Campylobacter jejuni undergoes a dramatic morphological transformation from a corkscrew-shaped rod to a coccoid form in response to unfavorable conditions. It has been speculated that the coccoid plays an important role in the survival and dissemination of C. jejuni but questions still remain regarding the viability of coccoid cells. Characterization of the genome of coccoid cells found that newly formed coccoid cells (i.e., 1-3 days) had a SmaI-digestion profile identical to that of spiral-shaped cells; however, there was a progressive degradation of the DNA with continued incubation at 37degreesC. Concomitant with genome degradation was the detection of DNA in supernatants of coccoid cells. In contrast, cells incubated at 4degreesC retained a spiral shape and their SmaI-digestion profile for 8 weeks and released little DNA into the medium. Thus, low temperature inhibited both coccoid formation and genome degradation. Collectively, these data support the theory that the coccoid form of C. jejuni is a manifestation of cellular degradation and spiral-shaped cells, or possibly coccoid cells formed at low temperature, are the most probable candidates for a viable but nonculturable form of this pathogen.