For the enumeration of vegetative cells and spores of
Bacillus cereus
in foods, a mannitol-egg yolk-phenol red-agar has been developed which exploits the failure of
B. cereus
to dissimilate mannitol, and the ability of most strains to produce phospholipase C. When a high degree of selectivity was required, polymyxin B sulfate in a concentration of 10 ppm appeared to be the most effective selective additive. Useful characteristics for the identification of presumptive isolates of
B. cereus
were found to be: morphology, dissimilation of glucose mostly to acetyl methyl carbinol under anaerobic conditions, hydrolysis of starch and gelatin, reduction of nitrate, and growth on 0.25% chloral hydrate agar.
Chapman's medium designed for the enumeration of Staphylococcus aureus showed three shortcomings: (i) micrococci and many Bacillus spp. grow well on this agar and may obscure colonies of Staph. aureus; (ii) some strains of Staph. aureus fail t o dissimilate mannitol; among 1164 strains examined, mostly from clinical sources, 4.4 yo appeared unable to dissimilate mannitol; (iii) the agar has inhibitory properties to sublethally impaired cells of Staph. uureus. The latter deficiency not being correctable, the suitability of the egg yolk-tellurite-glycine-pyruvate agar (ETGPA) of Baird-Parker (1962b) was evaluated. Of 522 strains of Staph. aureus of human and food origin 97.5 yo developed characteristically and generally quantitatively on Baird-Parker's agar.Raird-Parker's medium has proved valuable in ecological studies on the occurrence and development of Staph. aureus in foods incriminated in staphyloenterotoxicosis : among 165 atypical black colonies isolated from 450 food samples only 2% were Staph. aureua.
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