Enumeration of Staphylococcus aureus in Foods with Special Reference to Egg‐yolk Reaction and Mannitol Negative Mutants

Abstract

SUMMARY 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 to dissimilate mannitol; among 1164 strains examined, mostly from clinical sources, 4.4 % appeared unable to dissimilate mannitol; (iii) the agar has inhibitory properties to sublethally impaired cells of Staph. aureus. 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 % developed characteristically and generally quantitatively on Baird‐Parker's agar. Baird‐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. aureus.