Abstract
When 168 fresh clinical isolates of Staphylococcus aureus were 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 less than 0.01) and hospital origin (P less than 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'.