Modification of the Conventional Procedure for the Test of Staphylococcal Coagulase

The coagulase test is routinely used for the confirmation of suspect Staphylococcus aureus on Baird-Parker agar (BPA). Overnight (18 to 24 h) preincubation of suspect colonies in brain heart infusion (BHI) broth is generally practiced prior to the coagulase test. In order to shorten the test protocol, different preincubation times of S. aureus in BHI were evaluated. Stock cultures (257 strains) of S. aureus were subcultured on BPA, and single colonies of each strain on BPA were analyzed for coagulase activities by the conventional procedure except that the preincubation times in BHI were varied (0, 4, and 24 h). The formation of a clot was examined at 2-h intervals over a 6-h period and at 24 h, and any degree of clot formation (1+ to 4+) was considered a positive reaction. Low sensitivities were found for tests without preincubation in BHI prior to the enzyme test. However, for 4- and 24-h preincubation, there was no difference in the degree of clot formation and in the sensitivities of the coagulase tests with incubation periods from 6 (98.1%) to 24 h (99.6%). Compared with the conventional procedures which may need two days, the modified protocol (4-h preincubation in BHI) can identify a large majority (97.7-98.1 %) of suspect S. aureus on BPA within one working day.