In a recent study published in Zoonoses, researchers evaluate the prevalence, virulence, antimicrobial resistance, and molecular features of methicillin-resistant Staphylococcus aureus (MRSA) from ready-to-eat (RTE) food products.
Study: Antimicrobial Resistance, Virulence, and Genetic Characterization of Methicillin-Resistant Staphylococcus aureus Recovered from Ready-to-Eat (RTE) Food in China: A New Challenge for Food Safety. Image Credit: thebigland / Shutterstock.com
The risk of food poisoning from RTE products
S. aureus, which is found on the skin and in the upper respiratory tract, can cause severe and life-threatening diseases. The toxigenic strains synthesize virulence factors, such as Panton-valentine leucocidin, staphylococcal enterotoxins (SEs), and toxic shock syndrome toxin 1. SEs are the leading cause of staphylococcal food poisoning (SFP).
Although self-limiting and rarely life-threatening, SFP leads to significant discomfort and financial burden. SFP outbreaks present challenges to the food industry, public health, and catering businesses. Additionally, multidrug-resistant (MDR) strains, including MRSA, have been increasingly detected in food products, which can cause infections and limit the efficacy of current therapeutics.
Although RTE foods are popular, they are at an increased risk of microbial contamination during packaging, transport, storage, and sale. As a result, food poisoning may readily occur due to contamination by food-borne pathogens.
In 2015, S. aureus contamination was reported in over 4% of retail foods in China. Herein, RTE food products were the most contaminated, which raised public health concerns.
About the study
RTE foods were sampled from convenience stores, restaurants, retailers, supermarkets, and farm product markets across 25 Chinese provinces in 2018.
S. aureus was isolated from samples. All isolates were screened for MRSA and tested for their susceptibility to 13 antimicrobial agents, including penicillin, erythromycin, cefoxitin, oxacillin, clindamycin, tetracycline, chloramphenicol, ciprofloxacin, trimethoprim-sulfamethoxazole, gentamicin, daptomycin, vancomycin, and linezolid.
The genomic DNA of MRSA isolates was extracted and screened for virulence genes. MRSA lineages were identified based on multi-locus sequence typing (MLST), staphylococcal protein A (spa) typing, and staphylococcal chromosome cassette mec (SCCmec) typing. Sequence types and alleles were assigned, and clonal complexes were annotated.
Study findings
A total of 276 isolates of S. aureus were cultured from RTE foods, including cereal products, salads, sushi, sauced meat, snacks, ice cream, and sashimi. Of these, 30 isolates were MRSA. Over 90% of isolates exhibited resistance to at least one antimicrobial agent, whereas the remaining isolates were susceptible or exhibited intermediate susceptibility to all antimicrobials.
The highest resistance was observed for penicillin and erythromycin at 87.3% and 38.4%, respectively. All isolates were susceptible to daptomycin, vancomycin, and linezolid.
Seventy-three isolates were resistant to at least three antimicrobials. Sixteen isolates exhibited resistance to four classes of antimicrobials, whereas 13 were resistant to at least five antimicrobials.
All MRSA isolates were penicillin-resistant, 22 were MDR, and 12 resistance profiles were observed. Nine virulence genes were detected in 18 MRSA isolates, most of which had three or more virulence genes. The most common virulence gene was SE B (seb), in which four virulence gene profiles were observed.
MLST revealed 13 sequence types, with ST59, ST398, and ST1 being the most prevalent. The sequence types belonged to eight clonal complexes, and CC88 and CC398 were the most prevalent.
Similarly, 13 spa and four SCCmec types were detected. Notably, t011, t114, and t437 were the most prevalent spa types detected in seven, five, and seven isolates and were associated with CC398, CC1, and CC59 clonal complexes, respectively.
SCCmecIV was the most prevalent element detected in 18 isolates. Seventeen MRSA lineages were identified based on combined MLST, spa, and SCCmec typing analyses.
CC59-t437-SCCmecIV/V, CC398-t011-SCCmecV, and CC1-t114-SCCmecIV were the most prevalent lineages. Pulse-field gel electrophoresis of MRSA isolates revealed 18 patterns of 23 isolates grouped into six clusters with more than 76% genetic similarity.
Conclusions
A total of 250 isolates with resistance to at least one antimicrobial agent were identified, with resistance to penicillin most frequently observed. MDR prevalence was 26.1%, which was lower than previous estimates.
Thirty MRSA isolates were identified, and 18 had nine virulence genes. Over 50% of MRSA isolates had multiple virulence genes. MRSA isolates with similar genotypic diversity had similar virulence profiles and antimicrobial resistance patterns.
The study findings indicate that monitoring MRSA genotypes in RTE food products will allow researchers to trace future contamination issues and assess antimicrobial resistance and SFP risk, which can lead to improved food safety measures.
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