Mutations in Salmonella typhimurium conferring resistance to Felix O phage without loss of smooth character

Abstract

Several mutants obtained from smooth Salmonella typhimurium strains by selection for resistance to Felix O (FO) phage [whose receptor sites includes the N acetylglucosamine branch of the lipopolysaccharide (LPS) core] were smooth in cultural properties, antigenic character and phage sensitivity pattern (except for their FO resistance). However, the affected genes of several such 'FOR' (FO resistant) mutants were shown by transduction to map in the short cysE pyrE segment, which includes nearly all known rfa genes responsible for synthesis of LPS core. All of seven FOR mutants differed from their parents, and resembled rfa mutants with defects in the deeper part of the LPS core, by increased sensitivity to various antibiotics. One FOR mutant was non virulent (LD50 >107, compared with <100 for its parent); LT7 derivatives given this FOR gene by co transduction with cysE+ were likewise non virulent. It is inferred that FOR mutations affect the assembly of the inner part of the LPS core, perhaps causing incomplete blocks in glycosyl transferase reactions.