True identity of control Staphylococcus aureus strains and their performance in the tube coagulase test

One hundred laboratories were asked to submit their control Staphylococcus aureus strains to determine the true identity of strains presumed to be S. aureus NCTC 6571, and also to evaluate the performance of those strains being used as controls in the tube coagulase test (TCT). Of the 60 who replied, 55 laboratories sent at least one strain labelled as S. aureus NCTC 6571 (total of 64 strains). Of these, 84% were identified as S. aureus, and were indistinguishable from a fresh type strain by a combination of phenotypic methods including biotyping, antibiotic susceptibility testing and phage typing. Six-to-ten strains (9-16%), depending on the degree of stringency, were not identifiable as S. aureus NCTC 6571. The time since last retrieval from storage ranged from daily to > or = 3 years, but there was no correlation between this duration and the likelihood of differing from S. aureus NCTC 6571. Forty-seven laboratories submitted 51 strains used as controls in the TCT; these included 31 strains labelled as S. aureus NCTC 6571, eight wild strains, three other NCTC strains and nine strains of uncertain origin. Generally, the S. aureus NCTC 6571 strains produced weaker clots than the remainder. None of the S. aureus NCTC 6571 strains was found to be inoculum dependent but four of the other control strains were. The study demonstrates that some laboratories must improve procedures for ensuring that control S. aureus strains retain their true identity, particularly by avoiding repeated subcultures. Laboratories are divided in their use of strong or weak (S. aureus NCTC 6571) positive controls for the TCT. S. aureus NCTC 6571 is a more stringent control for the TCT than other control strains presently being used and is, therefore, to be preferred.