The in vitro susceptibility of 156 strains of Listeria monocytogenes isolated since 1958 from human cerebrospinal fluid or blood to twelve antibiotics was determined by an agar dilution technique. Erythromycin (0.05), trimethoprim (0.2), netilmicin (0.2), and penicillin (0.2) were the most active drugs on weight basis (MIC90 0.05–0.2 μg/ml). Ampicillin and imipenem had MICs for 90% of the strains of 0.4 μg/ml. Ceftazidime was inactive (MIC90 > 100 μg/ml). Comparison of susceptibility pattern between strains isolated in different years showed that the antimicrobial susceptibility of L. monocytogenes has not changed during the last 25 years. The minimal bactericidal concentration (MBC) of penicillin was determined by a macro tube dilution method in ten recent isolates. Penicillin was bactericidal for all the strains with a MBC of 0.4–3.1 μg/ml, i.e. one to three two‐fold dilutions above the MIC of 0.2–0.8 μg/ml, which means that no tolerant strains were found.
The in vitro activity of vancomycin against 40 clinical isolates of enterococci was determined by a macro‐tube dilution method and by quantitative killing curve procedures employing the standard medium of our department, i.e. a filtered ox broth. An attempt to remove the influence of technical factors on the MBC determination was made by using an inoculum in the early logarithmic growth phase and ensuring the exposure of all the organisms to the antibiotic. Vancomycin showed a good inhibitory activity for the enterococci tested (MIC90 of 1.6 μg/ml, 3.1 μ/ml and 1.6 μg/ml for S. faecalis, S. faecium and S. durans, respectively), but no bactericidal effect could be demonstrated as measured by the MBCs (> 100 μg/ml) and killing curve procedures.
An in vitro study was performed to evaluate the value of penicillinase in Stuart's transport medium. Swabs were taken from serum broth cultures of Staphylococcus aureus. Escherichia coli and Proteiis rettgeri, incubated with cefuroxime. Following 24 hours' storage in the transport medium, the recovery of P. rettgeri was significantly higher from Stuart's transport media containing penicillinase in a concentration of 100000 units/ml. than from swabs stored in Stuart's medium without penicillinase. Although susceptible to cefuroxime, we did not find the same effect on S. aureus and E. coli, presumably because these strains had higher MIC values than the P. reugeri strain tested.
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