A temperate phage, designated Sf6, has been isolated from Shigella flexneri 3a. Characterization of Sf6 revealed that it possesses the capacity for converting the S. flexneri 3,4 group antigen complex to group factor 6. Serological studies and chemical analysis of lipopolysaccharide from converted strains suggest that group factor 6 is a reflection of an acetylation of the preexisting 3,4 antigen complex. Evidence is provided that the 3,4 group antigen complex functions, at least in part, as a cell surface receptor site for Sf6 adsorption.

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Escherichia coli K-12 X971 was crossed with a smooth Salmonella typhimurium donor, HfrK6, which transfers early the ilv-linked rfa region determining lipopolysaccharide (LPS) core structure. Two ilv+ hybrids differing in their response to the LPS-specific phages FO and C21 were then crossed with S. typhimurium HfrK9, which transfers early the rfb gene cluster determining 0 repeat unit structure. Most recombinants selected for his+ (near rfb) were agglutinated by Salmonella factor 4 antiserum. Transfer of an F' factor (FS400) carrying the rfb-his region of S. typhimurium to the same two ilv+ hybrids gave similar results. LPS extracted from two ilv+,his+, factor 4-positive hybrids contained abequose, the immunodominant sugar for factor 4 specificity. By contrast, his+ hybrids obtained from X971 itself by similar HfrK9 and F'FS400 crosses were not agglutinated by factor 4 antiserum, indicating that the parental E. coli X971 does not have the capacity to attach Salmonella 0 repeat units to its LPS core. It is concluded that the Salmonella rfb genes are expressed only in E. coli X971 hybrids which have also acquired ilv-linked genes (presumably rfa genes affecting core structure or 0-translocase ability, or both) from a S. typhimurium donor. When E. coli X971 was crossed with a smooth E. coli donor, Hfr59, of serotype 08, which transfers his early, most his+ recombinants were agglutinated by E. coli 08 antiserum and lysed by the 08-specific phage, Q8. This suggests that, although the parental E. coli K-12 strain X971 cannot attach Salmonella-specific repeat units to its LPS core, it does have the capacity to attach E. coli 08-specific repeat units.