Staphylococcus aureusStrains That Express Serotype 5 or Serotype 8 Capsular Polysaccharides Differ in Virulence

Abstract

Most isolates of Staphylococcus aureus produce a serotype 5 (CP5) or 8 (CP8) capsular polysaccharide. To investigate whether CP5 and CP8 differ in their biological properties, we created isogenic mutants of S. aureus Reynolds that expressed CP5, CP8, or no capsule. Biochemical analyses of CP5 and CP8 purified from the isogenic S. aureus strains were consistent with published structures. The degree of O acetylation of each polysaccharide was similar, but CP5 showed a greater degree of N acetylation. Mice challenged with the CP5 ؉ strain showed a significantly higher bacteremia level than mice challenged with the CP8 ؉ strain. Similarly, the CP5 ؉ strain survived preferentially in the bloodstream and kidneys of infected mice challenged with a mixed inoculum containing both strains. The enhanced virulence of the CP5 ؉ strain in vivo correlated with its greater resistance to in vitro killing in whole mouse blood. Likewise, in vitro opsonophagocytic killing assays with human neutrophils and sera revealed greater survival of the Reynolds (CP5) strain, even though the kinetics of opsonization by C3b and iC3b was similar for both the CP5 ؉ and CP8 ؉ strains. Electron micrographs demonstrated C3 molecules on the cell wall beneath the capsule layer for both serotype 5 and 8 strains. Purified CP5 and CP8 stimulated a modest oxidative burst in human neutrophils but failed to activate the alternative complement pathway. These results indicate that CP5 and CP8 differ in a number of biological properties, and these differences likely contribute to the relative virulence of serotype 5 and 8 S. aureus in vivo.Staphylococcus aureus is a major bacterial pathogen that causes a wide spectrum of clinical infections, ranging from localized soft-tissue infections to life-threatening bacteremia and endocarditis (25). Many virulence factors contribute to the pathogenesis of staphylococcal infections, including surfaceassociated adhesins and secreted exoproteins and toxins (35). Like many invasive bacterial pathogens, S. aureus produces a capsular polysaccharide (CP) that enhances its resistance to clearance by host innate immune defenses. Most clinical isolates of S. aureus are encapsulated, and serotype 5 and 8 strains predominate (2,11,40). The type 5 (CP5) and type 8 (CP8) capsular polysaccharides have similar trisaccharide repeating units comprised of N-acetyl mannosaminuronic acid, N-acetyl L-fucosamine, and N-acetyl D-fucosamine (9, 28, 43). CP5 and CP8 are serologically distinct, and this can be attributed to differences in the linkages between the sugars and in the sites of O acetylation.Previous studies have correlated S. aureus capsule production with resistance to in vitro phagocytic uptake and killing (13, 41). Human neutrophils phagocytose capsule-negative mutants in the presence of nonimmune serum with complement activity, whereas serotype 5 isolates require both capsulespecific antibodies and complement for optimal opsonophagocytic killing (4, 41). Nilsson et al. (29) reported that peritoneal macrophages from mice phagocy...

MsaB activates capsule production at the transcription level in Staphylococcus aureus

“…This is particularly important during certain types of infection, such as bacteraemia or septicaemia, when the pathogen must survive in blood (Thakker et al, 1998;Watts et al, 2005;Nilsson et al, 1997;Kampen et al, 2005). In this study, we have demonstrated that the msaABCR operon is essential for capsule production in the two most clinically relevant capsule serotypes of S. aureus, CP5 and CP8.…”

“…These results suggest that the msaABCR operon is important in the bacterial resistance to phagocytosis and serum immune factors. Due to these phenotypes being associated with capsule production (O'Riordan & Lee, 2004;Thakker et al, 1998;Watts et al, 2005;Nilsson et al, 1997), we concluded that the increased susceptibility to phagocytosis and serum killing in the msaABCR and msaB deletion mutants is likely due to the lack of capsule production. This is supported by our finding that there was no significant difference between the msaABCR and the capA : : Tn mutants in survival assays (Fig.…”

“…Several studies have shown that the production of the capsule by S. aureus plays a role in its evasion of either phagocytosis or its elimination by components of the host immune system (O'Riordan & Lee, 2004;Thakker et al, 1998;Watts et al, 2005;Nilsson et al, 1997). We measured the susceptibility of the msaABCR and msaB mutants to phagocytosis by human PMNs in vitro.…”

“…The production of the capsule has been shown to play a major role in bacterial virulence during infection by facilitating the pathogen's survival inside the host as a way to escape phagocytosis (Karakawa et al, 1988;Thakker et al, 1998;Nanra et al, 2013). This is particularly important during the acute phase of certain types of infection, including bacteraemia Thakker et al, 1998;O'Riordan & Lee, 2004;Watts et al, 2005;Nilsson et al, 1997;Kampen et al, 2005). Several attempts have been made to produce an effective vaccine to protect individuals at risk of S. aureus infection.…”

“…After adherence to the host cell is achieved, the bacterium can then proceed to internalize within it, where it can readily escape elimination from the components of the host immune system (Tuchscherr et al, 2010). Regardless of the mechanism, it is clear that the production of the capsule is involved in bacterial pathogenesis by enhancing the ability of the bacterium to survive during the acute phase of infection and in the transition to the chronic stage of infection by either resisting phagocytosis or surviving after ingestion (O'Riordan & Lee, 2004;Thakker et al, 1998;Watts et al, 2005;Nilsson et al, 1997;Voyich et al, 2005;Tuchscherr et al, 2010;Kampen et al, 2005).…”