New Colorimetric Microdilution Method for In Vitro Susceptibility Testing of Borrelia burgdorferi Against Antimicrobial Substances

 A newly developed colorimetric microdilution method was used to analyze the activity of 12 antimicrobial agents against nine Borrelia burgdorferi isolates, including all three genospecies pathogenic for humans. In addition, in vitro antimicrobial resistance patterns of Borrelia valaisiana and Borrelia bissettii tick isolates were investigated. The applied test system is based upon color changes that occur in the presence of phenol red and result from the accumulation of nonvolatile acid produced by actively metabolizing spirochetes. After 72 h of incubation, minimal inhibitory concentrations (MICs) were determined from the decrease of absorbance by software-assisted calculation of growth curves. MIC values were lowest for azlocillin (MIC, ≤0.125 μg/ml), ceftriaxone (MIC range, ≤0.015–0.06 μg/ml), and azithromycin (MIC range, ≤0.015–0.06 μg/ml). Whereas tobramycin (MIC range, 8–64 μg/ml) exhibited little activity, spectinomycin (MIC range, 0.25–2 μg/ml) showed in vitro antimicrobial activity against Borrelia burgdorferi. The MICs of penicillin G for Borrelia afzelii isolates were ten times higher than those for Borrelia burgdorferi, Borrelia valaisiana, and Borrelia bissettii isolates (P<0.05) and 100 times higher than those for isolates belonging to the genospecies Borrelia garinii (P<0.05). Further significant differences with respect to the MIC values of the other antimicrobial agents tested were not noted. The colorimetric microdilution method offered the advantages of reliability, reproducibility, and convenience and could handle large numbers of isolates and antibiotics.     

Comparative Activity of Eighteen Antimicrobial Agents against Anaerobic Bacteria Isolated in South Africa

 The in vitro activity of 18 antimicrobial agents was determined against 378 anaerobic bacteria isolated in Bloemfontein, South Africa, during 1996/97. Against the gram-positive isolates, MICs of penicillin and cefoxitin were >0.5 μg/ml and >16 μg/ml, respectively, for five and three strains of non-perfringens Clostridium spp. Seventeen Peptostreptococcus anaerobius strains were resistant to penicillin (MIC≥2 μg/ml). All gram-positive anaerobes tested except one Peptostreptococcus sp. and one Clostridium sp. were susceptible to dalfopristin-quinupristin (MICs≤1 μg/ml). The carbapenems exhibited excellent activity against the gram-positive isolates and were effective against most gram-negative anaerobes, with the exception of the fusobacteria. Only seven strains exhibited decreased susceptibility to trovafloxacin (MICs>2 μg/ml). In mixed anaerobic/aerobic infections, carbapenems and the fourth-generation quinolone trovafloxacin were the agents most suitable for us as broad-spectrum monotherapy.     

Nationwide surveillance of antimicrobial resistance among Enterobacteriaceae in intensive care units in Taiwan

To determine the antimicrobial resistance profiles among clinical isolates of Enterobacteriaceae in Taiwanese intensive care units (ICUs), a national surveillance of antibiotic resistance among important Enterobacteriaceae was conducted from September 2005 through November 2005 at the ICUs of ten major teaching hospitals in Taiwan. A total of 574 Enterobacteriaceae isolates recovered from various clinical samples of our ICU patients were submitted for in vitro test. Minimum inhibitory concentrations (MICs) of these isolates to 18 antimicrobial agents were determined by the broth microdilution method. The prevalences of Enterobacteriaceae isolates with phenotypic extended-spectrum β-lactamase (ESBL) production were 26% in Klebsiella pneumoniae, 16% in Serratia marcescens, 14% in Escherichia coli, and 13% in Proteus mirabilis, in which a significantly rising prevalence of ESBL production among K. pneumoniae was noted (p = 0.002) when compared with a previous Taiwanese survey in 2000. Heterogeneous resistance to various fluoroquinolones was found among our Enterobacteriaceae isolates, except for Entetrobacter cloacae. Emergence of ertapenem-resistant isolates of E. coli, K. pneumoniae, E. cloacae, and S. marcescens was noted. Gradually increasing rates of drug-resistant Enterobacteriaceae were noted in Taiwanese ICUs. Periodic surveillance of the evolutionary trend of antimicrobial resistance among ICU isolates is crucial for starting appropriately empirical antimicrobial therapy in the future.     

In Vitro Activity of Gemifloxacin (SB-265805) Compared to Eleven Other Antimicrobial Agents Against Streptococcal Isolates, Excluding Streptococcus pneumoniae

 The purpose of the study presented here was to determine the in vitro activity of gemifloxacin compared with that of 11 other antimicrobial agents (5 of them quinolones) against 400 isolates of β-haemolytic and viridans group streptococci. The minimum inhibitory concentration values for gemifloxacin against 90% of the streptococci tested were as follows: Lancefield groups A, C and G, 0.06 μg/ml; Lancefield group B, Streptococcus mitis, Streptococcus mutans and Streptococcus bovis, 0.125 μg/ml; and Streptococcus milleri, 0.03 μg/ml. Resistance to penicillin, ampicillin and erythromycin was found mainly in the Streptococcus mitis isolates; tetracycline showed variable results, and no vancomycin resistance was encountered. Higher rates of ciprofloxacin resistance were identified in the Streptococcus bovis, mitis and mutans isolates. In conclusion, gemifloxacin was the most active quinolone tested followed by trovafloxacin, sparfloxacin, grepafloxacin, ciprofloxacin and levofloxacin, especially against isolates resistant to β-lactam agents, macrolides and tetracycline.     

Comparative in Vitro Activity of Voriconazole and Itraconazole Against Fluconazole-Susceptible and Fluconazole-Resistant Clinical Isolates of Candida Species from Spain

 The in vitro activity of voriconazole was compared with that of itraconazole against 299 fluconazole-susceptible (MIC≤8 μg/ml) and 130 fluconazole-resistant (MIC≥16 μg/ml) clinical isolates of Candida spp. An adaptation of the National Committee for Clinical Laboratory Standards reference method was employed for determination of MICs. Voriconazole showed more potent activity than either fluconazole and itraconazole, even against some Candida albicans, Candida glabrata, and Candida krusei isolates resistant to fluconazole. However, for fluconazole-resistant isolates, the MICs of itraconazole and voriconazole were proportionally higher than for fluconazole-susceptible isolates. These data may indicate cross-resistance.     

In vitro antimicrobial synergy testing of coagulase-negative staphylococci isolated from prosthetic joint infections using Etest and with a focus on rifampicin and linezolid

In recent years, coagulase-negative staphylococci (CoNS) have been increasingly recognised as causative agents of various infections, especially in immunocompromised patients and related to implanted foreign body materials. CoNS, and especially Staphylococcus epidermidis, transform into a stationary growth phase and produce biofilm when involved in a foreign body infection, making them difficult to eradicate with antimicrobials. Rifampicin has the ability to penetrate biofilm, but resistance may develop rapidly. To reduce the emergence of resistance, rifampicin should be combined with additional antimicrobials, of which several different ones have been proposed, including the relatively new class of antimicrobials, oxazolidinones, represented by linezolid. Thirty-seven CoNS isolates from patients with prosthetic joint infection were investigated by synergy testing using Etest. Nine antimicrobial combinations, based on either rifampicin or linezolid, were tested. For 16 (43%) of the isolates, a synergistic (n = 5), additive (n = 14) and/or antagonistic (n = 11) effect were identified. In conclusion, Etest is an objective and easily performed in vitro method for antimicrobial synergy testing. However, each isolate requires testing for the specific combination considered for treatment.     

Activity of doripenem against extended-spectrum β-lactamase-producing Enterobacteriaceae and Pseudomonas aeruginosa isolates

The in vitro activity of doripenem was evaluated against a recent collection of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae and Pseudomonas aeruginosa isolates (201 ESBL-producing Enterobacteriaceae [153 Escherichia coli and 48 Klebsiella pneumoniae] and 201 P. aeruginosa). Comparator agents included amikacin, tobramycin, ciprofloxacin, cefepime, cefotaxime, ceftazidime piperacillin-tazobactam, imipenem, and meropenem. Both doripenem and meropenem inhibited 100% of the ESBL-producing Enterobacteriaceae at ≤0.5 μg/mL. For these isolates, the MIC₉₀ of doripenem (0.12 μg/mL) was 4-fold lower than that of imipenem (0.5 μg/mL). Against P. aeruginosa, the MIC₉₀ of doripenem and meropenem was 2 μg/mL, 4-fold lower than that of imipenem (8 μg/mL). At an MIC of ≤2 μg/mL, doripenem, meropenem, and imipenem inhibited 90.5%, 89.6%, and 82.1% of P. aeruginosa isolates, respectively. Doripenem maintained activity against imipenem-nonsusceptible isolates of P. aeruginosa; at an MIC of ≤4 μg/mL, it inhibited 15 of the 25 isolates with MICs for imipenem of >4 μg/mL. Doripenem is active against ESBL-producing Enterobacteriaceae and P. aeruginosa isolates. Its activity is similar to that of meropenem and slightly better than that of imipenem. The results of this study suggest that doripenem could be an alternative therapeutic agent for infections caused by these organisms.     

Comparative In Vitro Activity of Gatifloxacin Against Stenotrophomonas maltophilia and Burkholderia Species Isolates Including Evaluation of Disk Diffusion and E Test Methods

 The in vitro activity of gatifloxacin, a new fluoroquinolone, was compared to that of five other fluoroquinolones against 105 Stenotrophomonas maltophilia isolates and 52 Burkholderia spp. isolates. The gatifloxacin MICs were determined using the broth microdilution method and the E test (AB Biodisk, Sweden); these methods were compared for test accuracy, and 5 μg disk zone diameters were compared for interpretive accuracy using the standardized disk diffusion method. In terms of potency, gatifloxacin was most similar to sparfloxacin and trovafloxacin against Stenotrophomonas maltophilia (MIC50, 0.5–1 mg/l) and Burkholderia spp. (MIC50, 1–2 mg/l). This activity was greater than that of ciprofloxacin, levofloxacin or ofloxacin (MIC50, ≥2 mg/l) against Stenotrophomonas maltophilia isolates but comparable to that of levofloxacin against the Burkholderia spp. (60% susceptible at ≤2 mg/l). The E test results compared well with the reference dilution test results (81–97% at ±1 log₂ dilution). The disk diffusion test using previously suggested breakpoints for other bacteria (≥18 mm or ≤2 mg/l for susceptible and ≤14 mm or ≥8 mg/l for resistant) also performed well, at >90% categorical agreement. The activity of gatifloxacin is comparable to that of other newer quinolones against isolates of Stenotrophomonas maltophilia and Burkholderia spp., and susceptibility testing using simple qualitative and quantitative methods appears to function well with these drug/organism combinations.     

Bacteremia caused by <Emphasis Type="Italic">Brevundimonas</Emphasis> species at a tertiary care hospital in Taiwan, 2000–2010

We investigated clinical and microbiological characteristics of 30 patients with Brevundimonas bacteremia treated at a tertiary care hospital in Taiwan during 2000–2010. All the 30 bacteria isolates were confirmed to the species level by 16S rRNA sequencing analysis. Minimum inhibitory concentrations (MICs) of 11 antimicrobial agents against these isolates were determined by the agar dilution method. Seventeen (57%) patients had underlying malignancy, 12 (40%) had undergone central catheter placement, and 13 (43%) had received chemotherapy within the previous three months. Eight (27%) patients had community-acquired bacteremia and the remaining 22 patients (73%) had healthcare-associated bacteremia. The overall 14-day and 30-day mortality rates were 13% and 17%, respectively. Among the 30 isolates, B. vesicularis constituted most commonly (n = 22, 63%), followed by B. nasdae (n = 5) and B. diminuta (n = 3). All isolates were susceptible to piperacillin-tazobactam and amikacin, while all were resistant to ciprofloxacin and colistin. Tigecycline (MICs at which 90% of isolates are inhibited [MIC₉₀] was 0.12 mg/L) and doripenem (MIC₉₀ of 1 mg/L) both possessed good in vitro activities. In conclusions, Brevundimonas should be considered a pathogen that can cause bacteremia in immunocompromised hosts. Piperacillin-tazobactam, amikacin, doripenem, and tigecycline exhibit good in vitro activities against these ciprofloxacin- and colistin-resistant Brevundimonas species.     

Clinical features,antimicrobial susceptibilities,and outcomes of <Emphasis Type="Italic">Elizabethkingia meningoseptica</Emphasis> (<Emphasis Type="Italic">Chryseobacterium meningosepticum</Emphasis>) bacteremia at a medical center in Taiwan, 1999–2006

A total of 118 patients with Elizabethkingia meningoseptica bacteremia at a medical center in Taiwan from 1999 to 2006 were studied. Minimum inhibitory concentrations (MICs) of 99 preserved isolates were determined. The incidence (per 100,000 admissions) of E. meningoseptica bacteremia increased from 7.5 in 1996 to 35.6 in 2006 (p = 0.006). Among them, 84% presented with fever, 86% had nosocomial infections, and 60% had acquired the infection in intensive care units (ICUs). The most common underlying diseases were malignancy (36%) and diabetes mellitus (25%). Seventy-eight percent of patients had primary bacteremia, followed by pneumonia (9%), soft tissue infection, and catheter-related bacteremia (6%). Forty-five patients (38%) had polymicrobial bacteremia. Overall, the 14-day mortality was 23.4%. Multivariate analysis revealed E. meningoseptica bacteremia acquired in an ICU (p = 0.048, odds ratio [OR] 4.23) and presence of effective antibiotic treatment after the availability of culture results (p = 0.049, OR 0.31) were independent predictors of 14-day mortality. The 14-day mortality was higher among patients receiving carbapenems (p = 0.046) than fluoroquinolones or other antimicrobial agents. More than 80% of the isolates tested were susceptible to trimethoprim-sulfamethoxzole, moxifloxacin, and levofloxacin. The MIC₅₀ and MIC₉₀ of the isolates to tigecycline and doxycycline were both 4 μg/mL and 8 μg/ml, respectively.     

A simplified method for testing Bordetella pertussis for resistance to erythromycin and other antimicrobial agents

Present methods of antimicrobial susceptibility testing of Bordetella pertussis are time consuming and require specialized media that are not commercially available. We tested 52 isolates of B. pertussis for resistance to erythromycin, trimethoprim-sulfamethoxazole, chloramphenicol, and rifampin by agar dilution with Bordet-Gengou agar (BGA) containing 20% horse blood (reference method), Etest using BGA and Regan-Lowe agar without cephalexin (RL-C), and disk diffusion using BGA and RL-C. The organisms tested included four erythromycin-resistant isolates of B. pertussis from a single patient, a second erythromycin-resistant strain of B. pertussis from an unrelated patient in another state, and 47 nasopharyngeal surveillance isolates of B. pertussis from children in the western United States. The results of agar dilution testing using direct inoculation of the organisms suspended in Mueller-Hinton broth were within +/-1 dilution of those obtained after overnight passage of the inoculum in Stainer-Scholte medium, which is the traditional method of testing B. pertussis. The Etest method produced MICs similar to those of the agar dilution reference method for three of the four antimicrobial agents tested; the trimethoprim-sulfamethoxazole results were lower with Etest, particularly when the direct suspension method was used. Most of the Etest MICs, except for that of erythromycin, were on scale. Disk diffusion testing using RL-C medium was helpful in identifying the erythromycin-resistant strains, which produced no zone of inhibition around the disk; susceptible isolates produced zones of at least 42 mm. Thus, the antimicrobial susceptibility testing of B. pertussis can be simplified by using the Etest or disk diffusion on RL-C to screen for erythromycin-resistant isolates of B. pertussis.     

In Vitro Activity of the New Echinocandin Antifungal, MK-0991, against Common and Uncommon Clinical Isolates of Candida Species

 A broth macrodilution method, performed as recommended by the National Committee for Clinical Laboratory Standards, was used for comparative testing of the new echinocandin antifungal agent MK-0991 and fluconazole against 50 yeast isolates belonging to 12 species of Candida. MK-0991 was shown to be highly effective against both fluconazole-susceptible and -resistant Candida spp., yielding minimum inhibitory concentrations ranging from ≤0.19 to 0.78 μg/ml. Fungicidal activity was exerted at ≤1.5 μg/ml for 73% of the isolates tested. This study suggests that MK-0991 has significant potential for clinical development.     

Use of the Polymerase Chain Reaction for Rapid Detection of High-Level Mupirocin Resistance in Staphylococci

 The minimum inhibitory concentrations (MICs) of mupirocin were determined by the E test (AB Biodisk, Sweden) and the agar dilution method for 107 staphylococci. The organisms consisted of 34 coagulase-negative staphylococci and 73 methicillin-resistant Staphylococcus aureus. Polymerase chain reaction (PCR) primers designed to amplify a 456 bp region of the plasmid-borne isoleucyl tRNA synthetase gene (ileS–2), responsible for high-level mupirocin resistance in staphylococci, were used on DNA preparations from these isolates. Isolates with high-level mupirocin resistance due to the ileS–2 gene should be PCR positive. There was close correlation between the E test and agar dilution MIC values, with only two strains differing by more than two serial dilutions. Most (51 of 54 strains) of the high-level resistant strains (MIC>256 μg/ml) were resistant to the highest concentration of mupirocin tested (1024 μg/ml). PCR correctly classified all but four (96%) of the isolates in accordance with the results of agar dilution. All four isolates that gave discrepant results were methicillin-resistant Staphylococcus aureus. Two of these were PCR positive, yet the MIC of mupirocin for these strains was <0.06 μg/ml; on prolonged incubation they produced halos within the inhibition zone on agar diffusion testing, suggesting that the phenotypic results may have been erroneous. One of 54 isolates for which the MIC exceeded 256 μg/ml was PCR negative when tested by the original methodology, but a 456 bp product was produced when retested using a lowered annealing temperature. One isolate for which the MIC of mupirocin was 16 μg/ml by agar dilution and 8 μg/ml by the E test was positive by PCR. PCR of the ileS–2 gene is a useful, rapid method for detecting high-level mupirocin resistance in staphylococci.     

Increased Adherence of Fluconazole-Resistant Isolates of Candida Species to Explanted Esophageal Mucosa

 The adherence of fluconazole-resistant and fluconazole-susceptible isolates of Candida albicans to explanted rabbit esophageal mucosa was examined in vivo. Among six Candida albicans isolates collected from HIV-infected patients, three fluconazole-resistant (MIC>64 μg/ml) isolates attached more avidly than three fluconazole-susceptible strains (MIC≤0.5 μg/ml) to esophageal mucosa (P≤0.05). When three strains each of six different Candida spp. were compared, the more inherently fluconazole-resistant isolates adhered more avidly in the following order: Candida glabrata>Candida krusei>Candida albicans fluconazole-sensitive >Candida tropicalis>Candida parapsilosis. Nonetheless, fluconazole-resistant Candida albicans demonstrated the greatest degree of adherence in comparison to all fluconazole-susceptible Candida albicans (P<0.001) and to all Candida spp. tested (P<0.001). Thus, the refractoriness of esophageal candidiasis in patients infected with fluconazole-resistant isolates may be related to both in vitro drug resistance and increased mucosal adherence.     

Carriage of Antibiotic-Resistant Streptococcus pneumoniae in Greek Infants and Toddlers

The prevalence, resistance patterns and serotypes of antibiotic-resistant Streptococcus pneumoniae strains recovered from Greek carriers under 24 months of age were studied. From February 1997 to April 1998, nasopharyngeal cultures were performed in 1269 children (ages 2–23 months, median 11 months) living in various areas of central and southern Greece. Resistance (including both intermediate and resistant isolates) to one or more antimicrobial agents was found in 132 of the 421 (31%) Streptococcus pneumoniae isolates, as follows: penicillin, 9% intermediate, 7.6% resistant; cefotaxime, 5.2% intermediate, 0.5% resistant; erythromycin, 0.7% intermediate, 18.1% resistant; clindamycin, 0.2% intermediate, 12.4% resistant; tetracycline, 0.7% intermediate, 16.4% resistant; chloramphenicol, 12.4% resistant; and trimethoprim-sulfamethoxazole, 3.8% intermediate, 14.3% resistant. The MICs of penicillin for 66% of the penicillin-nonsusceptible pneumococci were 1–4 μg/ml. Multidrug resistance was found in 64% of penicillin-nonsusceptible and 37% of penicillin-susceptible strains. Sixty-two percent of the penicillin-susceptible, multidrug-resistant strains belonged to serotype 6B and were resistant to all five non-β-lactam agents tested. This notable serotype 6B resistance pattern was described for the first time in a previous study performed from December 1995 to February 1996 in the city of Patras, southwestern Greece. Seventy-two percent of antibiotic-resistant isolates belonged to serotypes 6B, 9V, 14, 18C, 19F and 23F. These results document the spread of resistant pneumococcal strains in central and southern Greece, many of which are multidrug resistant.     

Impact of adopting minimum inhibitory concentration as the determinant of susceptibility to cephalosporins and carbapenems in multi-drug resistant Enterobacteriaceae

The Clinical and Laboratory Standards Institute has recommended that Enterobacteriaceae susceptibility to most cephalosporins and carbapenems be reported according to minimum inhibitory concentration (MIC) alone. We analyzed our record of multi-drug resistant Enterobacteriaceae to assess the impact of these changes. We compared susceptibilities of ceftriaxone-resistant Enterobacteriaceae when using the 2009 and new 2010 MIC standards. Vitek2? (BioMerieux), was used to assess the changes in susceptibility. Klebsiella pneumoniae, Proteus sp., and Escherichia coli were the major species from urine, sputum, blood, and other sterile sites. The new breakpoint for cephalosporins increased resistance in E. coli and P. mirabilis. Many Proteus categorized as resistant by extended-spectrum beta-lactamase (ESBL) detection or inferred resistance have MICs to ceftriaxone ≤1 mcg/ml. New carbapenem breakpoints increased resistant Klebsiella pneumoniae and Proteus mirabilis. The increased ceftriaxone resistance from lowering breakpoints was almost balanced by the loss of resistance in ESBL isolates with MICs ≤1 mcg/ml. MIC-based susceptibility for multi-drug resistant Enterobacteriaceae increases the number of resistant isolates. Inferring mechanisms of resistance has a disproportionate effect on the susceptibility of Proteus mirabilis to cephalosporins, and the MIC-based standard has an almost equivalent but opposite effect on Proteus mirabilis susceptibility to carbapenems.     

Serratia infections in a general hospital: characteristics and outcomes

We aimed to present our experience regarding infections caused by Serratia spp. in a region with relatively high antimicrobial resistance rates. We retrospectively reviewed the databases of the microbiological laboratory of the University Hospital of Heraklion, Crete (2/2004–12/2009). A total of 77 patients [67.5% men, mean age ± standard deviation (SD) = 56.9 ± 24.5 years) were identified; 37.7% were outpatients. Sixty-five (84.4%) of the 77 included patients had a Serratia marcescens isolate; the remaining 12 patients had a non-marcescens Serratia spp. The most frequently observed infections were respiratory tract infection (32.5%) and keratitis/endophthalmitis (20.8%). Seventy-three (94.9%) patients were cured. Four deaths were observed; three of them were considered as attributed to the Serratia infection. No difference was found regarding the characteristics and outcomes between patients with Serratia marcescens and non-marcescens infections. In addition, antipseudomonal penicillins and their combinations with beta-lactamase inhibitors, as well as carbapenemes, and fluoroquinolones exhibited high antimicrobial activity against both the tested Serratia marcescens and non-marcescens isolates. Our study adds useful information regarding the characteristics and outcomes of patients with Serratia infection, as well as the susceptibilities of the respective Serratia marcescens and non-marcescens isolates, in a region with relatively high levels of antimicrobial resistance.     

Evaluation of discrepancies between oxacillin and cefoxitin disk susceptibility for detecting methicillin-resistant <Emphasis Type="Italic">Staphylococcus aureus</Emphasis>

The Clinical Laboratory Standards Institute recommends that if both cefoxitin and oxacillin are tested against Staphylococcus aureus and either result is measured as resistant, the organism should be reported as oxacillin resistant. This indicates that discrepancies may be present between oxacillin and cefoxitin sensitivities in S. aureus. In this study, we aimed to investigate the discrepancy between oxacillin and cefoxitin susceptibility in S. aureus clinical isolates. Of 10,980 S. aureus isolates recovered from 2005 to 2010, 27 (0.3%) isolates with discordant results between oxacillin and cefoxitin were collected. Fourteen (oxacillin diameters 10–12 mm) of the 27 strains were susceptible (MICs = 0.5–2 μg/ml) and 13 (6–13 mm) were resistant (4–>256 μg/ml) to oxacillin. The cefoxitin MICs of 14 oxacillin-susceptible and 13 oxacillin-resistant strains ranged between 4 and 8 and 8 to 32 μg/ml, respectively. Discrepancies were present between oxacillin and cefoxitin in S. aureus, and these strains should be further tested for oxacillin MICs and for the mecA gene or β-lactamase activity.     

In vitro activity of tigecycline against methicillin-resistant Staphylococcus aureus, including livestock-associated strains

The in vitro activity of tigecycline was determined using a well-defined collection of methicillin-resistant Staphylococcus aureus (MRSA) isolates (n = 202), including 33 livestock-associated strains. Susceptibility testing was performed using the Etest system. Among the 202 MRSA strains, three (1.5%) had a minimum inhibitory concentration (MIC) value for tigecycline greater than 0.5 mg/l, which are considered to be resistant. When these strains were tested using Iso-Sensitest medium, the MICs were substantially lower and no resistance was found. This discrepancy warrants further investigations into the preferred test conditions for tigecycline. In conclusion, tigecycline showed good activity against MRSA strains in vitro.     

High prevalence of erythromycin-resistant Bordetella pertussis in Xi'an,China

Resistance of Bordetella pertussis, the causative agent of pertussis, to erythromycin is rare. Recently, several Chinese isolates were found to be erythromycin resistant. This study aimed to investigate the occurrence of pertussis in children suffering persistent cough and the prevalence of B. pertussis resistance to erythromycin in Xi'an, China. Three hundred and thirteen patients with suspected pertussis admitted to Xi'an Children's Hospital from January 2012 through to December 2013 were included and their nasopharyngeal (NP) swabs were taken for culture and PCRs (targeting IS481 and ptx-Pr). PCR-based sequencing was used to identify the A2047G mutation of B. pertussis 23S rRNA directly from the NP samples. Sixteen (5.1%) and 168 (53.7%) patients were positive for culture and IS481 PCR. Of the 168 samples positive for IS481 PCR, 122 (72.6%) and 100 (59.5%) were positive for ptx-Pr and 23S rRNA PCRs, respectively. All culture-positive samples were also positive for the three PCRs. Fourteen (87.5%) of the 16 B. pertussis isolates were found to be resistant to erythromycin (MICs > 256 mg/L). All the 14 isolates were confirmed to have a homogeneous A2047G mutation of 23S rRNA. Of the 100 samples positive for 23S rRNA PCR, 85 (85.0%) were found to have the A2047G mutation by sequencing. Our results indicate that in Xi'an, China, pertussis remains endemic in young children, and the circulating B. pertussis strains are mostly erythromycin resistant.