A comparison of selective media for the isolation of Salmonella from commercial egg white solids.

The determination of members of the genus Salmonella present in small numbers in egg solids has always been somewhat uncertain. Among other things, the nature of the product tends to reduce the selectivity of the various media used for their detection. Also some of the enrichment media tend to inhibit the growth of certain types of Salmonella (Ayres, 1953; Banwart and Ayres, 1953; Hurlev and Ayres, 1953). Banwart and Ayres (1953) found that tetrathionate broth was definitely inhibitory to Salmonella paratyphi and that, during the initial incubation period, inhibition and actual destruction of Salmonella anatis occurred. In this same investigation, brilliant green agar was found to support more growth of all the types of Salmonella studied, while bismuth sulfite was found to be inhibitory to S. typhimurium, S. anatis, S. worthington, and S. oranienburg. Selenite F broth has been recommended generally by most laboratories for enrichment of Salmonella in egg solids. However, the results obtained with selenite F broth are somewhat unsatisfactory because of overgrowth on the secondary plating medium. In some cases, the plates become so overgrown by acid producers that it is impossible to recognize a colony of Salmonella on brilliant green agar, where an acid base indicator is used. For this reason, 2 different agars, brilliant green agar and bismuth sulfite agar, have been employed. Bismuth sulfite agar is used because it is more inhibitory than brilliant green agar to organisms other than the Salmonella. Because of the loss of selective properties of enrichment broth upon the addition of egg products, Stokes and Osborne (1955) developed and evaluated 2 modified selenite broth media. The first was selenite F broth modified by the addition of yeast extract, substitution of mannitol for lactose, and use of a bile salt and brilliant green dye as selective agents in addition to sodium selenite. This broth was reported to be more effective in preventing growth of strains of Escherichia and Proteus than the commonly used selenite F broth. However, with the addition of egg solids this selectivity was largely lost. The second modification was made by the addition of sodium sulfapyridine to the first formula. This addition tended to neutralize the effect of egg products on selectivity.