Methicillin-resistant S. aureus strains prevalent in retail food in SG
There appear to be two clinically relevant methicillin-resistant Staphylococcus aureus (MRSA) strains—ST80 and ST6—in ready-to-eat food and food contact surfaces at retail in Singapore, and humans rather than animals are the likely sources of contamination, according to a team of investigators.
“Our limited findings suggest ready-to-eat food and food contact surfaces as potential environmental sources for colonisation and spread of MRSA in the community,” they added.
The team characterized a total of 227 coagulase-positive S. aureus strains isolated from local retail food and swabs collected from gloves of food handlers. These isolates were analysed using multi-locus sequence typing (MLST), staphylococcal Protein A (spa) typing and staphylococcal cassette chromosome (SCCmec) typing. Antibiotic susceptibility testing was additionally conducted, with the resulting data interpreted according to the Clinical and Laboratory Standards Institute guideline.
Of the S. aureus isolates, five (2.2 percent) from two food stalls were resistant to methicillin. They were identified as sequence type (ST) 80 (spa type t1198; from sliced onion, prawn fritters, fried egg) and ST6 (spa type t304; from food handler glove swabs). All five belonged to SCCmec type IV and were Panton-valentine leucocidin (pvl)-negative. [Antimicrob Resist Infect Control 2017;6:94]
“Most ST80 strains reported elsewhere belong to SCCmec type IV, are pvl-positive, and are usually associated with severe skin/soft tissue infections and necrotising pneumonia,” the investigators said. [Clin Microbiol Infect 2010;16:1077-83ST6]
Meanwhile, the SCCmec type IV, pvl-negative ST6 strain was previously isolated from patients overseas. This could mean that the spa type t304 isolated in the present study may be capable of causing human infection, the investigators added. [New Microbes New Infect 2014;2:100-5]
Food poisoning and S. aureus enterotoxins
On further investigation, the ST80 strains in the current study carried enterotoxin genes seb and sek and exfoliative toxin gene etd. The ST6 strains, on the other hand, carried the enterotoxin gene sea.
“The presence of enterotoxin genes (sea and seb) in these MRSA isolates suggests the isolates’ potential to produce toxins and cause staphylococcal food poisoning, if allowed to grow in sufficient numbers in food,” the investigators said.
Enterotoxins seb and sea have been reported to account for 90 percent of staphylococcal food poisoning worldwide. [Toxins (Basel) 2010;2:2177-97]
All five MRSA isolates were phenotypically susceptible to the following nonbeta-lactam agents: amikacin, ciprofloxacin, chloramphenicol, gentamicin, norfloxacin, tetracycline and trimethoprim/sulfamethoxazole.
However, they were found to be resistant to the beta-lactams tested (amoxycillin-clavulanic acid, ampicillin, cefoxitin, ceftriaxone and penicillin). ST80 isolates exhibited additional resistance to the macrolide azithromycin.
The investigators pointed out that MRSA with resistance to additional antibiotic classes is of particular concern. As this additional resistance may reflect the increasing use of such classes in the local clinical practice, further investigation is warranted.
“To date, little is known about the transmission of MRSA infections through food and food contact surfaces. However, their possible roles in the dissemination of specific MRSA lineages cannot be ruled out,” according to the investigators. In light of this, a more comprehensive and integrated (farm-to-hospital approach) surveillance of MRSA in Singapore and elsewhere should be conducted.