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Prevalence and characterization of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus isolated from retail yak butter in Tibet, China
Institute of Agricultural Product Quality Standard and Testing Research, Tibet Academy of Agricultural and Animal Husbandry Sciences, Lhasa Tibet 850032, China
This study aimed to investigate the prevalence, molecular characteristics and antibiotic resistance of Staphylococcus aureus isolates from yak butter in Tibet, China. A total of 218 yak butter samples were collected from retail stores in Tibet and screened for Staph. aureus. Furthermore, the virulence genes, resistance genes, antimicrobial susceptibility, and molecular typing [pulsed-field gel electrophoresis (PFGE), multilocus sequence typing, and staphylococcal protein A (spa) typing] of Staph. aureus isolates were detected. The results showed that 12.4% of yak butter samples were contaminated with Staph. aureus, including 5 samples positive for methicillin-resistant Staph. aureus (MRSA). Among all isolates, 96.3% harbored one or more virulence genes, including classical (sea and sec), novel enterotoxin-encoding genes (seh, sek, sel, and seq), and hemolysin genes (hla and hld). All isolates were resistant to at least 2 different antibiotic classes, and the isolates were most commonly resistant to sulfonamides, β-lactams, and erythromycin. For resistance genes, blaZ (74.1%) was most frequently detected, followed by dfrG (51.9%), erm(B) (22.2%), mecA (18.5%), tet(K) (14.8%), aph(2″)-Ia, aph(3′)-III, and ant(6)-Ia (11.1% for each), and erm(C) (7.4%). We detected 8 spa types, 6 sequence types (ST), and 5 clonal complex (CC) types. In addition, 1 isolate of Staph. aureus was nontypeable. We found that CC1-ST1-t559 (55.6%) was the most predominant clone, followed by CC59-ST59-t437 (11.1%), CC5-ST5-t002 (7.4%), CC1-ST1, CC1-ST1-t114, CC1-ST573-t4938, CC1-ST573-t8915, CC30-ST30-t021, and CC25-ST25-t167 (3.7% for each). For PFGE typing, a total of 5 clusters and 15 pulsotypes were generated, and some isolates from different samples showed indistinguishable pulsotypes. Our findings suggest that yak butter produced in Tibet, China, could be contaminated by Staph. aureus strains, including MRSA strains, carrying various virulence and resistance genes, representing multiple antimicrobial resistance phenotypes. The presence of potentially virulent and antibiotic-resistant Staph. aureus strains in yak butter poses a potential threat to consumers, and appropriate measures need to be taken in the production chain to reduce the occurrence of Staph. aureus in yak butter.
Yak butter is a traditional artisanal dairy product common in the Tibetan Plateau region. It is an important food for Tibetans because of its rich nutritional profile and unique flavor. Different from other butter in daily life, yak butter has a high fat content derived from yak milk, has a bright yellow color, and is fermented by natural inoculation (
Staphylococcus aureus is an important pathogen for both human and animal. In dairy yaks, Staph. aureus is regarded as one of the most frequent pathogens in mastitis (
). In addition, Staph. aureus persistently colonizes approximately 20 to 30% of the skin and nasopharynx of healthy populations, and intermittently colonizes about 60% of the skin and nasopharynx of healthy populations (
). As a handmade dairy product, food handlers have plenty of opportunities to directly contact yak butter. Food handlers may constitute a reservoir of virulent strains of Staph. aureus and may be potential vehicles for the transmission of Staph. aureus to foods (
In humans, ingesting milk and dairy products contaminated with Staph. aureus can cause severe toxin-mediated food poisoning, including nausea, violent vomiting, and abdominal cramping, with or without diarrhea (
). The pathogenicity of Staph. aureus is caused by exocrine toxin proteins, including enterotoxins (SE), hemolysin, exfoliative toxins, toxic shock syndrome toxin-1, Panton-Valentine leukocidin, and others (
). Enterotoxins are the main pathogenic factor of staphylococcal food poisoning. To date, 23 kinds of SE have been found, including the classical SE (SEA to SEE) and the novel SE (SEG to SET), whereas SE-like toxins (SElJ, SElU, SElV, SElX, and SElY) are non-emetic or have yet to be tested (
In recent years, the prevalence of antibiotic-resistant pathogens has increased, and the approval of new drugs has decreased, becoming an increasing public health concern worldwide (
). Moreover, foods and food-related environments can be potential vehicles for spreading multidrug resistance genes, which accelerates the thriving of global antibiotic resistance (
). The resistance of MRSA is mediated by a mobile genetic element called the staphylococcal chromosomal cassette (SCCmec), encoding penicillin-binding protein 2a and exhibiting multidrug resistance phenotypes to β-lactam antibiotics as well as non-β-lactam antimicrobial agents (
To our knowledge, there has thus far been a paucity of research reports on contamination of Staph. aureus and MRSA in yak butter. Therefore, the purpose of this study was to investigate the prevalence of Staph. aureus and MRSA in yak butter and to characterize these isolates by molecular typing, antimicrobial susceptibility testing, resistance genes, and toxin genes.
MATERIALS AND METHODS
Sample Collection and Isolation of Staph. aureus
From January to August 2014, 218 artisanal yak butter samples were collected from retail stores in 3 cities (Lhasa, Rikaze, and Naqu) in Tibet, China (Figure 1). In each city, 15 retail stores were selected. Four or five yak butter samples (approximately 0.5–1.0 kg for each sample) were randomly collected from the top of the yak butter display of each retail store. A total of 76, 72, and 70 yak butter samples were collected in Lhasa, Rikaze, and Naqu, respectively. Isolation and identification of Staph. aureus were conducted using the method described previously (
). First, the individually packaged yak butter samples were evenly divided into 4 pieces, and the surface layer of yak butter was put in a sterile sampling bag and mixed evenly. Then, 25 g of yak butter sample was placed into 225-mL sterile buffered peptone water (Qingdao Hope Bio-Technology Co. Ltd.) and cultured at 37°C for 18 to 24 h in a rotary shaker (TS-200DC; Tensuc). After pre-enrichment, a 3-mL aliquot was transferred into 30 mL of fresh 7.5% sodium chloride broth (Qingdao Hope Bio-Technology Co. Ltd.) for selective enrichment, and the solution was incubated at 37°C for 18 to 24 h in a rotary shaker. Then, a loop of each enriched culture was streaked onto Baird-Parker agar plates (Qingdao Hope Bio-Technology Co. Ltd.) with 5% egg yolk and tellurite. Presumptive colonies on Baird-Parker plates (black colonies surrounded by clear zones of 2–5 mm) was transferred to mannitol salt agar plates (Qingdao Hope Bio-Technology Co. Ltd.) for further verification. Then, presumptive colonies on mannitol salt agar (yellow colonies) were transferred to trypticase soy agar plates (Qingdao Hope Bio-Technology Co. Ltd.) for further purification. Colonies were defined as Staph. aureus by PCR detection of the thermonuclease gene (nuc) with primers nucF (5′-GCGATTGATGGTGATACGG TT-3′) and nucR (5′-AGCCAAGCCTTGACGAACTAAAGC-3′). Finally, all isolates were stored at −80°C in trypticase soy broth plus 50% (vol/vol) glycerol for further use.
Figure 1Distribution of yak butter sampling sites in Tibet, China.
), the antimicrobial susceptibility of 17 commonly used antibiotics were determined for the Staph. aureus isolates, and the results were evaluated after culture at 37°C for 16 to 18 h. The minimum inhibitory concentrations of the 17 antibiotics were as follows: penicillin ≥0.25 µg/mL, erythromycin ≥8 µg/mL, oxacillin ≥4 µg/mL, tetracycline ≥16 µg/mL, cefoxitin ≥8 µg/mL, ampicillin ≥0.5 µg/mL, ciprofloxacin ≥4 µg/mL, vancomycin ≥32 µg/mL, chloramphenicol ≥32 µg/mL, cefoperazone ≥64 µg/mL, amikacin ≥64 µg/mL, trimethoprim ≥16 µg/mL, sulfamethoxazole ≥512 µg/mL, rifampicin ≥4 µg/mL, gentamicin ≥16 µg/mL, amoxicillin/clavulanic acid ≥8/4 µg/mL, and linezolid ≥8 µg/mL. Staphylococcus aureus ATCC 29213 and Escherichia coli ATCC 25922 were used as quality control strains in each run.
DNA Extraction
Staphylococcus aureus isolates were inoculated onto trypticase soy agar plates (Beijing Land Bridge Technology Ltd.) and incubated at 37°C for 24 h. Subsequently, a single clone was transferred to 5 mL of trypticase soy broth (Beijing Land Bridge Technology Ltd.) and incubated at 37°C for 16 to 18 h in a rotary shaker. After pre-enrichment, a 1-mL aliquot was transferred to a 1.5-mL centrifuge tube and centrifuged at 9,500 × g for 10 min at room temperature. The supernatant was removed, and the bacterial pellet was washed with double-distilled H2O and repeated twice. Subsequently, DNA extraction was performed using bacterial genomic DNA extraction kits (Biospin Bacterial DNA Extraction Kit, Hangzhou Bioer Technology Co. Ltd.) following the manufacturer's instructions. Finally, the DNA template was stored at −40°C for further use.
Detection of Virulence and Resistance Genes
All Staph. aureus isolates were tested using uniplex PCR for 29 virulence genes and 23 resistance genes. Twenty-nine virulence genes were detected, including 5 classic enterotoxin genes (sea to see), 16 novel enterotoxin genes (seg to sev), toxic shock syndrome toxin-1 gene (tst), 4 hemolysin genes (hla, hlb, hld, and hlg), 2 exfoliative toxin genes (eta and etb), and Panton-Valentine leukocidin gene (pvl). Twenty-three antibiotic resistance genes were detected, including 3 erythromycin resistance genes (ermA, ermB, and ermC), 4 tetracycline resistance genes (tetK, tetM, tetO, and tetL), 4 aminoglycoside resistance genes [aac(6′)-Ia, aph(2″)-Ia, ant(4′)-Ia, and aph(3′)-III], 4 trimethoprim resistance genes (dfrD, dfrK, dfrG, and dfrS1), 3 chloramphenicol resistance genes (cat:pC211, cat:pC194, and cat:pC223), 2 β-lactam resistance genes (blaZ and mecA), and 3 glycopeptide-resistant genes (VanB, VanC1, and VanC2/3). The primers and annealing temperatures for the PCR assays are shown in Supplemental Table S1 (https://figshare.com/articles/dataset/Supplemental_Table_S1_doc/14773584,
). Amplification reactions were performed in a final volume of 25 μL, containing 1 μL of each primer (100 mM), 12.5 μL of Green Taq Mix (Vazyme Biotech Co. Ltd.), 5 μL of template DNA, and 5.5 μL of double-distilled H2O. The PCR conditions were as follows: incubation at 94°C for 10 min, followed by 35 cycles of 94°C for 30 s, an annealing temperature for 30 s, and 72°C for 45 s; and a final extension step at 72°C for 5 min. The PCR products were resolved in 1.0% (wt/vol) agarose gel electrophoresis in 1× Tris-Borate-EDTA buffer.
Molecular Typing
Multilocus sequence typing was determined by the sequencing of 7 housekeeping genes (arcC, aroE, glpF, gmk, pta, tpi, and yqiL) as described previously (
). Alleles and sequence types (ST) were assigned according to the multilocus sequence typing website for Staph. aureus (https://pubmlst.org/organisms/ staphylococcus-aureus). The ST were clustered into clonal complexes (CC) using eBURST (http://eburst.mlst.net). Among them, different ST with 5 or more identical alleles were defined as the same CC, as described previously (
). The sequencing results were analyzed through Ridom Staph software (Ridom Gmb) and the website https://www.spaserver.ridom.de/. The PCR and sequencing primers and conditions were determined using the spaserver website. The primers used in this study were synthesized by AuGCT DNA-SYN Biotechnology Co. Ltd. (Yangling, China).
Pulsed-field gel electrophoresis (PFGE) typing was performed to determine the genetic relatedness of Staph. aureus isolates as previously described (
). Briefly, agarose-embedded genomic DNA from each Staph. aureus isolate was digested with SmaI (TaKaRa). DNA fragments were separated using a CHEF III Mapper electrophoresis system (Bio-Rad Laboratories). The images of the gels stained with ethidium bromide were taken under UV transillumination. The images were used for cluster analysis by BioNumerics software version 3.0 (Applied Maths). Percent similarities were identified on a dendrogram derived by the unweighted pair group method using arithmetic averages and based on Dice coefficients. Band position tolerance and optimization were set at 1.5% and 0.5%, respectively. A similarity coefficient of 89.5% was selected to define the pulsed-field type clusters. Each frequency band difference within the PFGE type results in the addition of a numerical sequence to the pulsed-field cluster. Salmonella serotype Braenderup strain H9812 digested with XbaI was used as a molecular size marker.
Statistical Analysis
Statistical analysis was performed using SPSS 20.0 software (IBM Corp.). Differences among the detection frequencies of Staph. aureus-positive samples, virulence genes, drug resistance genes, and antibiotic-resistant isolates from different cities (Lhasa, Rikaze, and Naqu) were determined by chi-squared test. For these tests, P-values <0.05 were considered statistically significant.
RESULTS
Isolation and Prevalence of Staph. aureus and MRSA in Yak Butter
Overall, 12.4% (27/218) of yak butter samples were positive for Staph. aureus, including 15.8% (12/76) in Lhasa, 8.3% (6/72) in Rikaze, and 12.9% (9/70) in Naqu. Among 27 positive samples, 5 samples were positive for MRSA. A total of 27 Staph. aureus isolates (1 isolate per sample) were identified, including 5 MRSA isolates. The detection rate of Staph. aureus from yak butter did not significantly differ among Lhasa, Rikaze, and Naqu (P > 0.05).
Antimicrobial Susceptibility Testing
Antimicrobial susceptibility testing of Staph. aureus revealed that the isolates were generally resistant to sulfamethoxazole (100.0%, 27/27), followed by trimethoprim (96.3%, 26/27), ampicillin (81.5%, 22/27), erythromycin (37.0%, 10/27), penicillin (25.9%, 7/27), cefoxitin (18.5%, 5/27), tetracyclines, amoxicillin/clavulanic acid and gentamicin (11.1%, 3/27 for each), oxacillin and cefoperazone (3.7%, 1/27 for each). All isolates were susceptible to chloramphenicol, ciprofloxacin, rifampicin, vancomycin, amikacin, and linezolid. Furthermore, the Staph. aureus isolates from Naqu were more susceptible to antibiotics than those from Lhasa and Rikaze. To sum up, these isolates were mainly resistant to sulfonamides, β-lactams, and erythromycin antibiotics in Lhasa, Rikaze, and Naqu in Tibet, but they showed lower levels of resistance to other antibiotics (Table 1).
Table 1Antimicrobial resistance in Staphylococcus aureus isolates from commercial yak butter from different cities in Tibet, China
A total of 20 virulence genes and 9 resistance genes were identified in 27 isolates (Table 2, Table 3). Among them, the classical enterotoxin genes (sea and sec), the novel enterotoxin genes (seh, sek, sel, and seq), and the hemolysin genes (hla and hld) were frequently detected. These were followed by pvl (51.9%, 14/27); seg, sei, sem, sen, seo, and seu (22.2%, 6/27 for each); seb (11.1%, 3/27); sed, sej, and ser (7.4%, 2/27 for each); and sep and hlg (3.7%, 1/27 for each). Six virulence genes were not detected in the isolates, including see, ses, set, sev, tst, and hlb. In addition, most of the 27 isolates from yak butter samples carried a variety of virulence genes. We found no statistically significant difference (P > 0.05) in the detection rates from Lhasa, Rikaze, and Naqu. For resistance genes, only 5 types of antimicrobial resistance genes were detected, including tetracycline, aminoglycoside, trimethoprim, macrolide, and β-lactam antibiotic-related genes. The most common resistance gene was blaZ (74.1%, 20/27), followed by dfrG (51.9%, 14/27), erm(B) (22.2%, 6/27), mecA (18.5%, 5/27), tet(K) (14.8%, 4/27), aph(2″)-Ia, aph(3′)-III and ant(6)-Ia (11.1%, 3/27 for each), and erm(C) (7.4%, 2/27). The tet(K), aph(3′)-III, and ant(6)-Ia genes were detected only in isolates carrying the mecA gene. The carrying rate of resistance genes in Staph. aureus isolates varied with the location of isolation. The detection rates of aph(3′)-III and ant(6)-Ia genes in the isolates from Naqu were significantly higher than those from Lhasa (P < 0.05).
Table 2Virulence genes in Staphylococcus aureus isolates from commercial yak butter from different cities in Tibet, China
As shown in Table 4, all isolates showed 8 spa types, 6 ST, and 5 CC types. For spa typing, t559 (55.6%, 15/27) was the most predominant spa type. This was followed by t437 (11.1%, 3/27); t002 (7.4%, 2/27); and t021, t114, t167, t4938, and t8915 (3.7%, 1/27 for each). Two strains were not typeable. For ST, ST1 (63.0%, 17/27) was the most predominant, followed by ST59 (11.1%, 3/27), ST5 and ST537 (7.4%, 2/27 for each), and ST30 and ST25 (3.7%, 1/27 for each); 1 strain was not typeable. For CC typing, the 6 ST were divided into 5 groups, among which the first group contained 2 ST (ST1 and ST573), and 2 to 5 groups each contained 1 ST. The most predominant CC type was CC1 (70.4%, 19/27; composed of ST1 and ST573), followed by CC59 (11.1%, 3/27; composed of ST59), CC5 (7.4%, 2/27; composed of ST5), CC30 (3.7%, 1/27; composed of ST30), and CC25 (3.7%, 1/27; composed of ST25), and 1 strain was not typeable. In summary, the molecular types of Staph. aureus isolates from yak butter samples in Tibet were relatively simple, mainly representing CC1. However, ST1-t559 was commonly found in CC1 isolates, and the detection rates of ST1-t559 in Lhasa, Rikaze, and Naqu were 50.0%, 66.7%, and 55.6%, respectively.
Table 4Molecular characterization of Staphylococcus aureus isolates from commercial yak butter from different cities in Tibet, China
The PFGE typing results of the 27 isolates showed that all strains could be divided into 15 pulsotypes (designated as A–O) by SmaI, and 5 different clusters (designated as I-1 to I-5) were obtained when using an 89.5% similarity cutoff (Figure 2). Two major PFGE clusters were observed in 18 strains (66.7%, 18/27) grouped in I-2, and 4 strains (14.8%, 4/27) were observed in I-3, followed by 2 strains (7.4%, 2/27 for each) in I-1 and I-5, and 1 strain in I-4 (3.7%, 1/27). The 3 most predominant pulsotypes were observed in 8 strains (29.6%, 8/27) grouped in pulsotype E and 3 strains (11.1%, 3/27 for each) grouped in I and K. In general, the strains of pulsotypes E (Lhasa and Naqu) and K (Naqu and Rikaze), recovered from different samples at different sampling points, belonged to CC1-ST1-t559. The strains of pulsotype H (Lhasa) and I (Naqu), recovered from different samples at the same sampling points, belonged to CC5-ST5-t002 and CC30-ST59-t437, respectively.
Figure 2Dendrogram of pulsed-field gel electrophoresis (PFGE), clonal complex type (CC), sequence type (ST), staphylococcal protein A (Spa), pulsotype typing (PT), virulence gene profiles, resistance gene profiles, and antimicrobial resistance profiles of Staphylococcus aureus isolates from yak butter. NT = not typeable; AMP = ampicillin; ERY = erythromycin; TET = tetracycline; FOX = cefoxitin; OXA = oxacillin; FOP = cefoperazone; TMP = trimethoprim; SMZ = sulfamethoxazole; A/C = amoxicillin/clavulanic acid; PEN = penicillin; GEN = gentamicin. a–fNT: no detection.
). However, few reports exist regarding Staph. aureus, especially MRSA, contamination in yak butter. In this study, 12.4% of yak butter samples were positive for Staph. aureus. The detection rate of Staph. aureus-positive samples was similar to the 18% (9/50) of traditional butter samples in Iran (
). Overall, these results indicate that butter and its products have the potential to be contaminated by Staph. aureus. Further research will explore methods to prevent and control Staph. aureus contamination in yak butter in the future.
In recent years, the emergence of multidrug-resistant strains, especially MRSA, in milk and dairy products has also caused concerns (
Prevalence of Staphylococcus aureus and of methicillin-resistant S. aureus (MRSA) along the production chain of dairy products in north-western Greece.
). In this study, 2.3% of yak butter samples were positive for MRSA. The detection rate of MRSA-positive samples was higher than those reported in previous studies, where no MRSA was found in traditional butter samples (
Prevalence of Staphylococcus aureus and of methicillin-resistant S. aureus (MRSA) along the production chain of dairy products in north-western Greece.
A survey of the occurrence and properties of methicillin-resistant Staphylococcus aureus and methicillin-resistant Staphylococcus intermedius in water buffalo milk and dairy products in Turkey.
). At present, the effect of MRSA on food-related illness is very low, but the potential threat of MRSA spreading to people through the food chain cannot be ignored (
). Among them, the sec gene (81.5%) was detected frequently. This was in agreement with previous studies showing that Staph. aureus isolated from goat and cow mastitis or raw milk frequently carried the sec gene (
). Furthermore, the sea and sec genes were usually detected together in this study, which is different from the results of a previous study, which showed that sea and see are usually detected together in the isolates causing food poisoning (
Prevalence and characteristics of enterotoxin B-producing Staphylococcus aureus isolated from food sources: A particular cluster of ST188 strains was identified.
); this result was also verified in this study. For example, CC1-ST1-t559, CC59-ST59-t437, and CC5-ST5-t002 clones usually carried sea-sec-she-sek-sel-seq, seb-sek-seq, and sec gene profiles, respectively. In addition, some SE or SE-like genes are known to be located on mobile genetic elements, such as prophages, plasmids, Staph. aureus pathogenicity islands (SaPI), and others (
Characterization and expression analysis of Staphylococcus aureus pathogenicity island 3—Implications for the evolution of staphylococcal pathogenicity islands.
). The seg, sei, sem, sen, and seo genes belonged to the same enterotoxin gene cluster (egc) and were usually detected together. In this study, these genes were also usually detected together. Although new enterotoxins are not the main cause of food poisoning (
Molecular and epidemiological characterization of staphylococcal foodborne outbreak of Staphylococcus aureus harboring seg, sei, sem, sen, seo, and selu genes without production of classical enterotoxins.
Int. J. Food Microbiol.2017; 256 (28582663): 30-35
). Thus, Staph. aureus carrying classic and novel enterotoxin genes may constitute a potential risk to customer health.
In this study, all Staph. aureus isolates resisted at least one antibiotic, and 37.0% of isolates showed multiple drug resistance. Resistance to ampicillin, trimethoprim, and sulfamethoxazole were frequently detected among Staph. aureus isolates in our study (81.5%, 96.3%, and 100.0%, respectively). Reports of Staph. aureus resistance to ampicillin, trimethoprim, and sulfamethoxazole antibiotics have become common (Shim et al., 2017;
). Resistance to erythromycin in Staph. aureus has occurred as a consequence of animal and clinical antibiotic usage. In addition, 5 isolates carrying the mecA gene were detected, and all isolates showed multiple drug resistance (resistance to at least 3 types of antibiotics). Interestingly, most mecA-positive Staph. aureus isolates were resistant to cefoxitin, whereas most isolates were susceptible to oxacillin. This result is different from previous studies, which showed that MRSA isolates from humans, animals, and foods are frequently resistant to both oxacillin and cefoxitin in China (
Oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA), a hidden resistant mechanism among clinically significant isolates in the Wessex region/UK.
recently reported that malfunctions of the bla system leads to the constitutive expression of mecA. Therefore, the detection of a MRSA isolate should be combined with PCR detection of the mecA gene via the phenotypic resistance test, to avoid misdiagnosis by antibiotic phenotypic laboratory testing (
). In this study, the phenotypes and genotypes of the drug-resistant isolates are inconsistent. For example, the isolates carrying aph(2″)-Ia/aph(3′)-III did not show resistance to aminoglycoside drugs. This may be affected by external environmental factors, resulting in the inability to express related genes. Some isolates were also resistant to antibiotics but did not carry relevant resistance genes. These results indicate that resistance of isolates to antibiotics is a complex process. Isolates may possess other mechanisms of antibiotic resistance.
In this study, lineage CC1-ST1-t559 was identified as the most predominant lineage in yak butter. In addition, CCI-ST1-t559 strains from Lhasa, Rikaze, and Naqu showed the same PFGE patterns (cluster I-2 and I-3). To sum up, we suspect that the CC1-ST1-t559 clone was the most adaptive yak butter processing environment, and cross-contamination may occur. To our knowledge, this is the first report of CC1-ST1-t559 Staph. aureus in yak butter. Furthermore, another dominant CC59-ST59-t437 clone lineage of MRSA was detected. It is well known that ST59 is a predominant clonal lineage of community-associated MRSA in Asia (
Superantigen gene profiles and presence of exfoliative toxin genes in community-acquired methicillin-resistant Staphylococcus aureus isolated from Chinese children.
The changing pattern of population structure of Staphylococcus aureus from bacteremia in China from 2013 to 2016: ST239-030-MRSA replaced by ST59-t437.
have found that SEB contributes to systemic infection in the widespread community-associated MRSA lineage ST59, which may be the cause for the increased case-fatality rates of ST59 infections in China. Due to the high prevalence of infections in children, the ST59 clone lineage of Staph. aureus has become a growing public health concern (
). People pay more attention to the prevalence of ST59 in the community but ignore the prevalence in other sources. Meanwhile, CC5-ST5-t002, one of the most frequent hospital-associated Staph. aureus clones (
), was also detected in this study. It has been suggested that CC5 has become an increasingly frequent cause of MRSA infections as well as of bacteremia acquired by hospitalized patients (
Antimicrobial resistance and molecular epidemiology of Staphylococcus aureus causing bloodstream infections at Ruijin Hospital in Shanghai from 2013 to 2018.
). In addition, the CC5-ST5-t002 isolate carried 14 toxin genes and was resistant to 11 antibiotics. Because molecular characteristics of Staph. aureus isolates from yak butter overlapped with those of isolates causing human food poisoning and infections, their potential to cause food poisoning and infections in humans deserves attention and further exploration.
CONCLUSIONS
In this study, Staph. aureus and MRSA isolates were prevalent in yak butter in Tibet, China. The dominant clone types of Staph. aureus and MRSA were CC1-ST1-t559 and CC30-ST59-t437, respectively. In addition, the majority of the isolates found here exhibited multiple drug resistance and carried enterotoxin and antibiotic resistance-encoding genes as well. Cross-contamination of Staph. aureus was also identified in different yak butter samples. The presence of potentially virulent and antibiotic-resistant Staph. aureus strains in yak butter poses a potential threat to the consumers. Our findings provide useful information for potential assessment of health risks posed to consumers by Staph. aureus via yak butter and for enacting appropriate microbiological criteria. Therefore, to reduce the contamination levels of Staph. aureus and MRSA in the future, good sanitation and hygiene practices are necessary during yak butter processing. Moreover, further research is necessary to determine the contamination levels and growth characteristics of Staph. aureus, and its toxin production in yak butter, so as to regularly monitor the microbiological safety of yak butter.
ACKNOWLEDGMENTS
This research was supported by the National Natural Science Foundation of China (No. 31871894, U1703119 and 31271858; Beijing). No competing financial interests exist.
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Prevalence and characteristics of enterotoxin B-producing Staphylococcus aureus isolated from food sources: A particular cluster of ST188 strains was identified.
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Int. J. Food Microbiol.2017; 256 (28582663): 30-35
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