Crystal violet reactions of fresh clinical isolates ofStaphylococcus aureusfrom two British hospitals

Abstract

SUMMARYWhen 168 fresh clinical isolates ofStaphylococcus aureuswere examined for their reactions on a medium containing 1 part in 100 000 crystal violet 50·6% of strains produced a purple appearance, 39·3% produced a white appearance and 10·1% produced a yellow appearance. Purple-reacting isolates were significantly associated with both invasive infections (P < 0·01) and hospital origin (P < 0·001). There were no significant associations between the crystal violet reactions and either animal contact or other properties previously reported to be characteristic of white and yellow-reacting strains (beta haemolysin and bovine coagulase production). The results of phage typing showed associations between susceptibility to group III phages and purple-reacting strains and between phage group II susceptibility and white and yellow-reacting strains. There was also a highly significant association between white reactions on crystal violet agar and susceptibility to lysis by a combination of all three groups (that is, I + II + III) and white-reacting strains were significantly more susceptible to lysis by phages 94 and/or 96, whether as a restricted pattern or as part of a broader pattern. The purple reaction on crystal violet medium may be a reliable marker of the ‘ hospital Staphylococcus’.