Basis of virulence in a Panton-Valentine leukocidin-negative community-associated methicillin-resistant Staphylococcus aureus strain

β-Hemolysin, not agrA mutation, inhibits the hemolysis of α-hemolysin in Staphylococcus aureus laboratory and clinical strains

Staphylococcus aureus produces various hemolysins regulated by the Agr-QS system, except β-hemolysin encoded by the gene hlb. A classical laboratory S. aureus strain RN4220 displays only the β-hemolysin phenotype. It was suspected that the 8A mutation at the end of its agrA gene delayed the expressions of hla and RNAIII, then failed to express α- and δ-hemolysins. However, hla gene expression was detected at the later culture time without α-hemolysin phenotype, the reason for such a phenotype has not been clearly understood. We created hlb knockout and complementary mutants via homologous recombination in RN4220 and NRS049, two strains that normally produce β-hemolysin and carry agrA mutation. We found interestingly that the presence or absence of α-hemolysin phenotype in such strains depended on the expression of β-hemolysin instead of agrA mutations, which only inhibited δ-hemolysin expression. The hemolysis phenotype was verified by the Christie-Atkinson-Munch-Peterson (CAMP) test. Quantitative reverse transcription PCR was carried out to evaluate the relative gene expressions of hlb, hla, and RNAIII. The construction of mutants did not affect the agrA mutation status. We demonstrate that the absence of α-hemolysin in S. aureus RN4220 and NRS049 strains is attributed to their production of β-hemolysin instead of agrA mutation. Our findings broaden the understanding of the molecular mechanisms that control hemolysin expression in S. aureus that is crucial for the development of new therapeutic strategies to combat S. aureus infections.

IMPORTANCE

α-Hemolysin is a critical virulence factor in Staphylococcus aureus and its expression is largely controlled by the Agr-QS system. Nonetheless, the hemolysis phenotype and the regulation of the Agr-QS system in S. aureus still hold many mysteries. Our study finds that it is the expression of β- hemolysin rather than the agrA mutation that inhibits the function of the α-hemolysin in an important S. aureus strain RN4220 and a clinical strain presents a similar phenotype, which clarifies the misunderstood hemolytic phenotype and mechanism of S. aureus. Our findings highlight the interactions among different toxins and their biological roles, combined with QS system regulation, which is ultimately the true underlying cause of its virulence. This emphasizes the importance of considering the collaborative action of various factors in the infection process caused by this significant human pathogen.

Profiling daptomycin resistance among diverse methicillin-resistant Staphylococcus aureus lineages in China

Daptomycin (DAP) is effective against methicillin-resistant Staphylococcus aureus (MRSA). However, reduced susceptibility to DAP in MRSA may lead to treatment failures. We aim to determine the distribution of DAP minimum inhibitory concentrations (MICs) and DAP heteroresistance (hDAP) among MRSA lineages in China. A total of 472 clinical MRSA isolates collected from 2015 to 2017 in China were examined for DAP susceptibility. All isolates (n = 472) were found to be DAP susceptible, but 35.17% (166/472) of them exhibited a high DAP MIC (MIC >0.5 µg/mL). The high DAP MIC group contained a larger proportion of isolates with a higher vancomycin or teicoplanin MIC (>1.5 µg/mL) than the low DAP MIC group (19.3% vs 7.8%, P < 0.001; 22.3% vs 8.2%, P < 0.001). We compared the clonal complex (CC) distributions and clinical characteristics in MRSA isolates stratified by DAP MIC. CC5 isolates were less susceptible to DAP (MIC50 = 1 µg/mL) than CC59 isolates (MIC50 = 0.5 µg/mL, P < 0.001). Population analysis profiling revealed that 5 of 10 ST5 and ST59 DAP-susceptible MRSA isolates investigated exhibited hDAP. The results also showed that CC5 MRSA with an agrA mutation (I238K) had a higher DAP MIC than those with a wild-type agrA (P < 0.001). The agrA-I238K mutation was found to be associated with agr dysfunction as indicated by the loss of δ-hemolysin production. In addition, agr/psmα defectiveness was associated with hDAP in MRSA. Whole-genome sequencing analysis revealed mutations in mprF and walR/walK in DAP-resistant subpopulations, and most DAP-resistant subpopulations (6/8, 75%) were stable. Our study suggests that the increased DAP resistance and hDAP in MRSA may threaten the effectiveness against MRSA infections.

Analysis of the Presence of the Virulence and Regulation Genes from Staphylococcus aureus (S. aureus) in Coagulase Negative Staphylococci and the Influence of the Staphylococcal Cross-Talk on Their Functions

Coagulase-negative staphylococci (CoNS) are increasingly becoming a public health issue worldwide due to their growing resistance to antibiotics and common involvement in complications related to invasive surgical procedures, and nosocomial and urinary tract infections. Their behavior either as a commensal or a pathogen is a result of strict regulation of colonization and virulence factors. Although functionality of virulence factors and processes involved in their regulation are quite well understood in S. aureus, little is known about them in CoNS species. Therefore, the aim of our studies was to check if clinical CoNS strains may contain virulence factors and genes involved in resistance to methicillin, that are homologous to S. aureus. Moreover, we checked the presence of elements responsible for regulation of genes that encode virulence factors typical for S. aureus in tested isolates. We also investigated whether the regulation factors produced by one CoNS isolate can affect virulence activity of other strains by co-incubation of tested isolates with supernatant from other isolates. Our studies confirmed the presence of virulence factor and regulatory genes attributed to S. aureus in CoNS isolates and indicated that one strain with an active agr gene is able to affect biofilm formation and δ-toxin activity of strains with inactive agr genes. The cognition of prevalence and regulation of virulence factors as well as antibiotic resistance of CoNS isolates is important for better control and treatment of CoNS infections.

Identification and Validation of a Novel Antibacterial Compound MZ-01 against Methicillin-Resistant Staphylococcus aureus

The discovery of new classes of antibiotics is slow, and it is being greatly outpaced by the development of bacterial resistance. This disparity places us in an increasingly vulnerable position because we are running out of safe and effective therapeutic options to treat antibiotic-resistant infections. This is exemplified by the emergence and persistence of hospital-acquired and community-associated methicillin-resistant S. aureus (MRSA), which has markedly narrowed our options for treating life-threatening staph infections. Thus, there is an urgent need to develop novel, potent, preventive, and therapeutic agents. In our current study, we performed a whole-cell screening assay of synthetic libraries for antibacterial activity and identified a novel molecule, MZ-01. MZ-01 exhibited potent bactericidal activity against Gram-positive bacterial pathogens, including MRSA, Streptococcus pyogenes, and Streptococcus pneumoniae, at low concentrations. MZ-01 killed and lysed both the late exponential phase of an S. aureus population and bacteria inside mammalian cells. Furthermore, MZ-01 exhibited low cytotoxicity. These results indicate that MZ-01 is a promising scaffold to guide the development of novel, potent antibacterial agents against multidrug-resistant Gram-positive bacterial pathogens such as MRSA.

Prevalence and Characterization of PVL-Positive Staphylococcus aureus Isolated from Raw Cow’s Milk

The present study aimed to investigate the prevalence, antibiotic susceptibility profiles, and some toxin genes of Panton-Valentine leukocidin (PVL)-positive Staphylococcus aureus (S. aureus) in unpasteurized raw cow’s milk collected from retail outlets located at Mansoura, Dakahliya governorate, Egypt. In that context, a total of 700 raw cow’s milk samples were investigated for the presence of S. aureus, which was identified in 41.1% (288/700) of the samples. Among the S. aureus isolates, 113 PVL-positive S. aureus were identified and subjected for further analysis. The PVL-positive S. aureus were investigated for the existence of toxin-related genes, including hemolysin (hla), toxic shock syndrome toxin-1 (tst), and enterotoxins (sea, seb, sec, see, seg, sei, and selj). Genotypic resistance of PVL-positive strains was performed for the detection of blaZ and mecA genes. Among the PVL-positive S. aureus, sea, seb, and sec were detected in 44.2, 6.2%, and 0.9%, respectively, while the hla and tst genes were identified in 54.9% and 0.9%, respectively. The blaZ and mecA genes were successfully identified in 84.9 (96/113) and 32.7% (37/113) of the total evaluated S. aureus isolates, respectively. PVL-positive S. aureus displayed a high level of resistance to penicillin, ampicillin, and trimethoprim-sulfamethoxazole. Multidrug resistance (resistant to ≥3 antimicrobial classes) was displayed by all methicillin-resistant S. aureus (MRSA) and 38.2% of methicillin-sensitive S. aureus (MSSA) isolates. The obtained findings are raising the alarm of virulent PVL-positive MRSA clones in retail milk in Egypt, suggesting the requirement for limiting the use of β-lactam drugs in food-producing animals and the importance of implementing strong hygiene procedures in dairy farms and processing plants.

Comparative Genomic Analysis Provides Insights into the Evolution and Genetic Diversity of Community-Genotype Sequence Type 72 Staphylococcus aureus Isolates

Staphylococcus aureus sequence type (ST) 72, the predominant community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) lineage in South Korea, has emerged as a major cause of bloodstream infection in hospital settings. However, relatively little information is available regarding the genomic characteristics and dissemination of ST72. Here, we characterized the whole-genome sequence of 24 ST72 isolates from China, along with 83 ST72 genomes from global sources. Of these 107 ST72 isolates, 63 were MRSA and 44 were methicillin-susceptible S. aureus (MSSA). Phylogenetic analysis revealed four distinct clades (A, B, C, and D), of which clade D contained only MSSA isolates. By characterizing the evolutionary dynamics of the ST72 lineage, we found that the MRSA from China might not have developed from the MSSA in China. Furthermore, we observed both international transmission of ST72 isolates and interregional transmission within China. The distributions of the SCCmec and spa types of isolates differed among clades. Additionally, in silico analyses revealed that the distributions of resistance genes, virulence genes, and mobile genetic elements (MGEs) also differed among isolates of the four clades. This was especially true for clade D isolates, which had the lowest level of antimicrobial resistance and had obtained specific virulence genes such as tsst-1 by acquisition of specific MGEs. Notably, ST72 MRSA isolates were more antibiotic resistant than ST72 MSSA isolates, but comparably virulent. Our findings provide insight into the potential transmission and genotypic features of ST72 clones across the globe.

IMPORTANCE Understanding the evolution and dissemination of community-genotype ST72 Staphylococcus aureus isolates is important, as isolates of this lineage have rapidly spread into hospital settings and caused serious health issues. In this study, we first carried out genome-wide analysis of 107 global ST72 isolates to characterize the evolution and genetic diversity of the ST72 lineage. We found that the MSSA lineage in China might have evolved independently from the MRSA isolates from China, and that ST72 isolates have the potential to undergo both international transmission and interregional transmission within China. The diversity of isolates correlated with distinct acquisitions of SCCmec elements, antibiotic resistance genes, virulence genes, and mobile genetic elements. The comprehensive information on the ST72 lineage emerging from this study will enable improved therapeutic approaches and rapid molecular diagnosis.

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

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

The genetic feature and virulence determinant of highly virulent community-associated MRSA ST338-SCCmec Vb in China

ST59 is the predominant pathotype of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in China. As a variant of ST59, there is relatively little known about the detailed information of ST338. To address this issue, here, we described thirteen ST338 CA-MRSA strains isolated from severe bloodstream infection cases, and focused on their epidemiology, genetic features and virulence potential. Phylogenetic analysis showed the earliest isolated strain of this study is likely a predecessor of recent ST338 lineage (after year of 2014). Furthermore, the phylogenetic reconstruction and time estimation suggested that ST338 evolved from ST59 in 1991. Notably, the carrying patten of virulence factors of all ST338 strains were similar, and the genomic islands νSaα, νSaγ and SaPI and the core virulence factors like hla and psm were detected in ST338 isolates. However, all ST338 isolates lacked some adhesion factors such as clfA, clfB, eap, cna and icaD. Additionally, among these ST338 strains, one PVL-negative ST338 isolate was detected. Experiment on mice nose and human alveolar epithelial cell showed that the nasal colonization ability of ST338 was weaker than that of CA-MRSA MW2. In a mouse bloodstream infection model and skin infection model, PVL+ and PVL− strains had the similar virulence, which was dependent on upregulation of toxin genes rather than the presence of mobile genetic elements such as ΦSa2 carrying PVL. Our findings provide important insight into the epidemiology and pathogenicity of the novel and highly virulent ST338-SCCmec Vb clone.

Comparative Analysis of Virulence and Toxin Expression of Vancomycin-Intermediate and Vancomycin-Sensitive Staphylococcus aureus Strains

Previous studies on vancomycin-intermediate Staphylococcus aureus (VISA) have mainly focused on drug resistance, the evolution of differences in virulence between VISA and vancomycin-sensitive S. aureus (VSSA) requires further investigation. To address this issue, in this study, we compared the virulence and toxin profiles of pair groups of VISA and VSSA strains, including a series of vancomycin-resistant induced S. aureus strains—SA0534, SA0534-V8, and SA0534-V16. We established a mouse skin infection model to evaluate the invasive capacity of VISA strains, and found that although mice infected with VISA had smaller-sized abscesses than those infected with VSSA, the abscesses persisted for a longer period (up to 9 days). Infection with VISA strains was associated with a lower mortality rate in Galleria mellonella larvae compared to infection with VSSA strains (≥ 40% vs. ≤ 3% survival at 28 h). Additionally, VISA were more effective in colonizing the nasal passage of mice than VSSA, and in vitro experiments showed that while VISA strains were less virulent they showed enhanced intracellular survival compared to VSSA strains. RNA sequencing of VISA strains revealed significant differences in the expression levels of the agr, hla, cap, spa, clfB, and sbi genes and suggested that platelet activation is only weakly induced by VISA. Collectively, our findings indicate that VISA is less virulent than VSSA but has a greater capacity to colonize human hosts and evade destruction by the host innate immune system, resulting in persistent and chronic S. aureus infection.

Surgical Site Infections Caused by Highly Virulent Methicillin-Resistant Staphylococcus aureus Sequence Type 398, China

We identified 2 methicillin-resistant Staphylococcus aureus strains of sequence type 398 from surgical site infections in China. Genetic analysis and clinical data from these strains suggested that they were human-related but sporadic. Hemolysis analysis and mouse-skin infection models indicated a high virulence potential for these strains.

Comprehensive Virulence Gene Profiling of Bovine Non-aureus Staphylococci Based on Whole-Genome Sequencing Data

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from intramammary infection (IMI) in dairy cattle. Virulence factors (VFs) and mechanisms by which NAS cause IMI are not fully known. Herein, we analyzed the distribution of 191 VFs in 441 genomes of 25 NAS species, after classifying VFs into functional categories: adherence (n = 28), exoenzymes (n = 21), immune evasion (n = 20), iron metabolism (n = 29), and toxins (n = 93). In addition to establishing VF gene profiles, associations of VF genes between and among functional categories were computed, revealing distinctive patterns of association among VFs for various NAS species. Associations were also computed for low, medium, and high somatic cell count (SCC) and clinical mastitis (CM) isolates, demonstrating distinctive patterns of associations for low SCC and CM isolates, but no differences between high SCC and CM isolates. To determine whether VF distributions had any association with SCC or CM, various clustering approaches, including complete linkages, Ward clustering, and t-distributed stochastic neighbor embedding, were applied. However, no clustering of isolates representing low SCC, medium SCC, or high SCC or CM was identified. Regression analysis to test for associations with individual VF functional categories demonstrated that each additional toxin and host immune evasion gene increased the odds of having high SCC or CM, although an overall increase in the number of VFs was not associated with increased SCC or occurrence of CM. In conclusion, we established comprehensive VF gene profiling, determined VF gene distributions and associations, calculated pathogenic potentials of all NAS species, and detected no clear link between VF genes and mastitis.

IMPORTANCE Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Identification of Hemolysine Genes and their Association with Antimicrobial Resistance Pattern among Clinical Isolates of Staphylococcus aureus in West of Iran

Background

Staphylococcus aureus is expressing a broad range of different hemolysins enhancing its ability to establish and maintain infection in humans. The aim of this study was to identify the types of hemolysins in different clinical isolates of S. aureus and their association with antibiotic resistance patterns.

Materials and Methods

In this cross-sectional and descriptive study, clinical isolates of S. aureus were collected from Hamedan's hospitals during an 11-month period from June 2016 to January 2017 and identified by using biochemical tests. To determine the antibiotic resistance pattern, disk diffusion method and minimum inhibitory concentration (MIC) were conducted. Genomic DNA was extracted using extraction kit. The polymerase chain reaction was done with specific primers for identification of hla, hlb, hld, and hld genes.

Results

Among a total of 389 clinical samples, 138 isolates (35.45%) of S. aureus were identified, which 87 isolates (63.04%) were cefoxitin MIC of >4 μg/ml and resistant to methicillin. The highest frequency of antibiotic resistance was observed against erythromycin in 108 isolates (78.26%) and penicillin in 133 isolates (96.37%) and the lowest resistance was against gatifloxacin in 50 isolates (36.23%) and Cefazolin in 11 isolates (97.7%). Furthermore, the hla, hlb, hld, and hlg genes were detected among 11 (7.97%), 7 (5.07%), 16 (11.59%), and 4 (2.89%) isolates, respectively. There was a significant relationship between the presence of alpha and delta hemolysin-encoding genes and the antibiotic resistance pattern of isolates (P < 0.05).

Conclusion

The results exhibited that the association between the presence of the hemolysin genes and the antibiotic resistance pattern can be considered as a serious issue.

Current and future treatment options for community-associated MRSA infection

INTRODUCTION

Community-associated MRSA (CA-MRSA) represents a global epidemic which beautifully encapsulates the fascinating ability of bacterial organisms to adapt quickly on an evolutionary basis to the extreme selective pressure of antibiotic exposure. In stark contrast to Healthcare-associated MRSA (HA-MRSA), it has become apparent that CA-MRSA is less straight forward of a challenge in terms of controlling its transmission, and has forced clinicians to adjust empiric management of clinical syndromes such as skin and soft tissue infection (SSTI) as well as pneumonia.

AREAS COVERED

This review details the history and epidemiology of CA-MRSA, while covering both current and future treatment options that are and may be available to clinicians. The authors reviewed both historic and more recent literature on this ever-evolving topic.

EXPERT OPINION

While development of new anti-MRSA agents should be encouraged, the importance of antimicrobial stewardship in the battle to stay ahead of the curve with regards to the ongoing control of the MRSA epidemic should be emphasised.

Detection and analysis of methicillin-resistant human-adapted sequence type 398 allows insight into community-associated methicillin-resistant Staphylococcus aureus evolution

Background

Severe infections with highly virulent community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) are a global problem. However, the molecular events defining the evolution of CA-MRSA are still poorly understood. MRSA of sequence type (ST) 398 is known to frequently infect livestock, while ST398 isolates infecting humans are commonly methicillin-susceptible or represent MRSA originating from livestock-associated (LA)-MRSA.

Methods

We used whole genome sequencing of newly detected CA-MRSA ST398 isolates, in comparison to geographically matched LA-MRSA and methicillin-sensitive ST398, to determine their evolutionary history. Furthermore, we used phenotypic analyses including animal infection models to gain insight into the evolution of virulence in these CA-MRSA isolates. Finally, we determined methicillin resistance and expression of the methicillin resistance-conferring gene mecA and its penicillin-binding protein product, PBP2a, in a large series of CA-MRSA strains of divergent STs.

Results

We report several cases of severe and fatal infections due to ST398 CA-MRSA. The responsible isolates showed the typical genetic characteristics reported for human-adapted methicillin-sensitive ST398. Whole genome sequencing demonstrated that they evolved from human-adapted, methicillin-susceptible clones on several different occasions. Importantly, the isolates had not undergone consistent genetic alterations or changes in virulence as compared to their methicillin-susceptible predecessors. Finally, we observed dramatically and consistently lower methicillin resistance and expression of the resistance gene mecA, as compared to hospital-associated MRSA strains, in a diverse selection of CA-MRSA strains.

Conclusions

Our study presents evidence for the development of highly virulent human-adapted ST398 CA-MRSA isolates from methicillin-susceptible predecessors. Notably, our investigation indicates that, in contrast to widespread notions, the development of CA-MRSA is not necessarily associated with the acquisition of specific virulence genes or other virulence-increasing changes. Rather, our findings emphasize the importance of the CA-MRSA-characteristic staphylococcal cassette chromosome mec types, which provide only low-level methicillin resistance, for that process. Our findings are of particular importance for the diagnosis of CA-MRSA, inasmuch as they indicate that the presence of specific virulence genes cannot generally be used for that purpose.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-018-0514-9) contains supplementary material, which is available to authorized users.

Infection Prevention Strategy in Hospitals in the Era of Community-Associated Methicillin-Resistant Staphylococcus aureus in the Asia-Pacific Region: A Review

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has emerged as an important cause of healthcare-associated infection. CA-MRSA clones have replaced classic hospital MRSA clones in many countries and have shown higher potential in transmission and virulence than hospital MRSA clones. In particular, the emergence of CA-MRSA in the Asia-Pacific region is concerning owing to insufficient infection control measures in the region. The old strategies for infection prevention and control of MRSA comprised adherence to standard precaution and policy of active screening of MRSA carriers and decolonization, and it has been controversial which strategy is better in terms of outcome and cost-effectiveness. Epidemiological changes in MRSA has made the development of infection prevention strategy more complicated. Based on the literature review and the questionnaire survey, we considered infection prevention strategies for healthcare settings in the Asia-Pacific region in the era of CA-MRSA.

Virulence Factor Targeting of the Bacterial Pathogen Staphylococcus aureus for Vaccine and Therapeutics

BACKGROUND

Staphylococcus aureus is a major bacterial pathogen capable of causing a range of infections in humans from gastrointestinal disease, skin and soft tissue infections, to severe outcomes such as sepsis. Staphylococcal infections in humans can be frequent and recurring, with treatments becoming less effective due to the growing persistence of antibiotic resistant S. aureus strains. Due to the prevalence of antibiotic resistance, and the current limitations on antibiotic development, an active and highly promising avenue of research has been to develop strategies to specifically inhibit the activity of virulence factors produced S. aureus as an alternative means to treat disease.

OBJECTIVE

In this review we specifically highlight several major virulence factors produced by S. aureus for which recent advances in antivirulence approaches may hold promise as an alternative means to treating diseases caused by this pathogen. Strategies to inhibit virulence factors can range from small molecule inhibitors, to antibodies, to mutant and toxoid forms of the virulence proteins.

CONCLUSION

The major prevalence of antibiotic resistant strains of S. aureus combined with the lack of new antibiotic discoveries highlight the need for vigorous research into alternative strategies to combat diseases caused by this highly successful pathogen. Current efforts to develop specific antivirulence strategies, vaccine approaches, and alternative therapies for treating severe disease caused by S. aureus have the potential to stem the tide against the limitations that we face in the post-antibiotic era.

Characterization of a PVL-negative community-acquired methicillin-resistant Staphylococcus aureus strain of sequence type 88 in China

Sequence type 88 community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) strain SR434, isolated from an outpatient with skin and soft tissue infection, was subjected to whole genome sequencing, antimicrobial susceptibility testing, mouse skin infection model and hemolysis analysis to identify its virulence and resistance determinants. MRSA strain SR434 is resistant to clindamycin, erythromycin and fosfomycin. Four plasmids with resistance genes were identified in this strain, including a 20,658bp blaZ-carrying plasmid, a 2473bp ermC-carrying plasmid, a 2622bp fosB7-carrying plasmid (86% identity with plasmid in a ST2590 MRSA strain) and a 4817bp lnuA-carrying plasmid (99% identity with pLNU4 from bovine coagulase-nagetive Staphylococci). This strain contains staphylococcal cassette chromosome mec type IV and does not contain arginine catabolic mobile element or Panton-Valentine-Leukocidin. SR434 harbors genomic islands νSaα, νSaβ, νSaγ and ΦSa3 and pathogenicity islands νSa2 that carries genes encoding toxic shock syndrome toxin 1, superantigen enterotoxin C and superantigen enterotoxin L. Mouse skin infection model results show that SR434 had similar virulence potential causing invasive skin infection as a PVL-negative epidemic Korea clone HL1 (ST72). CA-MRSA strain of ST88 lineage might be a great concern for its high virulence. PVL has limited contribution to virulence phenotype among this lineage.

Genome comparisons of two Taiwanese community-associated methicillin-resistant Staphylococcus aureus ST59 clones support the multi-origin theory of CA-MRSA

Sequence type (ST) 59 is an epidemic lineage of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Asia. Two ST59 clones are prevalent in Taiwan: the Taiwan clone (TW) causes severe infections, whereas the Asian-Pacific clone (AP) is usually commensal. In this study, we sequenced the genome and transcriptome of the representative strains of these two clones and found their differences to focus on three mobile genetic elements: TW carries SCCmec Type V_T, Panton-Valentine leucocidin (PVL)-encoding prophage ΦSa2, whereas AP carries SCCmec Type IV and staphylokinase (SAK)-encoding prophage ΦSa3. The anti-virulent role of SAK was confirmed using murine skin and bloodstream infection models. ΦSa3 usually integrates into the hlb gene, but in AP was found to be integrated at the genomic island νSaβ. The mutation of the attB site "TGTATCCAAACTGG" to "TGTATCCGAATTGG" led to a failure in the integration of ΦSa3 in hlb, prompting atypical integration at other sites. The sak gene possessed remarkably different patterns of distribution among the different STs of S. aureus. We conclude that the atypical integration of ΦSa3 may help S. aureus adapt to the human host habitat and that the subsequent loss of ΦSa3 contributes toward the development of a virulent CA-MRSA lineage for wider horizontal transmission.

Dracorhodin Perochlorate attenuates Staphylococcus aureus USA300 virulence by decreasing α-toxin expression

α-Toxin, a pore-forming toxin secreted by most Staphylococcus aureus, plays critical role in the pathogenesis associated with various infectious diseases. The USA300 which is a major international epidemic methicilin-resisrant S. aureus has spread rapidly to multiple countries and become an emerging public health concern. In this study, the in vitro efficacy of Dracorhodin Perochlorate (DP) against USA300 virulence was evaluated. Using susceptibility testing, immunoblots, rabbit blood haemolytic assay and real-time RT-PCR, we observed that the α-toxin production was decreased when USA300 was co-cultured with different sub-inhibitory concentration of DP. Further, the protective effect of DP against USA300-mediated injury of human alveolar epithelial cells (A549) and MH-S cells was evaluated by cytotoxicity assays, and the result revealed that DP, at final concentration of 16 µg/ml, is a potent antagonist for USA300-mediated cell damage. Importantly, those beneficial effects might partially correlate with hla and RNAIII suppression by DP, leading to the inhibition of α-toxin production in culture supernatant. Overall, these results suggest that DP could attenuate the virulence of USA300 by decreasing α-toxin production without inhibiting bacterial growth, and this compound may represent an ideal candidate for the development of anti-virulence agent combating S. aureus infection.

Virulence determinants associated with the Asian community-associated methicillin-resistant Staphylococcus aureus lineage ST59

Understanding virulence is vital for the development of novel therapeutics to target infections with community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), which cause an ongoing epidemic in the United States and are on a global rise. However, what defines virulence particularly of global CA-MRSA lineages is poorly understood. Threatening a vast population, the predominant Asian CA-MRSA lineage ST59 is of major epidemiological importance. However, there have been no molecular analyses using defined virulence gene deletion mutants in that lineage as of yet. Here, we compared virulence in skin, lung, and blood infection models of ST59 CA-MRSA isolates with geographically matched hospital-associated MRSA isolates. We selected a representative ST59 CA-MRSA isolate based on toxin expression and virulence characteristics, and produced isogenic gene deletion mutants of important CA-MRSA virulence determinants (α-toxin, PSM α, Agr) in that isolate for in-vitro and in-vivo analyses. Our results demonstrate strongly enhanced virulence of ST59 CA-MRSA over hospital-associated lineages, supporting the notion that enhanced virulence is characteristic for CA-MRSA. Furthermore, they show strong and significant contribution of Agr, α-toxin, and PSMα to pathogenesis of ST59 CA-MRSA skin, lung, and blood infection, emphasizing the value of drug development efforts targeted toward those virulence determinants.

Phenol-Soluble Modulins Contribute to Early Sepsis Dissemination Not Late Local USA300-Osteomyelitis Severity in Rabbits

Introduction

In bone and joint infections (BJIs), bacterial toxins are major virulence factors: Panton—Valentine leukocidin (PVL) expression leads to severe local damage, including bone distortion and abscesses, while α-hemolysin (Hla) production is associated with severe sepsis-related mortality. Recently, other toxins, namely phenol-soluble modulins (PSMs) expressed by community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strain USA300 (LAC WT) were shown to have ex vivo intracellular cytotoxic activity after S. aureus invasion of osteoblasts, but their in vivo contribution in a relatively PVL-sensitive osteomyelitis model remains poorly elucidated.

Materials and Methods

We compared the outcomes of experimental rabbit osteomyelitises induced with pvl⁺hla⁺psms⁺ LAC WT and its isogenic Δpsm derivatives (LAC Δpsmα and LAC Δpsmαβhld) using an inoculum of 3 × 10⁸ CFUs. Mortality, hematogenous spread (blood culture, spleen and kidney), lung and bone involvements were assessed in two groups (non-survivors of severe sepsis and survivors sacrificed on day (D) 14).

Results

Severe sepsis-related mortality tended to be lower for Δpsm derivatives (Kaplan—Meier curves, P = .06). Non-survivors’ bone LAC-Δpsmα (6.9 log₁₀ CFUs/g of bone, P = .04) or -Δpsmαβhld (6.86 log₁₀ CFUs/g of bone, P = .014) densities were significantly higher than LAC WT (6.43 log₁₀ CFUs/g of bone). Conversely, lung Δpsmαβhld CFUs were significantly lower than LAC WT (P = .04). LAC Δpsmα, Δpsmαβhld and WT induced similar bone damage in D14 survivors, with comparable bacterial densities (respectively: 5.89, 5.91, and 6.15 log₁₀ CFUs/g of bone). Meanwhile, pulmonary histological scores of inflammation were significantly higher for LAC Δpsmα- and Δpsmαβhld-infected rabbits compared to LAC WT (P = .04 and .01, respectively) but with comparable lung bacterial densities.

Conclusion

Our experimental results showed that deactivating PSM peptides significantly limited bacterial dissemination from bone during the early phase of infection, but did not affect local severity of USA300 rabbit osteomyelitis.

Characteristics of the community-genotype sequence type 72 methicillin-resistant Staphylococcus aureus isolates that underlie their persistence in hospitals

Panton-Valentine leukocidin-negative methicillin-resistant Staphylococcus aureus (MRSA) clone ST72, known as a major community-associated MRSA in Korea, has emerged as an important pathogen in hospitals. To understand bacterial properties that underlie transformation of this clone into a nosocomial pathogen, we compared characteristics of the community-genotype ST72 MRSA isolates with those of ST5 and ST239 MRSA, which have been predominant nosocomial MRSA clones in Korea. Several genes associated with adhesion and virulence were absent or rarely found in ST72 isolates. Many ST72 isolates (70.1%) belonged to agr group I, but the agr group of other ST72 isolates could not be determined. As indicated by d-hemolysin production, ST72 isolates expressed fully functional agr, whereas agr dysfunction was observed in ST5 and ST239 isolates. In the biofilm formation assay, no upregulation of biofilm-forming activity of ST72 MRSA was detected. However, ST72 isolates demonstrated persistence under hypotonic and desiccating conditions (survival rates 72.3% and 33.9%, respectively), which was similar to characteristics of ST5 or ST239 isolates. ST72- MRSA isolates showed low virulence, but properties of their functional agr system could facilitate their spread in hospitals. In conclusion, tolerance to stressful environments, e.g., hypotonic and dry conditions, may also contribute to survival of the community-associated MRSA clones in healthcare facilities.

The Role of Staphylococcus aureus Virulence Factors in Skin Infection and Their Potential as Vaccine Antigens

Staphylococcus aureus (S. aureus) causes the vast majority of skin and soft tissue infections (SSTIs) in humans. S. aureus has become increasingly resistant to antibiotics and there is an urgent need for new strategies to tackle S. aureus infections. Vaccines offer a potential solution to this epidemic of antimicrobial resistance. However, the development of next generation efficacious anti-S. aureus vaccines necessitates a greater understanding of the protective immune response against S. aureus infection. In particular, it will be important to ascertain if distinct immune mechanisms are required to confer protection at distinct anatomical sites. Recent discoveries have highlighted that interleukin-17-producing T cells play a particularly important role in the immune response to S. aureus skin infection and suggest that vaccine strategies to specifically target these types of T cells may be beneficial in the treatment of S. aureus SSTIs. S. aureus expresses a large number of cell wall-anchored (CWA) proteins, which are covalently attached to the cell wall peptidoglycan. The virulence potential of many CWA proteins has been demonstrated in infection models; however, there is a paucity of information regarding their roles during SSTIs. In this review, we highlight potential candidate antigens for vaccines targeted at protection against SSTIs.

Genotypic Diversity of Staphylococcus aureus α-Hemolysin Gene (hla) and Its Association with Clonal Background: Implications for Vaccine Development

The α-hemolysin, encoded by the hla gene, is a major virulence factor in S. aureus infections. Changes in key amino acid residues of α-hemolysin can result in reduction, or even loss, of toxicity. The aim of this study was to investigate the diversity of the hla gene sequence and the relationship of hla variants to the clonal background of S. aureus isolates. A total of 47 clinical isolates from China were used in this study, supplemented with in silico analysis of 318 well-characterized whole genome sequences from globally distributed isolates. A total of 28 hla genotypes were found, including three unique to isolates from China, 20 found only in the global genomes and five found in both. The hla genotype generally correlated with the clonal background, particularly the multilocus sequence type, but was not related to geographic origin, host source or methicillin-resistance phenotype. In addition, the hla gene showed greater diversity than the seven loci utilized in the MLST scheme for S. aureus. Our investigation has provided genetic data which may be useful for future studies of toxicity, immunogenicity and vaccine development.