Microbiological and Genotypic Analysis of Methicillin-Resistant Staphylococcus aureus Bacteremia

Abstract

In a recent landmark trial of bacteremia caused by methicillin-resistant Staphylococcus aureus (MRSA) isolates, vancomycin MICs were >1 g/ml for only 16% of the isolates, and accessory gene regulator (agr) function as measured by delta-hemolysin activity was absent or reduced in only 28.1% of the isolates. This clinical study did not capture a population of MRSA isolates predictive of vancomycin treatment failure.Methicillin-resistant Staphylococcus aureus (MRSA) isolates with higher vancomycin MICs (Ͼ1 g/ml), reduced vancomycin bactericidal activity in vitro (5, 13), and attenuated agr function (1) have been associated with reduced vancomycin efficacy in bacteremia. Clinical studies targeting bacteremia caused by MRSA with these properties would be important to define the practical role of newer antimicrobials. We evaluated the properties of MRSA isolates from a recent study demonstrating the noninferiority of daptomycin compared to standard therapy in cases of S. aureus bacteremia and endocarditis (2).Eighty-nine baseline MRSA isolates from the previously published study comparing standard therapy (vancomycin plus low-dose gentamicin for the first 4 days) to daptomycin in MRSA bacteremia and endocarditis were analyzed (87 of 89 isolates were analyzed genotypically) (2).Vancomycin susceptibility testing was confirmed by CLSI broth microdilution methods. Susceptibility to other antimicrobials was performed by Cubist Pharmaceuticals as part of the prior study (2). agr function was determined semiquantitatively using delta-hemolysin activity as previously described (12). The genotype and staphylococcal cassette chromosome mec (SCCmec) type was determined by previously published PCR methods (3, 6, 11). spa X-repeat polymorphisms were determined by nucleotide sequencing as described previously (7,11). spa types were compared with results from previous studies (11) and with the Ridom spa server (4) to provisionally assign isolates to multilocus sequence typing-defined clonal complexes (CCs). Clonal complex 1 (CC1) equaled spa types 131 and 950; CC5 equaled spa types 2, 12, 23, 47, 65, 203, 302, 385, 402, and 437; CC8 equaled spa types 1, 4, 7, 363, and 954; CC30 equaled spa types 16 and 19; CC45 equaled spa types 15, 756, 951, and 953; CC59 spa equaled types 17 and 206. Note that four new spa types are included above as follows: spa types 950 (UJJJFFKBPE), 951 (XE3BMBKB), 953 (XE3BBKB), and 954 (YHGCO).

Factors Influencing Time to Vancomycin‐Induced Clearance of Nonendocarditis Methicillin‐ResistantStaphylococcus aureusBacteremia: Role of Platelet Microbicidal Protein Killing andagrGenotypes

“…This is in sharp contrast to the characteristics of a more recent patient cohort included as part of a clinical trial conducted during 2002–2005 comparing the efficacy of vancomycin plus 4-day gentamicin with daptomycin therapy for the treatment of MRSA bacteremia and endocarditis [32]. In this latter study, the isolates had lower vancomycin MIC and were more likely to have preserved agr function and be agr group I.…”

In vitro endothelial cell damage is positively correlated with enhanced virulence and poor vancomycin responsiveness in experimental endocarditis due to methicillin-resistant Staphylococcus aureus

“…Population analyses revealed no evidence of vancomycin-heteroresistant subpopulations (Seidl et al , submitted). These 10 isolates were originally selected for the study because they belonged to two distinct genetic backgrounds (CC45 and CC5) that have been associated with enhanced virulence in humans (Fusco et al , 2009, McCalla et al , 2008, Sakoulas et al , 2002, van Leeuwen et al , 2000). We recently analyzed these 10 strains in an experimental rabbit IE model for their comparative fitness and therapeutic response profiles to vancomyin (Table 2, Seidl et al, submitted; Antimicrob Agents Chemother ).…”

Relationship of agr Expression and Function with Virulence and Vancomycin Treatment Outcomes in Experimental Endocarditis Due to Methicillin-Resistant Staphylococcus aureus

“…Within the agr-I group, we selected five isolates genotyped as clonal complex 45 (CC45), because this genetic background has been associated with reduced vancomycin responses clinically (11). Among the agr-II isolates in the overall strain collection, all were genotyped as CC5, the most common CC type in MRSA bacteremia and which has been associated with complicated MRSA infections (10,24). Five isolates were randomly selected from the latter CC group.…”