Some Observations on Staphylococcal Pigmentation

Abstract

VARIOUS substances added to nutrient agar media are known to enhance colonial pigmentation of Staphylococcus aureus. These include sterile cow's milk^1,2, fermentable carbohydrate³, sodium chloride⁴, glycerophosphoric acid⁵ and glycerol monoacetate⁶. Not all these are equally effective, however, the most intense pigmentation being developed on milk agar and on glycerol monoacetate agar. Glycerol monoacetate agar has the additional advantage that strains of Staph. aureus growing on it can be readily differentiated into a number of colour varieties—yellow, orange, buff and white—after incubation for 48 h. In our hands, milk agar has given irregular results. Thus, although the medium has consistently induced strongly pigmented growth, some batches of it have shown little or no differentiation between the colour types, even after prolonged incubation, while other batches have developed striking colour differentiation after incubation for only 24 h. These inconsistencies led us to examine different components of cow's milk in order to determine the nature of its pigment-stimulating fraction.