The agar dilution method was used to determine the activities of gentamicin, erythromycin, streptomycin, chloramphenicol, ampicillin, sulfamethazine, cephalothin, penicillin G, and tetracycline against 73 strains belonging to the genus Listeria (L. innocua, L. seeligeri, and L. monocytogenes). All strains were isolated from raw milk, cheese, the dairy processing plant, poultry, and the poultry slaughterhouse. Gentamicin, ampicillin, and erythromycin, of which the MICs for 90% of the strains tested for all three species were .5.96 ,ug/ml, were found to be the most active agents studied. Most of the L. innocua strains isolated from poultry and the poultry slaughterhouse were resistant to tetracycline.Listeriosis is an infectious disease which affects both humans and animals. Most cases of human listeriosis appear to be sporadic, although a portion of these sporadic cases may be previously unrecognized common-source clusters (4). The source and route of infection are usually unknown. However, the recent ass §iciation of Listenia monocytogenes with several large food-borne outbreaks (8,11) suggests that contaminated food may be the primary source of the organism.Penicillin or ampicillin, alone or in combination with an aminoglycoside, seems to be the drug of choice (5); a trimethoprim-sulfamethoxazole combination has also been used (13). Although most clinical isolates of L. monocytogenes are susceptible in vitro to penicillin, ampicillin, tetracycline, and other antimicrobial agents, few quantitative studies have been carried out to determine the in vitro susceptibilities of Listeria spp. isolated from food. The purpose of the present study was to determine the susceptibilities of 73 strains of the genus Listeria, all isolated from food and food-processing plant environments, to nine antimicrobial agents: gentamicin, erythromycin, streptomycin, chloramphenicol, ampicillin, penicillin G, cephalothin, sodium sulfamethazine, and tetracycline.The study was carried out with 73 strains belonging to the genus Listeria. The strains were isolated by us from raw milk and cheese obtained from several markets, the environment from one dairy plant, the environment from one poultry slaughterhouse, and poultry from the same slaughterhouse. Numbers of strains of each Listeria species for each source and dates of isolation are shown in Listeria strains isolated from raw milk, cheese, the dairy plant, poultry, and the poultry slaughterhouse are shown in Table 2.The concentrations of gentamicin to which the Listeria innocua and Listeria seeligeri strains isolated in this study were susceptible ranged from 0.74 to 2.98 jig/ml. Results similar to those obtained in the present study have been reported for L. monocytogenes (3,10
