In vitro antimicrobial susceptibility, plasmid analysis, and serotyping of epidemic-associated Campylobacter jejuni.

Campylobacter jejuni strains from 11 outbreaks were characterized by antimicrobial susceptibility, plasmid profile, and serotyping by the methods of Lior et al. and Penner and Hennessy. All 31 strains were susceptible to erythromycin, clindamycin, chloramphenicol, kanamycin, tobramycin, streptomycin, and gentamicin. A total of 21 strains from nine outbreaks were resistant to one or more of the following antimicrobial agents: tetracycline, metronidazole, ampicillin, or carbenicillin. Of the 31 strains, 19 possessed plasmid DNA; 4 of the strains containing plasmids were sensitive to all antimicrobial agents tested. All of the strains that were resistant to tetracycline contained a 38-megadalton plasmid, and these plasmids shared common nucleic acid sequences. No other antimicrobial resistance was associated with the presence of plasmid DNA. Eight outbreaks appeared to have been caused by a single serotype, whereas in three outbreaks multiple serotypes were found. In two of the three outbreaks with multiple serotypes, plasmid profiles were also indicative of multiple strains of C. jejuni. Antimicrobial susceptibility and plasmid profile are potentially useful epidemiological markers for C. jejuni and may be used to supplement serotyping.