susceptibility testing criteria for both broth dilution and disk diffusion procedures using haemophilus test medium. Grepafloxacin-susceptible isolates are defined as those for which minimum inhibitory concentrations (MICs) are ⩽0.06 μg/ml and zones of inhibition are ⩾27 mm. PD 131628-susceptible strains of H. influenzae included those that exhibited MICs ⩽0.03 μg/ml, and the zones were ⩾32 mm. All MICs were well below concentrations that can be achieved in the blood and tissues of patients treated with either study drug. Criteria for defining a resistant category cannot be determined until strains with elevated MICs are available for study. The proposed quality control guidelines for H. influenzae ATCC 49247 and the disk test are 32–39 mm in diameter for grepafloxacin and 34–42 mm for PD 131628. The preliminary broth microdilution MIC control limits for this strain are 0.002–0.016 μg/ml for grepafloxacin and 0.002–0.008 μg/ml for PD 131628.