In vitro susceptibility of Listeria monocytogenes to some antibiotics and their combinations.

The Listeria monocytogenes strains isolated during recent years in our hospital have been tested for in vitro susceptibility against eleven antibiotics. The following tests were used: antibiotics discs method on Mueller-Hinton agar, determination of MICs by agar dilution method, study of the bactericidal potency of antibiotics combinations. Antibiotic discs method gives some too optimistic results, not confirmed by the MICs determined with agar dilution method. Ther are a few antibiotics used alone which are bactericidal upon Listeria monocytogenes at the concentration reached with usual therapeutic doses, and none regularly bactericidal. Bêtalactam-aminoglycoside combinations are the most effective ones but the ones involving gentamicin are more bactericidal than the ones involving streptomycin. Pristinamycin-aminoglycoside combinations have interesting activities and might be used as an alternative to betalactam-aminoglycoside ones in some peculiar and restricted indications. The other tested combinations are antagonistic or without interest.