Prevalence and Characteristics of Salmonella Isolated from Free-Range Chickens in Shandong Province, China

Compared with chickens raised in intensively managed breeding farms, free-range chickens in China are quite popular due to lower breeding density and less antibiotics usage. However, investigations about Salmonella enterica from free-range chickens are quite rare. The aim of the present study was to investigate prevalence and characteristics of Salmonella in free-range chickens in Shandong province, China. During the period of August and November 2015, 300 fresh fecal swabs from different broilers in three free-range chicken farms (100 samples per farm) were collected to isolate Salmonella, and then these isolates were subjected to serotyping, antibiotic sensitivity testing, enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR), and multilocus sequence typing (ST). A total of 38 Salmonella isolates (38/300, 12.7%) were recovered. The most common serotype was Enteritidis (81.6%), followed by Indiana (13.2%) and Typhimurium (5.3%). Twenty-two out of 38 isolates (57.9%) were resistant to ampicillin, the highest resistance rate, but resistance rates to cefazolin, cefotaxime, and ceftazidime were only 7.9%. The multidrug resistance (MDR) rate was 26.3%. Additionally, the Salmonella isolates could be classified into 25 genotypes by ERIC-PCR and were divided into three ST types (ST11, ST17, and ST19), with ST11 the highest isolation rate (81.6%). In summary, as with other poultry, free-ranging chickens may also serve as potential reservoir for antibiotic resistant Salmonella, thereby posing a threat to public health.


Introduction
Salmonella enterica is one of the most important pathogenic bacterial causes of food-borne diseases [1]. At present, more than 2,600 serotypes were identified, and most serotypes can cause food-borne infection [2]. Salmonella infections can result in gastrointestinal problems such as gastroenteritis, typhoid fever, and paratyphoid fever or even cause death in serious infections, especially for younger people and the elderly [3][4][5]. In China, bacterial food-borne disease cases are frequently caused by Salmonella, and Salmonella in foodproducing animals is a threat to public health [6,7]. Foodproducing animals especially chickens and pigs are regarded as the most important Salmonella carriers. Epidemiological investigations of Salmonella from chickens, pigs, and their meat products have shown that drug resistance of Salmonella has been increasing in China and other countries [7][8][9][10].
In recent years, free-range chicken farms are major contributors to organic food production in Shandong province, China. The free-range chicken farms are divided by spatially separated (chickens and crops are physically isolated), rotational (chickens can roam into field only after crops are harvested), and fully combined (chickens and crops are allowed to interact freely) farming. Importantly, although the chickens are purchased from local commercial poultry hatchery, fewer antibiotics are used for therapy or growth promotion. Compared with chickens raised in intensively managed breeding farms, free-range chickens in China are quite popular due to lower breeding density and less antibiotics usage.
However, investigations about Salmonella from freeranging chickens are quite rare in China. Therefore, the aim of the present study was to isolate Salmonella from freeranging chickens in Shandong province, China, and then these isolates were subjected to serotyping, antibiotic sensitivity testing, enterobacterial repetitive intergenic consensuspolymerase chain reaction (ERIC-PCR), and multilocus sequence typing (ST).

Sample Collection and Process. Between August and
November 2015, 300 fresh fecal swabs of chickens aged about 6 weeks (100 samples per farm) were obtained from three free-range chicken farms (Jinan, Linyi, and Laiwu) (about 500 chickens per farm) in Shandong province, China. Fresh fecal samples were collected from near the chickens using sterile cotton swabs. After sampling, the swabs were transported to our lab in an ice box and processed within 6 h. The cotton swabs were immediately put into vials containing 100 mL of germ-free BPW media and shaken sufficiently at 37 ∘ C for 18 to 20 h. Then 0.5 mL and 0.1 mL of the enrichment broth were added to 10 mL of Tetrathionate Broth (TT, Becton-Dickinson, USA) and Rappaport-Vassiliadis Medium (RV, Becton-Dickinson, USA), respectively, at 42 ∘ C ± 1, 100 ppm for 22 to 24 h. Then, the bacteria in TT and RV were inoculated to xylose lysine tergitol 4 (XLT4, Becton-Dickinson, USA) agar and incubated at 37 ∘ C ± 1 overnight. Suspected Salmonella colonies were then inoculated to trisaccharide agar slant and incubated at 35 ∘ C ± 1 for 24 h. The typical Salmonella colonies were identified by the VITEK system (BioMerieux, Marcy l'Etoile, France) and then verified by PCR amplification of inherent gene invA [11].

Salmonella Serotyping. Commercial serodiagnosis kits
for Salmonella (Ningbo Tianrun Bio-pharmaceutical Co., Ltd., China) were used to carry out the plate agglutination tests and identify serotypes of Salmonella. The serotyping scheme was referenced to the Kauffman-White salmonella serotyping scheme [12].

Discussion
Salmonella is an important pathogen of great importance in public health. Poultry are considered to be important carrier of Salmonella [17,18]. Numerous studies have been conducted on Salmonella isolated from intensive breeding chicken farms, slaughter houses, and chicken meats [2,[19][20][21], but little information is available on free-range chickens.
In the present study, the overall isolate rate of Salmonella in three free-range chickens was 12.7%, which was lower than that in intensive chicken farms in China (>35%) [22]. Numerous Salmonella serotypes are pathogenic, including S. enterica serovar Enteritidis and S. enterica serovar Typhimurium [23]. The most common serotype identified in the present study was S. enterica serovar Enteritidis (81.6%). It was consistent with investigation results from the intensively managed chicken farms in the Henan and Sichuan areas of China [24,25]. But the most common isolated Salmonella from the intensively managed chicken farms in Cambodia, Vietnam, and South Korea were S. enterica serovar Anatum, S. enterica serovar Infantis, and S. enterica serovar Hadar, respectively [17,26,27]. The difference of the Salmonella serotype distribution may mainly be related with area differences.
Compared with the resistant rate of Salmonella from intensive poultry farms in Shandong province, China, in 2012, in the present study, the resistant rate of these 38 Salmonella isolates against ampicillin, kanamycin, co-trimoxazole, and tetracycline was greatly lower (36.1-72.3% versus 26.3-57.9%) [28]. This apparent difference may be attributable to less drug use in free-range chickens. Of note, Salmonella isolates in this study were relatively sensitive to cephalosporin antibiotics Table 3: Antimicrobial resistance patterns of Salmonella isolated from three free-range chicken farms. Number of resistant isolates  Enteritidis  Indiana  Typhimurium  Total  AMK-AMP-CAZ-CFZ-CHL-CIP-CTX-DOX-GEN-K-NOR-RAD-SXT-TET  0  2  0  2  AMK-AMP-CHL-CIP-DOX-GEN-NOR-K-SXT-TET  0  1  0

Resistant patterns
(resistance rate, 7.9-13.2%), and these values are lower than the recorded values for cephalosporin antibiotics resistance in chickens raised in intensively managed farms in Shandong province, China (resistance rate, about 42% in 2012) [28]. Our findings showed that 26.3% of Salmonella isolates were MDR. This was lower than that in intensive poultry farms of Henan (46.0%) province in China [24]. Of note, in this study 2 out of 5 S. enterica serovar Indiana isolates were resistant to 14 antibiotics, and they were not only resistant to streptomycin and tetracycline but also resistant to chloramphenicol, fluoroquinolones, and cephalosporin antibiotics. The antibiotic resistance difference of Salmonella isolated from three freerange chicken farms may be due to the fact that the use of antibiotics in different regions is quite different and the distribution of the antibiotics in the environments, such as waters and soils, has a great effect on drug resistance of Salmonella.
The result of ERIC-PCR showed that the 38 Salmonella isolates were classified into 25 gene types, which indicates that the sources of Salmonella isolates in these flocks were diverse. Compared with Salmonella isolates from Laiwu, the isolates from Jinan and Linyi had lower genetic similarity. The difference should be further studied. The fact that isolates from Laiwu had higher genetic similarity may be associated with clonal spread of Salmonella. Thirty-eight Salmonella isolates were classified into 3 ST types (ST11, ST17, and ST19). All three types have also been isolated from human samples [29][30][31], suggesting that these Salmonella could be spread between human beings and chickens via food chain and threaten the human health.

Conclusions
Collectively, Salmonella isolated from free-ranging chickens showed relatively lower resistance rates than those raised in intensively managed chicken farms of China. These results also revealed that free-ranging chickens may serve as a potential reservoir for antibiotic resistance Salmonella, thereby posing a threat to public health.