Accessory Gene Regulator ( agr ) Locus in Geographically Diverse Staphylococcus aureus Isolates with Reduced Susceptibility to Vancomycin

Abstract

The majority of infections with glycopeptide intermediate-level resistant Staphylococcus aureus (GISA) originate in biomedical devices, suggesting a possible increased ability of these strains to produce biofilm. Loss of function of the accessory gene regulator (agr) of S. aureus has been suggested to confer an enhanced ability to bind to polystyrene. We studied agr in GISA, hetero-GISA, and related glycopeptide-susceptible S. aureus isolates. All GISA strains from diverse geographic origins belong to agr group II. All GISA strains were defective in agr function, as demonstrated by their inability to produce delta-hemolysin. Hetero-GISA isolate A5940 demonstrated a nonsense mutation in agrA that was not present in a pulsed-field gel electrophoresisindistinguishable vancomycin-susceptible isolate from the same patient. Various other agr point mutations were noted in several clinical GISA and hetero-GISA isolates. A laboratory-generated agr-null strain demonstrated a small but reproducible increase in vancomycin heteroresistance after growth in vitro in subinhibitory concentrations of vancomycin. This was not seen in the isogenic agr group II parent strain in which agr was intact. The in vitro bactericidal activity of vancomycin was attenuated in the agr-null strain compared to the parent strain. These findings imply that compromised agr function is advantageous to clinical isolates of S. aureus toward the development of vancomycin heteroresistance, perhaps through the development of vancomycin tolerance.Since 1997, several case reports have appeared describing Staphylococcus aureus clinical isolates with reduced susceptibility to glycopeptide antibiotics (5, 11, 13-15, 32, 35, 36). The mechanisms responsible for this low-level resistance are poorly understood but do not involve the vanA, vanB, vanC, vanD, vanE, or vanG genes that confer vancomycin resistance in enterococci (12-15). The preponderance of data suggests that the mechanism for intermediate-level vancomycin resistance in S. aureus may relate to sequestration of the antimicrobial agent by nonamidated muropeptides within a thickened cell wall (8,12,13).Our review of clinical case histories of several patients with glycopeptide intermediate-level resistant S. aureus (GISA) strains revealed that, in addition to prolonged exposure to vancomycin, most of these patients had infections originating in or involving biomedical devices such as artificial heart valves, central venous catheters, biliary stents, and dialysis catheters (5, 11, 13-15, 32, 35, 36).The accessory gene regulator (agr) locus of S. aureus is a quorum-sensing gene cluster of five genes (hld, agrB, agrD, agrC, and agrA) that upregulates production of secreted virulence factors, including the alpha-, beta-, and delta-hemolysins, and downregulates production of cell-associated virulence factors (16,23,25,26,28,31). Polymorphisms in agrD and agrC define four S. aureus agr groups (25). Published reports have noted that agr group I strains comprised a significant majority of clinical isolates (...

Reduced Vancomycin Susceptibility inStaphylococcus aureus, Including Vancomycin-Intermediate and Heterogeneous Vancomycin-Intermediate Strains: Resistance Mechanisms, Laboratory Detection, and Clinical Implications

“…In a number of these studies, determinations of agr activity were performed by the analysis of delta-hemolysin production on sheep blood agar; however, this is only a semiquantitative measure (296). Similar results were found by using microarray transcriptional analysis or real-time quantitative PCR to measure RNAIII transcripts in paired isolates (121,269).…”

“…agr group and hVISA/VISA. There initially appeared to be a link between the agr group II locus and VISA (296); however, subsequent studies have demonstrated the VISA phenotype in isolates from other agr groups (369). There does, however, appear to be a link between hVISA/VISA and the expression of agr in S. aureus and a potential association between the agr group and the response to vancomycin therapy (see below).…”

“…A number of studies have linked alterations in agr activation or function with vancomycin tolerance, an increased tendency to develop vancomycin resistance, and the presence of the hVISA or VISA phenotype (121,269,295,296,357). In a number of these studies, determinations of agr activity were performed by the analysis of delta-hemolysin production on sheep blood agar; however, this is only a semiquantitative measure (296).…”

Alterations in phage-typing patterns in vancomycin-intermediate Staphylococcus aureus

“…Vancomycin-intermediate expression in S. aureus is mediated by chromosomal mutation(s) (Avison et al, 2002;Pfeltz et al, 2000;Sakoulas et al, 2002; and is not due to horizontal gene transfer of vancomycin-resistance determinants (van) from organisms such as the vancomycin-resistant enterococci. VISA strains harbour a chromosomal mutation(s) that leads to alterations in cellwall synthesis and structure, which are thought to impart resistance by sequestering glycopeptide molecules away from their target (for review, see .…”

Role of two-component systems in the resistance ofStaphylococcus aureusto antibacterial agents

“…1,5 Two-component systems (TCSs) have recently been shown to affect susceptibility to several antibacterial agents. [6][7][8] TCSs are bacteria-specific signal transduction systems consisting of a sensor histidine kinase and cognate response regulator. In S. aureus, TCSs have been characterized mainly in terms of their effect on virulence factor expression and adaptation to environmental conditions.…”