In vitro susceptibility of Listeria monocytogenes isolated from human blood and cerebrospinal fluid. A material from the years 1958-1985

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 micrograms/ml). Ampicillin and imipenem had MICs for 90% of the strains of 0.4 micrograms/ml. Ceftazidime was inactive (MIC90 greater than 100 micrograms/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 micrograms/ml, i.e. one to three two-fold dilutions above the MIC of 0.2-0.8 micrograms/ml, which means that no tolerant strains were found.