Association between the agr locus and the presence of virulence genes and pathogenesis in Staphylococcus aureus using a Caenorhabditis elegans model

Abstract

Staphylococcus aureus is a commensal pathogen with a virulon that is under agr control. agr dysfunction has been seen in clinical strains that do not respond positively to treatment. This study aimed to establish the association between the genes in the virulon and the presence of agr and to determine the relationship between the presence or absence of agr and pathogenicity. PCR was used to identify the presence of the agr operon in 101 clinical S. aureus strains. δ-Haemolysin screening was conducted on all agr-positive strains using the blood agar assay. Singleplex and/or multiplex PCR was used to determine the presence of 31 virulence genes in the strains. Caenorhabditis elegans infectivity and lifespan assays were conducted using 30 CF512 nematodes per strain in triplicate. Significance associated with the carriage of virulence and agr genes was determined using the Chi-square test. Nematode survival was measured using Kaplan-Meier survival estimates and differences in survival were assessed using the log-rank test. The frequency of agr-negative strains was 20%. All groups of virulence genes were significantly associated with agr-positive strains: enterotoxin (p<0.001), toxins (p<0.001), capsule (p=0.036), and microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) (p=0.0026). The median lifespan (q=0.5) of agr-negative strains was 15.5 days and of agr-positive strains was 6.5 days. The log-rank test showed a significant difference in the survival rate of nematodes exposed to the two groups (p=0.006). There was a strong association between the carriage of virulence genes and the presence of the agr operon in clinical strains of S. aureus. Further, agr-positive strains were more pathogenic than agr-negative strains, suggesting a correlation between the presence of agr, carriage of virulence determinants, and pathogenicity.