Abstract
Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging pathogen that is difficult to treat due to the multiresistance of the bacteria upon infection. From 2011 to 2016, 1581 S. aureus strains were isolated from 4300 samples from retail foods covering most provincial capitals in China. To determine the prevalence of food-related MRSA and its genetic background in China, antibiotic resistance, staphylococcal toxin genes, staphylococcal cassette chromosome mec (SCCmec) typing, spa-typing and MLST were carried out in this study. In total, 108 (7.4%) isolates were confirmed for MRSA by phenotyping (cefoxitin) and genotyping (mecA/mecC gene). A total of 52.8% (57/108) of the MRSA isolates belonged to clonal complex 59 (CC59) (ST59, ST338, and ST3355), which was the predominant clone in this study. These CC59 isolates carried SCCmec elements of type IV, V, or III and exhibited spa type t437, t441, t543, t163, t1785, or t3485, and half of them carried major virulence genes, such as the Panton-Valentine leucocidin (PVL) gene. The secondary clones belonged to ST9 (15.7%, 17/108) with a type of t899-SCCmec III and showed a broader range of antimicrobial resistance. The remaining MRSA isolates (31.5%, 34/108) were distributed in 12 different STs and 18 different spa types. All isolates harbored at least one of the enterotoxin genes, whereas only 4 isolates (3.70%) were positive for the toxic shock syndrome toxin tsst alleles. For antibiotic susceptibility testing, all isolates were resistant to more than three antibiotics, and 79.6% of the isolates were resistant to more than 10 antibiotics. Amoxycillin/clavulanic acid, ampicillin, cefoxitin, penicillin, ceftazidime, kanamycin, streptomycin, clindamycin, and telithromycin was the most common antibiotic resistance profile (55.6%, 60/108) in the study. In summary, the results of this study implied that the major food-related MRSA isolate in China was closer to community-associated MRSA, and some of the remaining isolates (ST9-t899-SCCmec III) were supposed to livestock-associated MRSA. In addition, most MRSA isolates showed resistance to multiple drugs and harbored staphylococcal toxin genes. Thus, the pathogenic potential of these isolates cannot be ignored. In addition, further studies are needed to elucidate the transmission routes of MRSA in relation to retail foods and to determine how to prevent the spread of MRSA.
Highlights
Methicillin-resistant Staphylococcus aureus (MRSA) is a pathogen of increasing importance in hospitals as well as in the community and livestock
Of the 1581 S. aureus isolates from retail food in China, 108 (6.83%) isolates from 89 positive samples (2.1%, 89/4300) were confirmed as MRSA, which exhibited cefoxitin resistance, 99.1% of isolates (107/108) were positive for mecA genes, and none were positive for the mecC gene
Of the 108 MRSA isolates, 34 (7.25%) of the 469 isolates were from raw meat, 31 (6.07%) of the 511 isolates were from aquatic products, 20 (5.43%) of the 368 isolates were from quick-frozen products, 11 (7.43%) of the 148 isolates were from ready-to-eat food, 7 (16.67%) of the 42 isolates were from edible mushrooms, and 5 (16.67%) of the 30 isolates were from vegetables, whereas pasteurized milk was free of MRSA isolates (Table 2)
Summary
Methicillin-resistant Staphylococcus aureus (MRSA) is a pathogen of increasing importance in hospitals as well as in the community and livestock. For multilocus sequence typing (MLST), sequence type 5 (ST5), ST8, ST22, ST36, and ST45 spread successfully to different regions of the world and caused substantial nosocomial disease (Deleo et al, 2010), whereas CA-MRSA showed five lineages worldwide: ST1-IV (USA400), ST8-IV (USA300), ST30-IV (Pacific/Oceania), ST59-IV and V (USA1000, Taiwan) and ST80-IV (European) (Skov, 2009). LA-MRSA strains exhibit co-resistance to many nonβ-lactam antimicrobials (e.g., antibiotics and metals), including those commonly used in animal production, and many of them belong to CC398 or CC9, as determined by MLST (Bens et al, 2006; Cui and Li, 2009; Neela et al, 2009). The SCCmec elements of LA-MRSA are different from those carried by other MRSA genotypes commonly found in the community and healthcare settings (Li et al, 2011). The majority of LAMRSA isolates lack toxins, such as PVL and other enterotoxins (Hallin et al, 2011)
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