Chromogenesis by Staphylococcus aureus and typing according to pigmentation

The possibility of typing Staphylococcus aureus strains according to the color of colony pigments has been explored by a number of practical experiments. A technique has been developed for measuring the shade and intensity of pigments by using standard color cards and the results have been compared with those of phage typing and antibiograms. The data which were obtained from epidemics and isolated clinical infections illustrate situations in which strains were readily identified by pigment variations alone by the use of 2 or, in some instances, 3 methods. Results obtained by the pigment method are fairly comparable with those from phage typing and antibiograms when one considers outbreaks with only a few strains involved. On the other hand, when numerous strains are involved, analysis by this method may prove too insensitive. Experiments have shown that the intensity of color production decreases when the cultures are stored on stock culture agar in sealed test tubes. The decrease is stepwise, beginning after 5 to 12 months of storage. It is not synchronous in otherwise identical strains and, therefore, it seems to be influenced by factors which are not yet controllable. The method described therefore can be used only with strains recently isolated from their sources. However, this method may be convenient for screening, as it is simple, inexpensive, and relatively easy to standardize. Used together with antibiotic sensitivity tests, it should make a fairly good strain identification possible by most clinical bacteriology laboratories.