Capsular polysaccharide masks clumping factor A-mediated adherence of Staphylococcus aureus to fibrinogen and platelets.
ABSTRACT Clumping factor A (ClfA) is a Staphylococcus aureus cell wall-associated adhesin that mediates staphylococcal binding to fibrinogen and platelets. Our goals were to determine whether expression of capsular polysaccharide (CP) affected ClfA-mediated adherence of S. aureus and to assess whether the length of the ClfA repeat region influenced this interaction.
ClfA constructs with repeat regions of different lengths were introduced into isogenic S. aureus strains that expressed CP5, CP8, or no CP. S. aureus binding to fibrinogen was assessed in rabbit plasma and on fibrinogen-coated microtiter plates. Adherence of S. aureus strains to platelets was evaluated by flow cytometry and confocal microscopy.
As the length of the ClfA repeat region increased, binding of acapsular S. aureus to fibrinogen-coated microtiter plates was enhanced. By contrast, encapsulated S. aureus expressing the full-length ClfA were poorly adherent. The acapsular S. aureus mutant strain showed a 2-fold increase in platelet binding, compared with the isogenic encapsulated strains. By contrast, platelet aggregation was unaffected by CP production.
CP expression inhibits S. aureus ClfA-mediated binding to fibrinogen and platelets, and a full-length repeat region cannot overcome this inhibition. These findings have important implications for vaccine development, given that CP may mask surface adhesins.
[show abstract] [hide abstract]
ABSTRACT: Staphylococcus aureus, an important bacterial pathogen in the hospital and the community, has become increasingly resistant to multiple antibiotics. Non-antimicrobial approaches to controlling S. aureus are clearly needed. Because many individuals who are susceptible to staphylococcal infections are not competent to mount an effective immune response, passive as well as active immunisation strategies have been explored. A capsular polysaccharide-based vaccine (StaphVAX) showed promise in an initial phase III trial in haemodialysis patients, but was found to be ineffective in a confirmatory trial. Likewise, a human immunoglobulin G (IgG) preparation known as INH-A21 (Veronate) with elevated levels of antibodies to the staphylococcal surface adhesins ClfA and SdrG made it into phase III testing, where it failed to show a clinical benefit in neonates. A number of novel antigens are in pre-clinical trials, including cell-wall-anchored adhesins, surface polysaccharides and exotoxoids. Given the multiple and sometimes redundant virulence factors of S. aureus that enable it to be such a crafty pathogen, if a vaccine is to prove effective it will of necessity be multicomponent, incorporating a number of surface proteins, toxoids and surface polysaccharides.International Journal of Antimicrobial Agents 09/2008; 32 Suppl 1:S71-8. · 4.13 Impact Factor
Article: Antimicrobial activity of fibrinogen and fibrinogen-derived peptides - a novel link between coagulation and innate immunity.[show abstract] [hide abstract]
ABSTRACT: Fibrinogen is a key player in the blood coagulation system, and is upon activation with thrombin converted into fibrin that subsequently forms a fibrin clot. In the present study, we investigated the role of fibrinogen in the early innate immune response. Here we show that the viability of fibrinogen-binding bacteria is affected in human plasma activated with thrombin. Moreover, we found that the peptide fragment GHR28 released from the β-chain of fibrinogen has antimicrobial activity against bacteria that bind fibrinogen to their surface, whereas non-binding strains are unaffected. Notably, bacterial killing was detected in Group A Streptococcus bacteria entrapped in a fibrin clot, suggesting that fibrinogen and coagulation is involved in the early innate immune system to quickly wall off and neutralise invading pathogens.Thrombosis and Haemostasis 03/2013; 109(5). · 5.04 Impact Factor
Article: Different bacterial gene expression patterns and attenuated host immune responses are associated with the evolution of low-level vancomycin resistance during persistent methicillin-resistant Staphylococcus aureus bacteraemia.[show abstract] [hide abstract]
ABSTRACT: Low-level vancomycin resistance in Staphylococcus aureus (vancomycin-intermediate S. aureus (VISA) and hetero-VISA [hVISA]) emerges during persistent infection and failed vancomycin therapy. Up-regulation of genes associated with the "cell wall stimulon" and mutations in the vraSR operon have both been implicated in the development of resistance, however the molecular mechanisms of resistance are not completely understood. To further elucidate the mechanisms leading to resistance transcriptome comparisons were performed using multiple clinical pairs of vancomycin-susceptible S. aureus (VSSA) and hVISA/VISA (n = 5), and three VSSA control pairs from hospitalized patients with persistent bacteraemia that did not develop hVISA/VISA. Based on the transcriptome results multiple genes were sequenced and innate immune system stimulation was assessed in the VSSA and hVISA/VISA pairs. Here we show that up-regulation of vraS and the "cell wall stimulon" is not essential for acquisition of low-level vancomycin resistance and that different transcriptional responses occur, even between closely related hVISA/VISA strains. DNA sequencing of vraSR, saeSR, mgrA, rot, and merR regulatory genes and upstream regions did not reveal any differences between VSSA and hVISA/VISA despite transcriptional changes suggesting mutations in these loci may be linked to resistance in these strains. Enhanced capsule production and reduced protein A expression in hVISA/VISA were confirmed by independent bioassays and fully supported the transcriptome data. None of these changes were observed in the three control pairs that remained vancomycin-susceptible during persistent bacteremia. In a macrophage model of infection the changes in cell surface structures in hVISA/VISA strains were associated with significantly reduced NF-kappaB activation resulting in reduced TNF-alpha and IL-1beta expression. We conclude that there are multiple pathways to low-level vancomycin resistance in S. aureus, even among closely related clinical strains, and these can result in an attenuated host immune response. The persistent infections associated with hVISA/VISA strains may be a consequence of changes in host pathogen interactions in addition to the reduced antibiotic susceptibility.BMC Microbiology 02/2008; 8:39. · 3.04 Impact Factor