Evaluation of a Typing Scheme for Group A Streptococci Based Upon Bacteriocin-Like Inhibitor Production

A method of typing (P-typing) streptococci according to their production of inhibitory activity against a set of 9 indicator strains has been evaluated. Modifications to a previously described P-typing procedure have been made which improve the reproducibility of the method and also reduce the proportion of untypable strains. The test conditions, particularly medium composition, incubation temperature and aeration were found to be critical determinants of inhibitor production and must be carefully controlled. The use of certain commercial sources of Blood Agar Base and the presence of human blood in the typing medium are requirements for detection of some inhibitors, particularly for P-type 777 strains. The previously “standardized” conditions for P-typing specify incubation of the test strains aerobically at 32°C. However, in order to increase the proportion of typable strains an additional set of cultures should be incubated anaerobically. Anaerobic incubation was found to be essential for the production of bacteriocin-like inhibitors by M-type 49 strains and markedly increased inhibitor production by M-type 4 and M-type 57 strains. Also associated with anaerobic incubation was an increase in the amount of non-specific, acid-mediated inhibition, but this could be eliminated by buffering the typing medium with 0.5 % (w/v) calcium carbonate. The epidemiological validity of P-typing was tested in a survey of group A streptococcus carriage in a school and in 9 family groupings. Inhibitor production appeared to be a stable strain marker and the occurrence of some P-type subdivisions within clusters of strains having identical serotype patterns indicates that combined application of serotyping and P-typing may give improved strain discrimination.