Biochemical and genetic characteristics of atypical Campylobacter fetus subsp. fetus strains isolated from humans in the United States.

During a 2-year period, 14 biochemically atypical Campylobacter fetus subsp. fetus-like strains were received by the Campylobacter Reference Laboratory at the Centers for Disease Control. Sources of the isolates were blood, nine strains; stools, two strains; amniotic fluid, one strain; and abscesses, two strains. Atypical phenotypic characteristics exhibited by one or more strains were growth at 42 degrees C, 10 strains; no H2S by lead acetate paper, 3 strains; resistance to a 30-micrograms cephalothin disk, 2 strains; and nonmotility, 1 strain. By DNA-DNA hybridization, all 14 isolates and the type strain of C. fetus subsp. fetus (ATCC 27374) were 94 to 100% related in reassociation reactions at 50 degrees C, with 0.0 to 0.5% divergence, and were 86 to 100% related in reassociation reactions at 65 degrees C. Thus, all of these atypical strains were C. fetus subsp. fetus. MICs of 11 antimicrobial agents for these 14 strains were variable. All strains were susceptible to chloramphenicol, erythromycin, gentamicin, and tetracycline, and most were susceptible to ampicillin, clindamycin, and penicillin. Eleven strains were resistant to cephalothin (MIC greater than or equal to 16 micrograms/ml), nine were resistant to rifampin (MIC greater than or equal to 8 micrograms/ml), and all were resistant to nalidixic acid (MIC greater than 32 micrograms/ml) and vancomycin (MIC greater than 32 micrograms/ml). One can expect to see biochemical variability in C. fetus subsp. fetus strains and to encounter such strains from a variety of human sources, the most important of which appears to be blood.