Association between the accessory gene regulator (agr) locus and the presence of superantigen genes in clinical isolates of methicillin-resistant Staphylococcus aureus

Emergence and spread of ST5 methicillin-resistant Staphylococcus aureus with accessory gene regulator dysfunction: genomic insights and antibiotic resistance

The globally disseminated Staphylococcus aureus ST5 clone poses a major public health threat due to its multidrug resistance and virulence. Here, we identified an agr-dysfunctional (agrA-I238K) ST5 MRSA clone that has spread across East and Southeast Asia, with recent increases in China since its emergence in the 1970s. Comparative genomic analyses identified distinct single-nucleotide polymorphisms and mobile genetic elements linked to enhanced resistance and virulence. This clone exhibits resistance to seven antimicrobial classes, including third-generation tetracyclines and fusidic acid, and shares phenotypic and genetic similarities with the vancomycin-intermediate S. aureus Mu50 strain, including reduced susceptibility to vancomycin, teicoplanin, and daptomycin. The agrA-I238K mutation attenuates hemolytic activity, increases biofilm formation, and reduces daptomycin susceptibility, suggesting a key role in the clone's success. Our results demonstrate the important role of agrA-I238K mutation in the widespread distribution of agr-dysfunctional MRSA and highlight the importance of genomic surveillance in tracking the spread of agr-dysfunctional ST5 MRSA.

Direct Quantitative Immunochemical Analysis of Autoinducer Peptide IV for Diagnosing and Stratifying Staphylococcus aureus Infections

An immunochemical strategy to detect and quantify AIP-IV, the quorum sensing (QS) signaling molecule produced by Staphylococcus aureus agr type IV, is reported here for the first time. Theoretical calculations and molecular modeling studies have assisted on the design and synthesis of a suitable peptide hapten (AIPIVS), allowing to obtain high avidity and specific antibodies toward this peptide despite its low molecular weight. The ELISA developed achieves an IC₅₀ value of 2.80 ± 0.17 and an LOD of 0.19 ± 0.06 nM in complex media such as 1/2 Tryptic Soy Broth. Recognition of other S. aureus AIPs (I-III) is negligible (cross-reactivity below 0.001%), regardless of the structural similarities. A pilot study with a set of clinical isolates from patients with airways infection or colonization demonstrates the potential of this ELISA to perform biomedical investigations related to the role of QS in pathogenesis and the association between dysfunctional agr or the agr type with unfavorable clinical outcomes. The AIP-IV levels could be quantified in the low nanomolar range in less than 1 h after inoculating agr IV-genotyped isolates in the culture broth, while those genotyped as I-III did not show any immunoreactivity after a 48 h growth, pointing to the possibility to use this technology for phenotyping S. aureus. The research strategy here reported can be extended to the rest of the AIP types of S. aureus, allowing the development of powerful multiplexed chips or point-of-care (PoC) diagnostic devices to unequivocally identify its presence and its agr type on samples from infected patients.

Graphene-based fluorometric determination of agrD gene transcription in methicillin-resistant Staphylococcus aureus using exonuclease III-aided target recycling and DNA walker cascade amplification

A graphene-based bioassay is described for the fluorometric determination of agrD gene transcription (mRNA) in methicillin-resistant Staphylococcus aureus (MRSA). This method includes exonuclease III (Exo III)-assisted target recycling and DNA walker cascade amplification. Hairpin1 (HP1) consists of a capture probe (CP) and DNA walker sequence. In the absence of the target, 5'-amino modified hairpin2 (HP2) labeled with carboxyfluorescein (FAM) at its 3' terminus is covalently linked to graphene via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) catalysis, resulting in the quenching of the FAM signal. The stem-loop structure of HP1 opens when the target is added to form partially complementary DNA/RNA hybrids. Exo III then initiates the target recycling process by cleaving the CP and DNA walker cascade reaction by automatic walking. This iterative reaction causes the FAM to dissociate from the graphene, and the fluorescence can be measured at excitation/emission wavelengths of 480/514 nm. Therefore, the target can be assayed by fluorescence. This method has a linear relationship with the concentration of target within the range 1 fM to 100 pM with a detection limit of 1 fM. The developed bioassay was used to monitor biofilm formation and investigate the mechanism of drug action with satisfactory results. Schematic representation of the graphene-based fluorescent bioassay for agrD gene transcription in methicillin-resistant Staphylococcus aureus by using exonuclease III-aided target recycling and DNA walker cascade amplification.

Antibiotic susceptibility of human-associated Staphylococcus aureus and its relation to agr typing, virulence genes, and biofilm formation

BACKGROUND AND OBJECTIVE

Carriage of virulence factors confers some evolutionary benefit to bacteria, which favors the resistant strains. We aimed to analyze whether antibiotic susceptibility of Staphylococcus aureus strains is affected by agr typing, biofilm formation ability, and virulence profiles.

METHODS

A total of 123 S. aureus clinical isolates were subjected to antimicrobial susceptibility testing by disk diffusion method, biofilm formation by microtiter plate method, as well as polymerase chain reaction screening to identify virulence genes and the accessory gene regulator (agr) types I-IV. A P value < 0.05 was considered significant.

RESULTS

The most prevalent virulence gene was staphyloxanthin crtN, followed by hemolysin genes, capsular cap8H, toxic shock toxin tst, and enterotoxin sea, respectively. Resistant isolates were more commonly found in the agr-negative group than in the agr-positive group. Isolates of agr type III were more virulent than agr I isolates. Strong biofilm producers showed more antibiotic susceptibility and carried more virulence genes than non-strong biofilm producers. Associations were found between the presence of virulence genes and susceptibility to antibiotics. Carriage of the virulence genes and agr was higher in the inpatients; while, resistance and strong biofilms were more prevalent in the outpatients.

CONCLUSION

These findings indicated the presence of several virulence factors, biofilm production capacity, agr types and resistance to antibiotics in clinical S. aureus isolates. Considering the importance of S. aureus for human medicine, an understanding of virulence and resistance relationships would help to reduce the impact of S. aureus infections.

Regulation of virulence and β-lactamase gene expression in Staphylococcus aureus isolates: cooperation of two-component systems in bloodstream superbugs

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA)-bloodstream infections (BSI) are predominantly seen in the hospital or healthcare-associated host. Nevertheless, the interactions of virulence factor (VFs) regulators and β-lactam resistance in MRSA-BSI are unclear. This study aims to characterize the molecular relationship of two-component systems of VFs and the expression of the β-lactamase gene in MRSA-BSI isolates. In this study, 639 samples were collected from BSI and identified by phenotypic methods. We performed extensive molecular characterization, including SCCmec type, agr type, VFs gene profiles determinations, and MLST on isolates. Also, a quantitative real-time PCR (q-RT PCR) assay was developed for identifying the gene expressions.

RESULTS

Ninety-one (91) S. aureus and 61 MRSA (67.0%) strains were detected in BSI samples. The presence of VFs and SCCmec genes in MRSA isolates were as follows: tst (31.4%), etA (18.0%), etB (8.19%), lukS-PVL (31.4%), lukF-PV (18.0%), lukE-lukD (16.3%), edin (3.2%), hla (16.3%), hlb (18.0%), hld (14.7%), hlg (22.9%), SCCmecI (16.3%), SCCmecII (22.9%), SCCmecIII (36.0%), SCCmecIV (21.3%), and SCCmecV (16.3%). Quantitative real-time PCR showed overexpression of mecRI and mecI in the toxigenic isolates. Moreover, RNAIII and sarA genes were the highest expressions of MRSA strains. The multi-locus sequence typing data confirmed a high prevalence of CC5, CC8, and CC30. However, ST30, ST22, and ST5 were the most prevalent in the resistant and toxigenic strains.

CONCLUSION

We demonstrated that although regulation of β-lactamase gene expressions is a significant contributor to resistance development, two-component systems also influence antibiotic resistance development in MRSA-BSI isolates. This indicates that resistant strains might have pathogenic potential. We also confirmed that some MLST types are more successful colonizers with a potential for MRSA-BSI.

Clinical and molecular characteristics of Staphylococcus aureus isolated from Chinese children: association among the agr groups and genotypes, virulence genes and disease types

BACKGROUND

This study was aimed to investigate the clinical and molecular epidemiology of Staphylococcus aureus (S. aureus) isolated from Chinese children and determine the possible relationship among the accessory gene regulator (agr) groups and genotypes, as well as among the virulence genes and disease types.

METHODS

S. aureus strains were isolated from Beijing Children's Hospital between October 2017 and October 2019. The isolates and 19 virulence genes were characterized using multi-locus sequence typing, staphylococcal protein A (spa), staphylococcal cassette chromosome mec, and agr typing.

RESULTS

A total of 191 non-repetitive S. aureus clinical isolates were divided into 33 sequence types (STs), 18 clonal complexes (CCs), and 59 spa types. ST59 (39.8%), t437 (37.7%), and agr I (84.8%) were the predominant types. CC59, CC25, CC22, CC951, CC8, and CC398 belonged to agr I. CC5 and CC15 were assigned to agr II, and CC30 was characterized as agr III. CC121 was classified under agr IV. The eta, etb, and bbp genes were more prevalent in agr IV (P < 0.001 for each), while tst was more prevalent in agr group III compared to the other groups (P < 0.001). Nearly all isolates that harbored lukS/F-PV belonged to agr I (P = 0.005). However, the correlation between disease types and agr groups was not significant (P > 0.05).

CONCLUSIONS

An association among the agr groups and genotypes, as well as specific toxin genes, was observed among the S. aureus strains isolated from Chinese children. However, a statistical correlation was not found among the agr groups and disease types.