Early agr activation correlates with vancomycin treatment failure in multi-clonotype MRSA endovascular infections

The Purine Biosynthesis Repressor, PurR, Contributes to Vancomycin Susceptibility of Methicillin-resistant Staphylococcus aureus in Experimental Endocarditis

BACKGROUND

Staphylococcus aureus is the most common cause of life-threatening endovascular infections, including infective endocarditis (IE). These infections, especially when caused by methicillin-resistant strains (MRSA), feature limited therapeutic options and high morbidity and mortality rates.

METHODS

Herein, we investigated the role of the purine biosynthesis repressor, PurR, in virulence factor expression and vancomycin (VAN) treatment outcomes in experimental IE due to MRSA.

RESULTS

The PurR-mediated repression of purine biosynthesis was confirmed by enhanced purF expression and production of an intermediate purine metabolite in purR mutant strain. In addition, enhanced expression of the transcriptional regulators, sigB and sarA, and their key downstream virulence genes (eg, fnbA, and hla) was demonstrated in the purR mutant in vitro and within infected cardiac vegetations. Furthermore, purR deficiency enhanced fnbA/fnbB transcription, translating to increased fibronectin adhesion versus the wild type and purR-complemented strains. Notably, the purR mutant was refractory to significant reduction in target tissues MRSA burden following VAN treatment in the IE model.

CONCLUSIONS

These findings suggest that the purine biosynthetic pathway intersects the coordination of virulence factor expression and in vivo persistence during VAN treatment, and may represent an avenue for novel antimicrobial development targeting MRSA.

Gp05, a Prophage-Encoded Virulence Factor, Contributes to Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections represent a serious public health threat. We recently demonstrated that the presence of a novel prophage ϕSA169 was associated with vancomycin (VAN) treatment failure in experimental MRSA endocarditis. In this study, we assessed the role of a ϕSA169 gene, ϕ80α_gp05 (gp05), in VAN-persistent outcome using gp05 isogenic MRSA strain sets. Of note, Gp05 significantly influences the intersection of MRSA virulence factors, host immune responses, and antibiotic treatment efficacy, including the following: (i) activity of the significant energy-yielding metabolic pathway (e.g., tricarboxylic acid cycle); (ii) carotenoid pigment production; (iii) (p)ppGpp (guanosine tetra- and pentaphosphate) production, which activates the stringent response and subsequent downstream functional factors (e.g., phenol-soluble modulins and polymorphonuclear neutrophil bactericidal activity); and (iv) persistence to VAN treatment in an experimental infective endocarditis model. These data suggest that Gp05 is a significant virulence factor which contributes to the persistent outcomes in MRSA endovascular infection by multiple pathways. IMPORTANCE Persistent endovascular infections are often caused by MRSA strains that are susceptible to anti-MRSA antibiotics in vitro by CLSI breakpoints. Thus, the persistent outcome represents a unique variant of traditional antibiotic resistance mechanisms and a significant therapeutic challenge. Prophage, a critical mobile genetic element carried by most MRSA isolates, provides their bacterial host with metabolic advantages and resistance mechanisms. However, how prophage-encoded virulence factors interact with the host defense system and antibiotics, driving the persistent outcome, is not well known. In the current study, we demonstrated that a novel prophage gene, gp05, significantly impacts tricarboxylic acid cycle activity, stringent response, and pigmentation, as well as vancomycin treatment outcome in an experimental endocarditis model using isogenic gp05 overexpression and chromosomal deletion mutant MRSA strain sets. The findings significantly advance our understanding of the role of Gp05 in persistent MRSA endovascular infection and provide a potential target for development of novel drugs against these life-threatening infections.

MRSA Isolates from Patients with Persistent Bacteremia Generate Nonstable Small Colony Variants In Vitro within Macrophages and Endothelial Cells during Prolonged Vancomycin Exposure

Staphylococcus aureus (especially methicillin-resistant S. aureus [MRSA]) is frequently associated with persistent bacteremia (PB) during vancomycin therapy despite consistent susceptibility in vitro. Strategic comparisons of PB strains versus those from vancomycin-resolving bacteremia (RB) would yield important mechanistic insights into PB outcomes. Clinical PB versus RB isolates were assessed in vitro for intracellular replication and small colony variant (SCV) formation within macrophages and endothelial cells (ECs) in the presence or absence of exogenous vancomycin. In both macrophages and ECs, PB and RB isolates replicated within lysosome-associated membrane protein-1 (LAMP-1)-positive compartments. PB isolates formed nonstable small colony variants (nsSCVs) in vancomycin-exposed host cells at a significantly higher frequency than matched RB isolates (in granulocyte-macrophage colony-stimulating factor [GM-CSF], human macrophages PB versus RB, P < 0.0001 at 48 h; in ECs, PB versus RB, P < 0.0001 at 24 h). This phenotype could represent one potential basis for the unique ability of PB isolates to adaptively resist vancomycin therapy and cause PB in humans. Elucidating the molecular mechanism(s) by which PB strains form nsSCVs could facilitate the discovery of novel treatment strategies to mitigate PB due to MRSA.

Sequence Type 5 (ST5) as a Possible Predictor of Bacterial Persistence in Adult Patients with Methicillin-Resistant Staphylococcus aureus Pneumonia Treated with Vancomycin

Vancomycin remains the mainstay of treatment for methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. This study assessed risk factors for vancomycin failure in 63 patients with MRSA pneumonia through detailed clinical, microbiological, pharmacokinetic/pharmacodynamic, and genetic analyses of prospective multicenter studies conducted from February 2012 to July 2018. Therapeutic drug monitoring was performed during vancomycin treatment, and the 24-h area under the curve (AUC_0-24) was calculated. All baseline strains were collected for MIC determination, heterogeneous vancomycin-intermediate S. aureus (hVISA) screening, and biofilm determination. Whole-genome sequencing was performed on the isolates to analyze their molecular typing and virulence and adhesion genes. Clinical signs and symptoms improved in 44 patients (44/63, 69.8%), with vancomycin daily dose (P = 0.045), peak concentration (P = 0.020), and sdrC (P = 0.047) being significant factors. Isolates were eradicated in 51 patients (51/63, 81.0%), with vancomycin daily dose (P = 0.009), cardiovascular disease (P = 0.043), sequence type 5 (ST5; P = 0.017), tst (P = 0.050), and sec gene (P = 0.044) associated with bacteriological failure. Although the AUC_0-24/MIC was higher in the groups with bacterial eradication, the difference was not statistically significant (P = 0.108). Multivariate analysis showed that no variables were associated with clinical efficacy; ST5 was a risk factor for bacterial persistence (adjusted odds ratio, 4.449; 95% confidence interval, 1.103 to 17.943; P = 0.036). ST5 strains had higher frequencies of the hVISA phenotype, biofilm expression, and presence of some adhesion and virulence genes such as fnbB, tst, and sec than non-ST5 strains. Our study suggests that ST5 is a possible predictor of bacterial persistence in MRSA pneumonia treated with vancomycin. IMPORTANCE Few studies have simultaneously examined the influence of clinical characteristics of patients with pneumonia, the vancomycin pharmacokinetic/pharmacodynamic (PK/PD) index, and the phenotypic and genetic characteristics of methicillin-resistant Staphylococcus aureus (MRSA) strains. We assessed risk factors for vancomycin failure in patients with MRSA pneumonia by analyzing these influences in a prospective multicenter study. Sequence type 5 (ST5) was a possible predictor of bacterial persistence in adult patients with MRSA pneumonia (adjusted odds ratio, 4.449). We found that this may be related to ST5 strains having higher levels of vancomycin heterogeneous resistance, biofilms, and the presence of adhesion and virulence genes such as fnbB, tst, and sec.

Transcriptome Analyses of Prophage in Mediating Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections represent a significant subset of S. aureus infections and correlate with exceptionally high mortality. We have recently demonstrated that the lysogenization of prophage ϕSA169 from a clinical persistent MRSA bacteremia isolate (300-169) into a clinical resolving bacteremia MRSA isolate (301-188) resulted in the acquisition of well-defined in vitro and in vivo phenotypic and genotypic profiles related to persistent outcome. However, the underlying mechanism(s) of this impact is unknown. In the current study, we explored the genetic mechanism that may contribute to the ϕSA169-correlated persistence using RNA sequencing. Transcriptomic analyses revealed that the most significant impacts of ϕSA169 were: (i) the enhancement of fatty acid biosynthesis and purine and pyrimidine metabolic pathways; (ii) the repression of galactose metabolism and phosphotransferase system (PTS); and (iii) the down-regulation of the mutual prophage genes in both 300-169 and 301-188 strains. In addition, the influence of different genetic backgrounds between 300-169 and 301-188 might also be involved in the persistent outcome. These findings may provide targets for future studies on the persistence of MRSA.

Efficacy of antimicrobial peptide LL-37 against biofilm forming Staphylococcus aureus strains obtained from chronic wound infections

The antimicrobial peptide LL-37 showed inhibitory effects against Staphylococcus aureus strains, which often responsible for wound infections. Understanding the molecular mechanisms of biofilm-containing wound infections is important. Thus, this study aimed to investigate both the antimicrobial and biofilm efficacy of LL-37 against biofilm-positive methicillin-susceptible S. aureus (MSSA) strains and biofilm-positive methicillin-resistant S. aureus (MRSA) strains obtained from chronic wound infections and its effect on different quorum sensing and virulence genes at suboptimal concentrations. Fifteen biofilm-forming MRSA and 15 biofilm-forming MSSA strains were included in this study. The minimum inhibitory concentration (MIC) values and biofilm formation were tested by microdilution methods. Real-time PCR was performed to determine gene expression levels. MIC values for LL-37 were 89.6 mg/L and 132.3 mg/L for MSSA and MRSA strains, respectively. No statistically significant difference was found between MRSA and MSSA strains in terms of the effect of LL-37 on biofilm formation. A statistically significant difference was found between MRSA and MSSA strains for atlA, RNAIII, and agrA gene expression levels following exposure to a suboptimal concentration of LL-37. Ultimately, the required LL-37 antimicrobial concentration was quite high; however, LL-37 antibiofilm concentration may be acceptable for use in humans against biofilm-forming MRSA and MSSA strains. This is the first study to investigate to effect of a suboptimal LL-37 concentration on gene expression levels of biofilm-forming MSSA and MRSA strains. LL-37 affected quorum sensing and biofilm producing mechanisms, even at suboptimal MIC concentrations.

Citral modulates virulence factors in methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for high morbidity and mortality rates. Citral has been studied in the pharmaceutical industry and has shown antimicrobial activity. This study aimed to analyze the antimicrobial activity of citral in inhibiting biofilm formation and modulating virulence genes, with the ultimate goal of finding a strategy for treating infections caused by MRSA strains. Citral showed antimicrobial activity against MRSA isolates with minimum inhibitory concentration (MIC) values between 5 mg/mL (0.5%) and 40 mg/mL (4%), and minimum bactericidal concentration (MBC) values between 10 mg/mL (1%) and 40 mg/mL (4%). The sub-inhibitory dose was 2.5 mg/mL (0.25%). Citral, in an antibiogram, modulated synergistically, antagonistically, or indifferent to the different antibiotics tested. Prior to evaluating the antibiofilm effects of citral, we classified the bacteria according to their biofilm production capacity. Citral showed greater efficacy in the initial stage, and there was a significant reduction in biofilm formation compared to the mature biofilm. qPCR was used to assess the modulation of virulence factor genes, and icaA underexpression was observed in isolates 20 and 48. For icaD, seg, and sei, an increase was observed in the expression of ATCC 33,591. No significant differences were found for eta and etb. Citral could be used as a supplement to conventional antibiotics for MRSA infections.

Systematic review and meta-analysis of the epidemiology of vancomycin-resistance Staphylococcus aureus isolates

BACKGROUND

Vancomycin‑resistant Staphylococcus aureus (VRSA) is a serious public health challenging concern worldwide.

OBJECTIVES

Therefore, the objective of present study of 62 published studies was to evaluate the prevalence of VRSA based on different years, areas, isolate source, antimicrobial susceptibility testing, and the genetic determinants.

METHODS

We searched the relevant articles that focused on the prevalence rates of VRSA in PubMed, Scopus, Embase, and Web of Science from 2000 to 2019. Statistical analyses were conducted using STATA software (version 14.0).

RESULTS

The prevalence of VRSA was 2% before 2006, 5% in 2006-2014, and 7% in 2015-2020 that showed a 3.5-fold increase in the frequency of VRSA between before 2006 and 2020 years. The prevalence of VRSA was 5% in Asia, 1% in Europe, 4% in America, 3% in South America, and 16% in Africa. The frequencies of VRSA isolated from clinical, non-clinical, and mixed samples were 6%, 7%, and 14%, respectively. The prevalence of VRSA was 12% using disk diffusion agar method, 7% using MIC-base methods, and 4% using mixed-methods. The prevalence of vanA, vanB, and vanC1 positive were 71%, 26%, and 4% among VRSA strains. The most prevalent genotype was staphylococcal cassette chromosomemec (SCCmec) II, which accounted for 57% of VRSA. The most prevalent staphylococcal protein A (spa) types were t002, t030, and t037.

CONCLUSION

The prevalence of VRSA has been increasing in recent years particularly in Africa/Asia than Europe/America. The most prevalent of genetic determinants associated with VRSA were vanA and SCCmec II. This study clarifies that the rigorous monitoring of definite antibiotic policy, regular surveillance/control of nosocomial-associated infections and intensive surveillance of vancomycin-resistance are required for preventing emergence and further spreading of VRSA.

The Stringent Response Contributes to Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection Through the Purine Biosynthetic Pathway

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections represent a significant clinical-therapeutic challenge. Of particular concern is antibiotic treatment failure in infections caused by MRSA that are "susceptible" to antibiotic in vitro. In the current study, we investigate specific purine biosynthetic pathways and stringent response mechanism(s) related to this life-threatening syndrome using genetic matched persistent and resolving MRSA clinical bacteremia isolates (PB and RB, respectively), and isogenic MRSA strain sets. We demonstrate that PB isolates (vs RB isolates) have significantly higher (p)ppGpp production, phenol-soluble-modulin expression, polymorphonuclear leukocyte lysis and survival, fibronectin/endothelial cell (EC) adherence, and EC damage. Importantly, an isogenic strain set, including JE2 parental, relP-mutant and relP-complemented strains, translated the above findings into significant outcome differences in an experimental endocarditis model. These observations indicate a significant regulation of purine biosynthesis on stringent response, and suggest the existence of a previously unknown adaptive genetic mechanism in persistent MRSA infection.

Antibiofilm effects of N,O-acetals derived from 2-amino-1,4-naphthoquinone are associated with downregulation of important global virulence regulators in methicillin-resistant Staphylococcus aureus

Despite the existing antibiotics, antimicrobial resistance is a major challenge. Consequently, the development of new drugs remains in great demand. Quinones is part of a broad group of molecules that present antibacterial activity besides other biological properties. The main purpose of this study was to evaluate the antibiofilm activities of synthetic N,O-acetals derived from 2-amino-1,4-naphthoquinone [7a: 2-(methoxymethyl)-amino-1,4-naphthoquinone; 7b: 2-(ethoxymethyl)-amino-1,4-naphthoquinone; and 7c: 2-(propynyloxymethyl)-amino-1,4-naphthoquinone] against methicillin-resistant Staphylococcus aureus (MRSA). The derivatives 7b and 7c, specially 7b, caused strong impact on biofilm accumulation. This inhibition was linked to decreased expression of the genes fnbA, spa, hla and psmα3. More importantly, this downregulation was paralleled by the modulation of global virulence regulators. The substitution of 2-ethoxymethyl (7b) in comparison with 2-propynyloxymethyl (7c) enhanced sarA-agr inhibition, decreased fnbA transcripts (positively regulated by sarA) and strongly impaired biofilm accumulation. Indeed, 7b triggered intensive autolysis and was able to eliminate vancomycin-persistent cells. Consequently, 7b is a promising molecule displaying not only antimicrobial effects, but also antibiofilm and antipersistence activities. Therefore, 7b is a good candidate for further studies involving the development of novel and more rational antimicrobials able to act in chronic and recalcitrant infections, associated with biofilm formation.

Impact of the Novel Prophage ϕSA169 on Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections are life-threatening syndromes with few therapeutic options. The potential impact of bacteriophages on the persistent outcome has not been well studied. In this study, we investigated the role of a novel prophage (ϕSA169) in MRSA persistence by using a lysogen-free clinically resolving bacteremia (RB) isolate and comparing it to a derivative which was obtained by infecting the RB strain with ϕSA169, which has been lysogenized in a clinical persistent MRSA bacteremia (PB) isolate. Similar to the PB isolate, the ϕSA169-lysogenized RB strain exhibited well-defined in vitro and in vivo phenotypic and genotypic signatures related to the persistent outcome, including earlier activation of global regulators (i.e., sigB, sarA, agr RNAIII, and sae); higher expression of a critical purine biosynthesis gene, purF; and higher growth rates accompanied by lower ATP levels and vancomycin (VAN) susceptibility and stronger δ-hemolysin and biofilm formation versus its isogenic parental RB isolate. Notably, the contribution of ϕSA169 in persistent outcome with VAN treatment was confirmed in an experimental infective endocarditis model. Taken together, these results indicate the critical role of the prophage ϕSA169 in persistent MRSA endovascular infections. Further studies are needed to identify the mechanisms of ϕSA169 in mediating the persistence, as well as establishing the scope of impact, of this prophage in other PB strains.IMPORTANCE Bacteriophages are viruses that invade the bacterial host, disrupt bacterial metabolism, and cause the bacterium to lyse. Because of its remarkable antibacterial activity and unique advantages over antibiotics, for instance, bacteriophage is specific for one species of bacteria and resistance to phage is less common than resistance to antibiotics. Indeed, bacteriophage therapy for treating infections due to multidrug-resistant pathogens in humans has become a research hot spot. However, it is also worth considering that bacteriophages are transferable and could cotransfer host chromosomal genes, e.g., virulence and antimicrobial resistance genes, while lysogenizing and integrating into the bacterial chromosome (prophage), thus playing a role in bacterial evolution and virulence. In the current study, we identified a novel prophage, ϕSA169, from a clinical persistent MRSA bacteremia isolate, and we determined that ϕSA169 mediated well-defined in vitro and in vivo phenotypic and genotypic signatures related to the persistent outcome, which may represent a unique and important persistent mechanism(s).

Linezolid and Rifampicin Combination to Combat cfr-Positive Multidrug-Resistant MRSA in Murine Models of Bacteremia and Skin and Skin Structure Infection

Linezolid resistance mediated by the cfr gene in MRSA represents a global concern. We investigated relevant phenotype differences between cfr-positive and -negative MRSA that contribute to pathogenesis, and the efficacy of linezolid-based combination therapies in murine models of bacteremia and skin and skin structure infection (SSSI). As a group, cfr-positive MRSA exhibited significantly reduced susceptibilities to the host defense peptides tPMPs, human neutrophil peptide-1 (hNP-1), and cathelicidin LL-37 (P < 0.01). In addition, increased binding to fibronectin (FN) and endothelial cells paralleled robust biofilm formation in cfr-positive vs. -negative MRSA. In vitro phenotypes of cfr-positive MRSA translated into poor outcomes of linezolid monotherapy in vivo in murine bacteremia and SSSI models. Importantly, rifampicin showed synergistic activity as a combinatorial partner with linezolid, and the EC₅₀ of linezolid decreased 6-fold in the presence of rifampicin. Furthermore, this combination therapy displayed efficacy against cfr-positive MRSA at clinically relevant doses. Altogether, these data suggest that the use of linezolid in combination with rifampicin poses a viable therapeutic alternative for bacteremia and SSSI caused by cfr-positive multidrug resistant MRSA.

Bicarbonate Resensitization of Methicillin-Resistant Staphylococcus aureus to β-Lactam Antibiotics

Endovascular infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a major health care concern, especially infective endocarditis (IE). Standard antimicrobial susceptibility testing (AST) defines most MRSA strains as “resistant” to β-lactams, often leading to the use of costly and/or toxic treatment regimens. In this investigation, five prototype MRSA strains, representing the range of genotypes in current clinical circulation, were studied.

Endovascular infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a major health care concern, especially infective endocarditis (IE). Standard antimicrobial susceptibility testing (AST) defines most MRSA strains as “resistant” to β-lactams, often leading to the use of costly and/or toxic treatment regimens. In this investigation, five prototype MRSA strains, representing the range of genotypes in current clinical circulation, were studied. We identified two distinct MRSA phenotypes upon AST using standard media, with or without sodium bicarbonate (NaHCO₃) supplementation: one highly susceptible to the antistaphylococcal β-lactams oxacillin and cefazolin (NaHCO₃ responsive) and one resistant to such agents (NaHCO₃ nonresponsive). These phenotypes accurately predicted clearance profiles of MRSA from target tissues in experimental MRSA IE treated with each β-lactam. Mechanistically, NaHCO₃ reduced the expression of two key genes involved in the MRSA phenotype, mecA and sarA, leading to decreased production of penicillin-binding protein 2a (that mediates methicillin resistance), in NaHCO₃-responsive (but not in NaHCO₃-nonresponsive) strains. Moreover, both cefazolin and oxacillin synergistically killed NaHCO₃-responsive strains in the presence of the host defense antimicrobial peptide (LL-37) in NaHCO₃-supplemented media. These findings suggest that AST of MRSA strains in NaHCO₃-containing media may potentially identify infections caused by NaHCO₃-responsive strains that are appropriate for β-lactam therapy.

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.

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.

Sub-inhibitory concentrations of oxacillin modify the expression of agr locus in Staphylococcus aureus clinical strains belonging to different clonal complexes

Background

The ability of Staphylococcus aureus to invade tissues and cause an infectious disease is the result of a multi-factorial process supported by the huge number of virulence factors inherent to this microorganism tightly regulated by the accessory gene regulator (agr). During antimicrobial therapy bacteria may be exposed to sub-inhibitory concentrations (subMICs) of antibiotics that may trigger transcriptional changes that may have an impact on the pathogenesis of infection. The objective of this study was to investigate the effect of oxacillin sub-MICs on agr system expression as the key component in the regulation of virulence in methicillin-susceptible (MSSA) and -resistant S. aureus (MRSA) strains. Furthermore, we studied the genetic basis of the agr locus and their potential association with the expression levels.

Methods

We have examined the expression of RNAIII and agrA mRNA as biomarkers for agr expression in the presence and absence of oxacillin subMICs in 10 MSSA and 4 MRSA clinical strains belonging to 5 clonal complexes (CC45-agrI, CC8-agrI, CC5-agrII, CC15-agrII and CC30-agrIII) causing endovascular complications. The DNA sequences of agr locus were obtained by whole genome sequencing.

Results

Our results revealed that exposure to subMICs of oxacillin had an impact on agr locus expression modifying the relative levels of expression with increases in 11 strains and with decreases in 3 strains. Thereby, the exposure to subMICs of oxacillin resulted in higher levels of expression of agr in CC15 and CC45 and lower levels in CC30. We also observed the presence of mutations in agrC and agrA in 13/14 strains with similar mutation profiles among strains within individual CCs except for strains of CC5. Although, agr expression levels differed among strains within CCs, the presence of these mutations was associated with differences in agr expression levels in most cases.

Conclusions

Changes in agr expression induced by exposure to oxacillin subMICs should be considered because they could lead to changes in the virulence modulation and have an adverse effect on the course of infection, especially in certain clonal complexes.

Electronic supplementary material

The online version of this article (10.1186/s12879-018-3088-7) contains supplementary material, which is available to authorized users.

Infective endocarditis

Infective endocarditis (IE) is a rare, life-threatening disease that has long-lasting effects even among patients who survive and are cured. IE disproportionately affects those with underlying structural heart disease and is increasingly associated with health care contact, particularly in patients who have intravascular prosthetic material. In the setting of bacteraemia with a pathogenic organism, an infected vegetation may form as the end result of complex interactions between invading microorganisms and the host immune system. Once established, IE can involve almost any organ system in the body. The diagnosis of IE may be difficult to establish and a strategy that combines clinical, microbiological and echocardiography results has been codified in the modified Duke criteria. In cases of blood culture-negative IE, the diagnosis may be especially challenging, and novel microbiological and imaging techniques have been developed to establish its presence. Once diagnosed, IE is best managed by a multidisciplinary team with expertise in infectious diseases, cardiology and cardiac surgery. Antibiotic prophylaxis for the prevention of IE remains controversial. Efforts to develop a vaccine that targets common bacterial causes of IE are ongoing, but have not yet yielded a commercially available product.

The Global Regulon sarA Regulates β-Lactam Antibiotic Resistance in Methicillin-Resistant Staphylococcus aureus In Vitro and in Endovascular Infections

BACKGROUND

 The global regulator sarA modulates virulence of methicillin-resistant Staphylococcus aureus (MRSA) via regulation of principal virulence factors (eg, adhesins and toxins) and biofilm formation. Resistance of S. aureus strains to β-lactam antibiotics (eg, oxacillin) depends on the production of penicillin-binding protein 2a (PBP2a), encoded by mecA METHODS:  In the present study, we investigated the impact of sarA on the phenotypic and genotypic characteristics of oxacillin resistance both in vitro and in an experimental endocarditis model, using prototypic healthcare- and community-associated MRSA parental and their respective sarA mutant strain sets.

RESULTS

 All sarA mutants (vs respective MRSA parental controls) displayed significant reductions in oxacillin resistance and biofilm formation in vitro and oxacillin persistence in an experimental endocarditis model in vivo. These phenotypes corresponded to reduced mecA expression and PBP2a production and an interdependency of sarA and sigB regulators. Moreover, RNA sequencing analyses showed that sarA mutants exhibited significantly increased levels of primary extracellular proteases and suppressed pyrimidine biosynthetic pathway, argininosuccinate lyase-encoding, and ABC transporter-related genes as compared to the parental strain.

CONCLUSIONS

 These results suggested that sarA regulates oxacillin resistance in mecA-positive MRSA. Thus, abrogation of this regulator represents an attractive and novel drug target to potentiate efficacy of existing antibiotic for MRSA therapy.

Comparative efficacy of telavancin and daptomycin in experimental endocarditis due to multi-clonotype MRSA strains

BACKGROUND

MRSA strains of clonal complexes (CCs) 5, 8, 30 and 45 are leading causes of complicated endovascular infections associated with suboptimal clinical outcomes. Telavancin is a novel anti-MRSA agent that both inhibits bacterial cell wall synthesis and disrupts membranes by depolarization.

METHODS

In this study, we compared the in vitro susceptibility and in vivo efficacy of telavancin versus daptomycin in an experimental rabbit infective endocarditis (IE) model caused by four MRSA strains representing each of the above CC types.

RESULTS

All study strains were susceptible to telavancin (MICs of ≤0.12 mg/L) and daptomycin (MICs of ≤0.5 mg/L). In vitro time-kill analyses revealed that supra-MIC levels of telavancin were effective at preventing regrowth at 24 h of incubation. In the IE animal model for all CC types, treatment with telavancin produced significantly greater reductions in MRSA counts as compared with daptomycin-treated animals in all target tissues. Moreover, telavancin-treated animals had a significantly higher percentage of sterile tissue cultures versus daptomycin-treated animals (e.g. 78%-100% versus 0% sterile vegetations and 100% versus 0%-11% sterile kidneys and spleen, in the telavancin- and daptomycin-treated animals, respectively).

CONCLUSIONS

These results suggest that telavancin exhibits significantly greater efficacies versus daptomycin in treating experimental IE caused by MRSA clinical isolates across four common CC types.

Phloretin derived from apple can reduce alpha-hemolysin expression in methicillin-resistant Staphylococcus aureus USA300

Methicillin-resistant Staphylococcus aureus (MRSA) has become increasingly important because it is the most common cause of hospital-acquired infections, which have become globally epidemic. Our study specifically focused on the MRSA strain USA300, which was shown in 2014 to be responsible for the most current pandemic of highly virulent MRSA in the United States. We aimed to evaluate the in vitro effect of phloretin on USA300. Susceptibility testing, western blotting assays, hemolysis assays and real-time RT-PCR were employed to examine the in vitro effects of phloretin on alpha-hemolysin (Hla) production when the bacterium was co-cultured with phloretin. The protective effect of phloretin against the USA300-mediated injury of human alveolar epithelial cells (A549) was tested using the live/dead analysis and cytotoxicity assays. We showed that sub-inhibitory concentrations of phloretin have no effect on bacterial viability; however, they can markedly inhibit the production of Hla in culture supernatants and the transcriptional levels of hla (the gene encoding Hla) and agrA (the accessory gene regulator). Phloretin, at a final concentration of 16 µg/ml, could protect A549 cells from injury caused by USA300 in the co-culture system. Our study suggests that phloretin might have a potential application in the development of treatment for MRSA infections.