Staphylococcal Enterotoxin Genes in Coagulase-Negative Staphylococci—Stability, Expression, and Genomic Context

Abstract

In the current study, we screened a collection of coagulase-negative staphylococci (CoNS) isolates for orthologues of staphylococcal enterotoxins (SEs) involved in <i>S. aureus</i>-related staphylococcal food poisoning (SFP). The amplicons corresponding to SEs were detected in <i>S. chromogenes</i>, <i>S. epidermidis</i>, <i>S. haemolyticus</i>, <i>S. borealis</i>, <i>S. pasteuri</i>, <i>S. saprophyticus</i>, <i>S. vitulinus</i>, <i>S. warneri</i>, and <i>S. xylosus</i>. All amplicons were sequenced and identified as parts of known <i>S. aureus</i> or <i>S. epidermidis</i> SE genes. Quantitative real-time PCR allowed determining the relative copy number of each SE amplicon. A significant portion of the amplicons of the <i>sea</i>, <i>seb</i>, <i>sec</i>, and <i>seh</i> genes occurred at low copy numbers. Only the amplicons of the <i>sec</i> gene identified in three isolates of <i>S. epidermidis</i> displayed relative copy numbers comparable to <i>sec</i> in the reference enterotoxigenic <i>S. aureus</i> and <i>S. epidermidis</i> strains. Consecutive passages in microbiological media of selected CoNS isolates carrying low copy numbers of <i>sea</i>, <i>seb</i>, <i>sec</i>, and <i>seh</i> genes resulted in a decrease of gene copy number. <i>S. epidermidis</i> isolates harbored a high copy number of <i>sec</i>, which remained stable over the passages. We demonstrated that enterotoxin genes may occur at highly variable copy numbers in CoNS. However, we could identify enterotoxin genes only in whole-genome sequences of CoNS carrying them in a stable form at high copy numbers. Only those enterotoxins were expressed at the protein level. Our results indicate that PCR-based detection of enterotoxin genes in CoNS should always require an additional control, like analysis of their presence in the bacterial genome. We also demonstrate <i>S. epidermidis</i> as a CoNS species harboring SE genes in a stable form at a specific chromosome site and expressing them as a protein.