Staphylococcus aureus CC30 Lineage and Absence of sed, j, r-Harboring Plasmid Predict Embolism in Infective Endocarditis

Abstract

Staphylococcus aureus induces severe infective endocarditis (IE) where embolic complications are a major cause of death. Risk factors for embolism have been reported such as a younger age or larger IE vegetations, while methicillin resistance conferred by the mecA gene appeared as a protective factor. It is unclear, however, whether embolism is influenced by other S. aureus characteristics such as clonal complex (CC) or virulence pattern. We examined clinical and microbiological predictors of embolism in a prospective multicentric cohort of 98 French patients with monomicrobial S. aureus IE. The genomic contents of causative isolates were characterized using DNA array. To preserve statistical power, genotypic predictors were restricted to CC, secreted virulence factors and virulence regulators. Multivariate regularized logistic regression identified three independent predictors of embolism. Patients at higher risk were younger than the cohort median age of 62.5 y (adjusted odds ratio [OR] 0.14; 95% confidence interval [CI] 0.05-0.36). S. aureus characteristics predicting embolism were a CC30 genetic background (adjusted OR 9.734; 95% CI 1.53-192.8) and the absence of pIB485-like plasmid-borne enterotoxin-encoding genes sed, sej, and ser (sedjr; adjusted OR 0.07; 95% CI 0.004-0.457). CC30 S. aureus has been repeatedly reported to exhibit enhanced fitness in bloodstream infections, which might impact its ability to cause embolism. sedjr-encoded enterotoxins, whose superantigenic activity is unlikely to protect against embolism, possibly acted as a proxy to others genes of the pIB485-like plasmid found in genetically unrelated isolates from mostly embolism-free patients. mecA did not independently predict embolism but was strongly associated with sedjr. This mecA-sedjr association might have driven previous reports of a negative association of mecA and embolism. Collectively, our results suggest that the influence of S. aureus genotypic features on the risk of embolism may be stronger than previously suspected and independent of clinical risk factors.

Keywords: CC30; MRSA; S. aureus; enterotoxin; infective endocarditis; plasmid; stroke; superantigen.

Figure 1

Genotypic relationships and characteristics of 98 S. aureus isolates from endocarditis patients with and without embolism. Shown is a minimum spanning tree where connections between isolates are selected as to minimize the total number of genotypic differences in the tree, based on DNA arrays targeting 332 genes and alleles. Colored marks are used to indicate embolism-associated isolates and those harboring sedjr, a set of plasmid-borne enterotoxin-coding genes negatively associated with embolism in the cohort. Gray marks denote isolates belonging to rare clonal complexes (CCs). MRSA, methicillin-resistant S. aureus.

Figure 2

Stratification of the risk of embolism in S. aureus endocarditis patients based on independent clinical and microbiological predictors. Three predictors of embolism were identified using regularized logistic regression followed by analysis of deviance. Shown are the proportions of patients with embolism in each risk group with 95% binomial confidence interval. Mark size is proportional to group sample size.

Figure 3

Identification of predictors of embolism using random forests. Shown are the distributions of random forest importance measures for 118 microbiological predictors and 2 clinical covariates. Importance measures were obtained from 1,000-tree random forests and Boruta feature selection algorithm with 500 repetitions. Predictors whose importance was significantly higher than the maximum importance of shadow variables were classified as significantly important. For readability, labels are shown only for important predictors and shadow variables.