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

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 (1962 b ) 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.