Prevalence of antimicrobial resistance in Salmonellae isolated from market-age swine

Antimicrobial resistance levels were examined for 365 Salmonella isolates r ecovered from the lymph nodes (n = 224) and cecal contents (n = 141) of mar ket-age swine at slaughter. Antimicrobial resistance testing was performed by disk diffusion using 13 antibiotics common in the treatment of disease i n human and veterinary medicine. Although none of the antibiotics tested we re used subtherapeutically within the last 5 years on the farms sampled, re sistance to chlortetracycline, penicillin G, streptomycin, and sulfisoxazol e was common. Penicillin G resistance was significantly more frequent (P = 0.03) and sulfisoxazole resistance was significantly less frequent (P < 0.0 1) in lymph node versus cecal isolates. Multidrug resistance was observed a mong 94.7% of the lymph node isolates and 93.5% of the cecal isolates. The most frequent multidrug resistance pattern included three antibiotics-penic illin G, streptomycin, and chlortetracycline. Isolates in somatic serogroup 13, and more specifically, Salmonella Agona and Salmonella Schwarzengrund isolates, were often resistant to a greater number of antibiotics than were isolates in the other serogroups. Streptomycin, sulfisoxazole, ampicillin (lymph node isolates), and nitrofurantoin (cecal isolates) resistance level s differed significantly between somatic serogroups. The prevalence of peni cillin G-, streptomycin-, and sulfisoxazole-resistant isolates differed sig nificantly between serovars for both lymph node and cecal isolates. Results of this study suggest that a correlation exists between the somatic serogr oup or serovar of a Salmonella isolate and its antimicrobial resistance sta tus, which is specific to the antibiotic of interest and the source of the isolate (lymph node versus cecal contents).