Relationship between pNG2, an Emr plasmid in Corynebacterium diphtheriae, and plasmids in aerobic skin coryneforms.

Erythromycin-resistant (Emr) coryneforms from cutaneous lesions and erythromycin-susceptible (Ems) coryneforms from normal skin sites were screened for plasmids. Approximately one-third of the 40 isolates carried one or more plasmids ranging in mass from 2.5 to 36 megadaltons, all exhibiting different restriction enzyme digest patterns. In contrast, only Corynebacterium diphtheriae strains comprising a single cohort of apparently identical Emr, pNG2-carrying isolates have been identified as plasmid carriers. Homology was demonstrated between pNG2 and a number of fragments in restriction enzyme digests of plasmids from both Emr and Ems skin coryneforms under high-stringency conditions. However, none was detected between pNG2 and the genomic or plasmid DNAs of Emr staphylococci or streptococci isolated concurrently with the Emr coryneforms. One coryneform plasmid, pNG34, exhibited extensive homology with pNG2, and many comigrating fragments were observed. Very little relationship was observed between C. diphtheriae and the skin coryneforms when their genomic DNAs were hybridized. The origin and presence of pNG2 in Emr C. diphtheriae is discussed in relation to these findings.