Persistent Staphylococcus aureus isolates from two independent cases of bacteremia display increased bacterial fitness and novel immune evasion phenotypes

Intestinal newborn regulatory B cell antibodies modulate microbiota communities

The role of immunoglobulins produced by IL-10-producing regulatory B cells remains unknown. We found that a particular newborn regulatory B cell population (nBreg) negatively regulates the production of immunoglobulin M (IgM) via IL-10 in an autocrine manner, limiting the intensity of the polyreactive antibody response following innate activation. Based on nBreg scRNA-seq signature, we identify these cells and their repertoire in fetal and neonatal intestinal tissues. By characterizing 205 monoclonal antibodies cloned from intestinal nBreg, we show that newborn germline-encoded antibodies display reactivity against bacteria representing six different phyla of the early microbiota. nBreg-derived antibodies can influence the diversity and the cooperation between members of early microbial communities, at least in part by modulating energy metabolism. These results collectively suggest that nBreg populations help facilitate early-life microbiome establishment and shed light on the paradoxical activities of regulatory B cells in early life.

Berberine hydrochloride reduces staphyloxanthin synthesis by inhibiting fni genes in methicillin-resistant Staphylococcus aureus

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) poses a great health threat to humans. Looking for compounds that could reduce the resistance of S. aureus towards methicillin is an effective way to alleviate the antimicrobial resistance crisis.

METHODS AND RESULTS

Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), Time-killing growth curve, staphyloxanthin and penicillin-binding protein 2a (PBP2a) were detected. A quantitative polymerase chain reaction was used to measure the effect of BBH on the gene transcription profiles of MRSA. The MIC of MRSA-ST59-t437 towards oxacillin was 8 µg/ml, and MBC was 128 µg/ml. After adding a sub-inhibitory concentration of BBH, the MIC and MBC of MRSA-ST59-t478 towards oxacillin went down to 0.125 and 32 µg/ml respectively. The amount of PBP2a and staphyloxanthin were reduced after treatment with BBH. Moreover, the transcription levels of sarA, mecA and fni genes were downregulated.

CONCLUSIONS

It is for the first time reported that BBH could inhibit staphyloxanthin synthesis by inhibiting fni gene. Moreover, fni might be the target gene of sarA, and there might be another regulatory pathway to inhibit staphyloxanthin biosynthesis. BBH could effectively reduce the methicillin resistance of MRSA-ST59-t437 by downregulating fni, sarA and mecA genes.

Limited Adaptation of Staphylococcus aureus during Transition from Colonization to Invasive Infection

Staphylococcus aureus carriage is a risk factor for invasive infections. Unique genetic elements favoring the transition from colonizing to invasive phenotype have not yet been identified, and phenotypic adaptation traits are understudied. We therefore assessed phenotypic and genotypic profiles of 11 S. aureus isolate pairs sampled from colonized patients simultaneously suffering from invasive S. aureus infections. Ten out of 11 isolate pairs displayed the same spa and multilocus sequence type, suggesting colonization as an origin for the invasive infection. Systematic analysis of colonizing and invasive isolate pairs showed similar adherence, hemolysis, reproductive fitness properties, antibiotic tolerance, and virulence in a Galleria mellonella infection model, as well as minimal genetic differences. Our results provide insights into the similar phenotypes associated with limited adaptation between colonizing and invasive isolates. Disruption of the physical barriers of mucosa or skin was identified in the majority of patients, further emphasizing colonization as a major risk factor for invasive disease. IMPORTANCE S. aureus is a major pathogen of humans, causing a wide range of diseases. The difficulty to develop a vaccine and antibiotic treatment failure warrant the exploration of novel treatment strategies. Asymptomatic colonization of the human nasal passages is a major risk factor for invasive disease, and decolonization procedures have been effective in preventing invasive infections. However, the transition of S. aureus from a benign colonizer of the nasal passages to a major pathogen is not well understood, and both host and bacterial properties have been discussed as being relevant for this behavioral change. We conducted a thorough investigation of patient-derived strain pairs reflecting colonizing and invasive isolates in a given patient. Although we identified limited genetic adaptation in certain strains, as well as slight differences in adherence capacity among colonizing and invasive isolates, our work suggests that barrier breaches are a key event in the disease continuum of S. aureus.

Air pollution induces Staphylococcus aureus USA300 respiratory tract colonization mediated by specific bacterial genetic responses involving the global virulence gene regulators Agr and Sae

Exposure to particulate matter (PM), a major component of air pollution, is associated with exacerbation of chronic respiratory disease, and infectious diseases such as community-acquired pneumonia. Although PM can cause adverse health effects through direct damage to host cells, our previous study showed that PM can also impact bacterial behaviour by promoting in vivo colonization. In this study we describe the genetic mechanisms involved in the bacterial response to exposure to black carbon (BC), a constituent of PM found in most sources of air pollution. We show that Staphylococcus aureus strain USA300 LAC grown in BC prior to inoculation showed increased murine respiratory tract colonization and pulmonary invasion in vivo, as well as adhesion and invasion of human epithelial cells in vitro. Global transcriptional analysis showed that BC has a widespread effect on S. aureus transcriptional responses, altering the regulation of the major virulence gene regulators Sae and Agr and causing increased expression of genes encoding toxins, proteases and immune evasion factors. Together these data describe a previously unrecognized causative mechanism of air pollution-associated infection, in that exposure to BC can increase bacterial colonization and virulence factor expression by acting directly on the bacterium rather than via the host.

Microbiological and Molecular Features Associated with Persistent and Relapsing Staphylococcus aureus Prosthetic Joint Infection

Background: Persistent and relapsing prosthetic joint infection (PJI) due to Staphylococcus aureus presents a clinical challenge. This study aimed to provide an extensive description of phenotypic and genomic changes that could be related to persistence or relapse. Methods: Initial and second S. aureus isolates from 6 cases of persistent and relapsing PJI, along with clinical isolates from 8 cases, with favorable outcome were included. All isolates were studied by phenotypic and genotypic approaches. Results: Recurrent S. aureus isolates exhibited a significant increase in adhesive capacity, invasion and persistence compared to resolved isolates. No association was found for the presence or absence of certain genes with the persistence or relapse of PJI. All sequential isolates showed identical sequence type (ST). Resistance gene loss during the infection and a great diversity of variants in different virulence genes between the pair of strains, mainly in genes encoding adhesins such as fnbA, were observed. Conclusions: S. aureus-caused relapse and persistence PJI is associated with bacterial phenotypical and genotypical adaptation. The main paths of adaptation were persistence in the intracellular compartment, and the loss of antibiotic resistance genes and variant acquisition, especially in genes encoding adhesins.

The relation between the gut microbiome and osteoarthritis: A systematic review of literature

Background

Along with mechanical and genetic factors, emerging evidence suggests that the presence of low-grade inflammation has a role in the pathogenesis of osteoarthritis (OA) and seems to be related to the microbiome composition of the gut.

Purpose

To provide evidence whether there is clinical or preclinical evidence of gut-joint axis in the pathogenesis and symptoms of OA.

Methods

An extensive review of the current literature was performed using three different databases. Human, as well as animal studies, were included. The risk of bias was identified using ROBINS and SYRCLE tools, while the quality of evidence was assessed using GRADE and CAMADARES criteria.

Results

A total of nineteen articles were included. Multiple animal studies demonstrated that both obesity, and high-fat and high-sugar diets resulted in a gut dysbiosis status characterized by increased Firmicutes/Bacteroidetes (F/B) phyla ratio and increased permeability. These changes were associated with increased lipopolysaccharide serum levels, which consequently resulted in synovitis and OA severity. The administration of pre-and probiotics partially reversed this bacterial composition. In addition, in human studies, a decreased amount of gut Bacteroidetes, subsequent increased F/B ratio, have also been observed in OA patients.

Conclusions

Our review confirms preliminary yet sound evidence supporting a gut-joint axis in OA in primarily preclinical models, by showing an association between diet, gut dysbiosis and OA radiological severity and self-reported symptoms. Clinical studies are needed to confirm these findings, and to investigate whether interventions targeting the composition of the microbiome will have a beneficial clinical effect.

Gut permeability and osteoarthritis, towards a mechanistic understanding of the pathogenesis: a systematic review

Osteoarthritis (OA) is the most common condition affecting human joints. Along with mechanical and genetic factors, low-grade inflammation is increasingly supported as a causal factor in the development of OA. Gut microbiota and intestinal permeability, via the disruption of tight junction competency, are proposed to explain a gut-joint axis through the interaction with the host immune system. Since previous studies and methods have underestimated the role of the gut-joint axis in OA and have only focussed on the characterisation of microbiota phenotypes, this systematic review aims to appraise the current evidence concerning the influence of gut permeability in the pathogenesis of OA. We propose that the tight junction disruption may be due to an increase in zonulin activity as already demonstrated for many other chronic inflammatory disorders. After years of unreliable quantification, one study optimised the methodology, showing a positive validated correlation between plasma lipopolysaccharide (LPS), obesity, joint inflammation, and OA severity. Chemokines show a prominent role in pain development. Our systematic review confirms preliminary evidence supporting a gut-joint axis in OA pathogenesis and progression. Being modifiable by several factors, the gut microbiota is a promising target for treatment. We propose a pathogenetic model in which dysbiosis is correlated to the bipartite graph of tight junctions and bacterially-produced products, aiming to direct future studies in the search of other bacterial products and tight junction disassembly regulators.KEY MESSAGESPrevious studies and methods have underestimated the impact of the gut-joint axis in osteoarthritis and have focussed on the characterisation of microbiota phenotypes rather than clear molecular mediators of disease.Gut dysbiosis is related to higher levels of bacterial toxins that elicit cartilage and synovium inflammatory pathways.Future research may benefit from focussing on both tight junctions and bacterially-produced products.

The role of mprF mutations in "see-saw effect" of Daptomycin-resistant methicillin-resistant Staphylococcus aureus isolates

The emergence of daptomycin-resistant (DAP-R) Staphylococcus aureus strains has become a global problem. Point mutations in mprF are the main cause of daptomycin (DAP) treatment failure. However, the impact of these specific point-mutations in methicillin-resistant S. aureus (MRSA) strains associated with DAP resistance and the "see-saw effect" of distinct beta-lactams remains unclear. In this study, we used three series of clinical MRSA strains with three distinct mutated mprF alleles from clone complexes (CC) 5 and 59 to explore the "see-saw effect" and the combination effect of DAP plus beta-lactams. Through construction of mprF deletion and complementation strains of SA268, we determined that mprF-S295A, mprF-S337L and one novel mutation of mprF-I348del within the bifunctional domain lead to DAP resistance. Compared with wild-type mprF cloned from a DAP-susceptible (DAP-S) strain, these three mprF mutations conferred the "see-saw effect" to distinct beta-lactams in the SA268ΔmprF strains and mutated-mprF (I348del and S337L) did not alter the cell surface positive charge (P > 0.05). The susceptibility to beta-lactams increased significantly in DAP-R CC59 strains and the "see-saw effect" was found to be associated with distinct mutated mprF alleles and the category of beta-lactams. The synergistic activity of DAP plus oxacillin was detected in all DAP-R MRSA strains. Continued progress in understanding the mechanism of restoring susceptibility to beta-lactam antibiotics mediated by the mprF mutation and its impact on beta-lactam combination therapy will provide fundamental insights into treatment of MRSA infections.

Comparative analysis of genomic characteristics, fitness and virulence of MRSA ST398 and ST9 isolated from China and Germany

Methicillin-resistant Staphylococcus aureus (MRSA) of sequence types ST398 and ST9 are dominant lineages among livestock in Europe and Asia, respectively. Although both STs were commonly found as colonizers of the skin and the mucosal membranes, MRSA ST398, rather than MRSA ST9, has been reported to cause infections in humans and animals. Herein, we comparatively analyzed the genomic characteristics, fitness and virulence of MRSA ST398 and ST9 isolated from pigs in both China (CHN) and Germany (GER) to explore the factors that lead to differences in their epidemics and pathogenicity. We observed that the CHN-MRSA ST9 and the GER-MRSA ST9 have evolved independently, whereas the CHN-MRSA ST398 and GER-MRSA ST398 had close evolutionary relationships. Resistance to antimicrobial agents commonly used in livestock, the enhanced ability of biofilm formation, and the resistance to desiccation contribute to the success of the dominant clones of CHN-MRSA ST9 and GER-MRSA ST398, and the vwb^νSaα gene on the genomic island might in part contribute to their colonization fitness in pigs. All MRSA ST398 strains revealed more diverse genome structures, higher tolerance to acids and high osmotic pressure, and greater competitive fitness in co-culture experiments. Notably, we identified and characterized a novel hysA^νSaβ gene, which was located on the genomic island νSaβ of MRSA ST398 but was absent in MRSA ST9. The enhanced pathogenicity of the MRSA ST398 strains due to hysA^νSaβ might in part explain why MRSA ST398 strains are more likely to cause infections.

Genome-Wide Association Studies for the Detection of Genetic Variants Associated With Daptomycin and Ceftaroline Resistance in Staphylococcus aureus

Background

As next generation sequencing (NGS) technologies have experienced a rapid development over the last decade, the investigation of the bacterial genetic architecture reveals a high potential to dissect causal loci of antibiotic resistance phenotypes. Although genome-wide association studies (GWAS) have been successfully applied for investigating the basis of resistance traits, complex resistance phenotypes have been omitted so far. For S. aureus this especially refers to antibiotics of last resort like daptomycin and ceftaroline. Therefore, we aimed to perform GWAS for the identification of genetic variants associated with DAP and CPT resistance in clinical S. aureus isolates.

Materials/methods

To conduct microbial GWAS, we selected cases and controls according to their clonal background, date of isolation, and geographical origin. Association testing was performed with PLINK and SEER analysis. By using in silico analysis, we also searched for rare genetic variants in candidate loci that have previously been described to be involved in the development of corresponding resistance phenotypes.

Results

GWAS revealed MprF P314L and L826F to be significantly associated with DAP resistance. These mutations were found to be homogenously distributed among clonal lineages suggesting convergent evolution. Additionally, rare and yet undescribed single nucleotide polymorphisms could be identified within mprF and putative candidate genes. Finally, we could show that each DAP resistant isolate exhibited at least one amino acid substitution within the open reading frame of mprF. Due to the presence of strong population stratification, no genetic variants could be associated with CPT resistance. However, the investigation of the staphylococcal cassette chromosome mec (SCCmec) revealed various mecA SNPs to be putatively linked with CPT resistance. Additionally, some CPT resistant isolates revealed no mecA mutations, supporting the hypothesis that further and still unknown resistance determinants are crucial for the development of CPT resistance in S. aureus.

Conclusion

We hereby confirmed the potential of GWAS to identify genetic variants that are associated with antibiotic resistance traits in S. aureus. However, precautions need to be taken to prevent the detection of spurious associations. In addition, the implementation of different approaches is still essential to detect multiple forms of variations and mutations that occur with a low frequency.

Genotypic and Phenotypic Characterization of Staphylococcus aureus Isolates from the Respiratory Tract in Mechanically-Ventilated Patients

Staphylococcus aureus is a commensal and frequent colonizer of the upper respiratory tract. When mechanical ventilation disrupts natural defenses, S. aureus is frequently isolated from the lower airways, but distinguishing between colonization and infection is difficult. The objectives of this study were (1) to investigate the bacterial genome sequence in consecutive isolates in order to identify changes related to the pathological adaptation to the lower respiratory tract and (2) to explore the relationship between specific phenotypic and genotypic features with the patient’s study group, persistence of the clinical isolate and clinical outcome. A set of 94 clinical isolates were selected and corresponded to 34 patients that were classified as having pneumonia (10), tracheobronchitis (11) and bronchial colonization (13). Clinical strains were phenotypically characterized by conventional identification and susceptibility testing methods. Isolates underwent whole genome sequencing using Illumina HiSeq4000. Genotypic characterization was performed with an in-house pipeline (BacterialTyper). Genomic variation arising within-host was determined by comparing mapped sequences and de novo assemblies. Virulence factors important in staphylococcal colonization and infection were characterized using previously established functional assays. (1) Toxin production was assessed using a THP-1 cytotoxicity assay, which reports on the gross cytotoxicity of individual isolates. In addition, we investigated the expression of the major virulence factor, alpha-toxin (Hla) by Western blot. (2) Adhesion to the important extracellular matrix molecule, fibronectin, was determined using a standardized microtitre plate assay. Finally, invasion experiments using THP-1 and A539 cell lines and selected clinical strains were also performed. Repeated isolation of S. aureus from endotracheal aspirate usually reflects persistence of the same strain. Within-host variation is detectable in this setting, but it shows no evidence of pathological adaptation related to virulence, resistance or niche adaptations. Cytotoxicity was variable among isolates with 14 strains showing no cytotoxicity, with these latter presenting an unaltered Fn binding capacity. No changes on cytotoxicity were reported when comparing study groups. Fn binding capacity was reported for almost all strains, with the exception of two strains that presented the lowest values. Strains isolated from patients with pneumonia presented a lower capacity of adhesion in comparison to those isolated during tracheobronchitis (p = 0.002). Hla was detected in 71 strains (75.5%), with most of the producer strains in pneumonia and bronchial colonization group (p = 0.06). In our cohort, Hla expression (presence or absence) in sequential isolates was usually preserved (70%) although in seven cases the expression varied over time. No relationship was found between low cytotoxicity and intracellular persistence in invasion experiments. In our study population, persistent S. aureus isolation from airways in ventilated patients does not reflect pathological adaptation. There is an important diversity of sequence types. Cytotoxicity is variable among strains, but no association with study groups was found, whereas isolates from patients with pneumonia had lower adhesion capability. Favorable clinical outcome correlated with increased bacterial adhesion in vitro. Most of the strains isolated from the lower airways were Hla producers and no correlation with an adverse outcome was reported. The identification of microbial factors that contribute to virulence is relevant to optimize patient management during lower respiratory tract infections.

The Resistance to Host Antimicrobial Peptides in Infections Caused by Daptomycin-Resistant Staphylococcus aureus

Daptomycin is an important antibiotic for the treatment of infections caused by Staphylococcus aureus. The emergence of daptomycin resistance in S. aureus is associated with treatment failure and persistent infections with poor clinical outcomes. Here, we investigated host innate immune responses against clinically derived, daptomycin-resistant (DAP-R) and -susceptible S. aureus paired isolates using a zebrafish infection model. We showed that the control of DAP-R S. aureus infections was attenuated in vivo due to cross-resistance to host cationic antimicrobial peptides. These data provide mechanistic understanding into persistent infections caused by DAP-R S. aureus and provide crucial insights into the adaptive evolution of this troublesome pathogen.

Carvacrol Targets SarA and CrtM of Methicillin-Resistant Staphylococcus aureus to Mitigate Biofilm Formation and Staphyloxanthin Synthesis: An In Vitro and In Vivo Approach

Carvacrol is an essential oil traditionally used in culinary processes as spice due to its aromatic nature and also known for various biological activities. In the present study, the antivirulence efficacy of carvacrol against methicillin-resistant Staphylococcus aureus (MRSA) is explored. MRSA is an opportunistic pathogen capable of causing various superficial and systemic infections in humans. Biofilm formation and virulence factors of MRSA are responsible for its pathogenesis and resistance. Hence, the aim of this study was to explore the antibiofilm and antivirulence efficacy of carvacrol against MRSA. Carvacrol at 75 μg/mL inhibited MRSA biofilm by 93%, and it also decreased the biofilm formation on polystyrene and glass surfaces. Further, microscopic analyses revealed the reduction in microcolony formation and collapsed structure of biofilm upon carvacrol treatment. The growth curve analysis and the Alamar blue assay showed the nonfatal effect of carvacrol on MRSA. Further, carvacrol significantly reduced the production of MRSA biofilm-associated slime and extracellular polysaccharide. In addition, carvacrol strongly inhibited the antioxidant pigment staphyloxanthin and its intermediates' synthesis in MRSA. Inhibition of biofilm and staphyloxanthin by carvacrol enhanced the susceptibility of MRSA to oxidants and healthy human blood. Quantitative polymerase chain reaction (qPCR) analysis unveiled the downregulation of sarA-mediated biofilm gene expression and staphyloxanthin-associated crtM gene expression. The sarA-dependent antibiofilm potential of carvacrol was validated using S. aureus Newman wild-type and isogenic ΔsarA strains. In silico molecular docking analysis showed the high binding efficacy of carvacrol with staphylococcal accessory regulator A (SarA) and 4,4'-diapophytoene synthase (CrtM) when compared to positive controls. Furthermore, the in vivo efficacy of carvacrol against MRSA infection was demonstrated using the model organism Galleria mellonella. The results revealed the nontoxic nature of carvacrol to the larvae and the rescuing potential of carvacrol against MRSA infection. Finally, the current study reveals the potential of carvacrol in inhibiting the biofilm formation and staphyloxanthin synthesis of MRSA by targeting the global regulator SarA and a novel antivirulence target CrtM.

Intestinal methicillin-resistant Staphylococcus aureus causes prosthetic infection via 'Trojan Horse' mechanism: Evidence from a rat model

Aims

Methicillin-resistant Staphylococcus aureus (MRSA) can cause wound infections via a 'Trojan Horse' mechanism, in which neutrophils engulf intestinal MRSA and travel to the wound, releasing MRSA after apoptosis. The possible role of intestinal MRSA in prosthetic joint infection (PJI) is unknown.

Methods

Rats underwent intestinal colonization with green fluorescent protein (GFP)-tagged MRSA by gavage and an intra-articular wire was then surgically implanted. After ten days, the presence of PJI was determined by bacterial cultures of the distal femur, joint capsule, and implant. We excluded several other possibilities for PJI development. Intraoperative contamination was excluded by culturing the specimen obtained from surgical site. Extracellular bacteraemia-associated PJI was excluded by comparing with the infection rate after intravenous injection of MRSA or MRSA-carrying neutrophils. To further support this theory, we tested the efficacy of prophylactic membrane-permeable and non-membrane-permeable antibiotics in this model.

Results

After undergoing knee surgery eight or 72 hours after colonization, five out of 20 rats (25.0%) and two out of 20 rats (10.0%) developed PJI, respectively. Strikingly, 11 out of 20 rats (55.0%) developed PJI after intravenous injection of MRSA-carrying neutrophils that were isolated from rats with intestinal MRSA colonization. None of the rats receiving intravenous injections of MRSA developed PJI. These results suggest that intestinal MRSA carried by neutrophils could cause PJI in our rat model. Ten out of 20 (50.0%) rats treated with non-membrane-permeable gentamicin developed PJI, whereas only one out of 20 (5.0%) rats treated with membrane-permeable linezolid developed PJI.

Conclusion

Neutrophils as carriers of intestinal MRSA may play an important role in PJI development.Cite this article: Bone Joint Res. 2020;9(4):152-161.

Genetic Diversity of Composite Enterotoxigenic Staphylococcus epidermidis Pathogenicity Islands

The only known elements encoding enterotoxins in coagulase-negative staphylococci are composite Staphylococcus epidermidis pathogenicity islands (SePIs), including SePI and S. epidermidis composite insertion (SeCI) regions. We investigated 1545 Staphylococcus spp. genomes using whole-genome MLST, and queried them for genes of staphylococcal enterotoxin family and for 29 ORFs identified in prototype SePI from S. epidermidis FRI909. Enterotoxin-encoding genes were identified in 97% of Staphylococcus aureus genomes, in one Staphylococcus argenteus genome and in nine S. epidermidis genomes. All enterotoxigenic S. epidermidis strains carried composite SePI, encoding sec and sel enterotoxin genes, and were assigned to a discrete wgMLST cluster also containing genomes with incomplete islands located in the same region as complete SePI in enterotoxigenic strains. Staphylococcus epidermidis strains without SeCI and SePI genes, and strains with complete SeCI and no SePI genes were identified but no strains were found to carry only SePI and not SeCI genes. The systematic differences between SePI and SeCI regions imply a lineage-specific pattern of inheritance and support independent acquisition of the two elements in S. epidermidis. We provided evidence of reticulate evolution of mobile elements that contain elements with different putative ancestry, including composite SePI that contains genes found in other coagulase-negative staphylococci (SeCI), as well as in S. aureus (SePI-like elements). We conclude that SePI-associated elements present in nonenterotoxigenic S. epidermidis represent a scaffold associated with acquisition of virulence-associated genes. Gene exchange between S. aureus and S. epidermidis may promote emergence of new pathogenic S. epidermidis clones.

The msaABCR Operon Regulates the Response to Oxidative Stress in Staphylococcus aureus

Staphylococcus aureus has evolved a complex regulatory network that controls a multitude of defense mechanisms against the deleterious effects of oxidative stress stimuli, subsequently leading to the pathogen's survival and persistence in the hosts. Previously, we characterized the msaABCR operon as a regulator of virulence, antibiotic resistance, and the formation of persister cells in S. aureus Deletion of the msaABCR operon resulted in the downregulation of several genes involved in resistance against oxidative stress. Notably, those included carotenoid biosynthetic genes and the ohr gene, which is involved in resistance against organic hydroperoxides. These findings led us to hypothesize that the msaABCR operon is involved in resisting oxidative stress generated in the presence of both H₂O₂ and organic hydroperoxides. Here, we report that a protein product of the msaABCR operon (MsaB) transcriptionally regulates the expression of the crtOPQMN operon and the ohr gene to resist in vitro oxidative stresses. In addition to its direct regulation of the crtOPQMN operon and ohr gene, we also show that MsaB is the transcriptional repressor of sarZ (repressor of ohr). Taken together, these results suggest that the msaABCR operon regulates an oxidative stress defense mechanism, which is required to facilitate persistent and recurrent staphylococcal infections. Moving forward, we plan to investigate the role of msaABCR in the persistence of S. aureus under in vivo conditions.IMPORTANCE This study shows the involvement of the msaABCR operon in resisting oxidative stress by Staphylococcus aureus generated under in vitro and ex vivo conditions. We show that MsaB regulates the expression and production of a carotenoid pigment, staphyloxanthin, which is a potent antioxidant in S. aureus We also demonstrate that MsaB regulates the ohr gene, which is involved in defending against oxidative stress generated by organic hydroperoxides. This study highlights the importance of msaABCR in the survival of S. aureus in the presence of various environmental stimuli that mainly exert oxidative stress. The findings from this study indicate the possibility that msaABCR is involved in the persistence of staphylococcal infections and therefore could be a potential antimicrobial target to overcome recalcitrant staphylococcal infections.

Long-Term Intrahost Evolution of Methicillin Resistant Staphylococcus aureus Among Cystic Fibrosis Patients With Respiratory Carriage

Staphylococcus aureus is the most commonly identified airway colonizer of cystic fibrosis (CF) patients, and infections with methicillin-resistant S. aureus (MRSA) are associated with poor outcomes. Yet, little is known about the intrahost evolution of S. aureus among CF patients. We investigated convergent evolution and adaptation of MRSA among four CF patients with long-term respiratory carriage. For each patient, we performed whole-genome sequencing on an average of 21 isolates (range: 19–23) carried for a mean of 1,403 days (range: 903–1,679), including 25 pairs of isolates collected on the same day. We assessed intrahost diversity, population structure, evolutionary history, evidence of switched intergenic regions (IGRs), and signatures of adaptation in the context of patient age, antibiotic treatment, and co-colonizing microbes. Phylogenetic analysis delineated distinct multilocus sequence type ST5 (n = 3) and ST72 (n = 1) clonal populations in addition to sporadic, non-clonal isolates, and uncovered a putative transmission event. Variation in antibiotic resistance was observed within clonal populations, even among isolates collected on the same day. Rates of molecular evolution ranged from 2.21 to 8.64 nucleotide polymorphisms per year, and lineage ages were consistent with acquisition of colonization in early childhood followed by subsequent persistence of multiple sub-populations. Selection analysis of 1,622 core genes present in all four clonal populations (n = 79) found 11 genes variable in three subjects – most notably, ATP-dependent protease clpX, 2-oxoglutarate dehydrogenase odhA, fmtC, and transcription-repair coupling factor mfd. Only one gene, staphylococcal protein A (spa), was found to have evidence of gene-wide diversifying selection. We identified three instances of intrahost IGR switching events, two of which flanked genes related to quorum sensing. The complex microbial ecology of the CF airway poses challenges for management. We illustrate appreciable intrahost diversity as well as persistence of a dominant lineage. We also show that intrahost adaptation is a continual process, despite purifying selective pressure, and provide targets that should be investigated further for their function in CF adaptation.

Phenotypic and Genotypic Characteristics of Methicillin-Resistant Staphylococcus aureus (MRSA) Related to Persistent Endovascular Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus infection and correlates with poor clinical outcomes. MRSA isolates from patients with PB differ significantly from those of resolving bacteremia (RB) with regard to several in vitro phenotypic and genotypic profiles. For instance, PB strains exhibit less susceptibility to cationic host defense peptides and vancomycin (VAN) killing under in vivo-like conditions, greater damage to endothelial cells, thicker biofilm formation, altered growth rates, early activation of many global virulence regulons (e.g., sigB, sarA, sae and agr) and higher expression of purine biosynthesis genes (e.g., purF) than RB strains. Importantly, PB strains are significantly more resistant to VAN treatment in experimental infective endocarditis as compared to RB strains, despite similar VAN minimum inhibitory concentrations (MICs) in vitro. Here, we review relevant phenotypic and genotypic characteristics related to the PB outcome. These and future insights may improve our understanding of the specific mechanism(s) contributing to the PB outcome, and aid in the development of novel therapeutic and preventative measures against this life-threatening infection.

Can Methicillin-resistant Staphylococcus aureus Silently Travel From the Gut to the Wound and Cause Postoperative Infection? Modeling the "Trojan Horse Hypothesis"

OBJECTIVE

To determine whether intestinal colonization with methicillin-resistant Staphylococcus aureus (MRSA) can be the source of surgical site infections (SSIs).

BACKGROUND

We hypothesized that gut-derived MRSA may cause SSIs via mechanisms in which circulating immune cells scavenge MRSA from the gut, home to surgical wounds, and cause infection (Trojan Horse Hypothesis).

METHODS

MRSA gut colonization was achieved by disrupting the microbiota with antibiotics, imposing a period of starvation and introducing MRSA via gavage. Next, mice were subjected to a surgical injury (30% hepatectomy) and rectus muscle injury and ischemia before skin closure. All wounds were cultured before skin closure. To control for postoperative wound contamination, reiterative experiments were performed in mice in which the closed wound was painted with live MRSA for 2 consecutive postoperative days. To rule out extracellular bacteremia as a cause of wound infection, MRSA was injected intravenously in mice subjected to rectus muscle ischemia and injury.

RESULTS

All wound cultures were negative before skin closure, ruling out intraoperative contamination. Out of 40 mice, 4 (10%) developed visible abscesses. Nine mice (22.5%) had MRSA positive cultures of the rectus muscle without visible abscesses. No SSIs were observed in mice injected intravenously with MRSA. Wounds painted with MRSA after closure did not develop infections. Circulating neutrophils from mice captured by flow cytometry demonstrated MRSA in their cytoplasm.

CONCLUSIONS

Immune cells as Trojan horses carrying gut-derived MRSA may be a plausible mechanism of SSIs in the absence of direct contamination.

Antibiotic resistance and host immune evasion in Staphylococcus aureus mediated by a metabolic adaptation

Staphylococcus aureus is a notorious human bacterial pathogen with considerable capacity to develop antibiotic resistance. We have observed that human infections caused by highly drug-resistant S. aureus are more prolonged, complicated, and difficult to eradicate. Here we describe a metabolic adaptation strategy used by clinical S. aureus strains that leads to resistance to the last-line antibiotic, daptomycin, and simultaneously affects host innate immunity. This response was characterized by a change in anionic membrane phospholipid composition induced by point mutations in the phospholipid biosynthesis gene, cls2, encoding cardiolipin synthase. Single cls2 point mutations were sufficient for daptomycin resistance, antibiotic treatment failure, and persistent infection. These phenotypes were mediated by enhanced cardiolipin biosynthesis, leading to increased bacterial membrane cardiolipin and reduced phosphatidylglycerol. The changes in membrane phospholipid profile led to modifications in membrane structure that impaired daptomycin penetration and membrane disruption. The cls2 point mutations also allowed S. aureus to evade neutrophil chemotaxis, mediated by the reduction in bacterial membrane phosphatidylglycerol, a previously undescribed bacterial-driven chemoattractant. Together, these data illustrate a metabolic strategy used by S. aureus to circumvent antibiotic and immune attack and provide crucial insights into membrane-based therapeutic targeting of this troublesome pathogen.

Role of Purine Biosynthesis in Persistent Methicillin-Resistant Staphylococcus aureus Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus endovascular infections. In this study, we investigated potential genetic mechanisms underlying the persistent outcomes. Compared with resolving bacteremia (RB) isolates (defined as isolates associated with negative results of blood cultures 2-4 days after initiation of therapy), PB strains (defined as isolates associated with positive results of blood cultures ≥7 days after initiation of therapy) had significantly earlier onset activation of key virulence regulons and structural genes (eg, sigB, sarA, sae, and cap5), higher expression of purine biosynthesis genes (eg, purF), and faster growth rates, with earlier entrance into stationary phase. Importantly, an isogenic strain set featuring a wild-type MRSA isolate, a purF mutant strain, and a purF-complemented strain and use of strategic purine biosynthesis inhibitors implicated a causal relationship between purine biosynthesis and the in vivo persistent outcomes. These observations suggest that purine biosynthesis plays a key role in the outcome of PB and may represent a new target for enhanced efficacy in treating life-threatening MRSA infections.

Bacteria in the respiratory tract-how to treat? Or do not treat?

BACKGROUND

Acute and chronic respiratory tract infections are a common cause of inappropriate antimicrobial prescription. Antimicrobial therapy leads to the development of resistance and the emergence of opportunistic pathogens that substitute the indigenous microbiota.

METHODS

This review explores the major challenges and lines of research to adequately establish the clinical role of bacteria and the indications for antimicrobial treatment, and reviews novel therapeutic approaches.

RESULTS

In patients with chronic pulmonary diseases and structural disturbances of the bronchial tree or the lung parenchyma, clinical and radiographic signs and symptoms are almost constantly present, including a basal inflammatory response. Bacterial adaptative changes and differential phenotypes are described, depending on the clinical role and niche occupied. The respiratory tract has areas that are potentially inaccessible to antimicrobials. Novel therapeutic approaches include new ways of administering antimicrobials that may allow intracellular delivery or delivery across biofilms, targeting the functions essential for infection, such as regulatory systems, or the virulence factors required to cause host damage and disease. Alternatives to antibiotics and antimicrobial adjuvants are under development.

CONCLUSIONS

Prudent treatment, novel targets, and improved drug delivery systems will contribute to reduce the emergence of antimicrobial resistance in lower respiratory tract infections.

Myricetin protects Galleria mellonella against Staphylococcus aureus infection and inhibits multiple virulence factors

Staphylococcus aureus is an opportunistic pathogen related to a variety of life-threatening infections but for which antimicrobial resistance is liming the treatment options. We report here that myricetin, but not its glycosylated form, can remarkably decrease the production of several S. aureus virulence factors, including adhesion, biofilm formation, hemolysis and staphyloxanthin production, without interfering with growth. Myricetin affects both surface proteins and secreted proteins which indicate that its action is unrelated to inhibition of the agr quorum sensing system. Analysis of virulence related gene expression and computational simulations of pivotal proteins involved in pathogenesis demonstrate that myricetin downregulates the saeR global regulator and interacts with sortase A and α-hemolysin. Furthermore, Myr confers a significant degree of protection against staphylococcal infection in the Galleria mellonella model. The present findings reveal the potential of Myr as an alternative multi-target antivirulence candidate to control S. aureus pathogenicity.

Pathogenic Characteristics of Staphylococcus aureus Endovascular Infection Isolates from Different Clonal Complexes

Staphylococcus aureus is a major cause of bacteremia and, even with appropriate clinical management, causes high morbidity, and mortality due to its involvement in endovascular complications and metastatic infections. Through different pathogenic in vivo and in vitro models we investigated the behavior of S. aureus most relevant clonal complexes (CCs) causing endovascular complications. We analyzed 14 S. aureus strains representing CC5, CC8, CC15, CC30, and CC45 that caused endovascular complications, including methicillin susceptible and resistant isolates and strains with different functionality of the agr global regulator. Their adherence to collagen, interaction with the endothelium, resistance to immune attack, capacity to form biofilm and virulence in the Galleria mellonella model were analyzed. CC30 and CC45 showed greater adhesion to collagen and CC8 showed a trend towards higher rate of intracellular persistence in endothelial cells. All CCs exhibited similar tolerance to neutrophil antimicrobial peptide hNP-1 and were capable of forming biofilms under static conditions. The virulence assay in the G. mellonella model demonstrated that CC15 and CC30 were the most and least virulent, respectively. The analysis of the genomic sequences of the most relevant virulence genes identified some CC15 specific gene patterns (absence of enterotoxins and sak gene) and variants (mainly in leucocidins and proteases), but did not reveal any gene or variant that could be responsible for the increased virulence detected for CC15 strains. Even though all the CCs were capable of causing endovascular complications, our results showed that different CCs are likely to produce these complications through different mechanisms which, if confirmed in more sophisticated models, would indicate the need to more specific management and therapeutic approaches.

Cinnamaldehyde Inhibits Staphylococcus aureus Virulence Factors and Protects against Infection in a Galleria mellonella Model

Bacterial resistance to the available marketed drugs has prompted the search of novel therapies; especially in regards of anti-virulence strategies that aim to make bacteria less pathogenic and/or decrease their probability to become resistant to therapy. Cinnamaldehyde is widely known for its antibacterial properties through mechanisms that include the interaction of this compound with bacterial cell walls. However, only a handful of studies have addressed its effects on bacterial virulence, especially when tested at sub-inhibitory concentrations. Herein, we show for the first time that cinnamaldehyde is bactericidal against Staphylococcus aureus and Enterococcus faecalis multidrug resistant strains and does not promote bacterial tolerance. Cinnamaldehyde actions were stronger on S. aureus as it was able to inhibit its hemolytic activity on human erythrocytes and reduce its adherence to latex. Furthermore, cinnamaldehyde enhanced the serum-dependent lysis of S. aureus. In vivo testing of cinnamaldehyde in Galleria mellonella larvae infected with S. aureus, showed this compound improves larvae survival whilst diminishing bacterial load in their hemolymph. We suggest that cinnamaldehyde may represent an alternative therapy to control S. aureus-induced bacterial infections as it presents the ability to reduce bacterial virulence/survival without promoting an adaptive phenotype.

Comparative analysis of phenol-soluble modulin production and Galleria mellonella killing by community-associated and healthcare-associated meticillin-resistant Staphylococcusaureus strains

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) have emerged globally and have been associated with more severe disease than healthcare-associated MRSA (HA-MRSA). The purpose of this study was to determine whether laboratory measures of virulence can distinguish dominant CA-MRSA clones from HA-MRSA clones. We compared the production of phenol-soluble modulins (PSMs) and ability to kill Galleria mellonella caterpillars for a range of CA- and HA-MRSA strains. Twenty-two HA-MRSA strains [ST22-IV (EMRSA-15), ST36-II (EMRSA-16) and ST239-III] and 26 CA-MRSA strains [ST1-IV (PVL+ USA400), ST1-IV (PVL-), ST8-IV (USA300), ST22-IV (PVL+), ST30-IV, ST59-IV and ST80-IV] were analysed. PSM production was measured using and compared using t-tests and ANOVA. A G mellonella (caterpillar) pathogenicity model was performed, and differences were compared using survival analysis and the log-rank test. There was no significant difference in overall PSM production between HA and CA strains (P=0.090), but there was significant variation between clones (P=0.003). G. mellonella caterpillar killing varied significantly by clone (P<0.001), and overall killing was greater for HA compared with CA clones (P=0.007). The increased acute virulence phenotype of CA-MRSA clones in humans is not associated with increased PSM production in vitro or increased killing in an in vivo caterpillar pathogenicity model.

Staphylococcus aureus: the current state of disease, pathophysiology and strategies for prevention

Staphylococcus aureus is both a commensal organism and also an important opportunistic human pathogen, causing a variety of community and hospital-associated pathologies, such as bacteremia-sepsis, endocarditis, pneumonia, osteomyelitis, arthritis and skin diseases. The resurgence of S. aureus during the last decade in many settings has been facilitated not only by bacterial antibiotic resistance mechanisms but also by the emergence of new S. aureus clonal types with increased expression of virulence factors and the capacity to neutralize the host immune response. Prevention of the spread of S. aureus infection relies on the use of contact precautions and adequate procedures for infection control that so far have not been fully effective. Prevention using a prophylactic vaccine would complement these processes, having the potential to bring additional, significant progress toward decreasing invasive disease due to S. aureus.