Basic Antibiotic Resistance of Bacteria Isolated from Different Biotopes

Microbiology - Soil is not only a habitat of antibiotic-resistant microorganisms and a natural source of antibiotic resistance genes, but also an environment in which clinical determinants of...     

Antibiotic and Heavy Metal Resistance of Pseudomonas aeruginosa Isolated from Soils

1. 71 strains of Pseudomonas aeruginosa were isolated from soils and tested for resistance to antibiotics and heavy metals.
2. 6% were resistant both to 12/14 drugs and 9/12 heavy metals tested.     

Genetic and Molecular Properties of an Infectious Antibiotic Resistance (R) Factor Isolated from Klebsiella

A Klebsiella strain of human origin that was resistant to ampicillin, chloramphenicol, kanamycin, neomycin, streptomycin, and tetracycline was found to have all of these resistances associated with a R factor and a satellite molecular species of deoxyribonucleic acid (DNA) with an average buoyant density of 1.710 in cesium chloride gradients. There was no evidence of the existence of DNA with other buoyant densities. The strain bears two separable mutations for chloramphenicol resistance, both of which are associated with the R factor (KR9). Exposure of the Klebsiella strain to acridine derivatives or to ethidium bromide (which was more efficient) resulted in partial losses of resistance accompanied by the disappearance of the satellite DNA peak or shifts in its density. The R factor and its component genes were conjugally transmitted across generic boundaries and maintained in new hosts with different efficiencies. The basis of this difference lies not only in the efficiency of conjugal transfer but also in the stability of the components after transfer. All of the resistance genes and the resistance transfer factor were cotransducible by phage Plkc from Escherichia coli. Partially resistant strains could be reconstituted to full resistance or to a recombined pattern of partial resistance by conjugation with donors having complementary resistance patterns. This recombination serves as an efficient mechanism for rescuing superinfecting genes that are otherwise intracellularly excluded. KR9 is an fi⁺ type of R factor which in the natural state does not appear to be as repressed in conjugal transfer as other R factors.     

Incidence of Virulence Factors and Antibiotic Resistance among Enterococci Isolated from Food

The incidence of virulence factors among 48 Enterococcus faecium and 47 Enterococcus faecalis strains from foods and their antibiotic susceptibility were investigated. No strain was resistant to all antibiotics, and for some strains, multiple resistances were observed. Of E. faecium strains, 10.4% were positive for one or more virulence determinants, compared to 78.7% of E. faecalis strains. Strains exhibiting virulence traits were not necessarily positive for all traits; thus, the incidence of virulence factors may be considered to be strain specific.     

Differentiation of clinical Helicobacter pullorum isolates from related Helicobacter and Campylobacter species

Background. Helicobacter pullorum, first detected in the liver and intestinal contents of poultry, was defined as a new species in 1994. This organism has since been isolated from humans with gastroenteritis. Phenotypic as well as genotypic methods have been used to identify H. pullorum associated with cases of human disease. Materials and Methods. Clinical isolates were submitted for identification to the National Laboratory for Enteric Pathogens by Provincial Public Health Laboratories within Canada. Phenotypic characterization was conducted using a variety of growth and biochemical tests including oxidase, catalase, indoxyl acetate, H₂S production in triple sugar iron (TSI) agar, antimicrobial susceptibility testing, and fatty acid analysis. Genotypic identification was performed using a polymerase chain reaction–restriction fragment–length polymorphism (PCR‐RFLP) analysis of a 1‐kb fragment of the Helicobacter 16S rRNA gene. Results. During the last 7 years (1993–1999) a total of 11 isolates of H. pullorum were detected from patients with gastroenteritis for inclusion in this study. Typically, these isolates were oxidase and catalase positive, produced optimal growth at 42°C, and produced H₂S in TSI. Of these 11 isolates, 1 showed DNase activity, while another did not produce H₂S in TSI, and only 2 showed tolerance to 1% bile. Antimicrobial susceptibility assays indicated that 6 of the 11 strains were resistant to nalidixic acid. The fatty acid profiles of the isolates were similar to each other and provided a distinguishing profile from the other related species. Genetically identical and distinct species‐specific restriction fragment–length polymorphism (RFLP) patterns were produced using the restriction enzymes Bsr I and Dde I. Conclusion. Phenotypic and genotypic procedures were used to identify H. pullorum. Interspecies phenotypic variability was apparent and supported the use of a polyphasic approach for identification. Similarities to the more prominent human pathogens Campylobacter coli and C. lari were also noted. The use of a combination of phenotypic and, in particular, genotypic markers for H. pullorum should prove valuable both for epidemiological investigations and for the diagnosis of disease related to this emerging human pathogen.     

华东地区猪丹毒杆菌分离株的生物学特征及其耐药性分析

旨在了解华东地区猪场猪丹毒杆菌的生物学特征以及对临床常用抗菌药的耐药情况。通过对2017-2018 年华东地区猪场临床疑似猪丹毒感染的266份病料进行细菌分离、分析其生化特性、血清型、小鼠致病性等生物学特征,并进行16S rDNA 序列遗传进化分析和药物敏感性测定。经分离和鉴定共获得20株猪丹毒杆菌,血清型均为1a 型,能100%致死小鼠,具有猪丹毒杆菌的生化特性,16S rDNA 序列与国内外参考菌株、临床流行株的同源性达99%以上;现有商品化猪丹毒弱毒疫苗免疫小鼠后,分别以致死剂量的的5株猪丹毒杆菌分离株腹腔攻毒小鼠,免疫保护率均为100%。所有分离株对青霉素(PEN)、氨苄西林(AMP)、红霉素(ERY)、替米考星(TIL)、头孢噻呋(CEF)、头孢曲松(CRO)和氟苯尼考(FFC)敏感。然而,对林可酰胺类、喹诺酮类、氨基糖苷类、四环素类、磺胺类和多肽类抗生素有不同程度的多重耐药现象。本研究明确了华东地区猪丹毒杆菌的生物学特性和耐药情况,现有商品化猪丹毒弱毒疫苗对分离的强毒株可提供完全的免疫保护,为当地猪丹毒的有效防控提供了数据。     

Clonal Population Structure and Antimicrobial Resistance of Campylobacter jejuni in Chicken Meat from Belgium

Campylobacter jejuni is one of the most important causes of human diarrhea worldwide. In the present work, multilocus sequence typing was used to study the genotypic diversity of 145 C. jejuni isolates from 135 chicken meat preparations sampled across Belgium. Isolates were further typed by pulsed-field gel electrophoresis, and their susceptibilities to six antimicrobials were determined. Fifty-seven sequence types (STs) were identified; 26.8% of the total typed isolates were ST-50, ST-45, or ST-257, belonging to clonal complex CC-21, CC-45, or CC-257, respectively. One clonal group comprised 22% (32/145) of all isolates, originating from five different companies and isolated over seven sampling months. Additionally, 53.1% of C. jejuni isolates were resistant to ciprofloxacin, and 48.2% were resistant to tetracycline; 28.9% (42/145) of all isolates were resistant to both ciprofloxacin and tetracycline. The correlation between certain C. jejuni clonal groups and resistance to ciprofloxacin and tetracycline was notable. C. jejuni isolates assigned to CC-21 (n = 35) were frequently resistant to ciprofloxacin (65.7%) and tetracycline (40%); however, 90% (18/20) of the isolates assigned to CC-45 were pansusceptible. The present study demonstrates that certain C. jejuni genotypes recur frequently in the chicken meat supply. The results of molecular typing, combined with data on sample sources, indicate a possible dissemination of C. jejuni clones with high resistance to ciprofloxacin and/or tetracycline. Whether certain clonal groups are common in the environment and repeatedly infect Belgian broiler flocks or whether they have the potential to persist on farms or in slaughterhouses needs further investigation.Campylobacter jejuni is among the most common bacterial causes of human gastroenteritis worldwide (4, 23). Infected humans exhibit a range of clinical symptoms from mild, watery diarrhea to severe inflammatory diarrhea (14). In addition, C. jejuni has been identified as an important infectious trigger for Guillain-Barré syndrome, the most common cause of acute flaccid paralysis in polio-free regions (16). Another issue of concern regarding Campylobacter is the increase in antimicrobial resistance appearing in various regions around the world (1). Infection with an antimicrobial-resistant Campylobacter strain may lead to a suboptimal outcome of antimicrobial treatment or even to treatment failure (11).Consumption of contaminated water and raw milk has been implicated in campylobacteriosis outbreaks (23). However, the majority of human cases are sporadic, and consumption or mishandling of contaminated raw or undercooked poultry meat is believed to be an important source of infection. Risk assessment studies, outbreak investigations, and case-control reports all incriminate chicken meat as a major source, perhaps the major source, of food-borne transmission (14, 17, 32, 48). In Belgium in 1999, a controlled withdrawal of poultry products from sale due to alleged dioxin contamination resulted in a 40% reduction in the frequency of human campylobacteriosis (44). Thereafter and since the year 2000, the Campylobacter contamination of Belgian poultry carcasses and meat has been monitored by the Federal Agency for the Safety of the Food Chain, and the rate of positive samples is regarded as high. In 2006, 55.5% of cecal samples (n = 6,443) from Belgian broilers at slaughter tested positive for Campylobacter (3). In 2007, an industry-focused survey reported that 48% of Belgian chicken meat preparations (n = 656) were contaminated with Campylobacter (19).Molecular typing is an important tool in elucidating the diversity and transmission routes of Campylobacter isolates contaminating the food chain. In the United States, molecular analysis of Campylobacter spp. from poultry production and processing environments showed that many of the clones found within a flock are present in the final products, although the diversity of Campylobacter isolates in the final product was lower than that observed in the flock (22). Furthermore, numerous molecular epidemiological studies indicate that the genotypes of C. jejuni isolated from human cases overlap those of poultry origin (17, 47). Various molecular typing methods for the study of the population structure of Campylobacter are currently available (46). Among these, the multilocus sequence typing (MLST) approach is an emerging tool for research on the population structure and molecular epidemiology of Campylobacter. The technique is highly reproducible, portable, and easy to interpret, and results can be shared through a publicly accessible online database (31, 34). As such, MLST is becoming an important tool for studying the molecular epidemiology of Campylobacter in a global context. The accumulation of sequence typing data generated from different countries and settings could allow the creation of more-sophisticated models of the epidemiology and evolution of bacterial pathogens and the development of improved approaches for combating their spread (41).In Belgium, there is a paucity of information regarding the population structure of Campylobacter in the chicken meat supply. No population-based surveys have been conducted to investigate the molecular epidemiology of C. jejuni in chicken meat at points close to human consumption. In this study, MLST and pulsed-field gel electrophoresis (PFGE) were used to characterize the diversity of, and clonal relationships among, 145 C. jejuni isolates from Belgian chicken meat preparations. In addition, we characterized the antimicrobial resistance in this collection and correlated it with C. jejuni genotypes.     

金黄色葡萄球菌引起食物中毒的作用机制与其耐药性的研究进展

葡萄球菌广泛分布于自然界中,如空气、土壤、水以及物体的表面,在人和动物的皮肤表面部、鼻咽、肠道也常可发现葡萄球菌。大部分葡萄球菌是非致病菌,少数可引起人或动物致病,金黄色葡萄球菌（Staphylococcus aureus,金葡菌）即为最主要的致病性葡萄球菌。金葡菌是一种革兰氏阳性球菌,是医院感染常见的病原体之一,同时也是引起食品污染和细菌性食物中毒的一种重要细菌,其产生的毒素可使人中毒,带来非常严重的公共卫生负担。本文拟对金葡菌的病原与病理学特性,金葡菌与食物中毒,抗生素滥用与金葡菌耐药性等方面做简要综述。     

Antibiotic Resistance of Bacteria Isolated from the Internal Organs of Edible Snow Crabs

Antibiotic resistance and microbiota within edible snow crabs are important for the Chionoecetes (snow crab) fishing industry. We investigated these parameters using culture methods and antibiotic susceptibility tests with six internal organs from three species of Chionoecetes. Each sample revealed many unexpected microbial species within Chionoecetes internal organs. On the basis of 16S rRNA sequence analysis of 381 isolates, the most abundant genera identified in Chionoecetes opilio were Acinetobacter spp. (24%), Bacillus spp. (4%), Pseudomonas spp. (34%), Stenotrophomonas spp. (28%), and Agreia spp. (11%). In Chionoecetes sp. crabs, Acinetobacter spp. (23%), Bacillus spp. (12%), and Psychrobacter spp. (20%) were most prevalent, while Agreia spp. (11%), Bacillus spp. (31%), Microbacterium spp. (10%), Rhodococcus spp. (12%), and Agrococcus spp. (6%) were most abundant in C. japonicus. Our antibiotic resistance test found resistance to all nine antibiotics tested in 19, 14, and two of the isolates from C. opilio, Chionoecetes sp., and, C. japonicus respectively. Our results are the first to show that microbes with antibiotic resistance are widely distributed throughout the internal organs of natural snow crabs.     

Serotype Distribution,Antibiotic Resistance and Clonality of Streptococcus pneumoniae Isolated from Immunocompromised Patients in Tunisia

BackgroundPneumococcal disease, a major cause of morbidity and mortality globally, has higher incidence among young children, the elderly and the immunocompromised of all ages. In Tunisia, pneumococcal conjugate vaccines (PCVs) are not included in the national immunization program. Also, few studies have described the epidemiology of S. pneumoniae in this country and, in particular, no molecular typing studies have been performed. The aim of this study was to evaluate serotype distribution, antimicrobial resistance and clonality of Streptococcus pneumoniae isolated from neutropenic patients in Tunisia.MethodsFifty-nine S. pneumoniae were isolated from infection (n = 31) and colonization (n = 28) sites of patients (children and adults) attending the National Centre of Bone Marrow Transplantation in Tunis between 2005–2011. All isolates were characterized by serotype, antimicrobial resistance pattern and multilocus sequence typing (MLST).ResultsThe majority (66.1%) of the isolates belonged to five serotypes all included in PCVs: 6B, 9V, 14, 19F and 23F. The potential coverage of the 10-valent and 13-valent PCV was of 71.2% and 76.3% respectively. Resistance rates were very high and 69.5% of the isolates were multidrug resistant: non-susceptibility rates to penicillin, amoxicillin and cefotaxime were 66.1%, 40.7% and 27.1%, respectively; resistance rates to erythromycin, clindamycin, tetracycline, chloramphenicol and trimethoprim-sulfamethoxazole, were 69.5%, 61.0%, 37.3%, 22.0% and 67.8%, respectively. The most frequent serotypes had STs characteristic of multidrug resistant international clones known to be highly successful and important causes of pneumococcal infection: Spain 23F-ST81, France 9V/14-ST156, Spain 6B-ST90, 19F-ST320, and Portugal 19F-ST177.ConclusionsThe majority of S. pneumoniae strains recovered from immunocompromised patients in Tunisia are representatives of multidrug resistant pandemic clones that express serotypes targeted by PCVs. To contain the burden of pneumococcal disease and improve treatment choices among Tunisian immunocompromised patients PCVs should be offered to all of them.     

Functional and Molecular Surveillance of Helicobacter pylori Antibiotic Resistance in Kuala Lumpur

Background

Helicobacter pylori is the etiological agent for diseases ranging from chronic gastritis and peptic ulcer disease to gastric adenocarcinoma and primary gastric B-cell lymphoma. Emergence of resistance to antibiotics possesses a challenge to the effort to eradicate H. pylori using conventional antibiotic-based therapies. The molecular mechanisms that contribute to the resistance of these strains have yet to be identified and are important for understanding the evolutional pattern and selective pressure imposed by the environment.

Methods and Findings

H. pylori was isolated from 102 patients diagnosed with gastrointestinal diseases, who underwent endoscopy at University Malaya Medical Centre (UMMC). The isolates were tested for their susceptibility on eleven antibiotics using Etest. Based on susceptibility test, 32.3% of the isolates were found to have primary metronidazole resistance; followed by clarithromycin (6.8%) and fluoroquinolones (6.8%). To further investigate the resistant strains, mutational patterns of gene rdxA, frxA, gyrA, gyrB, and 23S rRNA were studied. Consistent with the previous reports, metronidazole resistance was prevalent in the local population. However, clarithromycin, fluoroquinolone and multi-drug resistance were shown to be emerging. Molecular patterns correlated well with phenotypic data. Interestingly, multi-drug resistant (MDR) strains were found to be associated with higher minimum inhibitory concentration (MIC) than their single-drug resistant (SDR) counterparts. Most importantly, clarithromycin-resistant strains were suggested to have a higher incidence for developing multi-drug resistance.

Conclusion

Data from this study highlighted the urgency to monitor closely the prevalence of antibiotic resistance in the Malaysian population; especially that of clarithromycin and multi-drug resistance. Further study is needed to understand the molecular association between clarithromycin resistance and multi-drug resistance in H. pylori. The report serves a reminder that a strict antibiotic usage policy is needed in Malaysia and other developing countries (especially those where H. pylori prevalence remained high).     

Antimicrobial Resistance of the Coral Pathogen Vibrio coralliilyticus and Caribbean Sister Phylotypes Isolated from a Diseased Octocoral

Vibrio coralliilyticus is a global marine pathogen that has been found to cause disease in several marine organisms, including corals. This study is the first report of the isolation of V. coralliilyticus from a diseased Caribbean octocoral, Pseudopterogorgia americana. Five sister phylotypes were positively identified using 16S rRNA gene sequencing, recA probes specific for V. coralliilyticus, and rep-PCR fingerprinting. The antimicrobial resistance was compared between pathogenic strains of V. coralliilyticus and the Caribbean strains. First, the antimicrobial resistance of V. coralliilyticus-type strain ATCC BAA-450 was determined using an agar-overlay antimicrobial bioassay at 24°C and 27°C, temperatures which are relevant to its known temperature-dependent virulence. From 108 distinct bacteria isolated from P. americana, 12 inhibited the V. coralliilyticus-type strain at 24°C and five at 27°C. Next, the phenotypic comparison of two Caribbean phylotypes and three V. coralliilyticus reference strains against a subset of 30 bacteria demonstrated a similar resistance trend. At both temperatures, the reference strains were inhibited by three bacteria isolates, while the Caribbean strains were inhibited by four to nine bacteria. Additionally, V. coralliilyticus-type strain ATCC BAA-450 and one of the Caribbean strains were inhibited by a higher number of bacteria at 24°C compared with 27°C. Together, these results highlight that V. coralliilyticus strains have antimicrobial resistance to the majority of coral-associated bacteria tested, which may be temperature-dependent in some strains. Furthermore, all V. coralliilyticus strains tested showed multi-drug resistance to a range of 11–16 (out of 26) commercial antibiotics. This study establishes V. coralliilyticus in association with a Caribbean octocoral and demonstrates its resistance to the antimicrobial activity of coral-associated bacteria and to commercial antibiotics.     

Inhibition of Phage Growth by an Antibiotic Rugulosin Isolated from Myrothecium verucaria

It was found that (?)rugulosin, an antibiotic isolated from Myrothecium verucaria, had a potent anti-phage effect on RNA phages (MS2, GA and Qβ) and DNA phages (δA and T4). The effect was almost independent of the host bacterial strains used. By a detailed investigation using MS2, it was revealed that the antibiotic did not affect the free phage or host bacterium alone but inhibited phage multiplication, and the degree of the inhibition depended on the multiplicity of infection. The inhibition was not mainly due to a drop in the burst size but rather was due to a decrease of the phage-producing cells during the early stages of phage infection and replication.     

Genetic Diversity and Antibiotic Resistance Patterns in a Campylobacter Population Isolated from Poultry Farms in Switzerland

The diversity and genetic interrelation of Campylobacter jejuni and C. coli isolated from Swiss poultry were assessed by three independent typing methods. Samples were derived prior to slaughter from 100 randomly selected flocks (five birds per flock) raised on three different farm types. The observed flock prevalence was 54% in total, with 50% for conventional and 69% for free-range farms. Birds held on farms with a confined roaming area had the lowest prevalence of 37%. Campylobacter isolates were characterized by amplified fragment length polymorphism (AFLP), restriction fragment length polymorphism of flaA PCR fragments (flaA-RFLP), and disk diffusion testing for eight antimicrobial agents that are commonly used in veterinary or human medicine in Switzerland. Analysis of the genotypic results indicates that the Campylobacter population in Swiss poultry is genetically highly diverse. Nevertheless, occasionally, isolates with identical or nearly identical characteristics were isolated from different farms or farm types in different locations. Genetic typing by AFLP and flaA-RFLP was found to be complementary. The majority of isolates (67%) were susceptible to all tested antibiotics; however, single, double, and triple resistances were observed in 7%, 23%, and 2% of the strains, respectively. There was no correlation between genotype and antibiotic resistance. Surprisingly, sulfonamide resistance was frequently found together with streptomycin resistance. Our findings illustrate the results of common genetic exchange in the studied bacterial population.     

Pretreatment Antibiotic Resistance in Helicobacter pylori Infection: Results of Three Randomized Controlled Studies

Background. Although combinations of antibiotics and antisecretory drugs are useful for treatment of Helicobacter pylori infection, treatment failure is common. The aim of this study was to evaluate the relation between pretreatment antibiotic resistance and outcome by using six different treatment regimens for H. pylori infection. Patients and Methods. Three hundred sixty-nine consecutive H. pylori–infected patients with dyspeptic symptoms were enrolled in three consecutive randomized, controlled, single-center clinical trials: trial A, 128 patients; trial B, 125 patients; trial C, 116 patients. Treatments consisted of (A) a 15-day course of dual therapy (omeprazole, 20 mg bid, and amoxicillin, 1 gm bid, or clarithromycin, 500 mg tid) (OA vs OC); (B) a 7-day triple therapy of omeprazole, 20 mg bid, plus metronidazole, 500 mg bid, and amoxicillin, 1,000 mg bid, or clarithromycin, 500 mg tid (OMA vs OMC); or (C) omeprazole, 20 mg bid, plus metronidazole, 500 mg bid, plus tetracycline, 500 mg qid, or doxycycline, 100 mg tid (OMT vs OMD). Diagnostic endoscopy was made in all patients before and 5 to 6 weeks after therapy. Six biopsies were taken from each patient for histology, rapid urease test, and H. pylori culture; antibiotic susceptibility testing was performed using the E-test method. Results. Overall cure rates were poor for both dual therapies OA and OC (38% and 37%, respectively) and for triple therapies OMA, OMC, and OMD (57%, 55%, and 58%, respectively). The OMT combination was successful in 91% (95% confidence interval [CI], 80.4%–97%). Metronidazole resistance was present in 29.7% (95% CI, 24%–35%), amoxicillin resistance was present in 26% (95% CI, 21%–32%), clarithromycin resistance was present in 23.1% (95% CI, 18%–29%), tetracycline resistance was present in 14% (95% CI, 10%–20%), and doxycycline resistance was present in 33.3% (95% CI, 21%–47%). Antibiotic resistance markedly reduced the cure rates and accounted for most of the poor results with the triple therapies: 89% versus 23%; 77% versus 26%; 100% versus 60%; and 67% versus 23% for OMC, OMA, OMT, and OMD, respectively. OMT appeared to be the best because of the high success rate with metronidazole-resistant H. pylori (71%) and in low-level tetracycline resistance. Conclusions. Pretreatment antibiotic-resistant H. pylori can, in part, explain the low cure rate of the infection and the variability in outcome in reported trials.     

Relationships Between Single Nucleotide Polymorphisms of the H-FABP Gene and Slaughter and Meat Quality Traits in Chicken

Using PCR-SSCP with five primer pairs, we detected six single nucleotide polymorphisms of the H-FABP gene: 332G → A, 534G → A, 783C → T, 835C → T, 1198T → C, and 2329C → T. Chi-square results showed significant differences (P < 0.05) in genotype frequency among breeds in Fragment 1 and extremely significant differences (P < 0.01) in Fragments 2–4. We found a significant association between Fragment 2 genotype and muscle fiber number, Arg and Thr (P < 0.05); between Fragment 3 genotype and living weight, carcass weight, breast muscle weight, abdominal fat weight, and abdominal fat percentage (P < 0.05); between Fragment 4 genotype and Thr, Phe, and inosinic acid (P < 0.05). It was concluded that H-FABP was the major gene influencing slaughter performance and meat quality or was linked with the major gene in these strains and that the C783T mutation could be used as a candidate molecular genetic marker for breeding selection. The combination M1C2–B2B2–D1D1 is an ideal model for breeding in these strains because it can improve slaughter and meat quality traits.     

Antibiotic Resistance Patterns and Detection of blaDHA-1 in Salmonella Species Isolates from Chicken Farms in South Korea

Fifteen nonrepetitive ampicillin-resistant Salmonella spp. were identified among 91 Salmonella sp. isolates during nationwide surveillance of Salmonella in waste from 131 chicken farms during 2006 and 2007. Additional phenotyping and genetic characterization of these 15 isolates by using indicator cephalosporins demonstrated that resistance to ampicillin and reduced susceptibility to cefoxitin in three isolates was caused by TEM-1 and DHA-1 β-lactamases. Plasmid profiling and Southern blot analysis of these three DHA-1-positive Salmonella serovar Indiana isolates and previously reported unrelated clinical isolates of DHA-1-positive Salmonella serovar Montevideo, Klebsiella pneumoniae, and Escherichia coli from humans and swine indicated the involvement of the large-size plasmid. Restriction enzyme digestion of the plasmids from the transconjugants showed variable restriction patterns except for the two Salmonella serovar Indiana isolates identified in this study. To the best of our knowledge, this is the first report of the presence of the DHA-1 gene among Salmonella spp. of animal origin.Nontyphoidal Salmonella (NTS) strains are a significant cause of gastrointestinal infections of food origin. These microbes are a heterogeneous group of medically important Gram-negative bacteria and can infect a wide range of animals, including humans (3, 6, 9-11, 25).Currently, no antimicrobial therapies are recommended for the treatment of NTS infection unless a patient is of extreme age, has an underlying disease, or is infected with an invasive Salmonella sp. However, the use of antibiotics in treatment of clinical enteric infection has been heavily compromised by emerging multidrug-resistant microbes (4, 17, 18, 23). In particular, resistance due to extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases is of special concern as these enzymes confer resistance to some of the front-line antibiotics used to treat enteric infection in humans and animals (4, 13, 14, 19).Four classes of β-lactamases are known to confer resistance to β-lactam antibiotics. Among these, plasmid-mediated class A and class C β-lactamases have been frequently reported, whereas class B and class D β-lactamases are relatively rare (4). TEM and SHV enzymes of class A β-lactamases are generally found in Gram-negative bacteria and are derived by one or more amino acid substitutions around the active site of the enzyme that is responsible for the ESBL phenotype (4). Recently, the CTX-M enzyme of class A β-lactamases has been increasingly reported from enteric microbes, like Salmonella and Escherichia coli (4, 5, 9, 15). These have greater activity against cefotaxime than do other oxyimino-β-lactam substrates, like ceftazidime, ceftriaxone, or cefepime (4, 5). Plasmid-mediated AmpC β-lactamases, like DHA and CMY, are not inhibited by clavulanic acid and have been isolated from a wide variety of clinical and community-acquired microbes (2, 4, 13, 14, 16). These β-lactamases are native to the chromosomes of many Gram-negative bacilli but are missing in some genera, like Salmonella (4). The majority of β-lactamases reported in Salmonella to date have been derived from human clinical isolates, and only limited information is available regarding Salmonella spp. derived from farm animals, although isolates from both humans and animals are of clinical and epidemiological importance (4, 15, 25).In light of this knowledge gap, our study focused on assessing the distribution of Salmonella serovars in poultry farms in South Korea. Subsequently, isolates were analyzed for resistance to antibiotics commonly used in farms. Phenotypic and genetic characteristics of ampicillin-resistant Salmonella isolates were tested to gain insight into what β-lactamases were prevalent among these strains. We also characterized DHA-1-associated plasmids in these Salmonella spp. and compared them with clinical isolates of Salmonella, Klebsiella pneumoniae, and Escherichia coli from humans and from swine.