Possible dual function of M protein: resistance to bacteriophage A25 and resistance to phagocytosis by human leukocytes.

Spontaneous phage A25-resistant (A25(R)) mutants of group A streptococci, strain K56, were isolated. The mutant cultures were unable to adsorb phage particles and hyperproduced M protein. Trypsin-digested A25(R) cells regained the ability to adsorb phage particles, but failed to become infectious centers. This failure indicated that the mutation created a double barrier to phage growth: (i) receptors were masked by M protein; (ii) irreversibly adsorbed phage were unable to multiply. Spontaneous variants of one A25(R) mutant, shown to be M negative (M(-)) by electron microscopy, serological tests, and sensitivity to phagocytosis, rapidly adsorbed phage and were able to become infectious centers. Therefore, it was concluded that the mutant phenotype, A25(R), arose by a single mutation and genes coding for this trait and M protein synthesis were either genetically linked, controlled by a common gene or were biochemically interdependent. The A25(R) phenotype was unstable and, as expected for plasmid-coded properties, acridine orange induced segregation of this phenotype. The parental M(+), A25-sensitive (A25(S)) cultures proved to be a mixed population. Infection at various multiplicities indicated that this culture was composed of phage A25(S) cells and cells more resistant to infection. Morphological comparison of thin sections of A25(R) and A25(S) cells by electron microscopy demonstrated striking differences. The A25(R) culture was composed entirely of cells uniformly covered with M protein, whereas the A25(S)M(+) wild-type culture was a mixed population, the majority of cells devoid of M protein. Phagocytosis by human blood enriched the culture for the latter cell type, suggesting that differences in phage sensitivity in the wild-type culture were also determined by the presence or absence of M protein. Thus M protein can serve a dual function for the streptococcal cell by allowing it to avoid infection by bacteriophage and ingestion by human leukocytes.