Staphylococcus aureus (S. aureus) is one of the main pathogens in mastitis of involved in intramammary infections in cows, goats, and sheep, which may lead to economic losses due to reduced milk production and poor milk quality (Aires-de-Sousa et al., 2007; Stapels et al., 2014). Contamination of raw milk with S. aureus has a highly potentially hazardous in China (Chao et al., 2015; Lan et al., 2017; Liu et al., 2017). Staphylococcus aureus can cause specific toxin-mediated conditions, such as scalded skin syndrome, staphylococcal food poisoning and toxic shock syndrome (Schmidt et al., 2017). Risk assessment for S. aureus in different raw milk samples should been conducted (Cortimiglia et al., 2015; Xu et al., 2015; Ding et al., 2016).

Staphylococcus aureus infections are related to the expression of virulence factors. Many potential virulence factors, such as enzymes and exotoxins, contribute to cause S. aureus diseases. Staphylococcus aureus can harbor different virulence genes encoding for enterotoxins, enterotoxin-like exfoliative toxin, toxic shock syndrome toxin-1 (TSST-1), and Panton-Valentine leukocidin (PVL) (Kot et al., 2016; Wang et al., 2016). Thermostable staphylococcal enterotoxins generally retain their biological activity after pasteurization treatment (Guimaraes et al., 2013). Staphylococcus aureus with toxin genes were present in about 60.0% of goat, ovine, caprine, and bubaline origin (Mork et al., 2010; Carfora et al., 2015). An efficient screening to detect the virulence genes in different raw milks is necessary.

The use of antibiotics in veterinary practice could cause the selection of antibiotic resistant S. aureus, which is a public health problem (Gomes and Henriques, 2016). Staphylococcus aureus is frequently resistant to antibiotics because it produces exopolysaccharide which forms a barrier to bacterial cell penetration by several antimicrobial agents (Johnston, 1998). Moreover, S. aureus can also develop acquired resistance to many other antimicrobial agents by carrying various resistance traits on plasmids or transposons (Chajecka-Wierzchowska et al., 2015). Some studies have reported that S. aureus which originated from goats, buffaloes, camels, and yak milks, showed high antimicrobial resistance patterns (Pamuk et al., 2012; Biswas et al., 2014; Cortimiglia et al., 2015; Redwan et al., 2016). Therefore, monitoring antimicrobial resistance in S. aureus from different raw milks is necessary to predict the risk on antimicrobial resistance of S. aureus, and to make decisions regarding antibiotic treatments on animals from a pre-harvest food safety standpoint.

The antimicrobial resistance and virulence characteristics of S. aureus from cow milk samples are well described. However, data regarding S. aureus from different raw milk samples in China are relatively limited. Therefore, the present work aimed to determine the prevalence, antimicrobial susceptibility and key virulence genes of S. aureus isolated from goat, buffalo, camel, and yak milk samples in China.

Staphylococcus aureus is one of the main pathogens in raw milk and dairy products, which may lead to economic losses (Aires-de-Sousa et al., 2007; Stapels et al., 2014). Staphylococcus aureus has a strong capability to produce a wide variety of enterotoxins (Liu et al., 2017). The enterotoxins produced by S. aureus are common causes of bovine mastitis pathogenesis and food-borne illnesses (Wang et al., 2016). However, little is known about the presence of S. aureus in goat, buffalo, camel, and yak milk in China. In the present study, the prevalence, antimicrobial susceptibility, and key virulence genes of S. aureus isolated from goat, buffalo, camel, and yak milk samples in China has been assessed.

The prevalence of S. aureus in different raw milk samples varies depending on the country. In our study, raw buffalo milk samples showed the highest prevalence of S. aureus (9/25, 36%), followed by goat milk (16/50, 32%), camel milk (6/25, 24%), and yak milk (5/25, 20%). These results are significantly higher than that in previous reports which have shown that the occurrence rate of S. aureus in buffalo milk was 17.5%, followed by goat milk (7.5%) and camel milk (3.4%) in Iran (Ebrahim and Forough, 2013), 31.43% in goat milk in Egypt (Elbagory, 2017), and 15% in camel milk in Sudan (Shuiep et al., 2009). In contrast, much higher occurrence rate of S. aureus was 43.1% in goat milk in Northern Italy (Cortimiglia et al., 2015), 54.02% in camel milk in Pakistan (Aqib et al., 2017), 47.36% in yak milk in India (Biswas et al., 2014), and 32% in camel milk in Kenya (Njage et al., 2013). Overall, the results indicate that S. aureus is common in different raw milks, including goat, buffalo, camel and yak milk in China. This suggests that more attention should be paid to explore the method of controlling S. aureus occurrence in goat, buffalo, camel, and yak milk.

The antibiotics resistance has correlation to many factors, including excessive use of antibiotics, spread of resistance, and the background antibiotic resistance levels (Hu et al., 2014; Low et al., 2016; Crofts et al., 2017). The poorly regulated use of antibiotics on dairy animals in the different raw milk production systems examined may be an important reason for the high antimicrobial resistance. In this study, the antimicrobial resistance was the frequently observed in S. aureus from goat milk (36.1%) and buffalo milks (16.7%). The results indicated that the antibiotics resistance of S. aureus isolated from goat milk in Shandong and Shaanxi province, buffalo milk in Guangxi Zhuang Autonomous Region were very serious in China, although less than raw cow milk (Liu et al., 2017). A high emission densities of the antibiotics in China were reported in Shandong province, Shaanxi province, and Guangxi Zhuang Autonomous Region (Zhang et al., 2015). It implied that the high antimicrobial resistance of S. aureus from goat and buffalo milk may be related to it.

In the current study, S. aureus showed a high percentage of antimicrobial resistance to penicillin G, tetracycline, gentamicin, and ampicillin. Similar findings were reported by Oliveira et al. (2011), who found that 100% of S. aureus from buffalo milks were resistant to penicillin G and ampicillin, and by Rola et al. (2015), who found that coagulase-positive staphylococcus isolated from goat milk were resistant to penicillin G and tetracycline, and by Njage et al. (2013), who found that S. aureus isolated from camel milk from Kenya showed a high percentage of antimicrobial resistance to ampicillin, gentamicin, and tetracycline. Moreover, a total of 26 strains (26/36, 72.2%) showed antibiotics resistance, and 19 strains (19/26, 52.8%) showed resistance to at least three antimicrobial classes in our study. Based on the high percentage of resistance, these antibiotics should be used with caution for mastitis caused by S. aureus in goat, buffalo, and camel milk in China.

MRSA has been identified as an emerging pathogen in livestock animals that is readily transferable to humans in contact with livestock (Liu et al., 2017). Numerous reports have described the prevalence of MRSA in bovine milk (Caruso et al., 2016; Basanisi et al., 2017), and the transmission of MRSA between people working on farms and dairy cattle (Antoci et al., 2013; Mole, 2013). On the contrary, little is known about the prevalence of MRSA in other ruminants (e.g., goats, camels, buffalo, yaks), although the consumption of milk from different animals species is more and more common. In the present study, 16.7% of S. aureus (6/36) isolated from goat milk were identified as methicillin-resistant strains (MRSA). The results showed that there is a high prevalence in goat milks. The prevalence of MRSA in Southern Italy was 1.23% in goat milk and sheep bulk tank milk (Caruso et al., 2016), 8.3% in cow milk samples and dairy products (Basanisi et al., 2017). Moreover, there was no MRSA strains detected in buffalo, yak, and camel milk samples in our study. There was difference in data regarding the prevalence of MRSA strains in buffalo milk samples. Pamuk et al. (2012) reported that 2.5% of the buffalo samples were contaminated with MRSA in Turkey. El-Ashker et al. (2015) found that 24.5% of S. aureus in buffalo milk were identified as MRSA in Dakahlia Governorate, Egypt.

In the present study, 42.3% of the strains (11/26) harbored at least one virulence genes, which was much lower than reported in other studies. Liu et al. (2017) found that 60.3% of S. aureus from cow milk harbored one or more virulence genes. Martins et al. (2017) found that 62.5% of S. aureus from sheep milk samples showed some toxin-encoding genes. There are also reports showing that 67% of the detected S. aureus exhibited at least one enterotoxin gene, many caprine, or ovine raw milk products may be contaminated with low levels of enterotoxigenic S. aureus (Merz et al., 2016). Liu et al. (2019) reported that 62 S. aureus (72.5%) carried virulence genes, including set, hlb, hld, lukED, ebp, clfA, and clfB. Moreover, the largest percentage of sec (31.8%) genes was found in our study. Several authors have also reported that enterotoxin genes sea and sec are the most common in S. aureus isolated from raw milk (Carfora et al., 2015; Liu et al., 2017). Staphylococcus aureus from different kinds of milk samples exhibited different virulence gene patterns. For goat milk, sec gene was the most frequent in the study, and also reported as the most common virulence genes in Brazil (Lyra et al., 2013; Cavicchioli et al., 2015; Salaberry et al., 2015). Other researchers have reported that the most prevalent enterotoxin genes were sec and sel from goat milk samples in Switzerland (Merz et al., 2016). For camel milk, the sec and pvl genes have been detected in the study. Njage et al. (2013) found that pvl gene was detected in S. aureus from raw and fermented camel milk from Kenya and Somali. However, three S. aureus from camel milk were positive for sec, seg, sei, sem, sen, and seo, respectively (Shuiep et al., 2009). Majority of S. aureus were co-carried many virulence genes in goat, yak, camle, and bufflo milk, and it may pose great potential risk to public health. In order to estimate the actual impact on public health, it was necessary to the detection of virulence genes in goat, buffalo, and camel milk samples.

In conclusion, the antimicrobial resistance and virulence genes of S. aureus in goat, buffalo, camel, and yak milk in China were assessed. Our data indicated that S. aureus, which exhibit a high percentage of antimicrobial resistant and carried multiple virulence genes, were commonly found in different raw milks, especially goat milk. Although the prevalence of MRSA in raw milk from china was low, MRSA may pose a potential threat for consumers of raw milk or dairy products made with raw milk and for people working or living in close contact with animals. Therefore, it was very important to monitor the usage of antimicrobial agents and the potential transfer of antimicrobial resistance genes in goat milk.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

HL, LD, and YZ designed and performed the researches. LM helped with the data analysis. CW and JW gave advices to the researchers. NZ gave the opinions on the researches design. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.