Differing mechanisms of surviving phagosomal stress among group BStreptococcusstrains of varying genotypes

Abstract

Group B Streptococcus (GBS), a leading cause of neonatal sepsis and meningitis, asymptomatically colonizes up to 30% of women and can persistently colonize even after antibiotic treatment. Previous studies have shown that GBS resides inside macrophages, but the mechanism by which it survives remains unknown. Here, we examined the ability of 4 GBS strains to survive inside macrophages and then focused on 2 strains belonging to sequence type (ST)-17 and ST-12, to examine persistence in the presence of antibiotics. A multiple stress medium was also developed using several stressors found in the phagosome to assess the ability of 30 GBS strains to withstand phagosomal stress. The ST-17 strain was more readily phagocytosed and survived intracellularly longer than the ST-12 strain, but the ST-12 strain was tolerant to ampicillin unlike the ST-17 strain. Exposure to sub-inhibitory concentrations of ampicillin and erythromycin increased the level of phagocytosis of the ST-17 strain, but had no effect on the ST-12 strain. In addition, blocking acidification of the phagosome decreased the survival of the ST-17 strain indicating a pHdependent survival mechanism for the ST-17 strain. Congruent with the macrophage experiments, the ST-17 strain had a higher survival rate in the multiple stress medium than the ST-12 strain, and overall, serotype III isolates survived significantly better than other serotypes. These results indicate that diverse GBS strains may use differing mechanisms to persist and that serotype III strains are better able to survive specific stressors inside the phagosome relative to other serotypes.

Intrinsic Maturational Neonatal Immune Deficiencies and Susceptibility to Group B Streptococcus Infection

“…Moreover, inhibition of the acidification of the phagosome significantly reduced the ability of GBS to survive in macrophages, suggesting that acidic pH is needed for GBS to survive phagosomal stress. This reduced survival, however, was not observed in all of the strains examined, suggesting that diverse strains of GBS are using alternative mechanisms to withstand phagosomal stress (114).…”

“…Contradictory to these results, one study found no significant difference in survival inside murine macrophages between WT and ΔpilB strains (146). One possible explanation for this difference could be that different strains were used, which may vary in the mechanism used to survive inside the phagosome (114). The pilus backbone protein specific for ST-17 lineages, Spb1, was also shown to enhance both phagocytosis and intracellular survival of GBS.…”

“…Because CR3 is important for opsonin-independent phagocytosis by macrophages, GBS was suggested to interact with CR3 in a C3-independent manner (113). Furthermore, the uptake of GBS requires actin (111) and varies by strain type (114). Besides the complement-binding domain, CR3 also contains a lectin domain that is able to bind the type III CPS to initiate phagocytosis in neutrophils (115).…”

Importance of strain lineages for Group B streptococcal survival

“…14 The current study demonstrated that distinct GBS lineages differed in their ability to be phagocytized by and survive within macrophages. 12 Specifically the GBS ST-17 isolate persisted longer in macrophages compared with ST-19 and ST-12 strains, despite having similar impact on macrophage viability. Interestingly following internalization, 24 hours post infection the ST-17 strain exhibited higher transcript levels of key virulence factors (cylE, scpB, lmb, and fbsA) compared with the ST-12 strain.…”

“…In this issue of Virulence, the authors of the article entitled "Differing mechanisms of surviving phagosomal stress among group B Streptococcus strains of varying genotypes" examined how lineage difference may impact GBS tolerance to antibiotics and various stress conditions, as well as intracellular persistence in phagocytic cells. 12 Phagocytic cells such as macrophages and neutrophils play an important role in innate immune defense against classic extracellular pathogens such as GBS. Pathogens that can persist within phagocytic cells and survive the phagolysosomal environment not only resistant phagocytic clearance, but may more readily disseminate into other bodily tissues.…”