A Staphylococcus aureus TIR domain protein virulence factor blocks TLR2-mediated NF-κB signaling

Signaling through Toll-like receptors (TLRs), crucial molecules in the induction of host defense responses, requires adaptor proteins that contain a Toll/interleukin-1 receptor (TIR) domain. The pathogen Staphylococcus aureus produces several innate immune-evasion molecules that interfere with the host's innate immune response. A database search analysis suggested the presence of a gene encoding a homologue of the human TIR domain in S. aureus MSSA476 which was named staphylococcal TIR domain protein (TirS). Ectopic expression of TirS in human embryonic kidney, macrophage and keratinocyte cell lines interfered with signaling through TLR2, including MyD88 and TIRAP, NF-κB and/or mitogen-activated protein kinase pathways. Moreover, the presence of TirS reduced the levels of cytokines MCP-1 and G-CSF secreted in response to S. aureus. The effects on NF-κB pathway were confirmed using S. aureus MSSA476 wild type, an isogenic mutant MSSA476ΔtirS, and complemented MSSA476ΔtirS +pTirS in a Transwell system where bacteria and host cells were physically separated. Finally, in a systematic mouse infection model, TirS promoted bacterial accumulation in several organs 4 days postinfection. The results of this study reveal a new S. aureus virulence factor that can interfere with PAMP-induced innate immune signaling in vitro and bacterial survival in vivo.

Staphylococcus aureus nasal isolates from healthy individuals cause highly variable host cell responses in vitro: the Tromsø Staph and Skin Study

Studies on Staphylococcus aureus populations colonizing the nasal cavity reveal that some bacterial strains are more common, while others are rarely found. This study included five isolates with the most common spa types and five isolates with rare spa types from healthy population. Selected phenotypic traits and genomic content among nasal S. aureus isolates were compared. Besides the rather similar growth rates, our data revealed a high diversity among isolates; that is, in biofilm formation, the ability to attach to and be internalized in keratinocytes as well as ability to induce pro- and anti-inflammatory cytokines. The results showed that S. aureus isolates from healthy hosts are phenotypically diverse and cause highly variable host cell responses. Therefore, generalizing the results from one S. aureus isolate to all is highly questionable.

Bacterial interference with canonical NFκB signalling

The human body is constantly challenged by a variety of commensal and pathogenic micro-organisms that trigger the immune system. Central in the first line of defence is the pattern-recognition receptor (PRR)-induced stimulation of the NFκB pathway, leading to NFκB activation. The subsequent production of pro-inflammatory cytokines and/or antimicrobial peptides results in recruitment of professional phagocytes and bacterial clearance. To overcome this, bacteria have developed mechanisms for targeted interference in every single step in the PRR–NFκB pathway to dampen host inflammatory responses. This review aims to briefly overview the PRR–NFκB pathway in relation to the immune response and give examples of the diverse bacterial evasion mechanisms including changes in the bacterial surface, decoy production and injection of effector molecules. Targeted regulation of inflammatory responses is needed and bacterial molecules developed for immune evasion could provide future anti-inflammatory agents.

Obesity and Staphylococcus aureus nasal colonization among women and men in a general population

BACKGROUND

Obesity and diabetes mellitus (DM) have been linked to increased risk of infections, and Staphylococcus aureus nasal colonization is a major risk factor for developing infections with the microbe. We therefore sought to find whether body mass index (BMI) and waist circumference (WC) could be associated with S. aureus colonization independent of DM.

METHODOLOGY

S. aureus colonization was assessed by nasal swab cultures among 2,169 women and 1,709 men, aged 30-87 years, in the population-based Tromsø Staph and Skin Study in 2007-08. Height (cm), weight (kg), WC (cm), and glycated haemoglobin (HbA1c,%) were measured. Multivariable logistic regression analyses including information on DM, HbA1c, hormonal contraceptive use and other potential confounders were used.

RESULTS

In the female population, each 2.5 kg/m(2) increase in BMI was associated with a 7% higher odds of S. aureus nasal colonization (P = 0.01). When comparing obese and lean women aged 30-43 years, we observed that BMI ≥32.5 versus <22.5 kg/m(2) and WC ≥101 versus <80 cm was associated with a 2.60 and 2.12 times higher odds of S. aureus colonization, respectively (95% confidence intervals 1.35-4.98 and 1.17-3.85). Among men, high WC was also associated with S. aureus nasal colonization. The associations did not change significantly when the analysis was restricted to participants without signs of pre-diabetes (HbA1c <6.0%) among women and men, and to non-users of hormonal contraceptives among women.

CONCLUSION

Our results support that obesity is a possible determinant for S. aureus nasal colonization independent of DM, in particular for premenopausal women. The role of obesity at different ages and by sex should be addressed in future prospective studies of S. aureus colonization.

Staphylococcus aureus mutants lacking cell wall-bound protein A found in isolates from bacteraemia, MRSA infection and a healthy nasal carrier

Staphylococcus aureus is a major human pathogen and a multitude of virulence factors enables it to cause infections, from superficial lesions to life-threatening systemic conditions. Staphylococcal protein A (SpA) is a surface protein contributing to S. aureus pathogenesis by interfering with immune responses and activating inflammation. Seven isolates with frameshift mutations in the spa repeat region were investigated to determine whether these mutations lead to truncation and secretion of SpA into the extracellular environment. Five isolates originated from blood cultures, one from an MRSA infection and one from a persistent nasal carrier. Full-length spa genes from the seven isolates were sequenced, and Western blot experiments were performed to localize SpA. Three isolates had identical deviating 25-bp spa repeats, but all isolates displayed different repeat successions. The DNA sequence revealed that the frameshift mutations created premature stop codons in all seven isolates, resulting in truncated SpA of different lengths, however, all lacking the XC region with the C-terminal sorting signal. SpA was detected by Western blot in six of the seven isolates, mainly extracellularly. Our findings demonstrate that S. aureus isolates with truncated SpA, not anchored to the cell wall, can still be found in bacteraemia, infection and among carriers.

Macrolide-resistant Streptococcus pyogenes in Norway: population structure and resistance determinants

A 2.7% prevalence of macrolide resistance in 1,657 Norwegian clinical Streptococcus pyogenes isolates was primarily due to erm(TR) (59%) and mef(A) (20%). Four clonal complexes comprised 75% of the strains. Macrolide resistance in S. pyogenes in Norway is imported as resistant strains or locally selected in internationally disseminated susceptible clones.

mef(A), mef(E) and a new mef allele in macrolide-resistant Streptococcus spp. isolates from Norway

OBJECTIVES

To type mef genes in a nationwide collection of clinical isolates of Streptococcus pneumoniae and Streptococcus pyogenes as well as pharyngeal carrier strains of viridans streptococci in Norway.

METHODS

Erythromycin-resistant mef-positive multilocus sequence-typed (MLST) clinical isolates of S. pneumoniae (n = 36) and S. pyogenes (n = 12) from the National Surveillance Program for Antimicrobial Resistance (NORM) as well as viridans streptococci (n = 20) from healthy adults were included. PCR-amplified mef genes were initially discriminated by BamHI digestion. Selected mef genes from representatives of different sequence types (STs) of S. pneumoniae (n = 11) and S. pyogenes (n = 4), and viridans group streptococcal species (n = 8) were typed by sequencing and their strains examined for co-resistances. Hydropathy plots of different mef-encoded proteins were performed.

RESULTS

A predominance of mef(A) was detected in S. pneumoniae (23/36) and S. pyogenes (9/12) due to the clonal spread of ST9 and ST39, respectively. mef(E) was the most widely distributed mef determinant occurring in nine different STs of S. pneumoniae and in four different viridans species. A new mef allele was identified in two STs of S. pyogenes.

CONCLUSIONS

mef(E) is the most widely distributed mef determinant in Norwegian clinical strains of S. pneumoniae and pharyngeal carrier strains of various viridans streptococci. However, mef(A) is more prevalent in S. pneumoniae and S. pyogenes due to clonal spread. A new mef allele was found in two different STs of S. pyogenes.

Molecular epidemiology of macrolide-resistant isolates of Streptococcus pneumoniae collected from blood and respiratory specimens in Norway

Norway has a low prevalence of antimicrobial resistance, including macrolide-resistant Streptococcus pneumoniae (MRSP). In a nationwide surveillance program, a total of 2,200 S. pneumoniae isolates were collected from blood cultures and respiratory tract specimens. Macrolide resistance was detected in 2.7%. M-type macrolide resistance was found in 60% of resistant isolates, and these were mainly mef(A)-positive, serotype-14 invasive isolates. The erm(B)-encoded macrolide-lincosamide-streptogramin B (MLS(B)) type dominated among the noninvasive isolates. One strain had an A2058G mutation in the 23S rRNA gene. Coresistance to other antibiotics was seen in 96% of the MLS(B)-type isolates, whereas 92% of the M-type isolates were susceptible to other commonly used antimicrobial agents. Serotypes 14, 6B, and 19F accounted for 84% of the macrolide-resistant isolates, with serotype 14 alone accounting for 67% of the invasive isolates. A total of 29 different sequence types (STs) were detected by multilocus sequence typing. Twelve STs were previously reported international resistant clones, and 75% of the macrolide-resistant isolates had STs identical or closely related to these clones. Eleven isolates displayed 10 novel STs, and 7/11 of these "Norwegian strains" coexpressed MLS(B) and tetracycline resistance, indicating the presence of Tn1545. The invasive serotype-14 isolates were all classified as ST9 or single-locus variants of this clone. ST9 is a mef-positive M-type clone, commonly known as England(14)-9, reported from several European countries. These observations suggest that the import of major international MRSP clones and the local spread of Tn1545 are the major mechanisms involved in the evolution and dissemination of MRSP in Norway.