In vitro activity of the new glycopeptide decaplanin

The activity of decaplanin, a new glycopeptide, was compared to that of vancomycin, teicoplanin and daptomycin. Decaplanin was two- to four-fold less active than vancomycin, teicoplanin and daptomycin againstStaphylococcus aureus andStaphylococcus epidermidis, with an MIC90 of 2 µg/ml for methicillin-susceptible and 4 µg/ml for methicillin-resistant isolates. Decaplanin had activity similar to that of vancomycin againstStreptococcus pyogenes, Streptococcus agalactiae, group C and G streptococci, with an MIC90 of 0.12 µg/ml. It was less active than the other agents against the viridans group streptococci (MIC90 4 µg/ml). The activity of decaplanin against enterococci (MIC90 4 µg/ml) was similar to that of vancomycin.Clostridium spp. were inhibited by 0.5 µg/ml, peptostreptococci and peptococci by 0.25 µg/ml. Decaplanin was active from pH 5.5 to 7.5. Inoculum size had a minimal effect on MICs, and increased concentrations of Ca²⁺ and Mg²⁺ and 50 % serum did not alter MICs or MBCs.     

In vitro susceptibility ofHelicobacter pylori to trospectomycin,pirlimycin (U-57930E), mirincamycin (U-24729A) and N-demethyl clindamycin (U-26767A)

The in vitro activity of trospectomycin, pirlimycin, mirincamycin and N-demethyl clindamycin was measured against 46 clinical isolates ofHelicobacter pylori using an agar dilution technique. The MIC50 and MIC90 were 4 and 64 µg/ml for pirlimycin and N-demethyl clindamycin, and 32 and 128 µg/ml for mirincamycin, respectively. All 46 strains were sensitive to trospectomycin with an MIC50 of 8 µg/ml and an MIC90 of 16 µg/ml. Of seven strains with the highest trospectomycin MICs (8 or 16 µg/ml) 100 % were found to be resistant to metronidazole. Among ten strains with low trospectomycin MICs (2 µg/ml or less) 100 % were sensitive to metronidazole. Possible explanations for the apparent correlation between the MICs of the two drugs are discussed. Since all metronidazole resistant strains were sensitive to trospectomycin, this drug may be useful in treating infection with metronidazole resistantHelicobacter pylori.     

Rapid decrease of free vancomycin in dense staphylococcal cultures

Bacterial numbers in broth cultures were determined by bioluminescence assay of intracellular bacterial ATP. Broth MICs for strains of Staphylococcus epidermidis (ATCC 14990 and 35984) and Staphylococcus aureus (ATCC 25923, 29213 and 6538) were determined for cultures with different inocula (10⁵–10⁸ bacteria/ml) after 24 h of incubation in supplemented Mueller–Hinton broth containing vancomycin. All of the tested strains except one were susceptible to methicillin, and all of the strains were susceptible to vancomycin. Free vancomycin concentrations in the broth cultures of all strains were determined with an agar well bioassay after 24 h of incubation. Free vancomycin concentrations and bacterial numbers of ATCC 35984 and ATCC 29213 were also determined after 0.5, 2, 4, and 8 h. In a low inoculum (10⁵ bacteria/ml), the broth MICs were 1–4 μg/ml. In a high inoculum (∼10⁸ bacteria/ml), the broth MICs increased two- to fourfold to 4–8 μg/ml. In dense inocula (∼10⁹–10¹⁰ bacteria/ml), the concentrations of free vancomycin in the broth were reduced, in most cases below the detection limit of the bioassay (≤0.5 μg/ml). This reduction of free vancomycin was fast, occurring in initially dense inocula in less than 30 min. No emergence of resistance was seen. These results show a rapid reduction of free vancomycin in the broth and a simultaneous increase in broth MICs in high inocula, without development of resistance. This indicates that the dosing regimen of vancomycin is of particular importance in staphylococcal infections with dense inocula, e.g. infective endocarditis.     

In vitro activity of cefcanel versus other oral cephalosporins

Cefcanel is a new orally absorbed cephalosporin. Its activity was compared with that of cefuroxime, cefaclor, cephalexin, and cefixime against grampositive and negative aerobic and anaerobic bacteria. Cefcanel had excellent activity against methicillin-susceptibleStaphylococcus aureus andStaphylococcus epidermidis, MIC90 1 µg/ml, superior to the other oral cephalosporins. However, methicillin-resistant staphylococci were resistant, MIC 16 µg/ml.Streptococcus pyogenes andStreptococcus pneumoniae were inhibited by 0.015–1 µg/ml, concentrations comparable to other cephalosporins.Clostridium spp. were inhibited by 0.25 µg/ml, 8- to 128-fold lower concentrations than were found for other agents, but the MICs were >64 µg/ml forBacteroides spp. The MIC90 forMoraxella catarrhalis was 1 µg/ml, similar to cefuroxime but 16-fold greater than the MICs of cefixime.Escherichia coli andKlebsiella pneumoniae which were high beta-lactamase producers were resistant, MICs >64 µg/ml, and 50 % ofEnterobacter cloacae andCitrobacter freundii were resistant. Cefcanel was hydrolyzed by TEM-1, TEM-3 and Moraxella Bro-1 beta-lactamases.Escherichia coli containing TEM-1, 2, 3, 5, 7, and 9 had cefcanel MICs of 16 µg/ml. Although cefcanel inhibited gram-positive species as well as or at lower concentrations than other cephalosporins, it lacked activity against gram-negative species that produced common plasmid beta-lactamase although it inhibitedHaemophilus influenzae carrying TEM-1.     

In vitro activity of amphotericin B,flucytosine and fluconazole against yeasts causing bloodstream infections

The in vitro activity of amphotericin B, flucytosine and fluconazole against 95 yeasts causing fungemia in a single institution over the last eight years was determined by a broth macromethod recommended by the National Committee for Clinical Laboratory Standards. All strains were inhibited by amphotericin B concentrations of 1 µg/ml. With flucytosine in most species the MIC50 was between 0.12 and 0.25 µg/ml and the MIC90 was between 0.25 and 1 µg/ml. One exception with flucytosine wasCandida krusei, with an MIC50 and MIC90 of 16 µg/ml and 32 µg/ml, respectively. Overall, 12 % of the isolates needed at least 8 µg/ml of fluconazole to be inhibited. Fluconazole was very active againstCandida albicans, Candida tropicalis andCryptococcus neoformans, with MIC50 ranging from 0.12 to 0.5 µg/ml and MIC90 of 1 µg/ml, and somewhat less active againstCandida parapsilosis (MIC50 of 1 µg/ml and MIC90 of 4 µg/ml). Fluconazole exhibited poor in vitro activity againstCandida krusei (MIC50 and MIC90 of 64 µg/ml) andTorulopsis glabrata (MIC50 of 4 µg/ml and MIC90 of 16 µg/ml). High MICs of fluconazole were found for four strains ofCandida albicans, one with an MIC of 4 µg/ml and three (5.7 %) with MICs of 16 µg/ml. Previous exposure to fluconazole could be demonstrated in two of these strains. Further work must be done in order to determine appropriate breakpoints of antifungal agents, to assess the clinical relevance of azole resistance in yeasts causing bloodstream infections and to identify risk factors for infections with azole-resistant yeasts.     

In vitro antimicrobial activity of the new antibiotic vermisporin

The antimicrobial activity of vermisporin, a new antibiotic produced by fermentation of the fungusOphiobolus vermisporis, was tested in vitro. Vermisporin inhibited 90 % ofBacteroides fragilis and otherBacteroides spp. at 1 µg/ml (range 0.25–1 µg/ml).Clostridium perfringens were inhibited by 1 µg/ml (range 0.25–2 µg/ml). Vermisporin inhibited 90 % ofStaphylococcus aureus, including methicillin-resistantStaphylococcus aureus, at 0.5 µg/ml (range 0.12–0.5 µg/ml). Vermisporin MICs for group A, B, C, F and G streptococci were < 1 µg/ml when tested in Haemophilus Test Medium but 8 µg/ml in the presence of blood. Vermisporin MICs forEnterobacteriaceae, Pseudomonas aeruginosa andHaemophilus influenzae exceeded 64 µg/ml. Inhibited organisms had MBCs 16- to 32-fold above the MICs.     

Determination of the range of antibacterial activity by use of viable counts

The activity of three aminoglycosides and six beta-lactam antibiotics on strains of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus, and enterococci was studied. The minimal inhibitory concentrations (MICs), the minimal bactericidal concentrations (MBCs), and the minimal antibiotic concentrations (MACs) were determined after 5 h of incubation in broth cultures by colony-forming-unit counts. The MICs were also determined by agar dilution after 24 h of incubation. The MICs on agar after 24 h of incubation were higher than those in broth after 5 h of incubation. The differences ranged from 1.1- to 14.2-fold, but in most cases were only three- to fivefold (P less than 0.05 to less than 0.001). The MBCs at 5 and 24 h were comparable in 71% of tests. For current practice, the MBC of enterococci can be determined after 5 h of incubation with antibiotics. The aminoglycosides showed MBCs which were closer to the MICs than were those of the beta-lactam antibiotics, which required a higher multiple of the MIC to show a bactericidal effect. The MBCs of oxacillin and cefamandole for S. aureus after 5 h of incubation were greater than 128 times the respective MICs. The MACs ranged from 1/1.5 to 1/7 of the 5-h MICs. The three endpoints, MIC, MBC, and MAC, indicate the antibacterial range of an antibiotic in terms of inhibition of growth and bacterial survival. The data suggest that the antibacterial range of an antibiotic is similar for most strains of a given species and is, to some extent, a characteristic of similar antibiotics.     

In vitro antimicrobial susceptibility of viridans streptococci isolated from blood cultures

The susceptibility of 211 viridans streptococci isolated from blood cultures to eight antimicrobial agents was determined. All the isolates were susceptible to cefotaxime, ceftriaxone, imipenem and vancomycin. Thirty eight percent of the isolates were resistant to penicillin (MICs 0.25 µg/ml). Tetracycline resistance was found in 41 % of the isolates and in 7 % of these strains tetracycline resistance was combined with erythromycin resistance. FiveStreptococcus mitis isolates exhibited increased (MIC 64 µg/ml and 128 µg/ml) or high-level (MIC 500 µg/ml) resistance to gentamicin, kanamycin and tobramycin. Four of these isolates were also resistant to penicillin (MICs 16–32 µg/m). In vitro synergy was not demonstrated for combinations of penicillin and gentamicin against threeStreptococcus mitis isolates with gentamicin MICs of 1000, 128 and 64 µg/ml. Results of this study indicate the importance of monitoring antibiotic resistance trends in viridans streptococci particularly with respect to penicillin and aminoglycoside resistance.     

Glycopeptide Resistance in Coagulase-Negative Staphylococci

 Coagulase-negative staphylococci (CNS) were the first organisms in which acquired glycopeptide resistance was recognized. Ever since the early reports, it has been apparent that resistance to teicoplanin is more common than that to vancomycin and that resistance occurs mostly in species such as Staphylococcus haemolyticus and Staphylococcus epidermidis. The minimum inhibitory concentrations (MICs) of teicoplanin for CNS usually fall over a wide range, and, especially in some methicillin-resistant isolates of the two above-mentioned species, they can reach and even exceed the resistance breakpoint, whereas vancomycin MICs tend to remain more stable over a narrower range within the limits of susceptibility. CNS strains intermediately susceptible and even resistant not only to teicoplanin but also to vancomycin have, however, been isolated, most frequently from patients subjected to prolonged glycopeptide treatment. Laboratory detection of glycopeptide-resistant CNS may be problematic, mainly because susceptibility tests, particularly those for teicoplanin, are influenced by various technical factors, and agar diffusion tests may yield false susceptibility data. In studies with experimental glycopeptides, some molecules have exhibited improved in vitro activity compared with teicoplanin and vancomycin, but these encouraging microbiological findings have not usually been followed by in vivo trials. Stepwise and single-step exposure to teicoplanin and vancomycin has allowed stable clones for which glycopeptide MICs are increased to be obtained from susceptible CNS strains, particularly strains of Staphylococcus haemolyticus and Staphylococcus epidermidis. In these studies, resistance to teicoplanin was generally easier to obtain than resistance to vancomycin, and the levels of teicoplanin resistance were higher. Population studies have demonstrated the usually heterogeneous nature of glycopeptide resistance in CNS. Although glycopeptide-resistant CNS have been shown to differ in several features from their glycopeptide-susceptible counterparts, the exact mechanism of staphylococcal glycopeptide resistance remains unknown.     

In vitro activity of sparfloxacin and three other fluoroquinolones against methicillin-resistantStaphylococcus aureus andStaphylococcus epidermidis

Fluoroquinolones are variably active against methicillin-resistantStaphylococcus aureus (MRSA) and methicillin-resistantStaphylococcus epidermidis (MRSE). The purpose of this study was to test the in vitro susceptibility of 50 isolates each of MRSA and MRSE to four of the new fluoroquinolones — sparfloxacin, ciprofloxacin, ofloxacin and norfloxacin — and to see if resistance could be induced in five susceptible strains of each species by serial passage with increasing concentrations of drug. A standard microdilution technique was used to determine minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of each drug for each isolate. Agar dilution plates were also prepared containing concentrations of drug varying from one-half of the reported MIC to 128 times the reported MIC, and microorganisms persisting were serially passaged. Initially, 98 % of the strains of MRSA were susceptible to the fluoroquinolones. MBCs were essentially identical to MICs. Similarly, 96 % of the strains of MRSE were susceptible. Following exposure to increasing concentrations of each fluoroquinolone, resistance appeared to emerge less rapidly to sparfloxacin and ofloxacin than to norfloxacin and ciprofloxacin.     

Characterization of Staphylococci with Reduced Susceptibilities to Vancomycin and Other Glycopeptides

During the last several years a series of staphylococcal isolates that demonstrated reduced susceptibility to vancomycin or other glycopeptides have been reported. We selected 12 isolates of staphylococci for which the vancomycin MICs were ≥4 μg/ml or for which the teicoplanin MICs were ≥8 μg/ml and 24 control strains for which the vancomycin MICs were ≤2 μg/ml or for which the teicoplanin MICs were ≤4 μg/ml to determine the ability of commercial susceptibility testing procedures and vancomycin agar screening methods to detect isolates with reduced glycopeptide susceptibility. By PCR analysis, none of the isolates with decreased glycopeptide susceptibility contained known vancomycin resistance genes. Broth microdilution tests held a full 24 h were best at detecting strains with reduced glycopeptide susceptibility. Disk diffusion did not differentiate the strains inhibited by 8 μg of vancomycin per ml from more susceptible isolates. Most of the isolates with reduced glycopeptide susceptibility were recognized by MicroScan conventional panels and Etest vancomycin strips. Sensititre panels read visually were more variable, although with some of the panels MICs of 8 μg/ml were noted for these isolates. Vitek results were 4 μg/ml for all strains for which the vancomycin MICs were ≥4 μg/ml. Vancomycin MICs on Rapid MicroScan panels were not predictive, giving MICs of either ≤2 or ≥16 μg/ml for these isolates. Commercial brain heart infusion vancomycin agar screening plates containing 6 μg of vancomycin per ml consistently differentiated those strains inhibited by 8 μg/ml from more susceptible strains. Vancomycin-containing media prepared in-house showed occasional growth of susceptible strains, Staphylococcus aureus ATCC 29213, and on occasion, Enterococcus faecalis ATCC 29212. Thus, strains of staphylococci with reduced susceptibility to glycopeptides, such as vancomycin, are best detected in the laboratory by nonautomated quantitative tests incubated for a full 24 h. Furthermore, it appears that commercial vancomycin agar screening plates can be used to detect these isolates.     

Accuracy of Commercial and Reference Susceptibility Testing Methods for Detecting Vancomycin-Intermediate Staphylococcus aureus

We compared the results obtained with six commercial MIC test systems (Etest, MicroScan, Phoenix, Sensititre, Vitek Legacy, and Vitek 2 systems) and three reference methods (agar dilution, disk diffusion, and vancomycin [VA] agar screen [VScr]) with the results obtained by the Clinical and Laboratory Standards Institute broth microdilution (BMD) reference method for the detection of VA-intermediate Staphylococcus aureus (VISA). A total of 129 S. aureus isolates (VA MICs by previous BMD tests, ≤1 μg/ml [n = 60 strains], 2 μg/ml [n = 24], 4 μg/ml [n = 36], or 8 μg/ml [n = 9]) were selected from the Centers for Disease Control and Prevention strain collection. The results of BMD with Difco Mueller-Hinton broth were used as the standard for data analysis. Essential agreement (percent ±1 dilution) ranged from 98 to 100% for all methods except the method with the Vitek Legacy system, for which it was 90.6%. Of the six commercial MIC systems tested, the Sensititre, Vitek Legacy, and Vitek 2 systems tended to categorize VISA strains as susceptible (i.e., they undercalled resistance); the MicroScan and Phoenix systems and Etest tended to categorize susceptible strains as VISA; and the Vitek Legacy system tended to categorize VISA strains as resistant (i.e., it overcalled resistance). Disk diffusion categorized all VISA strains as susceptible. No susceptible strains (MICs ≤ 2 μg/ml) grew on the VScr, but all strains for which the VA MICs were 8 μg/ml grew on the VScr. Only 12 (33.3%) strains for which the VA MICs were 4 μg/ml grew on VScr. The differentiation of isolates for which the VA MICs were 2 or 4 μg/ml was difficult for most systems and methods, including the reference methods.In January 2006, the Clinical and Laboratory Standards Institute (CLSI) published new interpretive criteria for vancomycin and Staphylococcus aureus. The breakpoints were lowered from ≤4 μg/ml to ≤2 μg/ml for susceptible, 8 to 16 μg/ml to 4 to 8 μg/ml for intermediate, and ≥32 μg/ml to ≥16 μg/ml for resistant (2). The vancomycin breakpoints for coagulase-negative staphylococci were not changed. The rationale for lowering the S. aureus intermediate breakpoint to 4 μg/ml was (i) that intermediate S. aureus isolates, although they are rare, likely represented a population of organisms that demonstrate heteroresistance, and (ii) limited outcome data suggested that infections with these isolates are likely to fail vancomycin therapy (9). The results of broth microdilution performed by use of the CLSI reference method were the primary S. aureus susceptibility data evaluated before the CLSI breakpoint change was made. We undertook the study described here to determine the accuracy of commercial systems and reference methods for the detection of decreased vancomycin susceptibility among isolates of S. aureus.(This work was presented in part at the 47th Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, IL, 17 to 20 September 2007.)     

In vitro activity of PD 117596-2, a broad-spectrum difluoroquinolone

The activity of PD 117596-2, a novel quinolone, was compared to that of other quinolones, ceftazidime, imipenem and gentamicin. PD 117596-2 inhibited mostEnterobacteriaceae at concentrations <0.25 µg/ml, being equal or superior in activity to ciprofloxacin and 2- to 4-fold more active than ofloxacin. It inhibited ceftazidime-resistantEnterobacter spp.,Citrobacter spp. andSerratia marcescens. The MIC90 forPseudomonas aeruginosa, including strains with imipenem MICs of 8 µg/ml and gentamicin MICs > 16 µg/ml, was 0.25 µg/ml. PD 117596-2 was more active than ciprofloxacin againstPseudomonas cepacia andPseudomonas maltophilia, and it inhibitedNeisseria gonorrhoeae andHaemophilus influenzae at < 0.03 µg/ml. PD 117596-2 inhibited staphylococci at 0.5 µg/ml, being 2-fold superior to other quinolones, and with an MIC of 0.25 µg/ml was more active against group A, B, C and G streptococci andStreptococcus pneumoniae. MICs forBacteroides spp. were 2 µg/ml compared to 8–32 µg/ml for other agents. The frequency of spontaneous resistance was low (< 10^–10). Differences in MICs and MBCs were within one dilution, and there was a minimal effect of inoculum size. Although PD 117596-2 was less active at pH 5.5, MICs were < 0.5 µg/ml.     

In vitro development and stability of tolerance to cloxacillin and vancomycin inStaphylococcus aureus

The stability of tolerance ofStaphylococcus aureus during subculturing at 37°C and development of this property after repeated exposure to cloxacillin or vancomycin were investigated in vitro. Four of five tolerant strains lost this property during repeated subculturing at 37°C for 50 days. Conversely, tolerance emerged in two of four nontolerant strains after repeated cycles of exposure to 25 µg of cloxacillin/ml or 10 µg of vancomycin/ml alternating with growth in antibiotic-free medium. Previous in vivo exposure to cloxacillin did not enhance the development of tolerance in vitro. MICs of both cloxacillin and vancomycin did not change significantly during this procedure. Whether the conversion of nontolerant strains to the tolerant state can also occur during antibiotic exposure in treatment of patients remains to be determined.     

Differences in vancomycin MIC among MRSA isolates by agar dilution and E test method

In this study, the correlation between vancomycin minimum inhibitory concentration (MIC) obtained by the E test technique and the Clinical And Laboratory Standards Institute (CLSI) agar dilution method was evaluated. A total of 53 Methicillin Resistant Staphylococcus aureus (MRSA) strains were tested by both the methods in the present study. MICs of vancomycin obtained by the E test method were consistently higher (+0.5 to 2 log2 dilutions) than those obtained by the agar dilution method. Out of 53 MRSA isolates, 49 isolates showed higher MIC results by E test than by agar dilution method. Three isolates showed same MIC result by both methods. Since many studies have demonstrated increased clinical failure with MRSA isolates for which vancomycin MICs are increased (>1 µg/ml) but still within the susceptibility range (≤ 2 µg/ml), our findings suggest the requirement to re-look into the breakpoints for vancomycin for determining sensitivity of MRSA isolates. Guidelines should also specify the method to be used for determining the MIC.     

Prevalence of antibiotic resistance among clinical isolates of methicillin-resistant staphylococci

The prevalence of methicillin-resistant and multiply antibiotic-resistant staphylococci causing infections in hospitalized patients was studied over a two-year period. Among 122 clinically significant staphylococci, the prevalence of methicillin resistance was 66 %, with a higher prevalence ofStaphyloccus haemolyticus (85 %) andStaphylococcus epidermidis (83 %) observed than ofStaphylococcus aureus (49 %). Multiple antibiotic resistance was observed more frequently among coagulase-negative staphylococci (52 to 70 %) than amongStaphylococcus aureus (17 %). All strains of methicillin-resistantStaphylococcus aureus were susceptible to glycopeptide antibiotics and to trimethoprim-sulfamethoxazole, whereas approximately 50 % of coagulase-negative staphylococci exhibited either moderate susceptibility or resistance to teicoplanin. For these latter strains, vancomycin remains the agent of choice, whereas teicoplanin cannot be recommended unless its efficacy is established by MIC determination.     

Comparative in vitro activity of the new oxazolidinones DuP 721 and DuP 105 against Staphylococci and Streptococci

The in vitro activity of DuP 721 and DuP 105, two orally active members of the oxazolidinones, was compared with that of glycopeptides and ciprofloxacin against 185 gram-positive isolates. Ninety percent ofStaphylococcus aureus isolates, including penicillin- and methicillin-resistant strains, were inhibited by DuP 721 at 1 µg/ml and by DuP 105 at 16 µg/ml; DuP 721 inhibited 90% of coagulasenegative staphylococci tested at 1 µg/ml. The MIC90 forStreptococcus faecalis was 4 µg/ml with DuP 721 and 16 µg/ml with DuP 105; DuP 721 inhibited 90% of -hemolytic streptococci of group A at 0.5 µg/ml. Similar results on selected strains were obtained by continuously recording the optical density of cultures.     

Effects of environmental factors on the in vitro potency of azithromycin

The effects of media, pH, cations, serum, CO₂ or anaerobic atmosphere, inoculum size and time of incubation on the in vitro potency of azithromycin were determined. The potency of azithromycin against all genera was particularly sensitive to changes in pH. The MIC forStaphylococcus aureus strains ranged from 50 µg/ml at pH 6 to 0.025 µg/ml at pH 8; for erythromycin the MIC change was less (1.6 to 0.05 µg/ml). Incubation for 18 h in 5 % CO₂ or an anaerobic atmosphere (10 % CO₂, 10 % H₂, 80 % N₂) lowered the pH by approximately 0.8 units with gram-negative organisms and 0.4 units with gram-positive organisms. This resulted in an MIC eight times greater than the aerobic MIC. In addition, the MIC100 for azithromycin and erythromycin againstBacteroides strains growing in Wilkins-Chalgren broth fell from 3.1 µg/ml in the anaerobic atmosphere to 0.2 and 0.4 µg/ml, respectively, when using the Oxyrase enzyme system to remove oxygen. With the Oxyrase system, the pH of the medium at the MIC remained at 7.2, while it fell to 6.7 in the anaerobic gas mixture. An increase in potency for both agents was also observed with other anaerobic species when using the Oxyrase system. The addition of serum produced an increase in potency of azithromycin and erythromycin that correlated with an increase in pH during incubation, despite the use of buffered media. Adding cations to Mueller-Hinton broth resulted in increased MICs for gram-negative organisms; the highest increases observed were four-fold forEscherichia coli. The activity of control antibiotics was not affected to the same degree as that of azithromycin. Increasing the incubation period from 24 to 48 h did not change the MIC values of azithromycin forStaphylococcus aureus orEscherichia coli; however, the MBC values were lower at 48 h and equalled the MIC values. Inoculum size or manner of preparation had no significant effect on the potency of azithromycin.     

In vitro antimicrobial susceptibility ofListeria monocytogenes isolated in the UK and otherListeria species

The MICs and MBCs of 21 antimicrobial agents were determined for 103 strains ofListeria monocytogenes isolated in the UK and 27 strains of otherListeria species. Ampicillin, penicillin, azlocillin, imipenem, gentamicin, netilmicin, amikacin, erythromycin, rifampicin, trimethoprim, clindamycin and vancomycin had good activity, while cephalothin, chloramphenicol, ciprofloxacin and ofloxacin were less active, and cefuroxime, enoxacin, norfloxacin and fosfomycin were the least active. Tetracycline had good activity against many strains, but the MIC was high for some. Unlike the otherListeria species tested,Listeria ivanovii was susceptible to fosfomycin. Inoculum size and media employed were shown to affect the MBC, tryptose phosphate broth yielding higher MBCs than Mueller-Hinton or Isosensitest broths.     

In vitro activity and beta-lactamase stability of the new oral cephalosporin Bay v 3522

The activity of the new oral cephalosporin Bay v 3522 was compared to that of six other beta-lactam agents. Bay v 3522 inhibited methicillin-susceptibleStaphylococcus aureus andStaphylococcus epidermidis at 2 µg/ml, compared to MICs of 8 µg/ml for the other cephalosporins tested. It was more active againstStreptococcus pyogenes (MIC 0.06 µg/ml) than cefuroxime, cefixime, cephalexin and cefaclor. Groups B, C and G streptococci were inhibited at 0.12 µg/ml, while the MIC90 forStreptococcus bovis and viridans streptococci was 0.5 and 2 µg/ml, respectively. The MIC90 for enterococci andListeria monocytogenes was 8 µg/ml.Clostridium perfringens was inhibited by 0.12 µg/ml, but mostBacteroides spp. were resistant. The MIC90 for beta-lactamase positiveEscherichia coli (producing primarily TEM-1) was >64 µg/ml and for beta-lactamase negative strains 16 µg/ml. The MIC90 for high-level beta-lactamase producingKlebsiella pneumoniae was >64 µg/ml versus 4 µg/ml for other isolates. The MIC90 forMoraxella catarrhalis was 2 µg/ml, forHaemophilus influenzae 1 µg/ml, and forNeisseria gonorrhoeae 4 µg/ml.Enterobacter cloacae, Citrobacter freundii, Proteus mirabilis, Providencia spp. andPseudomonas aeruginosa were resistant. Bay v 3522 was destroyed by TEM-1, SHV-1, TEM-3 and P99 beta-lactamases.