The anti-repressor MecR2 promotes the proteolysis of the mecA repressor and enables optimal expression of β-lactam resistance in MRSA

A neoteric antibacterial ceria-silver nanozyme for abiotic surfaces

Community-associated and hospital-acquired infections caused by bacteria continue to yield major global challenges to human health. Bacterial contamination on abiotic surfaces is largely spread via high-touch surfaces and contemporary standard disinfection practices show limited efficacy, resulting in unsatisfactory therapeutic outcomes. New strategies that offer non-specific and broad protection are urgently needed. Herein, we report our novel ceria-silver nanozyme engineered at a molar ratio of 5:1 and with a higher trivalent (Ce3+) surface fraction. Our results reveal potent levels of surface catalytic activity on both wet and dry surfaces, with rapid, and complete eradication of Pseudomonas aeruginosa, Staphylococcus aureus, and methicillin resistant S. aureus, in both planktonic and biofilm form. Preferential electrostatic adherence of anionic bacteria to the cationic nanozyme surface leads to a catastrophic loss in both aerobic and anaerobic respiration, DNA damage, osmodysregulation, and finally, programmed bacterial lysis. Our data reveal several unique mechanistic avenues of synergistic ceria-Ag efficacy. Ag potentially increases the presence of Ce3+ sites at the ceria-Ag interface, thereby facilitating the formation of harmful H2O2, followed by likely permeation across the cell wall. Further, a weakened Ag-induced Ce-O bond may drive electron transfer from the Ec band to O2, thereby further facilitating the selective reduction of O2 toward H2O2 formation. Ag destabilizes the surface adsorption of molecular H2O2, potentially leading to higher concentrations of free H2O2 adjacent to bacteria. To this end, our results show that H2O2 and/or NO/NO2-/NO3- are the key liberators of antibacterial activity, with a limited immediate role being offered by nanozyme-induced ROS including O2•- and OH•, and likely other light-activated radicals. A mini-pilot proof-of-concept study performed in a pediatric dental clinic setting confirms residual, and continual nanozyme antibacterial efficacy over a 28-day period. These findings open a new approach to alleviate infections caused by bacteria for use on high-touch hard surfaces.

Chalcogen-Varied Imidazolone Derivatives as Antibiotic Resistance Breakers in Staphylococcus aureus Strains

In this study, a search for new therapeutic agents that may improve the antibacterial activity of conventional antibiotics and help to successfully overcome methicillin-resistant Staphylococcus aureus (MRSA) infections has been conducted. The purpose of this work was to extend the scope of our preliminary studies and to evaluate the adjuvant potency of new derivatives in a set of S. aureus clinical isolates. The study confirmed the high efficacy of piperazine derivatives of 5-arylideneimidazol-4-one (7-9) tested previously, and it enabled the authors to identify even more efficient modulators of bacterial resistance among new analogs. The greatest capacity to enhance oxacillin activity was determined for 1-benzhydrylpiperazine 5-spirofluorenehydantoin derivative (13) which, at concentrations as low as 0.0625 mM, restores the effectiveness of β-lactam antibiotics against MRSA strains. In silico studies showed that the probable mechanism of action of 13 is related to the binding of the molecule with the allosteric site of PBP2a. Interestingly, thiazole derivatives tested were shown to act as both oxacillin and erythromycin conjugators in S. aureus isolates, suggesting a complex mode of action (i.e., influence on the Msr(A) efflux pump). This high enhancer activity indicates the high potential of imidazolones to become commercially available antibiotic adjuvants.

The depsidones from marine sponge-derived fungus Aspergillus unguis IB151 as an anti-MRSA agent: Molecular docking, pharmacokinetics analysis, and molecular dynamic simulation studies

Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging nosocomial pathogen among hospitalized patients, with high morbidity and mortality rates. The discovery of a novel antibacterial is urgently needed to address this resistance problem. The present study aims to explore the antibacterial potential of three depsidone compounds: 2-clorounguinol (1), unguinol (2), and nidulin (3), isolated from the marine sponge-derived fungus Aspergillus unguis IB1, both in vitro and in silico. The antibacterial activity of all compounds was evaluated by calculating the Minimum inhibitory concentration (MIC) and Minimum bactericidal concentration (MBC) against MRSA using agar diffusion and total plate count methods, respectively. Bacterial cell morphology changes were  studied for the first time using scanning electron microscopy (SEM). Molecular docking, pharmacokinetics analysis, and molecular dynamics simulation were performed to determine possible protein-ligand interactions and the stability of the targeting penicillin-binding protein 2a (PBP2a) against 2-clorounguinol (1). The research findings indicated that compounds 1 to 3 exhibited MIC and MBC values of 2 µg/mL and 16 µg/mL against MRSA, respectively. MRSA cells displayed a distinct shape after the addition of the depsidone compound, as observed in SEM. According to the in silico study, 2-chlorounguinol exhibited the highest binding-free energy (BFE) with PBP2a (-6.7 kcal/mol). For comparison, (E)-3-(2-(4-cyanostyryl)-4-oxoquinazolin-3(4H)-yl) benzoic acid inhibits PBP2a with a BFE less than -6.6 kcal/mol. Based on the Lipinski's rule of 5, depsidone compounds constitute a class of compounds with good pharmacokinetic properties, being easily absorbed and permeable. These findings suggest that 2-chlorounguinol possesses potential antibacterial activity and could be developed as an antibiotic adjuvant to reduce antimicrobial resistance.

In vivo and in vitro characterizations of melibiose permease (MelB) conformation-dependent nanobodies reveal sugar-binding mechanisms

Salmonella enterica serovar Typhimurium melibiose permease (MelBSt) is a prototype of the Na+-coupled major facilitator superfamily transporters, which are important for the cellular uptake of molecules including sugars and small drugs. Although the symport mechanisms have been well-studied, mechanisms of substrate binding and translocation remain enigmatic. We have previously determined the sugar-binding site of outward-facing MelBSt by crystallography. To obtain other key kinetic states, here we raised camelid single-domain nanobodies (Nbs) and carried out a screening against the WT MelBSt under 4 ligand conditions. We applied an in vivo cAMP-dependent two-hybrid assay to detect interactions of Nbs with MelBSt and melibiose transport assays to determine the effects on MelBSt functions. We found that all selected Nbs showed partial to complete inhibitions of MelBSt transport activities, confirming their intracellular interactions. A group of Nbs (714, 725, and 733) was purified, and isothermal titration calorimetry measurements showed that their binding affinities were significantly inhibited by the substrate melibiose. When titrating melibiose to the MelBSt/Nb complexes, Nb also inhibited the sugar-binding. However, the Nb733/MelBSt complex retained binding to the coupling cation Na+ and also to the regulatory enzyme EIIAGlc of the glucose-specific phosphoenolpyruvate/sugar phosphotransferase system. Further, EIIAGlc/MelBSt complex also retained binding to Nb733 and formed a stable supercomplex. All data indicated that MelBSt trapped by Nbs retained its physiological functions and the trapped conformation is similar to that bound by the physiological regulator EIIAGlc. Therefore, these conformational Nbs can be useful tools for further structural, functional, and conformational analyses.

Progress in the Prevalence, Classification and Drug Resistance Mechanisms of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus is a common human pathogen with a variety of virulence factors, which can cause multiple infectious diseases. In recent decades, due to the constant evolution and the abuse of antibiotics, Staphylococcus aureus was becoming more resistant, the infection rate of MRSA remained high, and clinical treatment of MRSA became more difficult. The genetic diversity of MRSA was mainly represented by the continuous emergence of epidemic strains, resulting in the constant changes of epidemic clones. Different classes of MRSA resulted in different epidemics and resistance characteristics, which could affect the clinical symptoms and treatments. MRSA had also spread from traditional hospitals to community and livestock environments, and the new clones established a relationship between animals and humans, promoting further evolution of MRSA. Since the resistance mechanism of MRSA is very complex, it is important to clarify these resistance mechanisms at the molecular level for the treatment of infectious diseases. We firstly described the diversity of SCCmec elements, and discussed the types of SCCmec, its drug resistance mechanisms and expression regulations. Then, we described how the vanA operon makes Staphylococcus aureus resistant to vancomycin and its expression regulation. Finally, a brief introduction was given to the drug resistance mechanisms of biofilms and efflux pump systems. Analyzing the resistance mechanism of MRSA can help study new anti-infective drugs and alleviate the evolution of MRSA. At the end of the review, we summarized the treatment strategies for MRSA infection, including antibiotics, anti-biofilm agents and efflux pump inhibitors. To sum up, here we reviewed the epidemic characteristics of Staphylococcus aureus, summarized its classifications, drug resistance mechanisms of MRSA (SCCmec element, vanA operon, biofilm and active efflux pump system) and novel therapy strategies, so as to provide a theoretical basis for the treatment of MRSA infection.

The Synergistic Antimicrobial Effect and Mechanism of Nisin and Oxacillin against Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for skin and soft tissue infections with multi-resistance to many antibiotics. It is thus imperative to explore alternative antimicrobial treatments to ensure future treatment options. Nisin (NIS), an antibacterial peptide produced by Lactococcus lactis, was selected to combine with Oxacillin (OX), to evaluate the antimicrobial effect and potential mechanism against MRSA. The synergistic antimicrobial effect of OX and NIS was verified by Minimal Inhibitory Concentration (MIC) assays, checkerboard analysis, time-kill curve, biofilm producing ability, and mice skin infection model in vivo. For the potential synergistic antimicrobial mechanism, the microstructure and integrity change of MRSA cells were determined by Scanning and Transmission Electron Microscope (SEM and TEM), intracellular alkaline phosphatase activity and propidium iodide staining were assayed; And transcription of mecA, main gene of MRSA resistant to OX, were detected by qRT-PCR. The results showed NIS could restore the sensitivity of MRSA to OX and inhibit biofilm production; OX + NIS can make MRSA cell deform; NIS may recover OX sensitivity by inhibiting the transcription of mecA. In vivo, mice skin infection models indicate that OX + NIS can substantially alleviate MRSA infections. As a safe commercially available biological compound, NIS and the combination of antibiotics are worth developing as new anti-MRSA biomaterials.

Retrospective analysis of drug resistance characteristics and infection related risk factors of multidrug-resistant organisms (MDROs) isolated from the orthopedics department of a tertiary hospital

Patients infected with multidrug-resistant organisms (MDROs) are known to exhibit longer hospital stays and a significantly poorer prognosis. We performed a 6-year retrospective analysis of nosocomial infections reported in the orthopedics department of our institution, to gain valuable insights into antibiotic sensitivity and infectious characteristics of MDROs, in order to deduce effective measures to control the occurrence of multidrug-resistant infections in clinical practice. A retrospective, single center surveillance study (January 2012-December 2017) was performed on the nosocomial infections recorded in the department of orthopedics. A nosocomial infection is defined as one that develops when a patient is residing in a hospital but was not present at the time of admission. All relevant data, including basic patient information, cultivated bacterial strains, and antimicrobial resistance, was obtained from the hospital information system. A total of 1392 strains of pathogenic bacteria were isolated; 358 belonged to MDROs (detection rate = 25.7%). All the isolated strains of MDROs were mostly from secretions (52.2%). The number of cases infected with MDROs were 144 (40.2%) and 129 (36.0%) in the trauma and spinal wards, respectively. MRSA showed high resistance to β lactam antibiotics, but was sensitive to quinolone antibiotics, linezolid and cotrimoxazole. ESBL-positive strains showed more sensitivity to carbapenem antibiotics (resistance rate < 10%). MDR nonfermenters showed high resistance to most antibiotics. Logistic multivariate analysis revealed age, open injury, and central nervous system injury as independent risk factors for multidrug resistant infections. A high antibiotic resistance rate and an increasing prevalence of infections with MDROs was identified in the orthopedics department. Patients with open injury, central nervous system injury and those aged ≥ 60 years, were more prone to multidrug-resistant infections. Clinicians should pay more attention to such patients in order to actively prevent and control the occurrence of infections caused by MDROs.

Reduced Ceftaroline Susceptibility among Invasive MRSA Infections in Children: a Clinical and Genomic Investigation

Ceftaroline represents an attractive therapy option for methicillin-resistant Staphylococcus aureus (MRSA). Little data is available, however, regarding the frequency of reduced susceptibility (RS) to ceftaroline among pediatric MRSA infections. We screened invasive MRSA isolates at a tertiary children's hospital for ceftaroline RS. Ceftaroline RS occurred in 2.9% of isolates and only among health care associated infections. Ceftaroline RS isolates were more often clindamycin-resistant. Sequencing data indicated the predominance of the CC5 lineage among ceftaroline RS isolates.

Cefazolin susceptibility of coagulase-negative staphylococci (CoNS) causing late-onset neonatal bacteraemia

BACKGROUND

CoNS bacteraemia causes significant neonatal morbidity. Previous work has suggested that β-lactam antibiotics vary in their binding affinity to PBP2a (produced by the mecA gene) present in most CoNS.

OBJECTIVES

We evaluated cefazolin MICs for CoNS isolated in an Australian neonatal ICU (NICU) and correlated them with isolate genotype and phenotype.

METHODS

Significant blood isolates from 2009 to 2017 were speciated and underwent broth microdilution testing for cefazolin, cefoxitin, oxacillin and flucloxacillin. Correlation with mecA presence and PBP2a expression was evaluated. A selection of Staphylococcus capitis isolates underwent WGS.

RESULTS

The CoNS (n = 99) isolates were confirmed as S. capitis (n = 57), Staphylococcus epidermidis (n = 32), Staphylococcus haemolyticus (n = 2) and Staphylococcus warneri (n = 8). The MIC of cefazolin was ≤2 mg/L for 30% of isolates and 75% had an MIC of ≤8 mg/L (MIC90 = 16 mg/L). This contrasted with MIC90s of cefoxitin, oxacillin and flucloxacillin, which were all ≥32 mg/L. WGS found a number of S. capitis isolates closely related to the globally established NRCS-A clone.

CONCLUSIONS

CoNS displayed distinctly lower MIC values of cefazolin than of other agents tested. MIC variation may be related to binding affinity of PBP2a or regulation of expression of mecA by mecR1-mecI functional genes. Further, NRCS-A S. capitis strains were present in this Australian NICU before and after the unit underwent physical relocation, which raised questions about a common environmental source. It is considered justified to conduct a randomized clinical trial that assesses cefazolin versus vancomycin for management of late-onset neonatal sepsis.

An experiment-informed signal transduction model for the role of the Staphylococcus aureus MecR1 protein in β-lactam resistance

The treatment of hospital- and community-associated infections by methicillin-resistant Staphylococcus aureus (MRSA) is a perpetual challenge. This Gram-positive bacterium is resistant specifically to β-lactam antibiotics, and generally to many other antibacterial agents. Its resistance mechanisms to β-lactam antibiotics are activated only when the bacterium encounters a β-lactam. This activation is regulated by the transmembrane sensor/signal transducer proteins BlaR1 and MecR1. Neither the transmembrane/metalloprotease domain, nor the complete MecR1 and BlaR1 proteins, are isolatable for mechanistic study. Here we propose a model for full-length MecR1 based on homology modeling, residue coevolution data, a new extensive experimental mapping of transmembrane topology, partial structures, molecular simulations, and available NMR data. Our model defines the metalloprotease domain as a hydrophilic transmembrane chamber effectively sealed by the apo-sensor domain. It proposes that the amphipathic helices inserted into the gluzincin domain constitute the route for transmission of the β-lactam-binding event in the extracellular sensor domain, to the intracellular and membrane-embedded zinc-containing active site. From here, we discuss possible routes for subsequent activation of proteolytic action. This study provides the first coherent model of the structure of MecR1, opening routes for future functional investigations on how β-lactam binding culminates in the proteolytic degradation of MecI.

Roles of lytic transglycosylases in biofilm formation and β-lactam resistance in methicillin-resistant Staphylococcus aureus

Staphylococcus aureus is responsible for numerous community outbreaks and is one of the most frequent causes of nosocomial infections with significant morbidity and mortality. While the function of lytic transglycosylases (LTs) in relation to cell division, biofilm formation, and antibiotic resistance has been determined for several bacteria, their role in S. aureus remains largely unknown. The only known LTs in S. aureus are immunodominant staphylococcal antigen A (IsaA) and Staphylococcus epidermidis D protein (SceD). Our study demonstrates that, in a strain of methicillin-resistant S. aureus (MRSA), IsaA and SceD contribute differently to biofilm formation and β-lactam resistance. Deletion of isaA, but not sceD, led to decreased biofilm formation. Additionally, in isaA-deleted strains, β-lactam resistance was significantly decreased compared to that of wild-type strains. Plasmid-based expression of mecA, a major determinant of β-lactam resistance in MRSA, in an isaA-deleted strain did not restore β-lactam resistance, demonstrating that the β-lactam susceptibility phenotype is exhibited by isaA mutant regardless of the production level of PBP2a. Overall, our results suggest that IsaA is a potential therapeutic target for MRSA infections.

5-Arylideneimidazolones with Amine at Position 3 as Potential Antibiotic Adjuvants against Multidrug Resistant Bacteria

Searching for new chemosensitizers of bacterial multidrug resistance (MDR), chemical modifications of (Z)-5-(4-chlorobenzylidene)-2-(4-methylpiperazin-1-yl)-3H-imidazol-4(5H)-one (6) were performed. New compounds (7⁻17), with fused aromatic rings at position 5, were designed and synthesized. Crystallographic X-ray analysis proved that the final compounds (7⁻17) were substituted with tertiary amine-propyl moiety at position 3 and primary amine group at 2 due to intramolecular Dimroth rearrangement. New compounds were evaluated on their antibiotic adjuvant properties in either Gram-positive or Gram-negative bacteria. Efflux pump inhibitor (EPI) properties towards the AcrAB-TolC pump in Enterobacter aerogenes (EA289) were investigated in the real-time efflux (RTE) assay. Docking and molecular dynamics were applied to estimate an interaction of compounds 6⁻17 with penicillin binding protein (PBP2a). In vitro ADME-Tox properties were evaluated for compound 9. Most of the tested compounds reduced significantly (4-32-fold) oxacillin MIC in highly resistant MRSA HEMSA 5 strain. The anthracene-morpholine derivative (16) was the most potent (32-fold reduction). The tested compounds displayed significant EPI properties during RTE assay (37⁻97%). The naphthyl-methylpiperazine derivative 9 showed the most potent "dual action" of both oxacillin adjuvant (MRSA) and EPI (E. aerogenes). Molecular modeling results suggested the allosteric mechanism of action of the imidazolones, which improved binding of oxacillin in the PBP2a active site in MRSA.

Factors Contributing to the Evolution of mecA-Mediated β-lactam Resistance in Staphylococci: Update and New Insights From Whole Genome Sequencing (WGS)

The understanding of the mechanisms of antibiotic resistance development are fundamental to alert and preview beforehand, the large scale dissemination of resistance to antibiotics, enabling the design of strategies to prevent its spread. The mecA-mediated methicillin resistance conferring resistance to broad-spectrum β-lactams is globally spread in staphylococci including hospitals, farms and community environments, turning ineffective the most widely used and efficient class of antibiotics to treat staphylococcal infections. The use of whole genome sequencing (WGS) technologies at a bacterial population level has provided a considerable progress in the identification of key steps that led to mecA-mediated β-lactam resistance development and dissemination. Data obtained from multiple studies indicated that mecA developed from a harmless core gene (mecA1) encoding the penicillin-binding protein D (PbpD) from staphylococcal species of animal origin (S. sciuri group) due to extensive β-lactams use in human created environments. Emergence of the resistance determinant involved distortion of PbpD active site, increase in mecA1 expression, addition of regulators (mecR1, mecI) and integration into a mobile genetic element (SCCmec). SCCmec was then transferred into species of coagulase-negative staphylococci (CoNS) that are able to colonize both animals and humans and subsequently transferred to S. aureus of human origin. Adaptation of S. aureus to the exogenously acquired SCCmec involved, deletion and mutation of genes implicated in general metabolism (auxiliary genes) and general stress response and the adjustment of metabolic networks, what was accompanied by an increase in β-lactams minimal inhibitory concentration and the transition from a heterogeneous to homogeneous resistance profile. Nowadays, methicillin-resistant S. aureus (MRSA) carrying SCCmec constitutes one of the most important worldwide pandemics. The stages of development of mecA-mediated β-lactam resistance described here may serve as a model for previewing and preventing the emergence of resistance to other classes of antibiotics.

Methicillin-resistant Staphylococcus aureus

Since the 1960s, methicillin-resistant Staphylococcus aureus (MRSA) has emerged, disseminated globally and become a leading cause of bacterial infections in both health-care and community settings. However, there is marked geographical variation in MRSA burden owing to several factors, including differences in local infection control practices and pathogen-specific characteristics of the circulating clones. Different MRSA clones have resulted from the independent acquisition of staphylococcal cassette chromosome mec (SCCmec), which contains genes encoding proteins that render the bacterium resistant to most β-lactam antibiotics (such as methicillin), by several S. aureus clones. The success of MRSA is a consequence of the extensive arsenal of virulence factors produced by S. aureus combined with β-lactam resistance and, for most clones, resistance to other antibiotic classes. Clinical manifestations of MRSA range from asymptomatic colonization of the nasal mucosa to mild skin and soft tissue infections to fulminant invasive disease with high mortality. Although treatment options for MRSA are limited, several new antimicrobials are under development. An understanding of colonization dynamics, routes of transmission, risk factors for progression to infection and conditions that promote the emergence of resistance will enable optimization of strategies to effectively control MRSA. Vaccine candidates are also under development and could become an effective prevention measure.

Associations between resistance phenotype and gene expression in response to serial exposure to oxacillin and ciprofloxacin in Staphylococcus aureus

This study was designed to delineate the relationship between resistance phenotypes and gene expression in wild-type (SA^WT ), oxacillin-induced (SA^OXA ), ciprofloxacin-induced (SA^CIP ) and clinically acquired antibiotic-resistant Staphylococcus aureus (SA^CA ) exposed to oxacillin (β-lactam) and ciprofloxacin (fluoroquinolone). The phenotypic response and gene expression were varied with the antibiotic exposure. SA^WT was highly resistant to oxacillin (MIC = 8 μg ml^-1 ) after serial exposure to oxacillin, while the oxacillin susceptibility was not changed in SA^WT when exposed to ciprofloxacin (MIC = 0·25 μg ml^-1 ). The clinical isolate, SA^CA , was highly resistant to all classes of antibiotics used in this study. The increased resistance of SA^OXA and SA^CIP to penicillinase-labile penicillins was attributed to the production of β-lactamase, which is in good agreement with the overexpression of blaZ (>2-fold). The overexpression of efflux pump-related genes (norA, norB, norC, mdeA, mepR, mgrA and lmrS) was associated with the increased resistance of SA^CIP and SA^CA to aminoglycosides and quinolones. This study confirmed that the linkage between resistance phenotypes and molecular genotypes highly varied depending on intrinsic resistance profile, response to antibiotic exposure and genes conferring resistance. This study provides useful information for understanding the mechanisms of methicillin resistance in S. aureus in association with phenotypic and genotypic resistance determinants.

SIGNIFICANCE AND IMPACT OF THE STUDY

The improvement in current standards is essential to accurately detect methicillin-resistant Staphylococcus aureus in consideration of various resistance phenotypes and genotypes. The varied and distinctive expression patterns of antibiotic resistance-related genes were observed in S. aureus exposed to oxacillin and ciprofloxacin. It is worth noting the relationship between resistance phenotype and resistance genotype in terms of MIC values and expression of antibiotic resistance determinants. This study provides useful information for understanding the mechanisms of methicillin resistance in S. aureus in association with phenotypic and genotypic resistance determinants.

Norgestimate inhibits staphylococcal biofilm formation and resensitizes methicillin-resistant Staphylococcus aureus to β-lactam antibiotics

Formation of bacterial biofilms on medical devices can cause severe or fatal infectious diseases. In particular, biofilm-associated infections caused by methicillin-resistant Staphylococcus aureus are difficult to eradicate because the biofilm is strongly resistant to antibiotics and the host immune response. There is no effective treatment for biofilm-associated infectionss, except for surgical removal of contaminated medical devices followed by antibiotic therapy. Here we show that norgestimate, an acetylated progestin, effectively inhibits biofilm formation by staphylococcal strains, including methicillin-resistant S. aureus, without inhibiting their growth, decreasing the selective pressure for emergence of resistance. 17-Deacetyl norgestimate, a metabolite of norgestimate, shows much weaker inhibitory activity against staphylococcal biofilm formation, indicating that the acetyl group of norgestimate is important for its activity. Norgestimate inhibits staphylococcal biofilm formation by inhibiting production of polysaccharide intercellular adhesin and proteins in the extracellular matrix. Proteome analysis of S. aureus indicated that norgestimate represses the expression of the cell wall-anchored protein SasG, which promotes intercellular adhesion, and of the glycolytic enzyme enolase, which plays a secondary role in biofilm formation. Notably, norgestimate induces remarkable changes in cell wall morphology, characterized by increased thickness and abnormal rippled septa. Furthermore, norgestimate increases the expression level of penicillin binding protein 2 and resensitizes methicillin-resistant S. aureus to β-lactam antibiotics. These results suggest that norgestimate is a promising lead compound for the development of drugs to treat biofilm-associated infections, as well as for its ability to resensitize methicillin-resistant S. aureus to β-lactam antibiotics.

A synthetic molecule related to the hormone progesterone might keep medical devices free of biofilms without promoting antibiotic resistance. Implanted devices that have become contaminated with biofilms generally must be surgically removed prior to treating the underlying infection with antibiotics. Ken-ichi Okuda and colleagues at The Jikei University School of Medicine in Tokyo, with co-workers elsewhere in Japan, found that the synthetic progesterone analog norgestimate inhibits biofilm formation without inhibiting bacterial growth. They regard this selective effect on biofilm formation as a significant advantage, as it reduces the risk of inducing resistance in the targeted bacteria. They demonstrated the effect using staphylococcal bacteria, including the problematic and highly dangerous methicillin-resistant Staphylococcus aureus (MRSA). The research also indicated that norgestimate can resensitize MRSA bacteria to some of the antibiotics they are resistant to.

Evolutionary Origin of the Staphylococcal Cassette Chromosome mec (SCCmec)

Several lines of evidence indicate that the most primitive staphylococcal species, those of the Staphylococcus sciuri group, were involved in the first stages of evolution of the staphylococcal cassette chromosome mec (SCCmec), the genetic element carrying the β-lactam resistance gene mecA However, many steps are still missing from this evolutionary history. In particular, it is not known how mecA was incorporated into the mobile element SCC prior to dissemination among Staphylococcus aureus and other pathogenic staphylococcal species. To gain insights into the possible contribution of several species of the Staphylococcus sciuri group to the assembly of SCCmec, we sequenced the genomes of 106 isolates, comprising S. sciuri (n = 76), Staphylococcus vitulinus (n = 18), and Staphylococcus fleurettii (n = 12) from animal and human sources, and characterized the native location of mecA and the SCC insertion site by using a variety of comparative genomic approaches. Moreover, we performed a single nucleotide polymorphism (SNP) analysis of the genomes in order to understand SCCmec evolution in relation to phylogeny. We found that each of three species of the S. sciuri group contributed to the evolution of SCCmec: S. vitulinus and S. fleurettii contributed to the assembly of the mec complex, and S. sciuri most likely provided the mobile element in which mecA was later incorporated. We hypothesize that an ancestral SCCmec III cassette (an element carried by one of the most epidemic methicillin-resistant S. aureus clones) originated in S. sciuri possibly by a recombination event in a human host or a human-created environment and later was transferred to S. aureus.

Antimicrobial resistance and virulence characterization of Staphylococcus aureus and coagulase-negative staphylococci from imported beef meat

BACKGROUND

The objectives of this study were to characterize the diversity and magnitude of antimicrobial resistance among Staphylococcus species recovered from imported beef meat sold in the Egyptian market and the potential mechanisms underlying the antimicrobial resistance phenotypes including harboring of resistance genes (mecA, cfr, gyrA, gyrB, and grlA) and biofilm formation.

RESULTS

The resistance gene mecA was detected in 50% of methicillin-resistant non-Staphylococcus aureus isolates (4/8). Interestingly, our results showed that: (i) resistance genes mecA, gyrA, gyrB, grlA, and cfr were absent in Staphylococcus hominis and Staphylococcus hemolyticus isolates, although S. hominis was phenotypically resistant to methicillin (MR-non-S. aureus) while S. hemolyticus was resistant to vancomycin only; (ii) S. aureus isolates did not carry the mecA gene (100%) and were phenotypically characterized as methicillin- susceptible S. aureus (MSS); and (iii) the resistance gene mecA was present in one isolate (1/3) of Staphylococcus lugdunensis that was phenotypically characterized as methicillin-susceptible non-S. aureus (MSNSA).

CONCLUSIONS

Our findings highlight the potential risk for consumers, in the absence of actionable risk management information systems, of imported foods and advice a strict implementation of international standards by different venues such as CODEX to avoid the increase in prevalence of coagulase positive and coagulase negative Staphylococcus isolates and their antibiotic resistance genes in imported beef meat at the Egyptian market.

Novel methicillin resistance gene mecD in clinical Macrococcus caseolyticus strains from bovine and canine sources

Methicillin-resistant Macrococcus caseolyticus strains from bovine and canine origins were found to carry a novel mecD gene conferring resistance to all classes of β-lactams including anti-MRSA cephalosporins. Association of β-lactam resistance with mecD was demonstrated by gene expression in S. aureus and deletion of the mecD-containing island in M. caseolyticus. The mecD gene was located either on an 18,134-bp M. caseolyticus resistance island (McRI_mecD-1) or a 16,188-bp McRI_mecD-2. Both islands were integrated at the 3′ end of the rpsI gene, carried the mecD operon (mecD-mecR1_m-mecI_m), and genes for an integrase of the tyrosine recombinase family and a putative virulence-associated protein (virE). Apart from the mecD operon, that shared 66% overall nucleotide identity with the mecB operon, McRI_mecD islands were unrelated to any mecB-carrying elements or staphylococcal cassette chromosome mec. Only McRI_mecD-1 that is delimitated at both ends by direct repeats was capable of circular excision. The recombined excision pattern suggests site-specific activity of the integrase and allowed identification of a putative core attachment site. Detection of rpsI-associated integrases in Bacillus and S. aureus reveals a potential for broad-host range dissemination of the novel methicillin resistance gene mecD.

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.

A Look into the Melting Pot: The mecC-Harboring Region Is a Recombination Hot Spot in Staphylococcus stepanovicii

Introduction

Horizontal gene transfer (HGT) is an important driver for resistance- and virulence factor accumulation in pathogenic bacteria such as Staphylococcus aureus.

Methods

Here, we have investigated the downstream region of the bacterial chromosomal attachment site (attB) for the staphylococcal cassette chromosome mec (SCCmec) element of a commensal mecC-positive Staphylococcus stepanovicii strain (IMT28705; ODD4) with respect to genetic composition and indications of HGT. S. stepanovicii IMT28705 was isolated from a fecal sample of a trapped wild bank vole (Myodes glareolus) during a screening study (National Network on “Rodent-Borne Pathogens”) in Germany. Whole genome sequencing (WGS) of IMT28705 together with the mecC-negative type strain CM7717 was conducted in order to comparatively investigate the genomic region downstream of attB (GenBank accession no. KR732654 and KR732653).

Results

The bank vole isolate (IMT28705) harbors a mecC gene which shares 99.2% nucleotide (and 98.5% amino acid) sequence identity with mecC of MRSA_LGA251. In addition, the mecC-encoding region harbors the typical blaZ-mecC-mecR1-mecI structure, corresponding with the class E mec complex. While the sequences downstream of attB in both S. stepanovicii isolates (IMT28705 and CM7717) are partitioned by 15 bp direct repeats, further comparison revealed a remarkable low concordance of gene content, indicating a chromosomal “hot spot” for foreign DNA integration and exchange.

Conclusion

Our data highlight the necessity for further research on transmission routes of resistance encoding factors from the environmental and wildlife resistome.

The Staphylococcus aureus Chaperone PrsA Is a New Auxiliary Factor of Oxacillin Resistance Affecting Penicillin-Binding Protein 2A

Expression of the methicillin-resistant S. aureus (MRSA) phenotype results from the expression of the extra penicillin-binding protein 2A (PBP2A), which is encoded by mecA and acquired horizontally on part of the SCCmec cassette. PBP2A can catalyze dd-transpeptidation of peptidoglycan (PG) because of its low affinity for β-lactam antibiotics and can functionally cooperate with the PBP2 transglycosylase in the biosynthesis of PG. Here, we focus upon the role of the membrane-bound PrsA foldase protein as a regulator of β-lactam resistance expression. Deletion of prsA altered oxacillin resistance in three different SCCmec backgrounds and, more importantly, caused a decrease in PBP2A membrane amounts without affecting mecA mRNA levels. The N- and C-terminal domains of PrsA were found to be critical features for PBP2A protein membrane levels and oxacillin resistance. We propose that PrsA has a role in posttranscriptional maturation of PBP2A, possibly in the export and/or folding of newly synthesized PBP2A. This additional level of control in the expression of the mecA-dependent MRSA phenotype constitutes an opportunity to expand the strategies to design anti-infective agents.

Oxacillin sensitization of methicillin-resistant Staphylococcus aureus and methicillin-resistant Staphylococcus pseudintermedius by antisense peptide nucleic acids in vitro

Background

Antibiotic resistance genes can be targeted by antisense agents, which can reduce their expression and thus restore cellular susceptibility to existing antibiotics. Antisense inhibitors can be gene and pathogen specific, or designed to inhibit a group of bacteria having conserved sequences within resistance genes. Here, we aimed to develop antisense peptide nucleic acids (PNAs) that could be used to effectively restore susceptibility to β-lactams in methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Staphylococcus pseudintermedius (MRSP).

Results

Antisense PNAs specific for conserved regions of the mobilisable gene mecA, and the growth essential gene, ftsZ, were designed. Clinical MRSA and MRSP strains of high oxacillin resistance were treated with PNAs and assayed for reduction in colony forming units on oxacillin plates, reduction in target gene mRNA levels, and cell size. Anti-mecA PNA at 7.5 and 2.5 μM reduced mecA mRNA in MRSA and MRSP (p < 0.05). At these PNA concentrations, 66 % of MRSA and 92 % of MRSP cells were killed by oxacillin (p < 0.01). Anti-ftsZ PNA at 7.5 and 2.5 μM reduced ftsZ mRNA in MRSA and MRSP, respectively (p ≤ 0.05). At these PNA concentrations, 86 % of MRSA cells and 95 % of MRSP cells were killed by oxacillin (p < 0.05). Anti-ftsZ PNAs resulted in swelling of bacterial cells. Scrambled PNA controls did not affect MRSA but sensitized MRSP moderately to oxacillin without affecting mRNA levels.

Conclusions

The antisense PNAs effects observed provide in vitro proof of concept that this approach can be used to reverse β-lactam resistance in staphylococci. Further studies are warranted as clinical treatment alternatives are needed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0599-x) contains supplementary material, which is available to authorized users.

Phosphorylation of BlaR1 in Manifestation of Antibiotic Resistance in Methicillin-Resistant Staphylococcus aureus and Its Abrogation by Small Molecules

Methicillin-resistant Staphylococcus aureus (MRSA), an important human pathogen, has evolved an inducible mechanism for resistance to β-lactam antibiotics. We report herein that the integral membrane protein BlaR1, the β-lactam sensor/signal transducer protein, is phosphorylated on exposure to β-lactam antibiotics. This event is critical to the onset of the induction of antibiotic resistance. Furthermore, we document that BlaR1 phosphorylation and the antibiotic-resistance phenotype are both reversed in the presence of synthetic protein kinase inhibitors of our design, restoring susceptibility of the organism to a penicillin, resurrecting it from obsolescence in treatment of these intransigent bacteria.

Modulation of ccrAB Expression and SCCmec Excision by an Inverted Repeat Element and SarS in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a notorious human pathogen that can cause a broad spectrum of infections. MRSA strains are resistant to almost the entire family of β-lactam antibiotics due to the acquisition of staphylococcal cassette chromosome mec (SCCmec). The chromosome cassette recombinases A and B, encoded by ccrAB genes located on SCCmec, play a key role in the excision of SCCmec. Studies have shown that ccrAB genes are expressed in only a minority of cells, suggesting the involvement of a subtle regulatory mechanism in ccrAB expression which has not been uncovered. Here, we found that an inverted repeat (IR) element, existing extensively and conservatively within the ccrAB promoter of different SCCmec types, played a repressive role in ccrAB expression and SCCmec excision in MRSA strain N315. Replacement of the IR sequence led to a significant increase in ccrAB expression and curing of SCCmec from strain N315 cells. In addition, we identified the transcriptional regulator SarS using DNA-affinity chromatography and further demonstrated that SarS can bind to the IR sequence and upregulate ccrAB expression and SCCmec excision. These findings reveal a molecular mechanism regulating ccrAB expression and SCCmec excision and may provide mechanic insights into the lateral transfer of SCCmec and spread of antibiotic resistance in S. aureus.

Inhibition of penicillin-binding protein 2a (PBP2a) in methicillin resistant Staphylococcus aureus (MRSA) by combination of ampicillin and a bioactive fraction from Duabanga grandiflora

Background

The inhibition of penicillin-binding protein 2a (PBP2a) is a promising solution in overcoming resistance of methicillin resistance Staphylococcus aureus (MRSA). A potential approach in achieving this is by combining natural product with currently available antibiotics to restore the activity as well as to amplify the therapeutic ability of the drugs. We studied inhibition effects of a bioactive fraction, F-10 (isolated from the leaves of Duabanga grandiflora) alone and in combination with a beta-lactam drug, ampicillin on MRSA growth and expression of PBP2a. Additionally, phytochemical analysis was conducted on F-10 to identify the classes of phytochemicals present.

Methods

Fractionation of the ethyl acetate leaf extract was achieved by successive column chromatography which eventually led to isolation of an active fraction, F-10. Both extract and F-10 were analyzed for the presence of major classes of phytochemicals in addition to obtaining a high performance liquid chromatography (HPLC) profile to reveal the complexity of the fraction F-10. Broth microdilution method was employed to determine minimum inhibitory concentration (MIC) of the extract and fractions against MRSA. Evaluation of synergistic activity of the active fraction with ampicillin was determined using checkerboard methodand kinetic growth experiments. Effect of combination treatments on expression of PBP2a, a protein that confers resistance to beta-lactam antibiotics, was elucidated with the Western blot assay.

Results

MIC of F-10 against MRSA was 750 mg/L which showed an improved activity by 4-fold compared to its crude extract (MIC = 3000 mg/L). Phytochemical analysis revealed occurrence of tannins, saponin, flavonoids, sterols, and glycosides in F10 fraction. In FIC index interpretation, the most synergistic activity was achieved for combinations of 1/64 × MIC ampicillin + 1/4 × MIC F-10. The combination also evidently inhibited MRSA growth in kinetic growth curve assay. As a result of this synergistic interaction, MIC of ampicillin against MRSA was reduced to 0.78 mg/L (64-fold) from initial value of 50 mg/L. Western blot analysis suggested inhibition of PBP2a in MRSA cultures grown in synergistic combination treatment in which no PBP2a band was expressed.

Conclusions

The results demonstrated synergism between fraction F-10 of D. grandiflora with ampicillin in suppressing MRSA growth via PBP2a inhibition.

Mechanisms of Methicillin Resistance in Staphylococcus aureus

Staphylococcus aureus is a major human and veterinary pathogen worldwide. Methicillin-resistant S. aureus (MRSA) poses a significant and enduring problem to the treatment of infection by such strains. Resistance is usually conferred by the acquisition of a nonnative gene encoding a penicillin-binding protein (PBP2a), with significantly lower affinity for β-lactams. This resistance allows cell-wall biosynthesis, the target of β-lactams, to continue even in the presence of typically inhibitory concentrations of antibiotic. PBP2a is encoded by the mecA gene, which is carried on a distinct mobile genetic element (SCCmec), the expression of which is controlled through a proteolytic signal transduction pathway comprising a sensor protein (MecR1) and a repressor (MecI). Many of the molecular and biochemical mechanisms underlying methicillin resistance in S. aureus have been elucidated, including regulatory events and the structure of key proteins. Here we review recent advances in this area.

Synergistic activity and mechanism of action of Stephania suberosa Forman extract and ampicillin combination against ampicillin-resistant Staphylococcus aureus

Background

Ampicillin-resistant S. aureus (ARSA) now poses a serious problem for hospitalized patients, and their care providers. Plant-derived antibacterial that can reverse the resistance to well-tried agents which have lost their original effectiveness are the research objectives of far reaching importance. To this aim, the present study investigated antibacterial and synergistic activities of Stephania suberosa extracts (SSE) against ARSA when used singly and in combination with ampicillin.

Results

The majority chemical compounds of SSE were alkaloid (526.27 ± 47.27 mg/1 g of dried extract). The Minimum inhibitory concentration (MICs) for ampicillin and SSE against all ARSA strains were >512 μg/ml and 4 mg/ml, respectively. Checkerboard assay revealed synergistic activity in the combination of ampicillin (0.15 μg/ml) and SSE (2 mg/ml) at fractional inhibitory concentration index (FICI) <0.5. The killing curve assay had confirmed that the viability of ARSA was dramatically reduced from 5x10⁵ cfu/ml to 10³ cfu/ml within 6 h after exposure to SSE (2 mg/ml) plus ampicillin (0.15 μg/ml) combination. Electron microscopic study clearly revealed that these ARSA cells treated with this combination caused marked morphological damage, peptidoglycan and cytoplasmic membrane damage, and average cell areas significant smaller than control. Obviously, Immunofluorescence staining and confocal microscopic images confirmed that the peptidoglycan of these cells were undoubtedly disrupted by this combination. Furthermore, the CM permeability of ARSA was also increased by this combination. Enzyme assay demonstrated that SSE had an inhibitory activity against β-lactamase in concentrations manner.

Conclusions

So, these findings provide evidence that SSE has the high potential to reverse bacterial resistance to originate traditional drug susceptibility of it and may relate to three modes of actions of SSE: (1) inhibits peptidoglycan synthesis, resulting in morphological damage, (2) inhibits β-lactamases activity, and (3) increases CM permeability. It is widely recognized that many types of drugs are derived from alkaloids. So, this SSE offers the prominent potential to develop a novel adjunct phytopharmaceutical to ampicillin for the treatment of ARSA. Further active ingredients study, toxicity of it, and the synergistic effect on blood and tissue should be performed and confirmed in an animal test or in humans.

A novel hybrid SCCmec-mecC region in Staphylococcus sciuri

OBJECTIVES

Methicillin resistance in Staphylococcus spp. results from the expression of an alternative penicillin-binding protein 2a (encoded by mecA) with a low affinity for β-lactam antibiotics. Recently, a novel variant of mecA known as mecC (formerly mecALGA251) was identified in Staphylococcus aureus isolates from both humans and animals. In this study, we identified two Staphylococcus sciuri subsp. carnaticus isolates from bovine infections that harbour three different mecA homologues: mecA, mecA1 and mecC.

METHODS

We subjected the two isolates to whole-genome sequencing to further understand the genetic context of the mec-containing region. We also used PCR and RT-PCR to investigate the excision and expression of the SCCmec element and mec genes, respectively.

RESULTS

Whole-genome sequencing revealed a novel hybrid SCCmec region at the orfX locus consisting of a class E mec complex (mecI-mecR1-mecC1-blaZ) located immediately downstream of a staphylococcal cassette chromosome mec (SCCmec) type VII element. A second SCCmec attL site (attL2), which was imperfect, was present downstream of the mecC region. PCR analysis of stationary-phase cultures showed that both the SCCmec type VII element and a hybrid SCCmec-mecC element were capable of excision from the genome and forming a circular intermediate. Transcriptional analysis showed that mecC and mecA, but not mecA1, were both expressed in liquid culture supplemented with oxacillin.

CONCLUSIONS

Overall, this study further highlights that a range of staphylococcal species harbour the mecC gene and furthers the view that coagulase-negative staphylococci associated with animals may act as reservoirs of antibiotic resistance genes for more pathogenic staphylococcal species.

Structure-function studies of the staphylococcal methicillin resistance antirepressor MecR2

Methicillin resistance in Staphylococcus aureus is elicited by the MecI-MecR1-MecA axis encoded by the mec locus. Recently, MecR2 was also identified as a regulator of mec through binding of the methicillin repressor, MecI. Here we show that plasmid-encoded full-length MecR2 restores resistance in a sensitive S. aureus mecR2 deletion mutant of the resistant strain N315. The crystal structure of MecR2 reveals an N-terminal DNA-binding domain, an intermediate scaffold domain, and a C-terminal dimerization domain that contributes to oligomerization. The protein shows structural similarity to ROK (repressors, open reading frames, and kinases) family proteins, which bind DNA and/or sugar molecules. We found that functional cell-based assays of three point mutants affecting residues participating in sugar binding in ROK proteins had no effect on the resistance phenotype. By contrast, MecR2 bound short double-stranded DNA oligonucleotides nonspecifically, and a deletion mutant affecting the N-terminal DNA-binding domain showed a certain effect on activity, thus contributing to resistance less than the wild-type protein. Similarly, a deletion mutant, in which a flexible segment of intermediate scaffold domain had been replaced by four glycines, significantly reduced MecR2 function, thus indicating that this domain may likewise be required for activity. Taken together, these results provide the structural basis for the activity of a methicillin antirepressor, MecR2, which would sequester MecI away from its cognate promoter region and facilitate its degradation.

Redefining the role of the β-lactamase locus in methicillin-resistant Staphylococcus aureus: β-lactamase regulators disrupt the MecI-mediated strong repression on mecA and optimize the phenotypic expression of resistance in strains with constitutive mecA expression

In response to β-lactam chemotherapy, Staphylococcus aureus has acquired two resistance determinants: blaZ, coding for β-lactamase, which confers resistance to penicillins only, and mecA, coding for an extra cell wall cross-linking enzyme with reduced affinity for virtually all other β-lactams. The transcriptional control of both resistance determinants is regulated by homologous repressors (BlaI and MecI, respectively) and sensor inducers (BlaR1 and MecR1, respectively). There is a cross-talk between the two regulatory systems, and it has been demonstrated that bla regulators stabilize the mecA acquisition. In a recent study, we have unexpectedly observed that in most MRSA strains, there was no significant change in the resistance phenotype upon the overexpression in trans of a MecI repressor, whereas in those few strains negative for the bla locus, there was a massive decrease of resistance (D. C. Oliveira and H. de Lencastre, PLoS One 6:e23287, 2011). Here, we demonstrate that, contrary to what is currently accepted, the bla regulatory system efficiently disrupts the strong MecI-mediated repression on mecA, enabling the optimal expression of resistance. This effect appears to be due to the formation of MecI::BlaI heterodimers that might bind less efficiently to the mecA promoter and become nonfunctional due to the proteolytic inactivation of the BlaI monomer. In addition, we have also observed that the presence of bla regulators may enhance dramatically the expression of β-lactam resistance in MRSA strains with constitutive mecA expression, compensating for the fitness cost imposed by the large β-lactamase plasmid. These observations point to important unrecognized roles of the bla locus for the expression of the methicillin-resistant S. aureus (MRSA) phenotype.

Mobile genetic element SCCmec-encoded psm-mec RNA suppresses translation of agrA and attenuates MRSA virulence

Community acquired-methicillin resistant Staphylococcus aureus (CA-MRSA) is a socially problematic pathogen that infects healthy individuals, causing severe disease. CA-MRSA is more virulent than hospital associated-MRSA (HA-MRSA). The underlying mechanism for the high virulence of CA-MRSA is not known. The transcription product of the psm-mec gene, located in the mobile genetic element SCCmec of HA-MRSA, but not CA-MRSA, suppresses the expression of phenol-soluble modulin α (PSMα), a cytolytic toxin of S. aureus. Here we report that psm-mec RNA inhibits translation of the agrA gene encoding a positive transcription factor for the PSMα gene via specific binding to agrA mRNA. Furthermore, 25% of 325 clinical MRSA isolates had a mutation in the psm-mec promoter that attenuated transcription, and 9% of the strains had no psm-mec. In most of these psm-mec-mutated or psm-mec-deleted HA-MRSAs, PSMα expression was increased compared with strains carrying intact psm-mec, and some mutated strains produced high amounts of PSMα comparable with that of CA-MRSA. Deletion of psm-mec from HA-MRSA strains carrying intact psm-mec increased the expression of AgrA protein and PSMα, and virulence in mice. Thus, psm-mec RNA suppresses MRSA virulence via inhibition of agrA translation and the absence of psm-mec function in CA-MRSA causes its high virulence property.

Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to various antibiotics, including β-lactams, thus causing serious clinical problems. Hospital-associated (HA)-MRSA infects immunocompromised patients in hospitals. Community-acquired (CA)-MRSA causes serious diseases in healthy people who have not had contact with hospitals in the United States, Canada, or Europe. CA-MRSA produces higher amounts of extracellular toxins and has higher virulence than HA-MRSA, although the reason for this is unclear. SCCmec is a foreign DNA integrated into the MRSA chromosome that contains several genes including the mecA gene that confers resistance against methicillin. The SCCmec of CA-MRSA does not contain the psm-mec gene that exists in the HA-MRSA SCCmec. In the present study, we found that the transcription product of psm-mec inhibits translation of the agrA gene encoding a positive transcription factor for many extracellular toxins by direct binding to the agrA mRNA, resulting in decreased extracellular toxin production. Furthermore, some HA-MRSA strains carry mutated psm-mec or no psm-mec and produce higher amounts of extracellular toxins than HA-MRSA strains carrying intact psm-mec. These findings suggest that psm-mec RNA negatively regulates agrA and mutation or absence of psm-mec leads to a high virulence capacity of MRSA.

Proteolysis of mecA repressor is essential for expression of methicillin resistance by Staphylococcus aureus

Recently, we have demonstrated that the cognate regulatory locus of the mecA gene in methicillin-resistant Staphylococcus aureus (MRSA) is in fact a three-component system containing the novel mecR2 gene coding for an antirepressor. MecR2 interacts with the repressor MecI, disturbing its binding to the mecA promoter and fostering its proteolysis. Here, we engineered a point mutation in the putative cleavage site of MecI and demonstrated that MecI proteolysis is strictly required for the optimal expression of β-lactam resistance.

A Staphylococcus xylosus isolate with a new mecC allotype

Recently, a novel variant of mecA known as mecC (mecA(LGA251)) was identified in Staphylococcus aureus isolates from both humans and animals. In this study, we identified a Staphylococcus xylosus isolate that harbors a new allotype of the mecC gene, mecC1. Whole-genome sequencing revealed that mecC1 forms part of a class E mec complex (mecI-mecR1-mecC1-blaZ) located at the orfX locus as part of a likely staphylococcal cassette chromosome mec element (SCCmec) remnant, which also contains a number of other genes present on the type XI SCCmec.