A simple medium for the rapid detection of Salmonella-like paracolon organisms.

Routine investigation of faeces frequently yields paracolon bacilli some of which after 24 hours' incubation produce the biochemical reactions of the salmonella group. Usually these " salmonellalike " paracolon bacilli can be distinguished from salmonellae by negative serological findings and, later, by the delayed fermentation of lactose, sucrose, or salicin. Occasionally, however, they may contain a specific salmonella antigen, or antigens, and although they usually fail to agglutinate with the appropriate antiserum to full titre a strong slide agglutination reaction may be obtained. As most presumptive reports of salmonella isolations are based on slide agglutination results coupled with favourable biochemical reactions, the difficulties presented by organisms of this type are appreciable. A considerable number of salmonella-like paracolon organisms isolated in this laboratory during the past five years have given the following biochemical reactions after 24 hours' incubation: acid and gas were produced from glucose, mannitol, maltose, and dulcitol but not from lactose, sucrose, or salicin; H2S was produced, indole and urease were not formed. After two to five days' incubation all strains fermented lactose and some also fermented salicin. Sucrose was not fermented even after 21 days' incubation. It would appear, therefore, that delayed fermentation of lactose is characteristic of such paracolon bacilli, and a medium enabling these organisms to ferment lactose within 24 hours would obviate the usual delay in distinguishing them from the salmonellae. Massini (1907) demonstrated that delayed fermentation of lactose by Bacterium coli mutabile was due to the slow production of rapid lactosefermenting mutant cells. These mutant cells appeared to retain this characteristic indefinitely. Many workers investigating delayed fermentation of substrates by organisms of the coli-typhoiddysentery group have confirmed the findings of Massini. Lewis (1934), using a synthetic medium containing lactose as the sole carbon source, found that the lactose-fermenting mutants of a mutabile type of colon bacterium occurred at a constant frequency of approximately one lactose-fermenting cell to 100,000 non-lactose-fermenting cells, and that these mutants occurred spontaneously when grown on media not containing lactose. He also showed that unless 50% of lactose-fermenting mutant cells were present in a population, fermentation of 1% lactose peptone water would not occur within 24 hours. With this and other experimental evidence available, it appears that delayed fermentation of substrates by various organisms usually depends on the presence of sufficient rapidly fermenting mutant cells. Kriebel (1934), investigating a number of latelactose-fermenting paracolon bacteria, found that fermentation occurred more rapidly in a medium containing 50% lactose than with greater or lesser amounts. Although the fermentation time of these organisms was shortened, a positive reaction was not obtained within 24 hours. Chilton and Fulton (1946) reported that 10,' lactose agar slants heavily inoculated with slow lactosefermenting paracolon organisms did in some instances give evidence of fermentation within 24 hours. No explanation of these phenomena was offered, but the investigations Kriebel described suggested that further work along these lines was the method most likely to yield a medium of the type sought.