Protective effect of heterologous gram-positive vaccine against lethal upper respiratory tract infection with type M50 group A streptococci in mice.

Abstract Type M50 group A streptococci are exceptional for their virulence in mice. However, intranasal (i.n.) vaccination with heat-killed group A streptococci, either of type M50 or M55, or an M12 strain deficient in M-protein, protected mice against i.n. challenge with M50 streptococci (82, 88 and 83% survival, respectively). Significant resistance against M50 streptococci was also noted by i.n. application of heat-killed Lactobacillus fermenti (81% survival) as well as two strains of pneumococci (50 and 79% survival). In contrast, no protective effect was obtained using heat-killed trypsin-treated M55 streptococci. Nor did vaccination with Escherichia coli and Pseudomonas aeruginosa induce protection against type M50. Thus, M protein was not required for immunity against type M50. The results call for a revision of the hitherto accepted view that M proteins are the only candidates for mucosal vaccines against group A streptococci.