Identification and molecular characterization of Salmonella spp. from unpasteurized orange juices and identification of new serotype Salmonella strain S. enterica serovar Tempe

Several Salmonella enterica serotypes were isolated from unpasteurized orange juice samples analysed as a follow-up to an outbreak in 1999 of S. enterica serotype Muenchen in the Pacific Northwest regions of United States. Eleven S. enterica strains were serotyped and identified as S. enterica serotype Muenchen (2), S. enterica serotype Hidalgo (2), S. enterica serotype Alamo (1), S. enterica serotype Gaminera (2), S. enterica serotype Javiana (2) and a new serotyped strain S. enterica serotype Tempe (2). The identity of the new serotype S. enterica serovar Tempe serotype 30:b:1,7:z33 was confirmed by the National Salmonella Reference Laboratory at NCID/CDC, Atlanta. These strains were sensitive to ampicillin, chloramphenicol, kanamycin, tetracycline, streptomycin and sulfisoxazole antibiotics. Isolates were screened for invasion (invA) and virulence (spvC) genes using specific primers for these two genes by polymerase chain reaction. All strains were positive for invA gene giving 321-bp fragment, however negative to virulence spvC gene. For pulsed-field gel electrophoresis (PFGE) analysis, Salmonella strain plugs were made and digested with XbaI and subjected to 18-h electrophoresis. The PFGE patterns were different for each S. enterica serotypes suggesting the several origins of contamination in outbreak. S. enterica serotype.     

Molecular characterization of Salmonella enterica serovar Saintpaul isolated from imported seafood, pepper, environmental and clinical samples

A total of 39 Salmonella enterica serovar Saintpaul strains from imported seafood, pepper and from environmental and clinical samples were analyzed for the presence of virulence genes, antibiotic resistance, plasmid and plasmid replicon types. Pulsed-field gel electrophoresis (PFGE) fingerprinting using the XbaI restriction enzyme and plasmid profiling were performed to assess genetic diversity. None of the isolates showed resistance to ampicillin, chloramphenicol, gentamicin, kanamycin, streptomycin, sulfisoxazole, and tetracycline. Seventeen virulence genes were screened for by PCR. All strains were positive for 14 genes (spiA, sifA, invA, spaN, sopE, sipB, iroN, msgA, pagC, orgA, prgH, lpfC, sitC, and tolC) and negative for three genes (spvB, pefA, and cdtB). Twelve strains, including six from clinical samples and six from seafood, carried one or more plasmids. Large plasmids, sized greater than 50 kb were detected in one clinical and three food isolates. One plasmid was able to be typed as IncI1 by PCR-based replicon typing. There were 25 distinct PFGE-XbaI patterns, clustered to two groups. Cluster A, with 68.5% similarity mainly consists of clinical isolates, while Cluster C, with 67.6% similarity, mainly consisted of shrimp isolates from India. Our findings indicated the genetic diversity of S. Saintpaul in clinical samples, imported seafood, and the environment and that this serotype possesses several virulent genes and plasmids which can cause salmonellosis.     

Antibiotic Resistance and Molecular Characterization of Seafood Isolates of Nontyphoidal Salmonella by PFGE

Emergence of multidrug resistant nontyphoidal Salmonella is a major health concern worldwide due to the predominant occurrence of Salmonella enterica sub-species enterica serovar Typhimurium phage type 104 (DT104) conferring resistance to ampicillin, chloramphenicol, streptomycin, sulphonamide and tetracycline. Apart from antibiotic resistance, the identification and genotypic characterization of pathogens is essential for epidemiological surveillance and outbreak investigations. In this study 39 isolates of Salmonella obtained from seafood samples were examined for their susceptibility to various antibiotics and subjected to PFGE analysis using the restriction enzyme Xba1. The highest percentage resistance was for erythromycin (100%) followed by nalidixic acid (15.38%), co-trimoxazole (15.38%), chloramphenicol (12.82%), ampicillin (12.82%) and tetracycline (10.25%). Six (15.38%) of the 39 isolates were multidrug resistant. The XbaI digested chromosomal DNA generated 7 clusters suggesting the presence of diverse Salmonella strains in seafood. The Discriminatory Index for PFGE obtained by XbaI restriction enzyme was 0.91. The PFGE has been found highly discriminatory for subtyping S. Weltevreden and S. Newport. The XbaI PFGE was not only discriminatory but could also distinguish multidrug-resistant strains from the sensitive ones as the two groups they belonged to different pulsotypes. The study also demonstrated multiple clones of S. Weltevreden, S. Newport and S. Oslo present in seafood from the south west coast of India. Genetic diversity among the similar seafood sources suggests the presence of different clones of Salmonella which further increases the risk of seafood being a potential source of highly pathogenic bacteria like Salmonella.     

Antimicrobial susceptibilities of Salmonella isolates obtained from layer chicken houses on a commercial egg-producing farm in Japan, 1997 to 2002

Susceptibility to antimicrobial agents was examined for 325 isolates of Salmonella enterica serotypes Cerro, Infantis, Livingstone, and Montevideo isolated from layer houses on a commercial egg-production farm in the western region of Japan between 1997 and 2002. No antimicrobials were used for therapeutic purposes on the farm during this period. From 1.8 to 3.1% of the isolates were resistant to ampicillin, chloramphenicol, and tetracycline. Resistance to streptomycin and sulfisoxazole was found in 52.9 and 65.5%, respectively, of Salmonella Montevideo isolates and in 0 to 13.2% of the isolates of the other serotypes. All the streptomycin-resistant isolates of Salmonella Montevideo also exhibited resistance to sulfisoxazole. Salmonella Montevideo isolates were first isolated in 1998, and 80.0% of the isolates obtained in this year were resistant to streptomycin and sulfisoxazole. The results suggest that Salmonella Montevideo isolates that had already acquired resistance may have been introduced into the layer houses, although the route and vehicle of transmission were uncertain. The proportion of Salmonella Montevideo isolates resistant to streptomycin and sulfisoxazole significantly decreased (P < 0.01) from 79.5% for 1998 through 1999 to 37.3% for 2000 through 2002. This decrease probably was due to the fact that no antimicrobials were used on the farm. Among 10 isolates from different serotypes obtained from 1997 to 2002 that were resistant to ampicillin, two and five isolates harbored 42- and 63-kb R plasmids and identical DraI restriction enzyme digest patterns, respectively, and carried the blaTEM gene. The results suggest that the ampicillin resistance determinants were transferred among different serotypes of Salmonella in the layer houses.     

Detection and characterization of Salmonella associated with tropical seafood

The prevalence of Salmonella in seafood samples collected from the southwest coast of India was studied by conventional culture and by a DNA based molecular technique, polymerase chain reaction (PCR). While conventional culture techniques detected Salmonella in only 20 out of the 100 samples analyzed, direct enrichment lysate PCR detected 52 as positive for Salmonella. A set of three different PCR primers viz., hns, invA and invE were used. It was observed that hns primer detected Salmonella in a significantly higher number of samples. Fourteen out of nineteen isolates belonged to serovar S. enterica Weltevreden. S. Weltevreden isolates were genotyped yielding 4 different patterns both by RAPD and ERIC-PCR but when combined, the overall results discriminated the isolates of S. Weltevreden into 6 different types. This suggests that genetically diverse Salmonella Weltevreden are prevalent in seafood.     

Characterization of antimicrobial-resistant Salmonella isolated from imported foods

Two-hundred eight Salmonella isolates recovered from over 5,000 imported foods entering the United States in 2001 were tested for antimicrobial susceptibilities and further characterized for quinolone resistance mechanisms, integron carriage, and genetic relatedness. Salmonella Weltevreden (20%), Salmonella Newport (6%), Salmonella Lexington (5%), and Salmonella Thompson (4%) were the four most common serotypes recovered. Twenty-three (11%) isolates were resistant to at least one antimicrobial, and seven (3.4%) to three or more antimicrobials. Resistance was most often observed to tetracycline (9%), followed by sulfamethoxazole (5%), streptomycin (4%), nalidixic acid (3%), and trimethoprim/sulfamethoxazole (2%). One Salmonella Schwarzengrund isolate recovered from squid imported from Taiwan exhibited resistance to eight antimicrobials, including ampicillin, chloramphenicol, gentamicin, kanamycin, nalidixic acid, sulfamethoxazole, tetracycline, and trimethoprim/sulfamethoxazole. Six isolates (Salmonella Bareilly, Salmonella Derby, Salmonella Ohio and three Salmonella Schwarzengrund) contained class 1 integrons, which carried several resistance genes including dhfrI/dhfrXII, aadA, pse-1, and sat1, conferring resistance to trimethoprim/sulfamethoxazole, streptomycin, ampicillin, and streptothricin, respectively. Five of six nalidixic acid-resistant isolates possessed DNA point mutations at either Ser83 or Asp87 in DNA gyrase. One ciprofloxacin-resistant isolate possessed double mutations in DNA gyrase at positions Ser83 and Asp87 as well as a single mutation at Ser80 in parC. The top three serotypes identified, Salmonella Weltevreden (n = 41), Salmonella Newport (n = 13), and Salmonella Lexington (n = 11), were further characterized for genetic relatedness by pulsed-field gel electrophoresis. Fifty-five distinct pulsed-field gel electrophoresis patterns were observed among the 65 isolates, indicating extensive genetic diversity among these Salmonella serotypes contaminating imported foods.     

Antimicrobial susceptibility of Salmonella isolated from various products, from 1999 to 2003

Foodborne salmonellosis continues to be a major health concern worldwide; thus, detection and tracking of antimicrobial resistance in Salmonella isolates is of interest. The U.S. Food and Drug Administration initiated antimicrobial sensitivity screening of Salmonella isolates from food and related samples in 1999. This paper summarizes the antimicrobial resistance data for Salmonella isolates obtained from 1999 to 2003. A total of 22,231 imported and domestic samples were analyzed for Salmonella, of which 1,319 (5.9%) yielded the pathogen. Since more than one culture was isolated from some samples, the total number of isolates obtained and tested for antimicrobial sensitivity was 1,382. Antimicrobial sensitivity screening was performed with the disc diffusion assay on 11 antimicrobial agents. Of the 1,108 food isolates screened, 42.1% (n = 467) were serotypes Weltevreden, Newport, Lexington, Senftenberg, Typhimurium, Saint Paul, Paratyphi, Enteritidis, Thompson, and Bareilly. A total of 249 (18.0%) isolates from all sources were resistant to two or more antimicrobials. Resistance to sulfisoxazole, streptomycin, and tetracycline was most common, whereas resistance to ciprofloxacin was least common. Weltevreden (n = 148) was the most common serotype isolated from food, but only nine (6.1%) of these isolates were resistant to two or more antimicrobials. In contrast, although Derby was recovered only 19 times, 11 (57.9 %) of these isolates were resistant to two or more antimicrobials. Of the 274 isolates from animal feed, dog treats and environmental swabs, 49.6% (n = 136) belonged to serotypes Infantis, Mbandaka, Anatum, Senftenberg, Typhimurium, Montevideo, Cerro, Enteritidis, and Bredeney, with 76 (27.7%) of these isolates resistant to two or more antimicrobials. Only limited trends in antimicrobial resistance were observed over time, with resistance to sulfisoxazole increasing, resistance to tetracycline decreasing, and resistance to streptomycin fluctuating.     

Antibiotic resistance profiles of Salmonella serovars isolated from retail pork and chicken meat in North Vietnam

The spread of antibiotic resistance via meat poses a serious public health concerns. During 2007-2009, a total of 586 retail meat samples (318 pork and 268 chicken meats) were collected from three provinces (Bac Ninh, Ha Noi and Ha Tay) of North Vietnam to determine the prevalence of Salmonella. Isolates were characterized by serotyping and antibiotic susceptibility testing. Approximately 39.6% (n=126) of pork and 42.9% (n=115) of chicken samples were Salmonella-positive, and 14 Salmonella serovars were identified. Anatum (15.8%) was the most common serovar, followed by Infantis (13.3%), Emek (10.4%), Derby and Rissen (9.5%), Typhimurium (9.1%), Reading (7.5%) and London (6.2%). The isolation frequency of serovars Enteritidis, Albany, Hadar, Weltevreden, Newport and Blockey ranged from 1.2%-5.8%. Resistance to at least one antibiotic agent was detected in 78.4% of isolates (n=189) and the most frequent resistance were to tetracycline (58.5%), sulphonamides (58.1%), streptomycin (47.3%), ampicillin (39.8%), chloramphenicol (37.3%), trimethoprim (34.0%) and nalidixic acid (27.8%). No Salmonella isolates were resistant to ceftazidime. Chicken isolates had higher resistance to antibiotic agents than pork isolates (P<0.05). It showed that 159 Salmonella isolates belong to the 14 serovars were multidrug resistant (MDR) and 50 MDR patterns were found. This study indicated that Salmonella serovars isolated from retail meat samples were resistant to multiple antibiotics and this resistance was widespread among different serovars. The widespread resistance may have arisen from misuse or overuse of antibiotics during animal husbandry in North Vietnam.     

Antimicrobial resistance and phage and molecular typing of Salmonella strains isolated from food for human consumption in Spain

The aims of this study were to ascertain the population structure and antimicrobial susceptibility of Salmonella enterica serovars isolated in 2002 from food in 16 Spanish regions. Serovars were characterized by serotyping, phage typing, antimicrobial susceptibility, and pulsed-field gel electrophoresis (PFGE) typing, and 264 nonrelated strains were selected for further analysis. The main sources were eggs and their derivatives (21.6% of strains), poultry and related products (16.6%), and seafood (16.3%). High serotype diversity was detected (51 serotypes); the most common were Enteritidis (n = 96, 36.3%) and Typhimurium (n = 53, 20.1%), followed by a miscellaneous group of 49 different serotypes (n = 115, 43.5%). A 15% increase in Salmonella Enteritidis isolation was observed. Common phage types for Salmonella Enteritidis were PT1 (41.6% of isolates), PT4 (9.4%), PT6 (9.4%), and PT6a (9.4%), and common types for Salmonella Typhimurium were DTU302 (18.8%), DT104 (15.1%), and DT104B (13.2%). Salmonella Enteritidis strains were categorized into eight PFGE types with a similarity of 81 to 96%, and 73.9% of the strains were grouped into just one cluster. Salmonella Typhimurium isolates were divided into 13 PFGE types with a similarity of 64 to 86%, and one predominant clone contained 41.5% of the strains. Resistance rates for Salmonella Enteritidis, Salmonella Typhimurium, and the miscellaneous group were, respectively, 8.3, 69.8, and 13.9% for ampicillin, 3.1, 52.8, and 59% for streptomycin, 40.6, 22.6, and 10.4% for nalidixic acid, 15.6, 71.7, and 31.1% for tetracycline, 7.3, 18.8, and 9.5% for trimethoprim-sulfamethoxazole, 0, 50.9, and 4.3% for chloramphenicol, and 6.2, 71.7, and 17.4% for multiple (at least four) antimicrobials. All the strains remained susceptible to other beta-lactams and fluoroquinolones. Surveillance of S. enterica isolated from food is strongly recommended to reduce community exposure to antimicrobial resistant strains.     

Possible horizontal transmission of Salmonella via reusable egg trays in Thailand

Salmonella contamination of eggshells, egg contents, reusable egg trays, and various environmental samples was assessed. Although the overall Salmonella contamination rate from egg farms was low (3.2%), over a quarter (26.7%) of egg trays from farms and more than one third (36.7%) of trays from the market were contaminated. Salmonella strains isolated from reusable egg trays were analyzed by serotyping, antimicrobial susceptibility test and XbaI pulsed-field gel electrophoresis (PFGE) typing. Five serovars (S. Braenderup, S. Emek, S. Weltevreden, S. Stanley, and S. Derby) were isolated, and half of the strains assessed were found to be resistant to one or more of the six antimicrobial agents examined. The overall resistance rates to nalidixic acid, trimethoprim-sulfamethoxazole, tetracycline, and ampicillin were 40.7%, 36.0%, 26.7% and 3.5%, respectively. The PFGE types were matched against sample location and drug resistance. S. Braenderup PFGE type A2 (susceptible to all tested drugs) was isolated from all sample sites; PFGE type A2 (resistant to nalidixic acid) was isolated from Farm C and the market. S. Braenderup PFGE type A1 (resistant to four drugs) was isolated from Farms A and C. S. Weltevreden PFGE type C3 (susceptible to all tested drugs) was isolated from Farms A and B and type C4 (susceptible to all tested drugs) was isolated from Farm A and the market. The distribution of the related genotypes and resistance patterns of Salmonella in egg farms and the market indicate drug-resistant strains of Salmonella may be spread on reusable egg trays.     

Characterization of nalidixic acid-resistant and fluoroquinolone-reduced susceptible Salmonella Typhimurium in swine

From 2001 to 2008, a total of 27 isolates of Salmonella enterica serovar Typhimurium were obtained from 930 swine. All 27 isolates were resistant to streptomycin and tetracycline. Seventeen isolates were multidrug resistant to more than three antimicrobial agents. Seven of these multidrug-resistant isolates were pentaresistant to ampicillin, chloramphenicol, streptomycin, tetracycline, and nalidixic acid. Among 27 isolates, 14 isolates (51.8 %) were nalidixic acid resistant (MIC, ≥128 μg/ml) and had reduced susceptibility to various quinolones (MIC, 0.125 to 2 μg/ml). When quinolone resistance-determining regions in the gyrA and gyrB genes of these isolates were sequenced, 13 isolates had Asp87→Tyr mutations and 1 isolate had Asp87→Gly mutation in the quinolone resistance-determining region of gyrA, whereas no mutation was found in gyrB. Genes for qnrA, qnrB, and qnrS were not detected by PCR with specific primers. Pulsed-field gel electrophoresis of genomic DNA digested with Xba I showed two patterns suggesting a clonal spread of Salmonella Typhimurium in swine in Korea.     

Effect of farm type on within-herd Salmonella prevalence, serovar distribution, and antimicrobial resistance

Salmonella represents a major challenge to the pig industry, as pork presents a risk for human salmonellosis. In this study, we have examined the effect of farm type on the prevalence of fattening pigs shedding Salmonella on 12 farms at risk for harboring Salmonella. On six open (grow-to-finish) and six closed (farrow-to-finish) farms, the prevalence of pigs shedding Salmonella was determined on two occasions approximately 2 months apart. The serovar, phage type, and antimicrobial resistance of the obtained Salmonella isolates were determined. On all farms, pigs shedding Salmonella were detected on at least one of the two sampling days. The mean within-herd prevalence was 7.8%. Closed farms were two times less likely to have pigs shedding Salmonella than open farms. On open farms, the odds of finding Salmonella shedding in pigs were 1.9 times higher when sampling was performed at slaughter age than when samples were taken halfway through the fattening period. Salmonella enterica serovar Typhimurium was the most predominant serotype, with a prevalence of 62 to 63% on both farm types. Of all the Salmonella Typhimurium isolates, 65% had the tetraresistant profile ASSuT (ampicillin, streptomycin, sulfonamide, and tetracycline) with or without additional resistance to trimethoprim-sulfonamide. Phage type DT120 seemed to be especially associated with this antimicrobial-resistant profile. The prevalence of Salmonella Typhimurium isolates showing resistance to ampicillin, streptomycin, tetracycline, sulfonamide, trimethoprim-sulfonamide, and lincomycin hydrochloride and spectinomycin sulfate tetrahydrate was significantly higher on open farms than on closed farms.     

Occurrence of Salmonella enterica serotype typhimurium DT104A in retail ground beef

Surveillance data of cattle and human isolates of Salmonella enterica serovar Typhimurium DT104 indicate that this pathogen emerged worldwide in the 1980s, particularly in cattle. Studies were conducted to determine the prevalence of Salmonella Typhimurium DT104 in ground beef. Samples were also tested for the presence of generic Escherichia coli. A total of 404 fresh ground beef samples obtained at retail stores from New York, San Francisco, Philadelphia, Denver, Atlanta, Houston, and Chicago were shipped overnight to Georgia for processing. Salmonella spp. were isolated from 14 (3.5%) samples. Eight different serotypes were identified among the isolates, including Salmonella Typhimurium (5), Salmonella Lille (3), Salmonella Montevideo (1), Salmonella Hadar (1), Salmonella Meleagridis (1), Salmonella Cerro (1), Salmonella Kentucky (1), and Salmonella Muenster (1). Antibiotic resistance profiles indicated that all five Salmonella Typhimurium isolates were resistant to ampicillin, streptomycin, sulfamethoxazole, ticarcillin, and tetracycline but that they were sensitive to chloramphenicol. Phage typing revealed that all five Salmonella Typhimurium isolates were DT104A, a subtype of DT104. All five Salmonella Typhimurium DT104A isolates were obtained from ground beef sampled from retail outlets in San Francisco. Pulsed-field gel electrophoresis (PFGE) genomic DNA profiles of the five Salmonella Typhimurium DT104A isolates from ground beef were indistinguishable from those of four control Salmonella Typhimurium DT104 penta-resistant isolates from cattle that were used for comparison. A total of 102 generic E. coli isolates were obtained, only three of which were multiresistant to antibiotics. In addition, three E. coli isolates were recovered from samples that were positive for Salmonella Typhimurium DT104A. No correlation of antibiotic resistance profiles was observed between Salmonella Typhimurium DT104A and generic E. coli, as two of the three E. coli isolates were susceptible to all of the antibiotics tested, and the third isolate was resistant only to cephalothin. These data indicate that Salmonella Typhimurium DT104A can be isolated from retail ground beef, and because there was little overlap in antibiotic resistance patterns between Salmonella Typhimurium DT104A and E. coli isolates from the same ground beef samples, these limited data suggest that the transfer of antibiotic resistance genes among enteric bacteria in ground beef may not be common. This latter observation is further supported by the limited isolation of multiantibiotic-resistant E. coli from retail ground beef.     

Incidence of highly genetically diversified Vibrio parahaemolyticus in seafood imported from Asian countries

Vibrio parahaemolyticus is an important pathogen of humans and aquacultured animals, especially in Asian countries. In this study, we examined 686 samples of seafood imported from Hong Kong, Indonesia, Thailand and Vietnam for V. parahaemolyticus. V. parahaemolyticus was recovered from 315 (45.9%) samples. The incidence of V. parahaemolyticus in products from Hong Kong and Thailand was markedly higher than the incidence in products from Indonesia and Vietnam. The incidence rates in shrimp, crab, snail, lobster, sand crab, fish and crawfish were 75.8, 73.3, 44.3, 44.1, 32.5, 29.3 and 21.1%, respectively. None of the isolates possessed the hemolysin genes (tdh, trh). The chromosomal DNA of 121 randomly selected imported isolates and three local environmental strains was digested with SfiI and analyzed by pulsed-field gel electrophoresis (PFGE). These isolates were then grouped into 96 different but mostly unique PFGE patterns. After hierarchical cluster analysis, these patterns could be arbitrarily grouped into twenty-two PFGE types (type A to V). A wide range of PFGE types were identified in isolates from different origin. Moreover, the PFGE types were not specifically associated with the origin or kind of seafood. These results reveal the high genetic diversity in V. parahaemolyticus isolated from seafood.     

A pulsed field gel electrophoresis (PFGE) study that suggests a major world-wide clone of Salmonella enterica serovar Enteritidis

Since human infections by Salmonella enterica serovar Enteritidis (Salmonella Enteritidis) have been increasing world-wide over the past years and epidemiological studies have implicated the consumption of meat, poultry, eggs and egg products, elucidation of the predominant subtypes for this Salmonella spp. is important. In this study, 107 poultry and food isolates of Salmonella Enteritidis obtained from Germany were analyzed by pulsed field gel electrophoresis (PFGE), and the subtypes were compared with those of the 124 human isolates obtained in Taiwan. Results showed that for these 107 poultry and food isolates, when XbaI, SpeI and NotI were used for chromosomal DNA digestion followed by PFGE analysis, a total of 19, 20 and 19 PFGE patterns, respectively, were identified. Of them, 51 (47.7%), 52 (48.6%) and 42 (39.3%) strains belong to a single pattern of X3, S3 and N3, respectively, and 34 strains belong to a pattern combination of X3S3N3, which was the major subtype. When PFGE patterns of these 107 German isolates were compared with those of the 124 human isolates obtained in Taiwan, pattern combination of X3S3N3 was found as the most common pattern shared by isolates from both areas. PT4 is a major phage type for German and Taiwan isolates. Although most of the X3S3N3 strains are of this phage type, some strains of other PFGE patterns are also of this phage type. Since strains used in this study were unrelated, i.e., they were isolated from different origins in areas geographically far apart from each other, the PFGE study suggests a major world-wide clone of S. enterica serovar Enteritidis.     

Molecular characterization of antibiotic-resistant salmonella isolates from retail meat from markets in northern Vietnam

A total of 118 Salmonella isolates were detected from 283 retail meat samples (135 pork and 148 chicken meat) purchased at retail markets in Northern Vietnam. Thirteen serovars, including Infantis, Anatum, Rissen, Reading, Emek, Typhimurium, Blockley, London, Newport, Derby, Weltevreden, Albany, and Hadar, were determined. Resistance to tetracycline (54.2%), sulfonamides (52.5%), streptomycin (41.5%), trimethoprim (36.4%), chloramphenicol (35.6%), and ampicillin (33.1%) was commonly seen in the Salmonella isolates. Fourteen [bla(TEM), bla(OXA-1), bla(PSE-1), aadA1, sul1, tetA, tetB, tetG, cmlA1, floR, dfrA1, dfrA12, aac(3)-IV, and aphA1-1AB ] of 17 resistance genes were detected from the isolates demonstrating resistance. Genes for plasmid-mediated quinolone resistance, such as qnrA, qnrB, qnrS, qepA, and acc(6')-1b-cr, were not detected in 23 quinoloneresistant isolates. The substitution TCC to TTC at codon 83 of gyrA was found in the 18 quinolone-resistant isolates. The data revealed that resistant Salmonella strains were widely distributed in Northern Vietnam via the food chain and that they might contain multiple genes specifying identical resistance phenotypes. Thus, further studies are necessary to clarify the mechanisms of antibiotic resistance in Salmonella strains and their spread in the livestock market.     

Contamination of Salmonella in retail meats and shrimps in the Mekong Delta, Vietnam

From March 2000 to September 2001, 608 samples of retail meat (136 pork, 70 beef, 202 chicken, and 200 ducks) and 110 samples of retail shrimp from six provinces of the Mekong Delta in Vietnam were collected individually and examined for the prevalence of Salmonella. Of the 718 samples examined, 243 (33.8%) were Salmonella positive. Salmonella was isolated from 69.9% of the pork samples, 48.6% of the beef samples, 21.0% of the chicken meat samples, 22.3% of the duck meat samples, and 24.5% of the shrimp samples. From 261 Salmonella isolates, 24 different serovars were identified. The predominant serovars of the isolates were Salmonella Derby, Salmonella Weltevreden, and Salmonella London in pork; Salmonella Weltevreden, Salmonella London, and Salmonella Dessau in beef; Salmonella Emek, Salmonella Typhimurium, and Salmonella Dessau in chicken meat; Salmonella Lexington, Salmonella Derby, and Salmonella Dessau in duck meat; and Salmonella Weltevreden, Salmonella Tennessee, and Salmonella Dessau in shrimps. Salmonella Bovismorbificans, Salmonella Derby, Salmonella Dessau, and Salmonella Weltevreden were the most common serovars in all the samples examined. These results indicate a high rate of contamination by Salmonella in retail meats and shrimps in the Mekong Delta, Vietnam.     

Prevalence and antimicrobial resistance of Salmonella in retail foods in northern China

A total of 387 retail meat, seafood and milk powder samples were collected from nine cities in northern China in 2005 and screened for the presence of Salmonella. Salmonella strains isolated were subjected to serotyping and antimicrobial susceptibility testing. Salmonella was isolated from 81 (20.9%, 81/387) samples and classified into 23 serotypes. The isolates were frequently resistant to sulfamethoxazole (86.4%), sulfamethoxazole/trimethoprim (48.1%), nalidixic acid (30.9%), tetracycline (19.8%), carboxybenzylpenicillin (17.3%), amoxicillin (17.3%) and ampicillin (16.0%). The multiple resistance (resistance to ≥ 3 antibiotics) was found in 29.6% (n = 24) isolates. Additionally, 4 isolates from chicken displayed the ACSSuTNx profile, resistant to ampicillin, chloramphenicol, streptomycin, sulfonamide, tetracycline and nalidixic acid, in particular, strain HBS084 showing the resistance to as many as 20 antibiotics. Salmonella from chicken showed the higher frequency of antimicrobial resistance. Our findings indicate that in northern China food products of animal origin can be a source of exposure for consumers to multiresistant Salmonella strains.     

Genetic diversity and virulence gene determinants of antibiotic-resistant Salmonella isolated from preharvest turkey production sources

This study evaluated the molecular diversity of 29 Salmonella serotypes isolated from turkey ceca and the production environment. Isolates were resistant to bacitracin (100%), erythromycin (100%), novobiocin (100%), rifampin (100%), streptomycin (62%), gentamicin (52%), spectinomycin (48%), tetracycline (31%), sulfamethoxazole/trimethoprim (SXT) (3%) and tobramycin (3%). The minimum inhibitory concentration (MIC) values ranged from 32 to >/=1024 microg/ml. The pulsed-field gel electrophoresis (PFGE) and ribotyping patterns were identical within each of the serotypes Heidelberg, Worthington and Muenster. The plasmid profiles were identical within each of the Salmonella serotypes. Two different clones of Salmonella anatum were differentiated by PFGE typing but not by ribotyping. Heidelberg isolates from nine turkey ceca and three drinker samples had identical antibiotic resistance, PFGE, ribotype and plasmid patterns, suggesting that transmission of this particular clone may have occurred between the birds and the drinkers. Identical PFGE, ribotype and plasmid patterns were observed in one Salmonella worthington isolate from turkey ceca in one flock and two S. worthington isolates from feeder contents and drinkers from a subsequent flock, suggesting transmission of this pathogen between flocks. Individual and multiple polymerase chain reaction (PCR) analyses revealed the presence of the virulence genes invA, aceK and sopB and the absence of the h-1i gene in all isolates. A combination of genotypic and phenotypic markers can be useful in studying genetic variation among natural salmonellae populations in turkey production and delineating possible transmission pathways.