Coagulase-Positive Staphylococci Isolated from Chicken Meat: Pathogenic Potential and Vancomycin Resistance

Abstract

Coagulase-positive staphylococci (CPS) cause staphylococcal food poisoning. Recently, these bacteria have received increasing attention due to their potential role in the dissemination of antibiotic resistance markers. The present study aimed to evaluate coagulase-positive staphylococci counts, species distribution, enterotoxin genes prevalence, and the antibiotic resistance profile of CPS isolated from in natura chicken meat. Fifteen frozen and 15 chilled industrialized, uncooked chicken parts or entire carcasses were used. Staphylococcal counts revealed that frozen chicken meat samples displayed the lowest CPS count compared with chilled chicken meat samples (p<0.01). Staphylococcus aureus (62%) was the most common species, followed by S. intermedius, S. delphini, and S. schleiferi subsp. coagulans (10% each) and S. hyicus (8%). The polymerase chain reaction identification of sea, seb, sec, sed, and see genes revealed that 70% of the isolates harbored at least one enterotoxin gene, with sea and sed being the most frequently encountered ones. Two of the 50 investigated strains harbored three different enterotoxin genes. A high frequency of isolates resistant to penicillin, teicoplanin, oxacillin, and clindamycin was observed, and 80% of CPS were found to be resistant to at least one of the 11 tested antimicrobials. Vancomycin-resistant S. aureus and S. intermedius showed minimum inhibitory concentrations of 512 and 64 μg/mL, respectively. These isolates might indicate the dissemination of vancomycin resistance in the community and imply food safety hazards.

Introduction

C oagulase-positive staphylococci (CPS) are a group of Gram-positive bacteria comprised of seven species (Euzéby, 2012), with Staphylococcus aureus being the most common species involved in illnesses. Staphylococcal food poisoning is a foodborne disease characterized by vomiting, abdominal pain, and diarrhea, and is engendered by the ingestion of staphylococcal enterotoxins (Holmberg and Blake, 1984). The Center for Disease Control and Prevention estimated that more than 185,000 cases of staphylococcal food poisoning occur in the United States per year (Mead et al., 1999). Milk, dairy products, and meats are the foods that are most frequently associated with outbreaks (Asao et al., 2003; Do Carmo et al., 2004). Staphylococci in foods not only cause diseases, but also can serve as a reservoir of resistance genes, favoring the transference of such determinants from nonpathogenic to pathogenic and opportunist bacteria (Sorum and L'Abee-Lund, 2002; Gundogan et al., 2005).

Staphylococci isolated worldwide have shown high rates of antibiotic resistance (Lowy, 2003), which is particularly alarming in relation to hospital-acquired infections. Methicillin-resistant S. aureus (MRSA) has received increasing attention because its treatment is more challenging and its frequency of isolation in hospitals is high (Tice and Rehm, 2010; Crivellaro et al., 2011). In recent years, vancomycin-resistant S. aureus (VRSA) has been recovered from human infections (Sievert et al., 2008); these isolates harbor the vanA gene, which is a genetic determinant from enterococci that provides vancomycin resistance. Since vancomycin is the drug of choice for the treatment of MRSA infections, the emergence of VRSA should be closely monitored in order to avoid the dissemination of these microorganisms (Foucault et al., 2009).

The lack of data regarding the pathogenic potential of CPS species in uncooked poultry meat warrants more studies to establish whether or not safety hazards related to CPS exist in this type of food. The present study aims to evaluate CPS counts, species distribution, enterotoxin genes prevalence, and the antibiotic resistance profile of CPS isolated from in natura chicken meat in southern Brazil.

Materials and Methods

Staphylococcal quantification and coagulase tests

From March to April 2011, 15 frozen and 15 chilled industrialized, uncooked chicken parts or entire carcasses were purchased in supermarkets in the cities of State of Rio Grande do Sul, in southern Brazil. Frozen chickens were maintained at −12°C while chilled chickens were kept at a temperature between −1 and 4°C. In both cases, the meat was within biological safety expiration dates. After being purchased, both types of chicken meat were immediately transported to the Microbiology Laboratory of Federal University of Rio Grande do Sul under refrigeration (<7°C) for analysis. Only one sample of meat was taken from each chicken part or carcass. Under aseptic conditions, each of the 30 samples was further processed into pieces. Those were combined so that a total of 25 g of each sample was used for the analysis. Baird Parker Agar (Himedia) was used for the detection and enumeration of staphylococci. Positive strains were confirmed by the coagulase test. All assays were performed according to the Bacteriological Analytical Manual of the U.S. Food and Drug Administration (Bennett and Lancette, 2001). Coagulase-positive colonies were stored in brain heart infusion (Himedia) broth 20% glycerol at −20°C. Statistical tests (t-test) were performed in order to evaluate chilled and frozen sample counts (Rosner, 2011).

Biochemical identification

Coagulase-positive staphylococci were characterized by Gram staining, catalase production, and salt mannitol agar (Himedia) growth. The trial assay included the following: the Voges-Proskauer test (VM VP; Himedia), the maltose and trehalose fermentation test, and the susceptibility to polymyxin B test (300 μg, NewProv) (MacFaddin, 2000). ATCC 25923 and ATCC 19095 were used as controls.

Antibiotic susceptibility test and minimum inhibitory concentrations

Antibiotic susceptibility was determined according to the recommendations of Clinical and Laboratory Standards Institute (CLSI, 2011) by the disc-diffusion method (Bauer et al., 1966). Eleven antimicrobials (NewProv) that are commonly used in the treatment of clinical infections and in broiler production were tested: penicillin 10 U, gentamicin 10 μg, clindamycin 2 μg, ciprofloxacin 5 μg, trimethoprim–sulfamethoxazole 25 μg, cephalothin 30 μg, rifampin 5 μg, chloramphenicol 30 μg, oxacillin 1 μg, vancomycin 30 μg, and teicoplanin 30 μg. ATCC 25923 was used as a control, and all tests were repeated three times. The minimum inhibitory concentrations (MICs) were determined by the broth microdilution method, also in accordance with CLSI guidelines, in order to confirm vancomycin resistance. Vancomycin concentrations of 512 μg/mL and subsequent 1:1 serial dilutions were tested. These tests were also repeated three times.

PCR assays

DNA extractions were performed as previously described (Fredricks and Relman, 1998). Polymerase chain reaction (PCR) assays were carried out in order to detect the presence of the coagulase coa gene (Hookey et al., 1998), staphylococcal enterotoxin (SE) sea, seb, sec, sed, and see genes (Moura et al., 2012), vancomycin resistance vanA and vanB genes, (Depardieu et al., 2004), vanC1 (Dutkamalen et al., 1995) and vanC2/3 (Satake et al., 1997), and the 16S-rDNA region of methicillin-resistant S. aureus (Forsman et al., 1997). ATCC 13565, ATCC 23235, ATCC 14458, ATCC 19095, and ATCC 27664 were used as controls.

Results

Staphylococcal isolation and characterization in Southern Brazilian chicken meat samples

The presence of staphylococcal species was determined in 15 frozen and 15 chilled industrialized, uncooked chicken parts or entire carcasses, originated from the cities of Cruz Alta and Porto Alegre (State of Rio Grande do Sul, southern Brazil). Staphylococcal counts revealed that frozen chicken meat samples displayed the lowest counting mean, 348 colony-forming units (CFU)/g (standard deviation [SD] of 295 CFU/g), when compared with chilled chicken meat samples, which displayed a mean of 5315 CFU/g (SD of 5670 CFU/g) (p<0.01). It was possible to recover a total of 50 CPS from 20 chicken meat samples. Only CPS were present in the remaining 10 chicken meat samples. A maximum of three different species was recovered per sample. Staphylococcus aureus was isolated from 13 of 20 samples. The coa gene was not identified by PCR in 2 of the 50 phenotypically CPS, while the other strains were positive for coa PCR amplification. The isolates were morphologically and biochemically characterized and classified as follows: 31 (62%) S. aureus, 5 (10%) S. intermedius, 5 (10%) S. delphini, 5 (10%) S. schleiferi subsp. Coagulans, and 4 (8%) S. hyicus. All species were homogeneously distributed chilled and frozen samples, except S. hyicus, which was encountered only in chilled chicken meats. Twenty-two CPS were isolated from frozen chicken meat samples, whereas only 28 CPS were isolated from chilled samples.

Widespread distribution of enterotoxin genes among staphylococcal isolates

Table 1 shows the distribution of enterotoxin genes among CPS species. Thirty-five (70%) of the 50 strains were positive for one or more enterotoxin-encoding genes. The sea gene was the most prevalent (64%), followed by sed (26%), seb (6%), and sec (4%); none of the isolates harbored see. While sea was homogeneously present among the five species, seb was encountered only in S. hyicus and S. delphini, sec only in S. aureus, and S. intermedius and sed in all staphylococcal species except S. delphini. Eleven strains (22%) exhibited two or more genes. One S. aureus was positive for sea, sec, and sed and one S. hyicus harbored sea, seb, and sed genes. No statistically significant difference in the prevalence of enterotoxin genes was encountered between frozen and chilled samples.

Table 1.

Distribution of Staphylococcal Enterotoxin Genes Among Coagulase-Positive Staphylococci (CPS) Species Isolated from Chicken Meat Samples in South Brazil

	Number of isolates
CPS species	a	c	d	ab	ad	abd	acd	None
Staphylococcus aureus	15	—	2	—	4	—	1	10
S. schleiferi subsp. coagulans	—	—	—	—	3	—	—	1
S. hyicus	—	—	—	—	1	1	—	2
S. delphini	1	—	—	2	—	—	—	2
S. intermedius	3	1	—	—	1	—	—	—

a, Staphylococcal enterotoxins A; b, staphylococcal enterotoxins B; c, staphylococcal enterotoxins C; d, staphylococcal enterotoxins D.

Antibiotic resistance

Antibiotic susceptibility of staphylococci strains is shown in Table 2. In the present study, a high prevalence of resistance to penicillin (72%), teicoplanin (30%), oxacillin (18%), and clindamycin (16%) was observed. Twenty percent of the isolates were sensitive to all antibiotics, while 40% were resistant to one class of antibiotics, 28% to two classes, and 12% to three, four, or five classes. Staphylococci resistant to three or more classes of antibiotics were classified as multiresistant.

Table 2.

Resistance Profile of Coagulase-Positive Staphylococci (CPS) Species Isolated from Chicken Meat Samples in South Brazil

	Antibiotic and number of CPS resistant
CPS species	PEN	GEN	CLI	CIP	SUT	CFL	RIF	CLO	OXA	VAN	TEC	None
Staphylococcus aureus (n=31)	27	—	5	1	1	1	2	1	3	1	10	3
S. delphini (n=5)	2	—	—	—	1	—	—	—	2	—	—	3
S. schleiferi subsp. coagulans (n=5)	2	—	—	—	2	—	—	—	2	—	—	2
S. intermedius (n=5)	2	2	3	—	—	—	1	—	2	2	2	2
S. hyicus (n=4)	3	—	—	—	—	—	—	—	—	—	3	—
Total	36	2	8	1	4	1	3	1	9	3	15	10
(%)	(72)	(4)	(16)	(2)	(8)	(2)	(6)	(2)	(18)	(6)	(30)	(20)

PEN, penicillin 10 UI; GEN, gentamicin 10 μg; CLI, clindamycin 2 μg; CIP, ciprofloxacin 5 μg; SUT, trimethoprim–sulfamethoxazole 25 μg; CFL, cephalothin 30 μg; RIF, rifampin 5 μg; CLO, chloramphenicol 30 μg; OXA, oxacillin 1 μg; VAN, vancomycin 30 μg; TEC, teicoplanin 30 μg.

All S. aureus isolates were susceptible to gentamicin, and S. intermedius presented resistance to penicillin, gentamicin, clindamycin, rifampin, oxacillin, vancomycin, and teicoplanin. S. delphini and S. scheiferi subsp. coagulans strains were resistant to penicillin, oxacillin, and trimethoprim–sulfamethoxazole, while S. hyicus was resistant to penicillin and teicoplanin. Nine isolates were resistant to oxacillin: two S. delphini, two S. scheiferi subsp. coagulans, two S. intermedius, and three S. aureus. Only one isolate recovered from chilled chicken meat was identified by PCR as methicillin-resistant S. aureus (MRSA).

This study unexpectedly identified three vancomycin-resistant staphylococci (VRS) belonging to two species recovered from different chilled chicken meat samples. These VRS were submitted to broth microdilution, and two vancomycin-resistant S. intermedius (VRSI) isolates presented an MIC of 64 μg/mL, while the MRSA and VRSA isolate presented an MIC of 512 μg/mL. All three of the VRS carried vanA, vanB, and vanC2/3 genes. No isolates were positive for the vanC1gene.

The VRSA isolate was also resistant to penicillin, oxacillin, clindamycin, rifampin, cephalothin, and teicoplanin and did not harbor any classical enterotoxin genes. Both VRSI harbored the sea gene and presented resistance to penicillin, gentamicin, clindamycin, oxacillin, and teicoplanin, differing only in rifampin resistance.

Discussion

Staphylococcal isolation, quantification, and biochemical characterization

To evaluate CPS counting, we performed isolation from frozen and chilled samples. As expected, the difference in counting means between frozen and chilled samples was observed, since lower temperatures more efficiently inhibit bacterial growth (Al-Jasser, 2012). These counts show that the presence of staphylococci in chicken meat is significant.

S. aureus was the most abundant species in both frozen and chilled chicken meat samples, corroborating previous results described by other authors (Aarestrup et al., 2000; Citak and Duman, 2011). The presence of S. aureus in poultry meat might indicate contamination by handlers or even failures during meat processing. Studies have found that S. aureus isolated from food-poisoning sources, healthy nasal carriers, and infected samples shared the same staphylococcal protein A type (Wattinger et al., 2012), which may represent a major focus of contamination, since S. aureus was found to colonize the nasal mucosa of 30% of the food handlers investigated (Acco et al., 2003). In Turkey, Altay et al. (2003) reported the presence of CPS species in chicken meat; of the 46 CPS isolated, 60.9% were S. aureus, 19.6% were S. delphini, 6.5% were S. intermedius, 4.3% were S. aureus subsp. anaerobius, 2.2% were S. schleiferi subsp. coagulans, 2.2% were S. hyicus, and 4.3% remained unidentified.

Another important set of data described in the present study demonstrated that the coagulase-enzyme phenotype was not correlated with the coa gene, since two isolates with coagulase activity did not carry the gene. The absence of amplicons in these two isolates might have been engendered either by mutations in the primer annealing region or by the presence of pseudocoagulase genes (Chomarat and Flandrois, 1984; Bulanda et al., 1988). Moura et al. (2012) also found staphylococci that were positive to the coagulase test but were negative to identification of the coa gene.

SE prevalence

The enterotoxigenicity of S. aureus isolated from diverse foods has been described by several authors. However, few studies have reported the presence of enterotoxigenic non-S. aureus CPS recovered from uncooked chicken meat. Kitai et al. (2005) analyzed enterotoxigenic S. aureus in retail uncooked chicken meat in Japan. Among them, seb was the most prevalent gene, followed by sea and sec. In the present study, the most prevalent genes were sea and sed. This fact is remarkable because, among the described SEs and SE-like proteins, SEA and SED are the enterotoxins that are most typically involved in foodborne outbreaks (Casman, 1965; Genigeorgis, 1989). Moreover, in our analysis, 70% of the isolates harbored at least one enterotoxin gene. Similarly, other studies also found a high prevalence of enterotoxin genes among S. aureus isolates, such as 60.1% (Guven et al., 2010), 67% (Morandi et al., 2007), 68.4% (Rall et al., 2008) and 69% (Pereira et al., 2009); sea was the most prevalent gene in the latter two studies.

Regarding non-S. aureus CPS, S. intermedius was responsible for an outbreak of food poisoning in the United States (Khambaty et al., 1994), and S. hyicus was capable of inducing emetic activity in monkeys (Adesiyun et al., 1984). S. delphini was the etiological agent of a diarrhea outbreak on a commercial mink farm in the United States, causing the death of 2000 animals (Sledge et al., 2010). However, one study reported that S. schleiferi subsp. coagulans isolated from bovine and ovine mastitis was not enterotoxigenic (Almeida, 2009).

Antibiotic resistance

Antibiotic resistance rates were high, especially in relation to penicillin, teicoplanin, oxacillin, and clindamycin. Similar results were found by other authors (Guven et al., 2010; Pu et al., 2011). On the other hand, only one MRSA was recovered. The isolation of MRSA from raw meats was rare in both the present study and in others (Lee et al., 2008; Pereira et al., 2009). The spread of antibiotic resistance among broilers might occur due to the use of antimicrobials for therapeutics and growth promotion in chicken breeding (Butaye et al., 2003).

In foods, resistant bacteria have very often been encountered (Valsangiacomo et al., 2000), and antibiotic-resistant specimens in foods are a safety hazard for many reasons. First, resistant bacteria in foods can cause diseases. For example, in a community-acquired MRSA case, a hospital patient was infected by ingesting a banana containing nasal carriage of MRSA. It was later discovered that the person who prepared the hospital meals was an asymptomatic carrier of MRSA (Kluytmans et al., 1995). Another case involved an outbreak caused by the ingestion of baked pork meat contaminated with MRSA by the food handler (Jones et al., 2002). Second, foods may be a reservoir of resistant bacteria. Gundogan et al. (2005) analyzed raw veal and lamb samples as well as uncooked chicken parts. Of 80 S. aureus isolates, 67.5% were resistant to methicillin, 53.8% to penicillin, and 87.5% to bacitracin. Finally, nonpathogenic bacteria can transfer resistance genes to pathogenic and opportunistic bacteria (Sorum and L'Abee-Lund, 2002). Nawaz et al. (2000) suggested that the transfer of genetic determinants from poultry staphylococci to human microbiota is possible.

To our knowledge, this is the first report on VRSA isolated from foods. In addition, this is also the first case of VRSI reported in this species. The findings of VRSA and VRSI in chicken meats are a motive of concern, as they may suggest the spread of such microorganisms or their genetic material outside clinical boundaries. The spread of staphylococcal antimicrobial resistance in foods constitutes a potential risk for the community. Furthermore, the possible correlation between strains present in hospitals and those encountered in foods must be considered. MRSA represents a challenge for the treatment of frequent S. aureus infections acquired in hospitals. In particular, vancomycin resistance stands out rather prominently in this scenario (Boucher et al., 2010).

Conclusions

Coagulase-positive staphylococci isolated from uncooked chicken meat might represent a food safety hazard. The finding of VRSA and VRSI harboring the vanA, vanB, and vanC genes and presenting MICs of 512 and 64 μg/mL, respectively, is particularly alarming, given the difficulties in the treatment of vancomycin-resistant staphylococci and the potential spread of these genetic determinants. However, not only should S. aureus species receive attention from public health authorities, but other CPS species should also be monitored. In the present study, non–S. aureus species demonstrated remarkable enterotoxigenicity and antibiotic resistance. Many studies do not properly distinguish S. aureus from other CPS species because their protocol for S. aureus identification is based on tests for which all CPS are positive. Therefore, we ask whether security issues regarding other CPS species have been underestimated due to the mistaken attribution of enterotoxin genes, antibiotic resistance, and other virulence factors to S. aureus instead of CPS. More studies are necessary to determine the risks that CPS species imply for food safety. Currently, Brazil and the United States are world leaders in chicken meat exportation (Brazil provides chicken meat for over 180 countries). In addition, most of the slaughterhouses in Brazil are located in the southern region. Therefore, these data must be considered.

Footnotes

Acknowledgments

We would like to thank the National Council of Technological and Scientific Development of Brazil (CNPq) for sponsoring this research (JF grant #300912/2012-9).

Disclosure Statement

No competing financial interests exist.

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