Use of tartaric acid isomers and citric acid in the biotyping of Salmonella typhimurium

The colour-change and lead acetate tests for fermentation of d–, l–and m–tartaric acids and citric acid used in the Kristensen scheme for biotyping Salmonella typhimurium were found to be unreliable because, whatever the conditions of culture, they gave different results in replicate tests of the same strains. Many genotypically non-fermenting strains gave inconsistent reactions due to the emergence of fermenting mutant bacilli in some of their test cultures. No reliable test was found for the fermentation of citric acid. A ‘turbidity’ test was found to give consistent and reliable results with the three tartaric acid isomers. It demonstrated fermentation by the significantly greater amount of growth obtained in a 24 hr. culture in Oxoid peptone water with added isomer than in a control culture without isomer. Lewis & Stocker's (1971) plate-inhibition test for fermentation of m–tartrate, which identifies m–tartrate-negative strains because m–tartrate inhibits their growth on citrate- or glycerol-containing minimal medium, was found to be as reliable as, and easier to read than, the turbidity test. Use of the turbidity test for d–and l–tartrates and the plate-inhibition test for m–tartrate in biotyping 1435 strains of S. typhimurium showed that many strains had previously been mistyped by the lead acetate test and distinguished 16 new biotypes in addition to the 22 biotypes already recognized.