Occurrence and antimicrobial susceptibility of Salmonella isolates recovered from the pig slaughter process in Romania

Introduction: Reported human salmonellosis cases have increased in Romania. Antibiotic susceptibility testing of Salmonella strains isolated from pork and chicken meat indicate a worrying multidrug resistance pattern. This study aimed to investigate the occurrence of Salmonella and to evaluate the antibiotic resistance of Salmonella strains in a pig slaughterhouse-processing complex, which receives animals from 30% of the large industrialized swine farms in Romania. Methodology: A total of 108 samples, including pork (n = 47), packaged pork products (n = 44), scald water sludge (n = 8), and detritus from the hair removal machine of the slaughterhouse (n = 9) were examined for the presence of Salmonella through standard methods. The antibiotic susceptibility of the isolated strains to 17 antibiotics was tested using the Vitek 2 system. Results: Twenty-six (24.1%) samples were found to be Salmonella positive; this included 25.5% of meat samples and 15.9% of packaged products, as well as samples from two different points of the slaughter (41.2%). Resistance was observed against tetracycline (61.5%), ampicillin (50%), piperacillin (50%), trimethoprim-sulfamethoxazole (34.6%), amoxicillin/clavulanic acid (26.9%), nitrofurantion (23.1%), cefazolin (15.4%), piperacillin/tazobactam (7.7%), imipenem (3.8%), ciprofloxacin (3.8%), and norfloxacin (3.8%). No resistance towards cefoxitin, cefotaxime, ceftazidime, cefepime, amikacin, and gentamicin was found. Conclusions: Our study demonstrated the occurrence of multidrug-resistant Salmonella strains in the investigated pork production complex and highlighted it as a potential source of human infections. The results demonstrate the seriousness of antibiotic resistance of Salmonella in Romania, while providing a useful insight for the treatment of human salmonellosis by specialists.


Introduction
Members of the genus Salmonella are recognized to be one of the most important zoonotic pathogens, causing serious foodborne illnesses in humans worldwide.Human salmonellosis outbreaks are related to the consumption of contaminated foodstuffs of animal origin, and the disease is characterized by acute gastroenteritis, fever, abdominal pain, and nausea [1,2].
Based on a recent European Union summary report, Salmonella has been the leading cause (26.6%) of reported foodborne outbreaks in 2011, with 95,548 confirmed human clinical cases [2].The results of several Salmonella surveys, carried out in pig slaughtering and processing plants, have shown that fresh pig meat and pork products thereof can be considered an important vehicle for the transmission of Salmonella to humans [3][4][5][6].The application of misused and prolonged antibiotic treatments in farm animals with therapeutic and prophylactic purposes can lead to the development of antimicrobial-resistant commensals and/or zoonotic foodborne bacterial pathogens, including Salmonella [7].This undesired consequence is frequently associated with the transmission of resistant bacteria populations to humans, which may pose significant therapeutic challenges for clinicians treating diseases.Therefore, the continuous surveillance of the antimicrobial resistance of zoonotic bacterial strains such as Salmonella isolated from food and production environments can contribute greatly to a successful control program.
Swine breeding under an intensive production system is considered to be the most important economic sector of the pig industry in Romania.The survey of the occurrence and antibiotic resistance pattern of Salmonella strains isolated from pork is limited to only one study carried out in various regions of the country [6].Therefore, additional studies in this regard are still needed.
The aim of this study was to investigate the occurrence of Salmonella and to evaluate the antibiotic resistance of Salmonella strains isolated in a pig slaughterhouse-processing plant complex, which receives animals from 30% of the large industrialized swine farms in Romania.

Methodology
In the first 24 hours after slaughter, randomly selected pig meat samples provided from different carcass regions (n = 37), including haunches (n=4), sow flaps (n = 4), and tongues (n = 2) in the cutting and deboning room of the slaughterhouse were collected during several sampling days.In addition, packaged products were also provided by the processing plant, which included minced meat (n = 14), traditional Romanian peasant sausages (n = 15), and mici pasta (n = 15).Sampling was performed weekly for a period of six months.Sludge from the water used in the scalding process (n = 8) (a vertical scalding operation point with regular changing of the scald water) and detritus from a hair-removing machine (n = 9) of the slaughterhouse were also collected on different sampling days and further processed for Salmonella detection.
The samples were individually sterile packed, stored under refrigeration and transported to the laboratory of Food Hygiene of the Faculty of Veterinary Medicine Timişoara, Romania.All samples were processed on the day of arrival and Salmonella strains were isolated according to the ISO 6579/2006 standard.
Briefly, for pre-enrichment, 25 g of each sample was homogenized in a Stomacher (bioMérieux, Marcy l'Etoile, France) (90 seconds) with 225 mL of peptone buffered solution (PBS) and incubated at 37°C for 18 hours.Next, 0.1 mL aliquot from the pre-enriched culture was transferred to 10 mL of Rappaport-Vassiliadis (Biokar Diagnostics, Beauvais, France) broth and incubated at 41.5°C for 24 hours.At the same time, 1 mL of the pre-enriched culture in PBS was further cultured on Müller-Kauffmann Tetrathionate Novobiocin Broth (MKTTn; Biokar Diagnostics) selective enrichment medium at 37°C for 24 hours.Next, the samples were streaked onto solid xylose lysine deoxycholate (XLD; Biokar Diagnostics) and Rambach (Biokar Diagnostics) mediums and incubated again at 37°C for 24 hours.Salmonella colonies from each positive plate were selected, transferred and cultured on brain-heart infusion (Biokar Diagnostics) agar at 37°C for 24 hours.After growth, the isolates were confirmed to be Salmonella on the basis of their biochemical properties including indole, hydrogen sulphide and urease production, the fermentation of glucose and lactose, and lysine decarboxylation.The serological properties were tested using the Salmonella antisera O poli group (Statens Serum Institut, Copenhagen, Denmark).
Additionally, all strains were molecularly tested for the presence of the Salmonella-specific invA gene (~285 bp) through the polymerase chain reaction (PCR) protocol according to Rahn et al. (1992) with slight modifications [8].Amplification products were analyzed on 1.5% agarose gel stained with ethidium bromide.

Results
Overall, 26 (24.1%; 95% confidence interval [CI] 16.5%-33.4%)of the 108 collected samples were found to be Salmonella positive through standard examination methods.All isolated bacterial strains were successfully amplified targeting the invA gene, confirming the results of the standard method.The obtained PCR products showed typical profiles for Salmonella genus in the 1.5% agarose gel.The distribution of the isolated Salmonella strains according to their origin is summarized in Table 1.Salmonella was recovered from pork samples provided from tongues (1/2; 50%), carcasses (10/37; 27%), and sow flaps (1/4; 25%), but no bacteria were found in haunches.From the packaged meat products, Salmonella was isolated from peasant sausages (4/15; 26.6%) and minced meat (3/14; 21.4%), while the mici pasta was found to be Salmonella-free.With regards to sludge and detritus collected from two different operation points of the slaughterhouse, Salmonella was isolated from both scalding (1/8; 12.5%) and hair removal (6/9; 66.6%) points.
The results of antibiotic resistance and susceptibility patterns of the isolated Salmonella strains are presented in Table 2.The Vitek 2 system proved to be an accurate, rapid (

Discussion
This is the first report about the occurrence and antibiotic susceptibility testing of Salmonella isolates recovered from the slaughter process in Romania.The current survey confirmed the presence of Salmonella strains in pork samples (12/47; 25.5%), packaged meat products (7/44; 15.9%), and at two different points along the swine slaughter line (7/17; 41.2%).The relatively high contamination rate (19/91; 20.9%; CI 13.3-30.9)with Salmonella of pig meat and products recorded in the current survey pointed out that these foodstuffs can be considered an important vehiclefor Salmonella transfer from animals to humans.
Results of other studies that aimed to detect Salmonella in fresh pig meat and pork products provided by slaughterhouses and processing plants have largely varied, depending on the study design, number of enrolled samples, or sensitivity of the diagnostic tests used.In accordance with results obtained in the current survey, data reported in the most recent European Union surveillance program showed the presence of Salmonella in fresh pig meat provided by slaughterhouses in Slovakia (3.3%) and Spain (7.5%).Also, Salmonella surveys conducted at cutting and processing plants yielded positive findings in Hungary (3.0%), Portugal (5.0%), Italy (0.7%) and Estonia (0.4% and 0.9%).In contrast, no Salmonellapositive samples were detected in pig meat provided from a slaughterhouse in Bulgaria and from cutting and processing plants in Finland [2].[13].These results highlight the need to improve the quality of raw materials provided by pork slaughterhouses in a screened complex, with particular emphasis on the most important parameters and procedures in each stages, such as timetemperature couples, handling, or general hygiene [9,14].
Considering that Salmonella on the skin of pigs entering the slaughter line is a common finding [3,15], the low Salmonella isolation rate recorded at the scalding point of the investigated slaughterhouse suggests the efficiency of scalding in Salmonella reduction, as was previously demonstrated by Davies et al. (1999).On the other hand, contrary to what would be expected, a relatively high frequency isolation rate of Salmonella was observed in the detritus from the hair removal equipment.As described by various authors [1,16], carcass contamination during the hair removal process can occur, especially through the squeezing of feces from the anus, followed by bacterial proliferation in the detritus and subsequent contamination of carcasses.Regarding this issue, significant improvement of the processing technology at this level should be encouraged and implemented.
Antimicrobial susceptibility testing showed multidrug resistance towards at least two tested antimicrobials for 22/26 (84.6%) isolated and tested Salmonella strains.Only three strains were susceptible to all tested antibiotics and another strain showed resistance towards one drug (Table 2).Antimicrobial resistance towards 11 tested drugs was observed, with a variable proportion from 3.8% to 61.5%.High resistance levels to tetracycline (61.5% of the isolates), ampicillin (50%), piperacillin (50%) and trimethoprim-sulfamethoxazole (34.6%) were found.In agreement with our results, the highest level of resistance to tetracycline has been reported in Portugal (70%; 17), Italy (56%; 18) and Brazil (70.7%; 13) for Salmonella strains isolated from different products and levels of the pre-and post-harvest pig meat chain.Also, high resistance frequency was obtained in various studies for ampicillin [7,17,19] and sulfonamides compounds [13,[17][18][19].Moreover, marked resistance of pig meat origin Salmonella isolates towards ampicillin (56.2%), sulfonamides (54.5%), and tetracyclines (52.8%) was published by the European Food Safety Authority (EFSA) in its most recent summary report [20].It is important to mention that in Romania, in a previous study conducted by Mihaiu et al. (2014), similar to our findings, high resistance levels were obtained for tetracycline, sulphonamides, ampicillin, and trimethoprim [6].These results suggest the possible over-usage of these drugs in the pig breeding industry in Romania.On the other hand, the lack of the resistance to cephalosporins (cefoxitin, cefotaxime, ceftazidime, cefepime) and aminoglicosides (amikacin, gentamicin) can provide useful insight for the treatment of human salmonellosis by public health specialists.

Conclusions
Multidrug-resistant Salmonella strains in the slaughterhouse and processing phases of the pig meat chain of the investigated meat production complex were found to be a potential source for human infections.The antimicrobial resistance pattern showed increased resistance to some antibiotics, but decreased or a lack of resistance to others, suggesting some value in the treatment of salmonellosis in humans.However, taking into account the methodological limitations of the current survey and in order to have a more comprehensive understanding of the epidemiology of Salmonella infections, including the complexity of transmission or the presence of clinically relevant strains, further studies based on serotyping on a larger scale are still needed.On the other hand, even if the number of enrolled samples is limited, the results of the present study indirectly support the increasing trend of reported human salmonellosis cases since 2007 in Romania [2].
18 hour), and relatively easy test to use for the determination of antimicrobial susceptibility of the bacterial strains.

Table 1 .
Distribution of the isolated Salmonella strains according to their origin [9]09)reported the presence of Salmonella ranging from 0.3% to 4.3% in porcine minced meat from Belgium[9], and Mürmann et al.(2009)reported that Salmonella was found in 24.4% (82/336) of fresh pork sausages from Brazil