Overproduction of Type 8 Capsular Polysaccharide AugmentsStaphylococcus aureusVirulence

Abstract

Type 8 capsular polysaccharide (CP8) is the most prevalent capsule type in clinical isolates of Staphylococcus aureus. However, its role in virulence has not been clearly defined. CP8 strains such as strain Becker produce a small amount of capsule on their surface in vitro. In contrast, CP1 strains such as strain M produce a large amount of capsule, which has been shown to be an important antiphagocytic virulence factor. The cap8 and cap1 operons, required for the synthesis of CP8 and CP1, respectively, have been cloned and sequenced. To test whether CP8 contributes to the pathogenesis of S. aureus, we replaced the weak native promoter of the cap8 operon in strain Becker with the strong constitutive promoter of the cap1 operon of strain M. The resultant strain, CYL770, synthesized cap8-specific mRNA at a level about sevenfold higher than that in the parent strain. Remarkably, the CYL770 strain produced about 80-fold more CP8. In a mouse infection model of bacteremia, the CP8-overproducing strain persisted longer in the bloodstream, the liver, and the spleen in mice than the parent strain. In addition, strain CYL770 was more resistant to ospsonophagocytosis in vitro by human polymorphonuclear leukocytes. These results indicate that CP8 is an antiphagocytic virulence factor of S. aureus.For many human pathogens, capsular polysaccharides (CPs) play an important role in bacterial evasion of host immune surveillance, thereby conferring virulence to the pathogens. In Staphylococcus aureus, more than 90% of the strains produce CPs. Among the 11 serotypes of CP identified to date, type 1 CP (CP1), CP2, CP5, and CP8, have been chemically characterized. The repeating units of CP5 and CP8 are almost identical except for the linkages between the amino sugars and the position of the O acetylation. Type 1 and type 2 strains produce a large quantity of capsule. These capsules have been shown to play an important role in virulence. However, type 1 and type 2 strains are rarely isolated clinically. In fact, ϳ53 and ϳ22% of clinical isolates produce CP8 and CP5, respectively. Type 5 and type 8 strains, on the other hand, produce a much smaller amount of capsule than type 1 or type 2 strains in vitro. The small amount of CP5 and CP8 produced by these strains has hampered efforts to define the role of these capsules in virulence. Indeed, several reports concluded that CP5 and CP8 did not confer virulence to S. aureus (see reference 13 for a review). However, recent studies using specific animal models and growth conditions to enhance CP production have shown that CP5 does play a role in the pathogenesis of S. aureus, most probably by evading bacterial uptake and killing by phagocytes (1,18,26). Similar experiments, however, have not been employed to assess the role of CP8 in virulence.The cap1gene cluster, which is required for the synthesis of CP1, has been cloned and sequenced (12, 15). Initial sequencing showed that the cap1 locus was composed of 13 closely linked genes. Recently, we identified two more closely linked cap1 genes...

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

“…Studies documenting the role of CP8 in virulence were lacking until recently, when Luong and Lee (26) showed that a CP8-overproducing mutant was more resistant to in vitro opsonophagocytic killing by human neutrophils than the parental strain Becker. Likewise, the CP8-overproducing strain persisted longer in the bloodstream, liver, and spleen of infected mice than strain Becker.…”

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).…”

Endophthalmitis

“…PVL lyses neutrophils at higher concentrations but is a potent stimulator of the release of proinflammatory cytokines such as IL-8 at lower concentrations (Otto, 2010). Some strains of S. aureus are also encapsulated, rendering them resistant to phagocytosis and more virulent in animal models of sepsis (Luong and Lee, 2002;Thakker et al, 1998). Most clinical isolates of S. aureus produce either Type 5 or Type 8 capsular polysaccharide (Roghmann et al, 2005).…”