There has been considerable controversy about the significance and correct method of reading the coagulase test. Research results have indicated that the weak coagulase activity represented by 1+, 2+, and 3+ reactions seldom corresponds with other criteria associated with S. aureus (4).
A consensus of peers has established that a 4+ coagulase reaction is necessary for unquestioned identification of S. aureus.
Those strains suspected of being S. aureus on the basis of coagulase reactions of less than 4+ should be confirmed by other tests, such as anaerobic glucose fermentation, lysostaphin sensitivity, and thermonuclease production.
Studies of colonial morphology on Baird-Parker agar, lysostaphin sensitivity, coagulase and thermonuclease production, and glucose and mannitol fermentation were conducted on 100 enterotoxigenic and 51 nonenterotoxigenic strains of S. aureus (3).
In all cases, the reactions of enterotoxigenic and nonenterotoxigenic strains varied by 12% or less.
This research indicates that none of these tests can be relied upon to differentiate toxic and nontoxic staphylococci.