Lipopolysaccharide O side chain of Yersinia enterocolitica O:3 is an essential virulence factor in an orally infected murine model

The rfb gene cluster of Yersinia enterocolitica O:3, responsible for the biosynthesis of the O side chain, was previously cloned, and a Y. enterocolitica O:3 O side chain-specific bacteriophage (φYeO3-12) was isolated (A. Al-Hendy, P. Toivanen, and M. Skurnik, Microb. Pathog. 10:47-59, 1991). This paper describes the isolation and characterization of the bacteriophage φYeO3-12-resistant mutant of Y. enterocolitica O:3, YeO3-R2. Lipopolysaccharide isolated from YeO3-R2 lacked the O side chain, as evidenced by silver staining and by immunoblots probed with a Y. enterocolitica O:3 O side chain-specific monoclonal antibody. The core was complete, as shown in immunoblots probed with an outer core-specific monoclonal antibody. In Southern blotting with the cloned Y. enterocolitica O:3 rfb region as a probe, there was no detectable difference in the hybridization pattern of chromosomal DNA isolated from YeO3-R2 and that isolated from wild-type Y. enterocolitica O:3. This suggests that a point mutation, rather than a large deletion, was responsible for the rough phenotype of YeO3-R2. The virulence of YeO3-R2 was determined in an orally infected desferal-attenuated murine model. The mutant was ~50-fold less virulent than the isogenic wild type. The ability of YeO3-R2 to reexpress O side chain, and hence full virulence, was reconstituted by complementing the chromosomal mutation in trans with the distal 6.5 kb of the Y. enterocolitica O:3 rfb region. This same 6.5-kb fragment transcomplemented a transposon mutation in the same area of the Y. enterocolitica O:3 rfb region when expressed in Escherichia coli. This transcomplementation implies that the rfb region of Y. enterocolitica O:3 is organized into at least two separate operons.