Antimicrobial resistance of Yersinia enterocolitica strains from human patients, pigs and retail pork in Switzerland.

To obtain basic data for future resistance monitoring programs, 386 Yersinia enterocolitica strains from human patients, raw retail pork and pig feces were tested for their susceptibilities to 16 antimicrobial agents and two antimicrobial growth promoters (carbadox and olaquindox). No strains were resistant to ceftriaxone, cefuroxime, ciprofloxacine, gentamicin, kanamycin, neomycin or polymyxin. Although in Switzerland carbadox and olaquindox were used as growth promoters for pigs for over 25 years, all strains were susceptible to them. In contrast, there were high levels of resistance to ampicillin, cefalothin and amoxicillin/clavulanic acid. Less than 10% of clinical isolates and strains from pig feces were resistant to streptomycin, sulfonamide, trimethoprim/sulfamethoxazole, tetracyclin, trimethoprim and chloramphenicol, but strains from retail pork were all susceptible to these antimicrobial agents. This finding suggested that pork is probably not a major source of Y. enterocolitica that cause human infections in Switzerland. A difference between clinical isolates and strains from pork was also shown by serotyping. Clinical isolates frequently belonged to the O3 and O9 groups whereas these two serotypes were not found in strains from pork. Resistance to multiple antimicrobial agents was rare. When examined by pulsed field gel electrophoresis (PFGE), two strains of fecal origin with an identical pattern of resistance to six antimicrobial agents were shown to be unrelated. Of four clinical isolates with resistances to five antimicrobial agents, two were of the same pulsotype. Retrospectively, it was found that these strains came from two members of the same household and thus represented a mini-outbreak.