Juurnd of General Micrubiulugy ( I 975), 87, 1-1 o Printed in Great Britain I Mutations in Salmonella typhimurium Conferring Resistance

Several mutants obtained from smooth SalmoneIla typhimurium strains by selec-tion for resistance to Felix 0 (FO) phage [whose receptor site 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 (LD,,> 107, compared with < IOO for its parent); L T ~ 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