INVITRO ANTIMICROBIAL ACTIVITY ON LISTERIA-MONOCYTOGENES DURING A HUMAN EPIDEMIC

In 1987 an epidemic of listeriosis associated with a cheese Vacherin Mont d'Or was noted in Switzerland. We determined the MIC's of 20 antimicrobials on 74 strains of L. monocytogenes isolated during this epidemic. All strains were susceptible, amongst others, to penicillins and trimethoprim + sulfamethoxazole. However they were resistant to cephalosporins. Time-kill curves were performed on 4 strains. None of the antimicrobials tested alone was bactericidal. Only the combinations ampicillin-gentamicin, amoxicillin-gentamicin, imipenem-gentamicin and trimethoprim-sulfamethoxazole were bactericidal. Our results show that bactericidal activity can also be achieved with combinations that are additive or indifferent. The combination ampicillin-rifampicin was nearly antagonistic. The good activity of the combinations ampicillin-gentamicin and trimethoprim-sulfamethoxazole is confirmed and justifies their clinical applications for the treatment of listeriosis. Imipenem-gentamicin as a possible new alternative is proposed.