Intracellular survival of Streptococcus pyogenes in polymorphonuclear cells results in increased bacterial virulence

Abstract

It has recently been shown that survival within phagocytic cells constitutes an additional strategy used by Streptococcus pyogenes to evade the host defenses. Here we provide evidence that S. pyogenes can escape from the phagosome into the cytoplasm of phagocytic cells. Furthermore, intracellular bacteria seem to undergo phenotypic switching that results in much more virulent microorganisms.

FIG. 1.

Transmission electron photographs of S. pyogenes-infected PMNs present in the spleens of mice at 48 h postinfection. (A) S. pyogenes can be found in large vacuoles within phagocytic cells (filled arrow). (B) S. pyogenes organisms in the process of escaping from the phagocytic vacuole into the cytoplasm of the PMNs (filled arrow) and already free in the cytoplasm of the PMN in the proximity of a phagocytic vacuole (open arrow). (C) Dividing S. pyogenes in the cytoplasm of a PMN. The different lobules of the nuclei of PMNs are indicated by N.

FIG. 2.

Photographs showing the phenotypic switching of intracellularly located S. pyogenes. (A) Two types of colonies raised after spleen homogenates from S. pyogenes-infected mice were plated. Shown are opaque, wrinkled colonies (white arrows) similar to broth-grown colonies and highly capsulated, glossy colonies (gray arrows). (B) Colonies emerging from broth cultures exhibited the opaque, wrinkled phenotype. (C) Colonies generated from intracellularly located bacteria obtained after gentamicin treatment of PMNs isolated from S. pyogenes-infected mice exhibited the highly capsulated, glossy phenotype. (D and E) Transmission electron photographs of highly encapsulated S. pyogenes recovered from glossy colonies before (D) and after (E) treatment with hyaluronidase. The capsule is indicated by *. (F and G) Transmission electron photographs of S. pyogenes recovered from opaque, wrinkled colonies before (F) and after (G) treatment with hyaluronidase.

FIG. 3.

Survival rates (A) and bacterial loads in the tissues (B) of mice intravenously inoculated with either gentamicin-treated spleen cells isolated from S. pyogenes-infected mice (▪, filled bars) or broth-grown S. pyogenes (□, open bars). Mice inoculated with PMNs purified from the spleens of uninfected donors were used as the control (▵). (C) Bacteremia in mice inoculated with similar numbers of either intracellularly or extracellularly located S. pyogenes organisms isolated from tissues of infected mice. Mice inoculated with a comparable number of broth-grown bacteria were used as controls. Bars represent the means ± standard errors of the means of results from experiments with five mice per group.