Possible Dual Function of M Protein: Resistance to Bacteriophage A25 and Resistance to Phagocytosis by Human Leukocytes

Abstract

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^SM⁺ 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.