Expression of antigenic factor O:54 is associated with the presence of a plasmid in Salmonella.

Salmonella serovar Tonev (antigenic formula, 21,54:b:e,n,x) is classified in the Salmonella group O:54, and contains a 7.5-kilobase plasmid, designated pIP1340. Tonev NV, a variant lacking pIP1340, failed to express O:54 (antigenic formula identical with that of serovar Minnesota, 21:b:e,n,x). Phenotypic tagging of plasmid pIP1340 by transposon Tn5 produced plasmid pIP1341. Factor O:54 expression in Tonev NV was restored after transformation by pIP1341 NDA or after co-transformation by a mixture of pBR322 and pIP1340 DNA. Similarly, the twelve other serovars presently classified in Salmonella group O:54 harboured a small plasmid which was not detectable in isogenic O:54-negative variants isolated from the parent strain. Transfer of plasmid pIP1341 into these O:54-negative variants and also into reference strains of serovars of Salmonella group O:4 (Banana and Typhimurium), group O:7 (Thompson and Mbandaka), group O:8 (Ferruch, Hadar and Kentucky), group O:3,10 (Orion) and group O:21 (Minnesota) supported the conclusion that plasmid pIP1340 encodes or regulates some functions required for factor O:54 expression by various serovars of Salmonella.