Prevalence,Antibiotic Resistance,and Molecular Typing of Staphylococcus aureus Isolated from Ready-to-Eat Foods in Guangdong,South China

Abstract

The increasing global popularity of ready-to-eat (RTE) foods for their convenience simultaneously brings along a risk, as these products can be contaminated with various microorganisms, including potentially harmful pathogens. We aimed to investigate the food contamination of Staphylococcus aureus (S. aureus) in RTE foods in Guangdong, South China. All S. aureus isolates were subjected to characterization through antimicrobial susceptibility tests, multilocus sequence typing (MLST), and PCR analysis for detecting mec and blaZ genes. A total of 824 RTE food samples were collected from 2017 to 2022, of which 73 (8.9%) were found to be contaminated with S. aureus. Contamination levels were mostly in the range of 0.3–1.0 most probable number (MPN)/g, with 10 samples exceeding 110 MPN/g. Of the 73 S. aureus isolates, 10 were identified as methicillin-resistant S. aureus (MRSA). In MRSA, resistance was most frequently observed to penicillin (100%, 10/10), followed by erythromycin (80.0%, 8/10) and tetracycline (70%, 7/10). And in methicillin-sensitive S. aureus (MSSA), resistance was most frequently observed to penicillin (98.4%, 62/63), followed by tetracycline (30.2%, 19/63) and erythromycin (23.8%, 15/63). Overall, 98.6% (72/73) of the isolates demonstrated resistance to at least one antimicrobial agent, whereas 31.5% (23/73) were resistant to three or more antimicrobials. Fifty-seven S. aureus isolates harbored the penicillin-resistant gene blaZ, and 10 isolates carried the mec gene. In addition, 30.1% of the isolates harbored genes for classical staphylococcal enterotoxins (SEs), with seb being the most frequently detected SE gene. MLST revealed that the 73 isolates belonged to 14 different sequence types (STs), the most prevalent of which was ST7. In MRSA, the most common prevalent clone is ST6, and in MSSA, ST7 was the most common isolates. The prevalent multidrug resistance indicates that the resistance situation of foodborne S. aureus in Guangdong is severe, posing a potential threat to consumer safety and health.

Introduction

S taphylococcus aureus (S. aureus) is a common zoonotic pathogen that can cause severe diseases in humans. It is widespread in nature and is one of the main pathogens causing bacterial food poisoning. It frequently contaminates food and can produce various enterotoxins, increasing the risk of food poisoning, and it is regarded as one of the most prominent culprits of the global food poisoning outbreak (Kadariya et al., 2014; Wendlandt et al., 2013). S. aureus can cause a spectrum of symptoms, ranging from minor skin infections to life-threatening conditions, such as septicemia and pneumonia. The severity of these infections often leads to prolonged hospitalizations, placing a significant burden on health care systems (Cheung et al., 2021).

Considering the popularity of ready-to-eat (RTE) foods such as cooked meats, cold vegetable dishes, milk-infused tea, and baked goods among consumers, it is crucial to understand the extent of S. aureus contamination and the potential risk to public health. The risk of S. aureus contamination in RTE foods is exacerbated by factors such as the lack of additional cooking or processing, frequent handling which may introduce contaminants, and suboptimal storage conditions (De Vogli et al., 2014; Mengistu et al., 2022).

In China, the extensive use of antibiotics in human medicine and livestock breeding presents a significant challenge (Li, 2014). Due to the overuse of antibiotics, antibiotic-resistant bacteria have become a significant factor threatening global public health safety. S. aureus is prone to developing antibiotic resistance, especially with the emergence of methicillin-resistant S. aureus (MRSA), which has high resistance to penicillin and is often accompanied by multiple drug resistance phenomena, posing enormous challenges for the treatment of related infectious diseases (Andersson and Hughes, 2017; Lakhundi and Zhang, 2018; Vestergaard et al., 2019). S. aureus acquires antibiotic resistance in various ways, such as mobile elements and efflux pumps (Vestergaard et al., 2019). MRSA infections are linked to higher mortality rates compared with infections caused by methicillin-susceptible strains.

The mobile genetic element of staphylococcal cassette chromosome mec (SCCmec) is carried by MRSA isolates, which encodes low-affinity penicillin-binding protein (PBP) akin to PBP2a or PBP2′. The identity of the nucleotide sequence of mec genes is shared to a minimum of 70% with the mecA gene (Cheung et al., 2021; Ito et al., 2012). Multilocus sequence typing (MLST) is a useful tool for tracing the source of bacterial contamination, allowing for comparison and analysis of pathogens across different countries and time periods (Maiden et al., 2013). According to current statistics, there are nearly 4000 sequence types (STs) of S. aureus identified globally (Rong et al., 2018). In China, the distribution of foodborne S. aureus STs exhibits distinct regional patterns. For instance, in Beijing, ST398, ST25, and ST15 are the prevalent strains (Li et al., 2021). In contrast, ST7, ST6, and ST59 are the dominant strains in Yunnan Province (Liao et al., 2018). Furthermore, ST59, ST9, ST1, and ST398 are the primary STs associated with foodborne MRSA in China (Wu et al., 2019). Therefore, MLST can assist in monitoring the molecular profile of S. aureus contamination in food.

However, comprehensive data, particularly on RTE foods in South China, are scant. This study aims to systematically investigate the contamination of S. aureus in RTE food and conduct resistance analysis and MLST on the isolated strains, with a particular focus on the Guangdong region, South China, and provide a data reference for risk assessment related to S. aureus contamination.

Materials and Methods

Sample collection and bacterial isolates

From February 2017 to December 2022, we collected 824 RTE food samples from retail markets and supermarkets in Foshan, Guangdong Province. This collection process adhered to the general guidelines set forth by the National Food Safety Standard for Sample Collection (Ministry of Health, PRC, 2010). The samples included cooked meats (212 samples); Poon Choi (33 samples)—a traditional Cantonese dish from Guangdong, China, which is a multilayered stew made of seafood, various meats, and vegetables; cold vegetable dishes (350 samples); milk-infused tea (103 samples); and baked goods (126 samples). Each individual sample was placed in a separate sterile bag, promptly transported to the laboratory under refrigeration within a 2-day period, and continuously stored below 4°C.

Quantitative detection of S. aureus was performed as outlined in the GB 4789.10-2010 document (Ministry of Health of the People's Republic of China, 2010). A 25-g sample was randomly collected from each RTE food and placed into a sterile bag containing 225 mL of sterile physiological saline solution. After homogenization, 7.5% sodium chloride broth (Huankai, Guangzhou, China) was inoculated in triplicate with 1 mL aliquots of decimal dilutions of each sample. Samples were incubated at 36°C for 24 h. After which loopfuls of the resulting cultures were streaked onto Baird-Parker agar plates (Huankai) and incubated at 36°C for 48 h. Putative strains were confirmed as S. aureus through Gram staining, coagulase test, and matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) system (Autobio600, Zhengzhou, China). S. aureus ATCC 25923 and Staphylococcus epidermidis ATCC 12228 were used as control strains. The most probable number (MPN) was determined by counting the number of positive tubes in each of the three sets, using the MPN table.

All identified isolates were subsequently frozen and preserved in a brain–heart infusion medium with 40% glycerol at −80°C for further experiments.

Antibiotic susceptibility testing

The microbroth dilution method was utilized to evaluate the antibiotic susceptibility of S. aureus. In total, 16 antibiotics were subjected to testing, encompassing penicillin (PEN), cefoxitin (FOX), oxacillin (OXA), gentamicin (GM), ciprofloxacin (CIP), levofloxacin (LEV), moxifloxacin (MXF), erythromycin (EM), clindamycin (CLI), quinupristin/dalfopristin (QDA), linezolid (LZD), vancomycin (VAN), tetracycline (TET), tigecycline (TGC), furantoin (FT), and trimethoprim–sulfamethoxazole (SXT). S. aureus ATCC 25923 were used as the quality control strains. The results of the drug sensitivity tests were interpreted according to the 2022 standards of the Clinical and Laboratory Standards Institute (CLSI) (Clinical and Laboratory Standards Institute, 2022). If the isolate was resistant to cefoxitin, it was determined to be MRSA. Isolates were considered multidrug resistant (MDR) when found to be resistant to three or more antibiotics from different families (Magiorakos et al., 2012).

Preparation of DNA template and detection of antibiotic resistance genes and staphylococcal enterotoxin genes

Bacterial DNA was extracted by a bacterial genomic DNA extraction kit (Bacterial DNA Kit D3350; Omega Bio-Tek, Guangzhou, China), and the blaZ, mec, nucA and five classical staphylococcal enterotoxin (SE) genes (sea to see) were detected using PCR assays. The nucA gene was used as an internal reference gene. The primers (Sangon Biotech, Shanghai, China) and PCR protocol were based on previous research (Lv et al., 2021; Song et al., 2016).

Multilocus sequence typing

MLST of S. aureus was conducted by sequencing conserved regions of seven housekeeping genes: arcC, aroE, glpF, gmk, pta, tpi, and yqiL. The STs and clonal complexes (CCs) were subsequently performed through analysis in the dedicated S. aureus MLST database (https://pubmlst.org/S. aureus). The primers (Sangon Biotech) and PCR procedure were based on previous research (Zhang et al., 2023).

Data processing and analysis

Statistical processing was conducted using SPSS 14.0 software. Pearson's chi-square tests or Fisher's exact tests were used to analyze the data. p < 0.05 was considered statistically significant. A phylogenetic tree was constructed using BioNumerics 8.1.1 (Applied Maths, Sint-Martens-Latem, Belgium).

Ethical statement

Human and animal testing is unnecessary in our study.

Results

Contamination level of S. aureus in RTE foods

In this study, 73 of the 824 RTE foods (8.9%) were positive for S. aureus. Among the various food types, Poon Choi had the highest contamination rate of 15.2% (5/33), followed by cooked meats (26/212, 12.3%), cold vegetable dishes (34/350, 9.7%), baked goods (5/126, 4.0%), and milk-infused tea (3/103, 2.9%) (Table 1). Furthermore, through MPN quantitative detection, we found that 57.5% (42/73) of the positive samples had an MPN value of 0.3–1.0 MPN/g, 15.1% (11/73) had an MPN value of 1.0–10 MPN/g, and 13.7% (10/73) had an MPN value of 10–110 MPN/g, and 10 samples exceeded 110 MPN/g.

Table 1.

Prevalence and Contamination Level of Staphylococcus aureus in Different Ready-to-Eat Foods

Type	Contamination rate (%)	No. of positive samples by quantitative methods/(MPN/g)
Type	Contamination rate (%)	0.3–1.0	1–10	10–110	≥110
Cold vegetable dishes	34/350 (9.7)	25	5	2	2
Poon Choi	5/33 (15.2)	3	1	1	0
Milk-infused tea	3/103 (2.9)	1	1	1	0
Cooked meats	26/212 (12.3)	10	3	5	8
Baked goods	5/126 (4.0)	3	1	1	0
Total	73/824 (8.9)	42	11	10	10

“Poon Choi,” a traditional Cantonese dish originating from Guangdong, China, is a multilayered stew featuring an assortment of ingredients such as seafood, various meats, and vegetables.

Antibiotic resistance level of S. aureus

Among the 16 antibiotics tested, all the 73 S. aureus isolates were sensitive to MXF, QDA, VAN, LZD, and TGC. The resistance rates to PEN, TET, EM, CLI, GM, FOX, OXA, CIP, LEV, SXT, and FT were 98.6% (72/73), 35.6% (26/73), 31.5% (23/73), 21.9% (16/73), 17.5% (11/73), 13.7% (10/73), 13.7% (10/73), 11.0% (8/73), 9.6% (7/73), 7.9% (5/73), and 2.7% (2/73), respectively. As shown in Table 2, of the 73 isolates, 72 (98.6%) showed resistance to at least 1 antibiotic, and 23 (31.5%) isolates were MDR.

Table 2.

Antibiotic Susceptibility Results of the 73 Strains of Staphylococcus aureus

Antimicrobial agents	Resistance rate (%)			Resistance break point (μg/mL)
Antimicrobial agents	Overall (N = 73)	MRSA (n = 10)	MSSA (n = 63)	S	I	R
PEN	72 (98.6)	10 (100)	62 (98.4)	≤0.12	—	≥0.25
FOX	10 (13.7)	10 (100)	0 (0.0)	≤4	—	≥8
OXA	10 (13.7)	10 (100)	0 (0)	≤2	—	≥4
GM	11 (17.5)	3 (30.0)	8 (12.7)	≤4	8	≥16
CIP	8 (11.0)	4 (40.0)^**	4 (6.3)	≤1	2	≥4
LEV	7 (9.6)	2 (20.0)	4 (6.3)	≤1	2	≥4
MXF	0 (0)	0 (0)	0 (0)	≤0.5	1	≥2
EM	23 (31.5)	8 (80.0)^**	15 (23.8)	≤0.5	1–4	≥8
CLI	16 (21.9)	6 (60.0)^**	10 (15.9)	≤0.5	1–2	≥4
QDA	0 (0)	0 (0)	0 (0)	≤1	2	≥19
LZD	0 (0)	0 (0)	0 (0)	≤4	—	≥8
VAN	0 (0)	0 (0)	0 (0)	≤2	4–8	≥16
TET	26 (35.6)	7 (70.0)^*	19 (30.2)	≤4	8	≥16
TGC	0 (0)	0 (0)	0 (0)	≤0.12	—	≥2
FT	2 (2.7)	1 (10.0)	1 (1.6)	≤16	—	≥32
SXT	5 (7.9)	2 (20.0)	3 (3.2)	≤2/38	—	≥4/76
Any resistance	72 (98.6)	10 (100)	62 (98.4)
MDR	23 (31.5)	10 (100)	13 (20.6)

Any resistance: resistance to at least one antibiotic. MDR: resistance to three or more antibiotics.

p < 0.05, ^** p < 0.01.

CIP, ciprofloxacin; CLI clindamycin; EM, erythromycin; FOX, cefoxitin; FT, furantoin; GM, gentamicin; LEV, levofloxacin; LZD, linezolid; MDR, multidrug-resistant; MRSA, methicillin-resistant S. aureus; MSSA, methicillin-sensitive S. aureus; MXF, moxifloxacin; OXA, oxacillin; PEN, penicillin; QDA, quinupristin/dalfopristin; SXT, trimethoprim–sulfamethoxazole; TET, tetracycline, TGC, tigecycline; VAN, vancomycin.

All MRSA isolates were sensitive to MXF, QDA, VAN, LZD, and TGC, with 100% resistance rates to PEN and OXA. Resistance rates for other antibiotics were as follows: EM (80.0%), TET (70.0%), CLI (60.0%), CIP (40.0%), GM (30.0%), LEV and SXT (20.0%), and FT (10.0%). The 63 methicillin-sensitive S. aureus (MSSA) isolates were sensitive to MXF, QDA, VAN, LZD, and TGC and also showed low resistance rates to FT, SXT, CIP, and LEV, at 1.6% (1/63), 3.2% (2/63), 6.3% (4/63), and 6.3% (4/63), respectively. The highest resistance rate was to PEN (98.6%), followed by TET (30.2%), EM (23.8%), and CLI (15.9%). MRSA showed significantly higher resistance rates to EM, TET, CLI, LEV, and CIP compared with MSSA (EM: 80% vs. 23.8%, chi-square = 10.157, p < 0.01; TET: 70% vs. 30.2%, chi-square = 4.363, p < 0.05; CLI: 60% vs. 15.9%, chi-square = 7.410, p < 0.01; CIP: 40.0% vs. 6.3%, chi-square = 6.863, p < 0.01). There was no statistically significant difference in resistance rates to PEN, GM, LEV, FT, and SXT. The detailed antibiotic resistance profiles have been shown in the Supplementary Material.

Presence of mec, blaZ, and SEs

A total of 13.7% (10/73) of isolates were positive for the mec gene, and 78.1% (57/73) were positive for the blaZ gene. Of the 73 isolates, 22 carried one or more classical enterotoxin genes. The detection frequencies for sea, seb, sec, sed, and see genes were 24.7% (18/73), 27.4% (20/73), 17.8% (13/73), 21.9% (16/73), and 2.7% (2/73), respectively. S. aureus isolated from cold vegetable dishes, cooked meats, and Poon Choi had a relatively high probability of carrying SE gene (Table 3).

Table 3.

Prevalence of mecA, blaZ, and Staphylococcal Enterotoxins in Staphylococcus aureus Isolated from Ready-to-Eat Foods

Genes	Percentage of S. aureus isolates positive for the detected genes
Genes	Total (N = 73)	Cold vegetable dishes (n = 34)	Poon Choi (n = 5)	Milk-infused tea (n = 3)	Cooked meats (n = 26)	Baked goods (n = 5)
Mec	10 (13.7)	3 (8.8)	1 (20.0)	0 (0.0)	6 (23.1)	0 (0.0)
blaZ	57 (78.1)	25 (73.5)	4 (80.0)	1 (33.3)	24 (92.3)	3 (60.0)
Sea	18 (24.7)	9 (26.5)	2 (40.0)	0 (0.0)	7 (23.1)	0 (0.0)
Seb	20 (27.4)	10 (29.4)	2 (40.0)	0 (0.0)	8 (30.8)	0 (0.0)
Sec	13 (17.8)	5 (14.7)	2 (40.0)	0 (0.0)	5 (19.2)	1 (20.0)
Sed	16 (21.9)	8 (23.5)	2 (40.0)	0 (0.0)	6 (23.1)	0 (0.0)
See	2 (2.7)	1 (2.9)	0 (0.0)	0 (0.0)	1 (3.8%)	0 (0.0)
Any SEs	22 (30.1)	18 (52.9)	3 (60.0)	0 (0.0)	16 (61.5)	1 (20.0)

SEs, staphylococcal enterotoxins.

STs and CCs of S. aureus isolates

A total of 14 STs were identified among the 73 S. aureus isolates, which were divided into 4 CCs and 5 singletons. The CCs and singletons included CC1 (ST1, ST9, ST188, and ST2139), CC5 (ST5, ST6, and ST2239), CC8 (ST72), CC15 (ST15), ST59, ST398, ST7, ST25, and ST2221. The major genotypes were ST7 (26.0%, 19/73), ST9 (16.4%, 12/73), and ST398 (11.0%, 8/73). The dominant CCs were CC1 (34.2%, 25/73), ST7 (26.0%, 19/73), and CC5 (15.1%, 11/73) (Table 4).

Table 4.

Molecular Characteristics of Sequence Types in Staphylococcus aureus Isolated from Ready-to-Eat Foods

CCs	ST Number	Cold vegetable dishes	Poon Choi	Milk-infused tea	Cooked meats	Baked goods	MRSA	MSSA
CC1	ST1	3	0	0	3	0	0	6
	ST9	7	1	0	2	2	1	11
	ST188	5	1	0	0	0	1	5
	ST2139	0	0	0	1	0	0	1
CC5	ST5	0	1	1	2	0	0	4
	ST6	1	0	0	4	0	3	2
	ST2239	0	0	0	1	1	0	2
CC8	ST72	2	0	1	0	0	0	3
ST7	ST7	9	1	1	6	2	2	17
CC15	ST15	1	0	0	0	0	0	1
ST25	ST25	1	0	0	0	0	0	1
ST59	ST59	2	1	0	1	0	1	3
ST398	ST398	3	0	0	5	0	2	6
ST2221	ST2221	0	0	0	1	0	0	1
Total		34	5	3	26	5	10	63

CC, clonal complexes; MRSA, methicillin-resistant S. aureus; MSSA, methicillin-sensitive S. aureus; ST, sequence type.

The phylogenetic relationship diagram drawn using the BioNumerics software based on the genotypes of the seven housekeeping genes of the strains is shown in Figure 1. The size of the circle in the figure represents the number of ST strains, the line between the circles represents the closeness of the phylogenetic relationship, and the value on the line represents the relative genetic distance.

FIG. 1.

Phylogenetic tree of 73 S. aureus isolates in RTE foods based on MLST allelic profiles. Each circle represents ST, and the size and color depth of each circle correspond to the number of sample. MLST, multilocus sequence typing; RTE, ready-to-eat; ST, sequence type.

STs in MRSA and MSSA isolates

Among the 10 MRSA isolates, 6 were from cooked meats, 3 were from cold vegetable dishes, and 1 (MRSA-ST7) was from Poon Choi (Table 5). The predominant STs of MRSA isolates were ST6 (30%, 3/10), followed by ST398 (20.0%, 2/10), ST7 (20.0%, 2/10), ST9 (10.0%, 1/10), ST188 (10.0%, 1/10), and ST59 (10.0%, 1/10). ST7 (27.0%, 17/63) was the most common in MSSA isolates, followed by ST9 (17.5%, 11/63), ST398 (9.5%, 6/63), ST1 (9.5%, 6/63), ST188 (7.9%, 5/63), ST5 (6.3%, 4/63), ST59(4.8%, 3/63), ST72 (4.8%, 3/63), ST6 (3.2%, 2/63), and ST2236 (3.2%, 2/63). ST2139, ST15, ST25, ST2221 each accounted for 1.6% (1/63) (Table 4).

Table 5.

Distribution and Molecular Typing of Staphylococcus aureus Isolates from Ready-to-Eat Food Samples

Isolation types	MLST types	MLST-CC	MRSA/MSSA	No. of isolates
Cold vegetable dishes	ST1	CC1	MSSA	3
	ST9	CC1	MSSA	7
	ST188	CC1	MRSA	1
	ST188	CC1	MSSA	4
	ST6	CC5	MSSA	1
	ST72	CC8	MSSA	2
	ST7	ST7	MSSA	9
	ST15	CC15	MSSA	1
	ST25	ST25	MSSA	1
	ST59	ST59	MRSA	1
	ST59	ST59	MSSA	1
	ST398	ST398	MRSA	1
	ST398	ST398	MSSA	2
Poon Choi	ST9	CC1	MSSA	1
	ST188	CC1	MSSA	1
	ST5	CC5	MSSA	1
	ST7	ST7	MRSA	1
	ST59	ST59	MSSA	1
Milk-infused tea	ST7	ST7	MSSA	1
	ST72	CC8	MSSA	1
	ST5	CC5	MSSA	1
Cooked meats	ST1	CC1	MSSA	3
	ST9	CC1	MRSA	1
	ST9	CC1	MSSA	1
	ST2139	CC1	MSSA	1
	ST5	CC5	MSSA	2
	ST6	CC5	MRSA	3
	ST6	CC5	MSSA	1
	ST2239	CC5	MSSA	1
	ST7	ST7	MRSA	1
	ST7	ST7	MSSA	5
	ST59	ST59	MSSA	1
	ST398	ST398	MRSA	1
	ST398	ST398	MSSA	4
	ST2221	ST2221	MSSA	1
Baked goods	ST9	CC1	MSSA	2
	ST2239	CC5	MSSA	1
	ST7	ST7	MSSA	2

CC, clonal complexes; MLST, multilocus sequence typing; MRSA, methicillin-resistant S. aureus; MSSA, methicillin-sensitive S. aureus; ST, sequence type.

Discussion

S. aureus, a foodborne pathogen with the capability of causing various symptoms, is present in a diverse range of food types. In our research, we detected S. aureus in 8.9% (73/824) of RTE food samples. This discovery highlights the possible threat RTE foods may pose to consumers. Our study's level of contamination exceeds that of Switzerland (7.0%) (Baumgartner et al. 2014) and South Korea (8.6%) (Oh et al., 2007) but falls below that of the United States (27.9%) (Ge et al., 2017), Malaysia (26%) (Suat et al., 2017), and Algeria (23.2%) (Mekhloufi et al., 2021). In our study, 86.3% of the samples had S. aureus concentrations below 110 MPN/g. Taking into account the Ministry of Health, PRC's recommendation of a 100 cfu/g threshold for S. aureus in RTE meat (Ministry of Health of the People's Republic of China, 2014). Therefore, overall, the contamination level of S. aureus in Guangdong RTE foods was not exceedingly high.

This study found that 98.6% (72/73) of S. aureus were resistant to at least one antibiotic, with high resistance to PEN (98.6%, 72/73), TET (35.6%, 26/73), and EM (31.5%, 23/73), and 78.1% (57/73) of them holding the blaZ gene. The resistance was 98.4% for PEN, 43.5% for TET, and 17.8% for EM in national-wide survey of RTE foods (Yang et al., 2016). In Shaanxi, the resistance rate to TET was 40.6% and EM was 78.1% (Xing et al., 2014), whereas in Yunnan, the resistance rate to PEN, TET, and EM were 29.5%, 46.6%, and 51.0%, respectively (Liao et al., 2018). A comparison of domestic studies on the subject reveals significant variation in the drug resistance of foodborne S. aureus across the country, with regional differences that may be attributed to the selective pressure of antibiotic use.

Studies suggest that 95% of staphylococcal food poisoning are linked to SEs (Kokan and Bergdoll, 1987); thus, we focused on five classical SEs (sea, seb, sec, sed, see). Our findings revealed that 30.1% (22/73) of the S. aureus isolates carried at least one SE gene, aligning with prevalences observed in other regions such as China (29.7%) and the United States (25.8%) (Ge et al., 2017; Wang et al., 2017).

While MRSA is commonly observed in clinical isolates, its prevalence in food samples is relatively lower. In this study, we identified MRSA in 1.2% (10/824) of food samples. These results align with findings from similar studies in the United States (1.9%) and China (1.3%) (Ge et al., 2017; Yang et al., 2016). Our findings also indicate that MRSA exhibited significantly higher resistance to EM, TET, CLI, LEV, and CIP compared with MSSA, which is in line with previous research (Gan et al., 2021). It is worth noting that MDR bacteria were identified in 31.5% (23/73) of the samples in this study, with all MRSA isolates showing MDR.

Although this result is considerably lower than the detection rate of retail foods (94.6%) reported by Wu et al. (2018), it is higher than the detection rate of milk samples (10.3%) reported by Zhang et al., (2016). It is speculated that the high proportion of MDR observed in clinical MRSA (94.6%) may indicate the continual acquisition and transfer of new drug resistance genes (Yu et al., 2012). It is imperative to exercise stringent regulation over the utilization of antibiotics, proactively guard against the erroneous use of antibiotics, and rigorously enforce the proscription of antibiotics in livestock management to impede the emergence of antibiotic-resistant “superbugs.”

The molecular characteristics of the S. aureus isolates were analyzed through MLST. The isolates were divided into four CCs and five singletons; in this study, CC1 (ST1, ST9, ST188, and ST2139) was the main CC, accounting for 34.2% (25/73) of the strains. ST1 is a globally prevalent strain and is also a common ST of foodborne S. aureus in China (Gan et al., 2021; Nesaraj et al., 2023; Tang et al., 2017). ST9 contamination is often found in retail raw meat products, with a wide distribution in pork and detection in beef and chicken as well (He et al., 2013; Wan et al., 2013; Yan et al., 2016). ST188 is the predominant genotype in raw meat processing in China, and it is also a prevalent strain of community-associated S. aureus (Dong et al., 2020; Song et al., 2016). ST2139 is relatively uncommon in foodborne S. aureus in China, with only one case found in this study. ST7 was the main single ST in this survey, accounting for 26.0% (19/73).

ST7 is the dominant type in animal-related, clinical, and community-associated infections in the Asia–Pacific region (Dong et al., 2020; Gu et al., 2023). Ten MRSA were obtained in this study. The predominant STs of MRSA were ST6, ST398, and ST7, which is consistent with the epidemiological characteristics of foodborne MRSA in China. ST398 is a globally prevalent strain in livestock and the primary strain of meat-origin MRSA in China (Buyukcangaz et al., 2013; Tang et al., 2017; Yan et al., 2014). ST6 is the main prevalent strain of foodborne MRSA in Yunnan province (Liao et al., 2018). In this study, MRSA-ST398, MRSA-ST6, and MRSA-ST7 were all isolated from cooked meats, which might indicate that the contamination of these strains came from animals or the people who feed and process them.

Conclusions

This study represents the first exhaustive investigation into the prevalence of S. aureus and MRSA in retail RTE foods in Guangdong, South China. Our findings revealed a significant prevalence of S. aureus, with contamination detected in 8.9% of the samples. Among these, 57.5% exhibited a contamination level within the range of 0.3–1.0 MPN/g. Antibiotic resistance analysis of the identified S. aureus strains showed that 98.2% were resistant to at least one antibiotic, and 31.5% were MDR. Of the isolates, 13.7% tested positive for the mec gene, 78.1% for the blaZ gene, and 30.1% carried at least one SE gene. The MLST yielded 14 distinct genotypes, with ST7 emerging as the predominant strain and CC1 identified as the primary CC. The genotypic and enterotoxin diversity observed in this study highlights the substantial health risks posed by S. aureus in RTE foods.

Footnotes

Authors' Contributions

C.Z., L.Z., and J.Z. conceived the project and designed the experiments. C.Z. and L.Z. performed the experiments. C.Z. and Y.Q. analyzed the data Y.Q., B.H., and Z.S. contributed reagents and materials and complemented the writing. All authors have read and agreed to the published version of the article.

Acknowledgments

We sincerely thank all the participants who took part in this study.

Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by Guangdong Provincial Market Supervision Administration Project (No. 2022CZ03).

Supplementary Material

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