PCR detection of staphylococcal enterotoxin genes and exfoliative toxin genes in methicillin-resistant and methicillin-susceptible Staphylococcus aureus strains from raw human breast milk

Abstract

Background & aims: Human milk is known to be the best food for infants, as it contains all of the nutrients they need and also helps to protect them against infection. Staphylococcus aureus is one of the most common clinical and foodborne pathogens. It produces a variety of extracellular protein toxins, including enterotoxins, exfoliative toxin (ET), haemolysins and Panton-Valentine leukocidin (PVL). This study was carried out to evaluate the Xpert MRSA/SA nasal (Cepheid) PCR assay for the detection of Methicillin-Resistant S. aureus (MRSA) and Methicillin-susceptible S. aureus (MSSA) isolates, and to analyse the frequency of genes encoding the classical antigenic staphylococcal enterotoxins (SE) and exfoliative toxins (ET) in S. aureus strains isolated from raw human breast milk (HBM). Methods: A total of 72 milk samples were collected from mothers who had delivered in the hospital between 16 February 2014 and 24 April 2016. Samples were cultured and bacterial colonies were identified phenotypically by standard bacteriological methods. All staphylococci strains isolated by routine tests were examined with MALDI-TOF-MS. Then PCR Xpert MRSA/SA nasal was performed on the closed GeneXpert® random access platform (Cepheid), then the house-PCR to detect SE genes and ET genes. Results: S. aureus was identified in 86.2% (62/72) of the 72 Staphylococcus spp isolated from raw milk culture. PCR results showed that 30 of the 62 S. aureus strains (48.3%) harboured genes coding for toxins. sea was the most prevalent virulence gene (24.1%), followed by see (12.9%) and eta (12.9%) genes. Conclusions: Xpert MRSA/SA nasal complete assay allows rapid and accurate identification of MRSA and MSSA. This assay is very easy to perform and is useful for the diagnosis of milk contamination by S. aureus in human milk banks. These results suggest the potential infant health threat related to S. aureus contamination of HBM. Efforts are therefore required to improve safety standards to prevent staphylococcal food poisoning in these infants.

Keywords

• Staphylococcus aureus
• Staphylococcus lugdunensis
• MALDI-TOF-MS
• GeneXpert®
• Staphylococcal enterotoxins
• Exfoliative toxins