Staphylococcus aureus Capsular Polysaccharide Types 5 and 8 Reduce Killing by Bovine Neutrophils In Vitro

Abstract

Isogenic variants of Staphylococcus aureus strain Reynolds expressing either no capsule or capsular polysaccharide (CP) type 5 (CP5) or type 8 (CP8) were used to assess the effect of CP on bacterial killing and the respiratory burst of bovine neutrophils. The effects of antisera specific for CP5 and CP8 were also evaluated. The killing of live bacteria by isolated neutrophils was quantified in a bactericidal assay, while the respiratory burst after stimulation with live bacteria in whole blood was measured by flow cytometry. The expression of a CP5 or CP8 capsule protected the bacteria from being killed by bovine neutrophils in vitro (P < 0.001), and the capsule-expressing variants did not stimulate respiratory burst activity in calf whole blood. The addition of serotype-specific antisera increased the killing of the capsule-expressing bacteria and enhanced their stimulating effect in the respiratory burst assay (P < 0.01). When the S. aureus variants were grown under conditions known not to promote capsule expression, there were no significant differences between them. The present study demonstrates that the expression of S. aureus CP5 or CP8 confers resistance to opsonophagocytic killing and prevents the bacteria from inducing respiratory burst of bovine neutrophils in vitro and that these effects can be reversed by the addition of serotype-specific antisera.Many bacterial species have well-characterized capsules which are important virulence factors that potentiate infections (20). In addition to being a major human pathogen, Staphylococcus aureus is the cause of a variety of infections in animals. Most important is mastitis of dairy cows, which compromises animal welfare and causes large economic losses for the dairy industry. S. aureus produces various surface polysaccharides (18,19,22,40), and most strains express capsular polysaccharides (CPs) in vivo or under defined culture conditions (24). Although at least 11 serotypes based on CPs have been proposed, only four CPs (CP1, CP2, CP5, and CP8) have been chemically characterized (15).Phagocytosis and killing by neutrophil granulocytes play a key role in defense against S. aureus infections. CP1 and CP2 have been shown to have antiphagocytic properties (9,17,25,41), but such heavily capsulated strains are rarely isolated clinically (2,31,43). Most clinical isolates from both human and bovine infections produce either CP5 or CP8, although considerable geographic variations exist in the prevalence of these types in bovine isolates (26,32,37).The relatively small amount of CP produced by serotype 5 and 8 strains has made it difficult to define the role of these capsules in virulence. However, the antiphagocytic properties of the S. aureus CP5 capsule, as measured in in vitro opsonophagocytic killing assays, have been shown to play an important role in the pathogenesis of S. aureus infections, most likely by allowing the organism to resist uptake and killing by phagocytes (21, 35).Opsonization is of major importance for effective phagocytosis of most bacterial ...

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.…”

“…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).…”

σ B and the σ B -Dependent arlRS and yabJ-spoVG Loci Affect Capsule Formation in Staphylococcus aureus

“…Although 11 serologically distinct CPs were identified in S. aureus, the majority of clinical isolates produce CPs of serotype 5 (CP-5) or serotype 8 (CP-8). CPs protect S. aureus against opsonophagocytic killing by polymorphonuclear leukocytes (16,17,25,53,56) and enhance virulence in a number of animal models of staphylococcal infection (34,40,53,54,57). Expression of CPs is known to be influenced by various environmental signals in vitro and in vivo (reviewed in references 35 and 56), and transcription of the cap operon was shown to be modulated by regulatory elements, such as arlRS, agr, ccpA, mgr, sae, and sarA (7,8,23,24,26,27,39,48,52,55).…”

Comparison of Capsular Typing of Staphylococcus aureus with Bacteriophage Typing: A Study in Gulbarga, India

“…Among those factors considered for typing, capsular polysaccharides expressed by S. aureus are one of them, since they are also important in the pathogenesis of staphylococcal infections. They enhance staphylococcal virulence by impeding phagocytosis, resulting in bacterial persistence in the bloodstream of infected hosts [2][3][4][5][6]. Most S. aureus isolates are encapsulated and so far eleven capsular serotypes have been described.…”