Recurrent infections and immune evasion strategies of Staphylococcus aureus

Staphylococcus aureus causes purulent skin and soft tissue infections (SSTIs) that frequently reoccur. Staphylococal SSTIs can lead to invasive disease and sepsis, which are among the most significant causes of infectious disease mortality in both developed and developing countries. Human or animal infections with S. aureus do not elicit protective immunity against staphylococcal diseases. Here we review what is known about the immune evasive strategies of S. aureus that enable the pathogen's escape from protective immune responses. Three secreted products are discussed in detail, staphylococcal protein A (SpA), staphylococcal binder of immunoglobulin (Sbi) and adenosine synthase A (AdsA). By forming a complex with V(H)3-type IgM on the surface of B cells, SpA functions as a superantigen to modulate antibody responses to staphylococcal infection. SpA also captures pathogen-specific antibodies by binding their Fcγ portion. The latter activity of SpA is shared by Sbi, which also associates with complement factors 3d and factor H to promote the depletion of complement. AdsA synthesizes the immune signaling molecule adenosine, thereby dampening innate and adaptive immune responses during infection. We discuss strategies how the three secreted products of staphylococci may be exploited for the development of vaccines and therapeutics.     

Antimicrobial resistance in Staphylococcus aureus

Recognized since 1883 as a common cause of infection, Staphylococcus aureus' preantimicrobial-era bacteremia mortality rate was 82%. The mortality of that era threatens to return as evidence of growing vancomycin resistance undermines the utility of vancomycin therapy. Successful treatment of S. aureus infections requires knowledge of its antimicrobial resistance capacity.     

Lead resistance and sensitivity in Staphylococcus aureus

Abstract Five lead-resistant strains of Staphylococcus aureus were isolated. Plasmid-free lead-sensitive variants were obtained from the three plasmid-bearing strains. Lead-resistant strains tolerated an approximately 600 × higher Pb(NO₃)₂ concentration than lead-sensitive strains. Both types of strains initially bound lead, but only the resistant strains accumulated the metal as an intracellular lead-phosphate.     

Bacitracin sensing and resistance in Staphylococcus aureus

Bacterial two-component systems (TCSs) have been demonstrated to be associated with not only the expression of virulence factors, but also the susceptibility to antibacterial agents. In Staphylococcus aureus, 16 types of TCSs have been identified. We previously found that the inactivation of one uncharacterized TCS (designated as BceRS, MW gene ID: MW2545-2544) resulted in an increase in susceptibility to bacitracin. In this study, we focused on this TCS and tried to identify the TCS-controlled factors affecting the susceptibility to bacitracin. We found that two ABC transporters were associated with the susceptibility to bacitracin. One transporter designated as BceAB (MW2543-2542) is downstream of this TCS, while another (formerly designated as VraDE: MW2620-2621) is separate from this TCS. Both transporters showed homology with several bacitracin-resistance factors in Gram-positive bacteria. Inactivation of each of these two transporters increased the susceptibility to bacitracin. Expressions of these transporters were significantly increased by the addition of bacitracin, while this induction was not observed in the TCS-inactivated mutant. These results indicate that this TCS senses bacitracin, and also positively regulates the expression of two ABC transporters.     

Conjugative trimethoprim resistance in Staphylococcus aureus

A multiply resistant Staphylococcus aureus isolate, WBG7410, harbours plasmids of 38, 26, 2.8, 2.4 and 1.9 kb and transfers trimethoprim and kanamycin resistance at high frequencies by conjugation. The transconjugants contained the 38-kb plasmid, pWBG707, and the 2.8-kb plasmid. Plasmid pWBG707 was shown to encode trimethoprim resistance, was conjugative and mobilised at high frequencies the 2.8-kb plasmid which presumably encodes kanamycin resistance. Plasmid pWBG707 was isolated mostly in the open circular form and analysis with EcoRI restriction endonuclease suggests that pWBG707 is a new conjugative plasmid distinct from the other conjugative plasmids reported in S. aureus.     

Plasmid-determined bleomycin resistance in Staphylococcus aureus

A 1580-bp fragment of Staphylococcus aureus plasmid pUB110 encoding resistance to the DNA synthesis inhibitor bleomycin has been cloned and sequenced. A DNA sequence containing an open reading frame of 405 bp was subcloned into several expression vectors and bleomycin resistance was expressed at high level in Escherichia coli under the control of lambda PL promoter. On induction, a ca. 14,000-Da protein was detected by gel electrophoresis. The bleomycin resistance determinant of the gram-positive plasmid pUB110 was compared to that of the enterobacterial transposon Tn5; limited regions of close relatedness could be identified.     

Stress resistance in Staphylococcus aureus.

Staphylococcus aureus is a major human pathogen of increasing importance as a result of the spread of antibiotic resistance. It causes a wide range of diseases and survives outside the host by virtue of its adaptability and resistance to environmental stress. Several cellular components involved in Staphylococcus aureus stress resistance have begun to be characterized.     

Early expression of SCIN and CHIPS drives instant immune evasion by Staphylococcus aureus

Chemotaxis inhibitory protein of staphylococci (CHIPS) and Staphylococcal complement inhibitor (SCIN) are small, excreted molecules that play a crucial role in the staphylococcal defence against the human innate immune system. Here we show that they both counteract crucial acute responses of our immune system such as complement activation, neutrophil chemotaxis and neutrophil activation. By studying gene expression via promoter-green fluorescent protein fusions, Northern blots and protein expression analyses, we show that SCIN and CHIPS are produced during the early (exponential) growth stages. Although the SCIN and CHIPS genes are expressed simultaneously, they are differently regulated by various Staphylococcus aureus regulatory loci. However, the sae locus is crucial for upregulation of both SCIN and CHIPS. This is the first study that presents the expression of two extracellular S. aureus proteins early during growth. Because SCIN and CHIPS are both efficient modulators of neutrophil chemotaxis, phagocytosis and killing, their early expression is necessary for efficient modulation of the early immune response.     

Staphylococcus aureus metalloprotease aureolysin cleaves complement C3 to mediate immune evasion

Complement is one of the first host defense barriers against bacteria. Activated complement attracts neutrophils to the site of infection and opsonizes bacteria to facilitate phagocytosis. The human pathogen Staphylococcus aureus has successfully developed ways to evade the complement system, for example by secretion of specific complement inhibitors. However, the influence of S. aureus proteases on the host complement system is still poorly understood. In this study, we identify the metalloprotease aureolysin as a potent complement inhibitor. Aureolysin effectively inhibits phagocytosis and killing of bacteria by neutrophils. Furthermore, we show that aureolysin inhibits the deposition of C3b on bacterial surfaces and the release of the chemoattractant C5a. Cleavage analyses show that aureolysin cleaves the central complement protein C3. Strikingly, there was a clear difference between the cleavages of C3 in serum versus purified conditions. Aureolysin cleaves purified C3 specifically in the α-chain, close to the C3 convertase cleavage site, yielding active C3a and C3b. However, in serum we observe that the aureolysin-generated C3b is further degraded by host factors. We pinpointed these factors to be factor H and factor I. Using an aureolysin mutant in S. aureus USA300, we show that aureolysin is essential and sufficient for C3 cleavage by bacterial supernatant. In short, aureolysin acts in synergy with host regulators to inactivate C3 thereby effectively dampening the host immune response.     

Transfer of erythromycin resistance in Staphylococcus aureus.

In the course of an outbreak of enteritis and conjunctivitis, Staphylococcus aureus was isolated from newborn infants. Strains cultured at a later phase of the outbreak differed from those found at the beginning in being resistant to several antibiotics, showing resistance to typing phages and releasing phages of the same lysis spectrum (10(9) p.f.u./ml after heating at 56 degrees C for 2 min). Transduction experiments with a strain and its cell-free lysate showed that inducible erythromycin resistance was transferable to strains isolated at the beginning of the outbreak and to laboratory strains. Plasmid origin of resistance was confirmed by (i) high transduction frequency; (ii) transduction to RN981 rec- mutants; (iii) kinetics of transduction; (iv) elimination of resistance. Mixed culture experiments yielded transductants at high frequency with resistance to erythromycin, streptomycin and tetracycline.