The resistance to 12 commonly used antimicrobial agents of 144 foodborne isolates belonging to the genus Listeria (23 L. monocytogenes , 54 L. innocua , 66 L. seeligeri , and 1 L. welshimeri ) was tested by using the agar disc-diffusion assay. The Listeria strains were isolated from dairy products (different varieties of unripened fresh and bacteria-ripened hard cheeses made from ewe's, cow's, and goat's milk) and meat products ( longaniza , a type of pork sausage). A total of 84 (93%) and 54 (100%) Listeria strains isolated from cheese and pork sausage, respectively, were resistant to multiple antimicrobial agents. More than 80% of the Listeria strains of both food origins were found to be susceptible to penicillin G and ampicillin, whereas the proportion of isolates resistant to the cephalosporins cefotaxime and cefoxitin was nearly 100%. The prevalence of resistance was much higher for isolates from pork sausage (73.8% on average) than for isolates from cheese (20.9%). This marked difference was statistically significant (P < 0.05; chi-square test) for all antibiotics except ampicillin, cefotaxime, and cefoxitin. The strains of the foodborne pathogen L. monocytogenes isolated from cheese were all susceptible to 9 of the 12 antimicrobial agents evaluated. In contrast, more than 80% of the L. monocytogenes strains isolated from pork sausage exhibited resistance to cefotaxime, cefoxitin, tobramycin, amikacin, chloramphenicol, tetracycline, and erythromycin. The appearance of substantial resistance to antibiotics in foodborne Listeria isolates suggests the need for more prudent use of antibiotics by farmers, veterinarians, and physicians.
Keywords: spp.; Antibiotic resistance; cheese; pork sausage.