Use of Antibiotic Susceptibility Patterns and Pulsed-Field Gel Electrophoresis To Compare Historic and Contemporary Isolates of Multi-Drug-Resistant Salmonella enterica subsp. enterica Serovar Newport

Recently, multi-drug-resistant (MDR) Salmonella enterica subspecies enterica serovar Newport reemerged as a public and animal health problem. The antibiotic resistance of 198 isolates and the pulsed-field gel electrophoresis patterns (PFGE) of 139 isolates were determined. Serovar Newport isolates collected between 1988 and 2001 were included in the study. One hundred seventy-eight isolates were collected from the San Joaquin valley in California and came from dairy cattle clinical samples, human clinical samples, bulk tank milk samples, fecal samples from preweaned calves, and waterways. Twenty clinical isolates from humans from various regions of the United States were also included in the study. Resistance to 18 antibiotics was determined using a disk diffusion assay. PFGE patterns were determined using a single enzyme (XbaI). The PFGE and antibiogram patterns were described using cluster analysis. Although the antibiotic resistance patterns of historic (1988 to 1995) and contemporary (1999 to 2001) isolates were similar, the contemporary isolates differed from the historic isolates by being resistant to cephalosporins and florfenicol and in their general sensitivity to kanamycin and neomycin. With few exceptions, the contemporary isolates clustered together and were clearly separated from the historic isolates. One PFGE-antibiogram cluster combination was predominant for the recent isolates, which were taken from human samples from all parts of the United States, as well as in the isolates from California, indicating a rapid dissemination of this phenotypic strain. The data are consistent with the hypothesis that the reemergence of MDR serovar Newport is not simply an acquisition of further antibiotic resistance genes by the historic isolates but reflects a different genetic lineage.     

Antimicrobial resistance and virulence determinants in European Salmonella genomic island 1-positive Salmonella enterica isolates from different origins

Salmonella genomic island 1 (SGI1) contains a multidrug resistance region conferring the ampicillin-chloramphenicol-streptomycin-sulfamethoxazole-tetracycline resistance phenotype encoded by bla(PSE-1), floR, aadA2, sul1, and tet(G). Its increasing spread via interbacterial transfer and the emergence of new variants are important public health concerns. We investigated the molecular properties of SGI1-carrying Salmonella enterica serovars selected from a European strain collection. A total of 38 strains belonging to S. enterica serovar Agona, S. enterica serovar Albany, S. enterica serovar Derby, S. enterica serovar Kentucky, S. enterica serovar Newport, S. enterica serovar Paratyphi B dT+, and S. enterica serovar Typhimurium, isolated between 2002 and 2006 in eight European countries from humans, animals, and food, were subjected to antimicrobial susceptibility testing, molecular typing methods (XbaI pulsed-field gel electrophoresis [PFGE], plasmid analysis, and multilocus variable-number tandem-repeat analysis [MLVA]), as well as detection of resistance and virulence determinants (PCR/sequencing and DNA microarray analysis). Typing experiments revealed wide heterogeneity inside the strain collection and even within serovars. PFGE analysis distinguished a total of 26 different patterns. In contrast, the characterization of the phenotypic and genotypic antimicrobial resistance revealed serovar-specific features. Apart from the classical SGI1 organization found in 61% of the strains, seven different variants were identified with antimicrobial resistance properties associated with SGI1-A (S. Derby), SGI1-C (S. Derby), SGI1-F (S. Albany), SGI1-L (S. Newport), SGI1-K (S. Kentucky), SGI1-M (S. Typhimurium), and, eventually, a novel variant similar to SGI1-C with additional gentamicin resistance encoded by aadB. Only minor serovar-specific differences among virulence patterns were detected. In conclusion, the SGI1 carriers exhibited pathogenetic backgrounds comparable to the ones published for susceptible isolates. However, because of their multidrug resistance, they may be more relevant in clinical settings.     

Identification of specific gene sequences conserved in contemporary epidemic strains of Salmonella enterica

Genetic elements specific to recent and contemporary epidemic strains of Salmonella enterica were identified using comparative genomic analysis. Two epidemic multidrug-resistant (MDR) strains, MDR Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) and cephalosporin-resistant MDR Salmonella enterica serovar Newport, and an epidemic pansusceptible strain, Salmonella serovar Typhimurium DT160, were subjected to Salmonella gene microarray and suppression subtractive hybridization analyses. Their genome contents were compared with those of coexisting sporadic strains matched by serotype, geographic and temporal distribution, and host species origin. These paired comparisons revealed that epidemic strains of S. enterica had specific genes and gene regions that were shared by isolates of the same subtype. Most of these gene sequences are related to mobile genetic elements, including phages, plasmids, and plasmid-like and transposable elements, and some genes may encode proteins conferring growth or survival advantages. The emergence of epidemic MDR strains may therefore be associated with the presence of fitness-associated genetic factors in addition to their antimicrobial resistance genes.     

Diversity of multidrug-resistant salmonella enterica strains associated with cattle at harvest in the United States

The prevalence and diversity of multidrug-resistant (MDR) Salmonella enterica strains associated with cattle at harvest in the United States were examined. Hides and carcasses of cattle were sampled at processing plants (n = 6) located in four geographically distant regions from July 2005 to April 2006. The mean prevalences of Salmonella on hides, preevisceration carcasses (immediately after hide removal), and postintervention carcasses (in the chiller and after the full complement of interventions) were 89.6%, 50.2%, and 0.8%, respectively. The values for MDR Salmonella enterica strains (defined as those resistant to two or more antimicrobials) as percentages of Salmonella prevalence were 16.7% (95% confidence interval [CI], 8.3 to 25.1%; median percent prevalence, 6.9%), 11.7% (95% CI, 4.4 to 19.0%; median, 4.8%), and 0.33% (95% CI, -0.3 to 0.70%; median, 0%), respectively. In this study, 16,218 Salmonella hide and carcass isolates were screened for antimicrobial resistance. Of these, 978 (6.0%) unique MDR S. enterica isolates were identified and serotyped and their XbaI pulsed-field gel electrophoresis (PFGE) profiles determined. The predominant MDR S. enterica serotypes observed were Newport (53.1%), Typhimurium (16.6%), and Uganda (10.9%). Differences in MDR S. enterica prevalence were detected, and PFGE analysis revealed both epidemic clusters (profiles found in plants in multiple regions/seasons) and endemic clusters (profiles observed in plants in limited regions/seasons) within several of the MDR serotypes examined. Despite these differences, multiple-hurdle processing interventions employed at all plants were found to be quite effective and decreased Salmonella carcass contamination by 98.4% (95% CI, 97.6 to 99.7%).     

Molecular epidemiology of Salmonella enterica serovar typhimurium isolates from cattle in hokkaido, Japan: evidence of clonal replacement and characterization of the disseminated clone

The molecular epidemiology of 545 Salmonella enterica serovar Typhimurium isolates collected between 1977 and 2009 from cattle in Hokkaido, Japan, was investigated using pulsed-field gel electrophoresis (PFGE). Nine main clusters were identified from 116 PFGE patterns. Cluster I comprised 248 isolates, 243 of which possessed a sequence specific to definitive phage type 104 (DT104) or U302. The cluster I isolates were dominant in 1993 to 2003, but their numbers declined beginning in 2004. Beginning in 2002, an increase was observed in the number of cluster VII isolates, consisting of 21 PFGE patterns comprising 165 isolates. A total of 116 isolates representative of the 116 PFGE profiles were analyzed by multilocus variable-number tandem-repeat analysis (MLVA). Other than two drug-sensitive isolates, 19 isolates within cluster VII were classified in the same cluster by MLVA. Among the cluster VII isolates, an antibiotic resistance type showing resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, kanamycin, cefazolin, and sulfamethoxazole-trimethoprim and a resistance type showing resistance to ampicillin, streptomycin, sulfonamides, tetracycline, and kanamycin were found in 23 and 125 isolates, respectively. In the 19 isolates representative of cluster VII, the bla(TEM-1) gene was found on a Salmonella serotype Typhimurium virulence plasmid, which was transferred to Escherichia coli by electroporation along with resistance to two to four other antimicrobials. Genomic analysis by subtractive hybridization and plasmid analysis suggested that the bla(TEM-1)-carrying virulence plasmid has a mosaic structure composed of elements of different origin. These results indicate an emerging multidrug-resistant S. Typhimurium clone carrying a virulence-resistance plasmid among cattle in Hokkaido, Japan.     

Acquisition of resistance to extended-spectrum cephalosporins by Salmonella enterica subsp. enterica serovar Newport and Escherichia coli in the turkey poult intestinal tract

Salmonella enterica subsp. enterica serovar Newport resistant to the extended-spectrum cephalosporins (ESCs) and other antimicrobials causes septicemic salmonellosis in humans and animals and is increasingly isolated from humans, animals, foods, and environmental sources. Mechanisms whereby serovar Newport bacteria become resistant to ESCs and other classes of antimicrobials while inhabiting the intestinal tract are not well understood. The present study shows that 25.3% of serovar Newport strains isolated from the turkey poult intestinal tract after the animals were dosed with Escherichia coli harboring a large conjugative plasmid encoding the CMY-2 beta-lactamase and other drug resistance determinants acquired the plasmid and its associated drug resistance genes. The conjugative plasmid containing the cmy-2 gene was transferred not only from the donor E. coli to Salmonella serovar Newport but also to another E. coli serotype present in the intestinal tract. Laboratory studies showed that the plasmid could be readily transferred between serovar Newport and E. coli intestinal isolates. Administration of a single dose of ceftiofur, used to prevent septicemic colibacillosis, to 1-day-old turkeys did not result in the isolation of ceftiofur-resistant E. coli or Salmonella serovar Newport. There was a remarkable association between serotype, drug resistance, and plasmid profile among the E. coli strains isolated from the poults. This study shows that Salmonella serovar Newport can become resistant to ESCs and other antibiotics by acquiring a conjugative drug resistance plasmid from E. coli in the intestines.     

Survival of Salmonella enterica serovar Newport in manure and manure-amended soils

Salmonella enterica serovar Newport has undergone a rapid epidemic spread in dairy cattle. This provides an efficient mechanism for pathogen amplification and dissemination into the environment through manure spreading on agricultural land. The objective of this study was to determine the survival characteristics of Salmonella serovar Newport in manure and manure-amended soils where the pathogen may be amplified. A multidrug-resistant (MDR) Salmonella serovar Newport strain and a drug-susceptible (DS) strain, both bovine isolates, were inoculated into dairy manure that was incubated under constant temperature and moisture conditions alone or after being mixed with sterilized or nonsterilized soil. Salmonella serovar Newport concentrations increased by up to 400% in the first 1 to 3 days following inoculation, and a trend of steady decline followed. With manure treatment, a sharp decline in cell concentration occurred after day 35, possibly due to microbial antagonism. For all treatments, decreases in Salmonella serovar Newport concentrations over time fit a first-order kinetic model. Log reduction time was 14 to 32 days for 1 log(10), 28 to 64 days for 2 log(10), and 42 to 96 days for 3 log(10) declines in the organisms' populations from initially inoculated concentrations. Most-probable-number monitoring data indicated that the organisms persisted for 184, 332, and 405 days in manure, manure-amended nonsterilized soil, and manure-amended sterilized soil, respectively. The MDR strain and the DS strain had similar survival patterns.     

Multidrug-resistant Salmonella enterica serovar paratyphi A harbors IncHI1 plasmids similar to those found in serovar typhi

Salmonella enterica serovars Typhi and Paratyphi A cause systemic infections in humans which are referred to as enteric fever. Multidrug-resistant (MDR) serovar Typhi isolates emerged in the 1980s, and in recent years MDR serovar Paratyphi A infections have become established as a significant problem across Asia. MDR in serovar Typhi is almost invariably associated with IncHI1 plasmids, but the genetic basis of MDR in serovar Paratyphi A has remained predominantly undefined. The DNA sequence of an IncHI1 plasmid, pAKU_1, encoding MDR in a serovar Paratyphi A strain has been determined. Significantly, this plasmid shares a common IncHI1-associated DNA backbone with the serovar Typhi plasmid pHCM1 and an S. enterica serovar Typhimurium plasmid pR27. Plasmids pAKU_1 and pHCM1 share 14 antibiotic resistance genes encoded within similar mobile elements, which appear to form a 24-kb composite transposon that has transferred as a single unit into different positions into their IncHI1 backbones. Thus, these plasmids have acquired similar antibiotic resistance genes independently via the horizontal transfer of mobile DNA elements. Furthermore, two IncHI1 plasmids from a Vietnamese isolate of serovar Typhi were found to contain features of the backbone sequence of pAKU_1 rather than pHCM1, with the composite transposon inserted in the same location as in the pAKU_1 sequence. Our data show that these serovar Typhi and Paratyphi A IncHI1 plasmids share highly conserved core DNA and have acquired similar mobile elements encoding antibiotic resistance genes in past decades.     

Evolution and population structure of Salmonella enterica serovar Newport

Salmonellosis caused by Salmonella enterica serovar Newport is a major global public health concern, particularly because S. Newport isolates that are resistant to multiple drugs (MDR), including third-generation cephalosporins (MDR-AmpC phenotype), have been commonly isolated from food animals. We analyzed 384 S. Newport isolates from various sources by a multilocus sequence typing (MLST) scheme to study the evolution and population structure of the serovar. These were compared to the population structure of S. enterica serovars Enteritidis, Kentucky, Paratyphi B, and Typhimurium. Our S. Newport collection fell into three lineages, Newport-I, Newport-II, and Newport-III, each of which contained multiple sequence types (STs). Newport-I has only a few STs, unlike Newport-II or Newport-III, and has possibly emerged recently. Newport-I is more prevalent among humans in Europe than in North America, whereas Newport-II is preferentially associated with animals. Two STs of Newport-II encompassed all MDR-AmpC isolates, suggesting recent global spread after the acquisition of the bla(CMY-2) gene. In contrast, most Newport-III isolates were from humans in North America and were pansusceptible to antibiotics. Newport was intermediate in population structure to the other serovars, which varied from a single monophyletic lineage in S. Enteritidis or S. Typhimurium to four discrete lineages within S. Paratyphi B. Both mutation and homologous recombination are responsible for diversification within each of these lineages, but the relative frequencies differed with the lineage. We conclude that serovars of S. enterica provide a variety of different population structures.     

Clonal expansion may account for high levels of quinolone resistance in Salmonella enterica serovar enteritidis

We have observed a high incidence of isolated nalidixic acid resistance in Salmonella enterica serovar Enteritidis isolates in Ireland, particularly isolates of phage type 1 (PT1). A group of nalidixic acid-resistant (n = 22) and nalidixic acid-susceptible (n = 28) isolates of serovar Enteritidis from multiple sites in Ireland were selected. Isolates were typed by pulsed-field gel electrophoresis (PFGE) with XbaI, and the MICs for nalidixic acid and ciprofloxacin were determined. Mutations associated with nalidixic acid resistance in clinical isolates and laboratory mutants of serovar Enteritidis and 32 nalidixic acid-resistant isolates of 15 other salmonella serovars were identified. PFGE had limited discriminatory power. A specific point mutation (G246T) associated with amino acid substitution Asp87Tyr in the quinolone resistance determining region of the gyrA gene accounted for 95% of all mutations in serovar Enteritidis and for all mutations in PT1 isolates. Greater diversity of mutations was observed among all non-Enteritidis salmonella serovars studied. Rates of nalidixic acid resistance in serovar Enteritidis may predominantly reflect clonal expansion after infrequent mutation or selection events.     

Novel virulence gene and clustered regularly interspaced short palindromic repeat (CRISPR) multilocus sequence typing scheme for subtyping of the major serovars of Salmonella enterica subsp. enterica

Salmonella enterica subsp. enterica is the leading cause of bacterial food-borne disease in the United States. Molecular subtyping methods are powerful tools for tracking the farm-to-fork spread of food-borne pathogens during outbreaks. In order to develop a novel multilocus sequence typing (MLST) scheme for subtyping the major serovars of S. enterica subsp. enterica, the virulence genes sseL and fimH and clustered regularly interspaced short palindromic repeat (CRISPR) loci were sequenced from 171 clinical isolates from nine Salmonella serovars, Salmonella serovars Typhimurium, Enteritidis, Newport, Heidelberg, Javiana, I 4,[5],12:i:-, Montevideo, Muenchen, and Saintpaul. The MLST scheme using only virulence genes was congruent with serotyping and identified epidemic clones but could not differentiate outbreaks. The addition of CRISPR sequences dramatically improved discriminatory power by differentiating individual outbreak strains/clones. Of particular note, the present MLST scheme provided better discrimination of Salmonella serovar Enteritidis strains than pulsed-field gel electrophoresis (PFGE). This method showed high epidemiologic concordance for all serovars screened except for Salmonella serovar Muenchen. In conclusion, the novel MLST scheme described in the present study accurately differentiated outbreak strains/clones of the major serovars of Salmonella, and therefore, it shows promise for subtyping this important food-borne pathogen during investigations of outbreaks.     

Molecular analyses of Salmonella enterica isolates from fish feed factories and fish feed ingredients

Isolates of the most commonly observed salmonella serovars in Norwegian fish feed factories from 1998 to 2000 (Salmonella enterica serovar Agona, S. enterica serovar Montevideo, S. enterica serovar Senftenberg, and S. enterica serovar Kentucky) were studied by pulsed-field gel electrophoresis (PFGE) and plasmid profile analysis and compared to isolates of the same serovars from fish feed ingredients, humans, and other sources (a total of 112 isolates). Within each serovar, a variety of distinct PFGE types (with similarity levels less than 90%) were observed in the feed ingredients and other sources, while only two distinct types of each serovar were identified in the factories. The combined results of PFGE and plasmid analyses showed that each factory harbored only a few S. enterica clones. Some of these clones persisted for at least 3 years in the factories, indicating that there was long-lasting contamination probably due to inadequate decontamination procedures.     

Comparison of Salmonella enterica serovar distribution and antibiotic resistance patterns in wastewater at municipal water treatment plants in two California cities

AIM: To determine Salmonella enterica serovars and antibiotic resistance (ABR) in the human waste stream. METHODS AND RESULTS: Sampling of influent wastewater at municipal treatment plants in two California cities was performed by collecting composite samples, over a 24-h period, from the treatment plants on five to six occasions. Serial water quantities were filtered and cultured with a Salmonella selective method and an oxytetracycline-supplemented Salmonella selective method. Antibiotic susceptibilities to 12 antibiotics were determined and the isolates were grouped based on ABR patterns. From 983 S. enterica isolated, 102 represented unique sampling-serovar-ABR patterns. Thirty-five different serovars were identified to be distributed over 17 different ABR patterns. The serovar distribution differed between the sampling sites, whereas there was no significant trend in levels of multiple ABR. CONCLUSIONS: Salmonella enterica was recovered with ease from small sample volumes of wastewater received by municipal water treatment plants. A large variety of serovars and ABR profiles were represented in the recovered Salmonella. SIGNIFICANCE AND IMPACT OF THE STUDY: The ease of sampling and recovery of Salmonella from municipal wastewater from treatment plants makes it a valuable sampling approach for monitoring the presence of Salmonella in the human population.     

Microarray-based analysis of IncA/C plasmid-associated genes from multidrug-resistant Salmonella enterica

In the family Enterobacteriaceae, plasmids have been classified according to 27 incompatibility (Inc) or replicon types that are based on the inability of different plasmids with the same replication mechanism to coexist in the same cell. Certain replicon types such as IncA/C are associated with multidrug resistance (MDR). We developed a microarray that contains 286 unique 70-mer oligonucleotide probes based on sequences from five IncA/C plasmids: pYR1 (Yersinia ruckeri), pPIP1202 (Yersinia pestis), pP99-018 (Photobacterium damselae), pSN254 (Salmonella enterica serovar Newport), and pP91278 (Photobacterium damselae). DNA from 59 Salmonella enterica isolates was hybridized to the microarray and analyzed for the presence or absence of genes. These isolates represented 17 serovars from 14 different animal hosts and from different geographical regions in the United States. Qualitative cluster analysis was performed using CLUSTER 3.0 to group microarray hybridization results. We found that IncA/C plasmids occurred in two lineages distinguished by a major insertion-deletion (indel) region that contains genes encoding mostly hypothetical proteins. The most variable genes were represented by transposon-associated genes as well as four antimicrobial resistance genes (aphA, merP, merA, and aadA). Sixteen mercury resistance genes were identified and highly conserved, suggesting that mercury ion-related exposure is a stronger pressure than anticipated. We used these data to construct a core IncA/C genome and an accessory genome. The results of our studies suggest that the transfer of antimicrobial resistance determinants by transfer of IncA/C plasmids is somewhat less common than exchange within the plasmids orchestrated by transposable elements, such as transposons, integrating and conjugative elements (ICEs), and insertion sequence common regions (ISCRs), and thus pose less opportunity for exchange of antimicrobial resistance.     

Diversity and Antimicrobial Resistance of Salmonella enterica Isolates from Surface Water in Southeastern United States

A study of prevalence, diversity, and antimicrobial resistance of Salmonella enterica in surface water in the southeastern United States was conducted. A new scheme was developed for recovery of Salmonella from irrigation pond water and compared with the FDA''s Bacteriological Analytical Manual (8th ed., 2014) (BAM) method. Fifty-one isolates were recovered from 10 irrigation ponds in produce farms over a 2-year period; nine Salmonella serovars were identified by pulsed-field gel electrophoresis analysis, and the major serovar was Salmonella enterica serovar Newport (S. Newport, n = 29), followed by S. enterica serovar Enteritidis (n = 6), S. enterica serovar Muenchen (n = 4), S. enterica serovar Javiana (n = 3), S. enterica serovar Thompson (n = 2), and other serovars. It is noteworthy that the PulseNet patterns of some of the isolates were identical to those of the strains that were associated with the S. Thompson outbreaks in 2010, 2012, and 2013, S. Enteritidis outbreaks in 2011 and 2013, and an S. Javiana outbreak in 2012. Antimicrobial susceptibility testing confirmed 16 S. Newport isolates of the multidrug resistant-AmpC (MDR-AmpC) phenotype, which exhibited resistance to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (ACSSuT), and to the 1st, 2nd, and 3rd generations of cephalosporins (cephalothin, amoxicillin-clavulanic acid, and ceftriaxone). Moreover, the S. Newport MDR-AmpC isolates had a PFGE pattern indistinguishable from the patterns of the isolates from clinical settings. These findings suggest that the irrigation water may be a potential source of contamination of Salmonella in fresh produce. The new Salmonella isolation scheme significantly increased recovery efficiency from 21.2 (36/170) to 29.4% (50/170) (P = 0.0002) and streamlined the turnaround time from 5 to 9 days with the BAM method to 4 days and thus may facilitate microbiological analysis of environmental water.     

Cost-effective application of pulsed-field gel electrophoresis to typing of Salmonella enterica Serovar Typhimurium

Salmonella enterica serovar Typhimurium is frequently isolated from humans and animals. Phage typing is historically the first-line reference typing technique in Europe. It is rapid and convenient for laboratories with appropriate training and experience, and costs of consumables are low. Phage typing and pulsed-field gel electrophoresis (PFGE) were performed on 503 isolates of serovar Typhimurium. Twenty-nine phage types and 53 PFGE patterns were observed. Most isolates of phage types DT104, DT104b, and U310 are not distinguishable from other members of their phage type by PFGE. By contrast, PFGE of isolates of phage types DT193 and U302 shows great heterogeneity. Analysis of experience with PFGE and phage typing can facilitate the selective application of PFGE to maximize the yield of epidemiologically relevant additional information while controlling costs.     

Characterization of Salmonella enterica serovar typhimurium from marine environments in coastal waters of Galicia (Spain)

Twenty-three Salmonella enterica serovar Typhimurium isolates from marine environments were characterized by phage typing, pulsed-field gel electrophoresis (PFGE) analysis, plasmid analysis, and antibiotic resistance, and the distribution of the different types in the coastal waters were subsequently analyzed. Five phage types were identified among the isolates (PT41, PT135, PT99, DT104, and DT193). PT135 isolates were exclusively detected during the winter months from 1998 to 2000, whereas DT104 and PT41 isolates were detected exclusively in the summer months from 2000 to 2002. XbaI PFGE analysis revealed 9 PFGE types, and plasmid profiling identified 8 plasmid types (with 1 to 6 plasmids) among the isolates. Only three isolates presented multidrug resistance to antibiotics. Two DT104 isolates were resistant to 8 and 7 antibiotics (profiles ACCeFNaSSuT and ACeFNeSSuT), whereas a PT193 isolate presented resistance to 6 antibiotics (profile ACFSSu). In addition, four PT41 isolates were resistant to a single antibiotic. The detection of multidrug-resistant phage types DT104 and DT193 in shellfish emphasizes the importance of monitoring the presence of Salmonella in routine surveillance of live bivalve molluscs.     

Genetic and phenotypic variability among Salmonella enterica serovar Typhimurium isolates from California dairy cattle and humans

Fifty-six human and 24 adult dairy cattle isolates of Salmonella enterica serovar Typhimurium from a single county in California were compared using ribotyping, insertion sequence typing (IS200), pulsed-field gel electrophoresis, plasmid typing, phage typing, and antimicrobial resistance testing. The majority of the isolates fell into one of two groups which were phage types DT104 and DT193. Combining the information from all typing methods, a total of 45 different "clusters" were defined, with 35 of those including only a single isolate. The library of isolates had a high degree of variability, but antibiotic resistance and plasmid typing each defined single clusters in which human or bovine isolates predominated (chi2, P < 0.05).     

Identification of Specific Gene Sequences Conserved in Contemporary Epidemic Strains of Salmonella enterica

Genetic elements specific to recent and contemporary epidemic strains of Salmonella enterica were identified using comparative genomic analysis. Two epidemic multidrug-resistant (MDR) strains, MDR Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) and cephalosporin-resistant MDR Salmonella enterica serovar Newport, and an epidemic pansusceptible strain, Salmonella serovar Typhimurium DT160, were subjected to Salmonella gene microarray and suppression subtractive hybridization analyses. Their genome contents were compared with those of coexisting sporadic strains matched by serotype, geographic and temporal distribution, and host species origin. These paired comparisons revealed that epidemic strains of S. enterica had specific genes and gene regions that were shared by isolates of the same subtype. Most of these gene sequences are related to mobile genetic elements, including phages, plasmids, and plasmid-like and transposable elements, and some genes may encode proteins conferring growth or survival advantages. The emergence of epidemic MDR strains may therefore be associated with the presence of fitness-associated genetic factors in addition to their antimicrobial resistance genes.     

Salmonella enterica serotype Bredeney: antimicrobial susceptibility and molecular diversity of isolates from Ireland and Northern Ireland.

Salmonella enterica serotype Bredeney has emerged as the third most commonly identified serotype among human clinical isolates referred to the Irish National Salmonella Reference Laboratory in the years 1998 to 2000. A collection of 112 isolates of S. enterica serotype Bredeney collected during the period 1995 to 1999 from animal, food, and human sources from both Ireland and Northern Ireland were studied. Antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), and DNA amplification fingerprinting (DAF) were performed on all isolates. Plasmid profiles were examined on a subset of 33 isolates. A high proportion (74%) of isolates were susceptible to all antimicrobial agents tested. Resistance to both sulfonamide and trimethoprim was observed in 21% of isolates, and resistance to multiple (five) antimicrobial agents was observed in a single isolate (0.9%). Eight different PFGE patterns were obtained, with 87% of isolates grouping as PFGE type A. PFGE type A was predominant in animals, food, and humans. There was good overall concordance between the groups identified by PFGE and DAF. Overall results indicate that most S. enterica serotype Bredeney isolates in Ireland and Northern Ireland from animal and human sources are clonally related.