Streptococcal and staphylococcal tissue infections : therapeutic challenges and opportunities

Abstract

Streptococcus pyogenes and Staphylococcus aureus have been leading causes of human infections throughout history. S. pyogenes is of the top-ten pathogens responsible for most death globally, 0.5 million deaths per year. S. aureus is carried asymptomatically by half the population at any point in time and S. aureus bacteremia is probably the most common life-threatening infection worldwide. These bacteria colonize us, cause mild self-limiting infections such as impetigo and pharyngitis but also rare grave conditions such as streptococcal/staphylococcal toxic shock syndrome (STSS) and necrotizing soft tissue infections (NSTI).

In STSS, patients are recommended to receive adjunctive intravenous immunoglobulin (IVIG) to dampen the mitogenic superantigen-response in T-lymphocytes. In NSTI, the benefit of IVIG treatment is unclear. The first randomized controlled trial of IVIG in NSTI by all microbiological etiologies showed no benefit, but the subgroup dominated by S. pyogenes and S. aureus infections indicated a trend of improved outcome. Here, we assessed plasma samples from these patients, demonstrating that a dose of 25g IVIG is effective at neutralizing toxins from most S. pyogenes strains. The neutralizing capacity of patient plasma correlated with the IVIG dose administered.

In NSTI, the antibiotic treatment recommendations include a -lactam antibiotic such as penicillin, and a toxin-dampening antibiotic such as clindamycin. Using an organotypic 3D model of human skin, we treated S. pyogenes tissue infections with this standard treatment and observed only a minor effect on reduction of bacterial viability. When we added the antibiotic rifampicin as adjunctive treatment, we observed a significant reduction of bacterial viability and metabolism. Bacterial biofilm formation has been recognized as a complicating microbiological feature of S. pyogenes NSTI, and this could be the reason behind the treatment failure and high morbidity and mortality associated with the infections.

S. aureus biofilm formation is regulated by the Accessory gene regulator or Agr system. Using an Agr-silent mutant, we measured biofilm formation by methicillin-resistant S. aureus (MRSA). We observed impaired biofilm dispersal in the Agr-silent MRSA strain, resulting in sustained biofilm formation on polystyrene surfaces. When grown on collagen-coated surfaces, biofilm increased by both strains. In our skin tissue model, both isolates formed biofilm, but the Agr-silent strain did not affect the epithelial integrity while the Agr-signaling strain caused epithelial damage and disseminated into the deeper layers of the tissue.

Host-pathogen interactions are complicated due to the multitude of cells and molecules involved. In this thesis, we have studied bacterial pathogens in their natural habitat: near human cells. Although not as complex as real tissue, our model systems are relevant by mimicking important features of the clinical setting. Our research questions are clinical, and our setup is experimental.