Increasing incidence and comparison of nalidixic acid-resistant Salmonella enterica subsp. enterica serotype typhimurium isolates from humans and animals

Antibiotic use in animal feed and its impact on human healt

Antibiotic resistance in bacteria that cause disease in man is an issue of major concern. Although misuse of antibiotics in human medicine is the principal cause of the problem, antibiotic-resistant bacteria originating in animals are contributory factors, with some types of resistance in some species of bacteria. Antibiotics are added to animal feeds to treat and prevent infections and to improve growth and production. Until recently, the major concerns about incorporation of antibiotics in animal feeds related to antibiotic residues in products from treated animals. Although, in 1969, the Swann (1969) report drew attention to the potential for antibiotic-resistant bacteria to spread from treated animals via the food chain, there was little response until the detection of vancomycin-resistant enterococci in animals fed a related glycopeptide, avoparcin. Subsequently, attention started to focus on the issue and other examples of transfer of resistant bacteria through the food chain, such as enterococci resistant to quinupristin-dalfopristin or to everninomicin, fluoroquinolone-resistant campylobacters and multiresistant Escherichia coli, and salmonella such as Salmonella typhimurium DT104. Reviews and committees in many countries have highlighted the need for better control of licensing of antibiotics, and codes for prudent use of antibiotics by veterinary practitioners and farmers. The continued use of antibiotic growth promoters has been questioned and there is a need to ensure that antibiotics important in human medicine are not used therapeutically or prophylactically in animals.

Antimicrobial resistance of 100 Salmonella strains isolated from Gallus gallus in 4 wilayas of Algeria

This study aims at identifying serotypes and surveying the antimicrobial resistance and plasmid support of resistance of 100 Salmonella strains, which were isolated from 96 out of 506 (18.97%) samples taken from different production farms in the wilayas (i.e., Algerian states) of Tizi-Ouzou, Bouira, Bejaïa, and Boumerdes in 2007. The highest percentage of Salmonella (48%) was recorded in Bouira. Thirteen serotypes were identified among the 100 Salmonella strains used in this study. The most prevalent ones were Salmonella Heidelberg (24%), Salmonella Enteritidis (20%), Salmonella Albany (16%), and Salmonella Typhimurium (9%). The strains showed resistance to 8 of the 34 antibiotics tested. Fifty-three percent of strains were resistant to at least one antibiotic, among which 15.09% were multiresistant. The most frequently observed resistance was to quinolones (58.49%), with a contribution of 94.74% of Salmonella Heidelberg resistant strains. The plasmid transfer performed on 53 strains showed that only 11 exhibited one or more markers of resistance, the most frequent being ampicillin, followed by tetracycline, then cotrimoxazole, sulphonamides, and kanamycin, in that order. The tetracycline characteristics were present in 72.72% of transconjugants, those of the β-lactams and sulphonamides in 27.27% each and those of the aminosides in 9.09%. The incompatibility groups of plasmids belong to the F1me and Com1 classes, and the molecular weight of the plasmid DNA was greater than 100 kb. The phenotypic and genotypic results indicate a clonal dissemination in the Gallus gallus species in this particular study; this phenomenon could generate resistant bacteria and transferable genes of resistance to humans.

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.

DNA gyrase and topoisomerase IV mutations in an in vitro fluoroquinolone-resistant Coxiella burnetii strain

The etiological agent of Q fever, Coxiella burnetii, is an obligate intracellular bacterium that multiplies within a vacuole with lysosomal characteristics. Quinolones have been used as an alternative therapy for Q fever. In this study, quinolone-resistance-determining regions of the genes coding for DNA gyrase and topoisomerase IV were analyzed by DNA sequencing from an in vitro fluoroquinolone-resistant C. burnetii strain (Q212). Sequencing and aligning of DNA gyrase encoding genes (gyrA and gyrB) and topoisomerase IV genes (parC and parE) revealed one gyrA mutation leading to the amino acid substitution Asp87Gly (Escherichia coli numbering), two gyrB mutations leading to the amino acid substitutions Ser431Pro and Met518Ile, and three parC mutations leading to the amino acid substitutions Asp69Asn, Thr80Ile, and Gly104Ser. The corresponding alignment of the C. burnetii Q212 reference strain, the in vitro developed fluoroquinolone-resistant C. burnetii Q212 strain, and E. coli resulted in the identification of several other naturally occurring mutations within and outside the quinolone-resistance-determining regions of C. burnetii providing indications of possible natural resistance to fluoroquinolones. The present study adds additional potential mutations in the DNA topoisomerases that may be involved in fluoroquinolone resistance in C. burnetii due to their previous characterization in other bacterial species.

Role of novel gyrA mutations in the suppression of the fluoroquinolone resistance genotype of vaccine strain Salmonella Typhimurium vacT (gyrA D87G)

OBJECTIVES

This study was aimed at characterizing the gyrA locus and determining its impact on fluoroquinolone susceptibility, DNA supercoiling degree and growth rate of Salmonella Typhimurium live vaccine strain vacT in comparison with its parent M415. Furthermore, the role of multiple drug resistance efflux in the susceptibility of vacT to fluoroquinolones and macrolides was investigated.

METHODS

DNA sequences were determined for genes gyrA, gyrB, parC and parE of M415 and three consecutive mutants Nal2ori, Nal2passage and vacT. The impact of gyrA mutations on the fluoroquinolone susceptibilities and relative DNA supercoiling degrees was investigated by a complementation assay using wild-type gyrA (gyrA+) and a reporter gene system, respectively. Doubling times of the strains and MICs of different antibiotics in the absence and presence of an efflux pump inhibitor (EPI) were determined.

RESULTS

Besides the gyrA mutation D87G, two novel mutations (G75A and A866S) were identified in the three mutants and a third novel mutation W59R in vacT. Fluoroquinolone susceptibilities and DNA supercoiling degrees of all three mutants were reduced compared with those of M415. Introduction of the gyrA+ allele restored fluoroquinolone susceptibilities of the two intermediate strains to the wild-type level; however, for vacT, MICs of fluoroquinolones were reduced below those of M415. VacT had a higher susceptibility to macrolides and the EPI compared with M415.

CONCLUSIONS

The data point to a combination of at least one non-gyrA mutation and novel gyrA mutation(s) as the basis for the unusual fluoroquinolone susceptibility of vacT.

Analysis of Salmonella spp. with resistance to extended-spectrum cephalosporins and fluoroquinolones isolated in North America and Latin America: report from the SENTRY Antimicrobial Surveillance Program (1997–2004)

Emerging antimicrobial-resistant Salmonella spp. requires increased efforts to appropriately test susceptibility. The SENTRY Antimicrobial Surveillance Program monitored Salmonella spp. and detected nalidixic acid-resistant strains with elevated fluoroquinolone minimum inhibitory concentration (MIC) results and strains with extended-spectrum beta-lactamase (ESBL) "phenotypes" over the last 8 years. A total of 786 stool and bloodstream isolates from North American and Latin American medical centers (2001-2003) were tested by reference broth microdilution methods. Genetic analysis was used to further characterize the resistance mechanisms. Twenty-one sites forwarded 89 (11.3%) nalidixic acid-resistant (MIC, > or =32 microg/mL) strains. Nineteen of these isolates were studied to determine mutations in the quinolone resistance-determining region (QRDR). Among the nalidixic acid-resistant Salmonella spp. isolates, fluoroquinolone MIC values were also elevated (8- to 32-fold) compared with "wild-type" strains. Ciprofloxacin and gatifloxacin (MIC(90), 0.5 microg/mL) were more potent than levofloxacin and garenoxacin (1 microg/mL) against nalidixic acid-resistant strains. Single gyrA mutations were responsible for elevated fluoroquinolone MIC values and included D87Y (5), S83F (7), D87N (5), and S83Y (2). During 2001, 9 sites contributed 11 (2.9%) strains that met ESBL screening criteria (> or =2 microg/mL) for aztreonam or ceftazidime or ceftriaxone. ESBL confirmation was evaluated by Etest (AB BIODISK, Solna, Sweden) ESBL strips and the enzymes were characterized by polymerase chain reaction and gene sequencing. The ESBL phenotype isolates had the following MIC patterns: ceftazidime (> or =16 microg/mL), aztreonam (4 to >16 microg/mL), and ceftriaxone (8-32 microg/mL). All strains were susceptible to cefepime, carbapenems, gentamicin, and fluoroquinolones. No strains were inhibited by clavulanic acid consistent with all isolates producing the identified CMY-2, AmpC-like enzyme. Fluoroquinolones may be compromised among isolates with QRDR mutations detected using nalidixic acid as a screening agent. Salmonella spp. with ESBL phenotypes were likely to harbor CMY-2 (not an ESBL) and remain susceptible to cefepime, carbapenems, and fluoroquinolones, which can be used for serious invasive Salmonella spp. infections. Compared with the stool culture isolates, the blood culture isolates had higher QRDR mutations, but remained susceptible to the fluoroquinolones. The blood culture isolates were more susceptible to penicillins (ampicillin and ticarcillin) and not significantly different for ceftriaxone or trimethoprim/sulfamethoxazole susceptibility patterns. No QRDR trends over time were detected in North America, but increased resistance was observed in Latin America.

Veterinary Drug Usage and Antimicrobial Resistance in Bacteria of Animal Origin

In the production of food animals, large amounts of antimicrobial agents are used for therapy and prophylaxis of bacterial infections and in feed to promote growth. There are large variations in the amounts of antimicrobial agents used to produce the same amount of meat among the different European countries, which leaves room for considerable reductions in some countries. The emergence of resistant bacteria and resistance genes due to the use of antimicrobial agents are well documented. In Denmark it has been possible to reduce the usage of antimicrobial agents for food animals significantly and in general decreases in resistance have followed. Guidelines for prudent use of antimicrobial agents may help to slow down the selection for resistance and should be based on knowledge regarding the normal susceptibility patterns of the causative agents and take into account the potential problems for human health. Current knowledge regarding the occurrence of antimicrobial resistance in food animals, the quantitative impact of the use of different antimicrobial agents on selection of resistance and the most appropriate treatment regimes to limit the development of resistance is incomplete. Programmes monitoring the occurrence and development of resistance and consumption of antimicrobial agents are strongly desirable, as is research into the most appropriate ways to use antimicrobial agents in veterinary medicine.

The importance of active efflux systems in the quinolone resistance of clinical isolates of Salmonella spp

The aim of this study was to determine the importance of the active elimination of antibiotics by active efflux systems, in the decrease in fluoroquinolone sensitivity of clinical isolates of Salmonella spp. as well as the intrinsic antibiotic activity of certain active efflux system inhibitors. The effect of the active efflux system on the decrease in sensitivity to nalidixic acid, ciprofloxacin, ofloxacin and sparfloxacin was studied by investigating the variation in the in vitro activity of these compounds when assayed in association with reserpine and MC 207.110. The active efflux systems inhibited by reserpine displayed low activity in the elimination of these compounds, whereas those inhibited by MC 207.110 showed high activity in the elimination of nalidixic acid and sparfloxacin, but were less effective in the elimination of ofloxacin and ciprofloxacin. These two compounds did not exhibit intrinsic inhibitory activity against Salmonella spp. at the concentrations assayed. These mechanisms of resistance to antibiotics are complex and vary depending on the chemical composition of the antibiotics used, and perhaps the inhibitors of these systems, although they do not exhibit any intrinsic antibiotic activity, may be used as adjuvants to increase the activity of certain antibiotics. These mechanisms complement the mutations in the gyrA gene and this supports the thesis that it is necessary to lower the breakpoint established by the NCCLS for ciprofloxacin, since the strains studied have resistance mechanisms that reduce the activity of this drug and may favour the emergence of resistant mutants during treatment.

Mutations in topoisomerase genes of fluoroquinolone-resistant salmonellae in Hong Kong

A total of 88 salmonella isolates (72 clinical isolates for which the ciprofloxacin MIC was >0.06 microg/ml, 15 isolates for which the ciprofloxacin MIC was < or =0.06 microg/ml, and Salmonella enterica serotype Typhimurium ATCC 13311) were studied for the presence of genetic alterations in four quinolone resistance genes, gyrA, gyrB, parC, and parE, by multiplex PCR amplimer conformation analysis. The genetic alterations were confirmed by direct nucleotide sequencing. A considerable number of strains had a mutation in parC, the first to be reported in salmonellae. Seven of the isolates sensitive to 0.06 micro g of ciprofloxacin per ml had a novel mutation at codon 57 of parC (Tyr57-->Ser) which was also found in 29 isolates for which ciprofloxacin MICs were >0.06 micro g/ml. Thirty-two isolates had a single gyrA mutation (Ser83-->Phe, Ser83-->Tyr, Asp87-->Asn, Asp87-->Tyr, or Asp87-->Gly), 34 had both a gyrA mutation and a parC mutation (29 isolates with a parC mutation of Tyr57-->Ser and 5 isolates with a parC mutation of Ser80-->Arg). Six isolates which were isolated recently (from 1998 to 2001) were resistant to 4 micro g of ciprofloxacin per ml. Two of these isolates had double gyrA mutations (Ser83-->Phe and Asp87-->Asn) and a parC mutation (Ser80-->Arg) (MICs, 8 to 32 microg/ml), and four of these isolates had double gyrA mutations (Ser83-->Phe and Asp87-->Gly), one parC mutation (Ser80-->Arg), and one parE mutation (Ser458-->Pro) (MICs, 16 to 64 micro g/ml). All six of these isolates and those with a Ser80-->Arg parC mutation were S. enterica serotype Typhimurium. One S. enterica serotype Typhi isolate harbored a single gyrA mutation (Ser83-->Phe), and an S. enterica serotype Paratyphi A isolate harbored a gyrA mutation (Ser83-->Tyr) and a parC mutation (Tyr57-->Ser); both of these isolates had decreased susceptibilities to the fluoroquinolones. The MICs of ciprofloxacin, levofloxacin, and sparfloxacin were in general the lowest of those of the six fluoroquinolones tested. Isolates with a single gyrA mutation were less resistant to fluoroquinolones than those with an additional parC mutation (Tyr57-->Ser or Ser80-->Arg), while those with double gyrA mutations were more resistant.

Topoisomerase IV mutations in quinolone-resistant salmonellae selected in vitro

The development of high-level fluoroquinolone resistance has rarely been observed in salmonellae and, in contrast to other Gram-negative bacteria mutations affecting topoisomerase IV, a known secondary target of quinolones in Escherichia coli has not been described except for one recent report. The present study used quinolone-susceptible field isolates representing epidemiologically relevant serovars and phage types Salmonella Hadar and Salmonella Typhimurium DT104 and DT204c to select fluoroquinolone-resistant mutants in vitro. Three selection steps were necessary to obtain high-level fluoroquinolone-resistant mutants (MICCip > or = 8 microg/ml). All first-step mutants examined had a single gyrA mutation (affecting either Ser83 or Asp87). Additional topoisomerase mutations affecting gyrA (Asp87), gyrB (Ser464), and parC (Gly78) were detected in second- and third-step mutants. Introducing into the respective mutants the corresponding plasmid-coded quinolone-susceptible allele of either gyrA, gyrB, or parC resulted in reduction of quinolone resistance, indicating a role for these mutations in quinolone resistance. In the presence of an inhibitor of RND-type efflux pumps, the susceptibilities to ciprofloxacin and chloramphenicol of second- and third-step mutants increased by two to four serial dilution steps, providing evidence that an efflux-mediated resistance mechanism contributes to the development of high-level fluoroquinolone resistance in salmonellae.

Reevaluating fluoroquinolone breakpoints for Salmonella enterica serotype Typhi and for non-Typhi salmonellae

Salmonella enterica infections cause considerable morbidity and mortality worldwide. Antimicrobial therapy may be life-saving for patients with extraintestinal infections with S. enterica serotype Typhi or non-Typhi salmonellae. Because antimicrobial resistance to several classes of traditional first-line drugs has emerged in the past several decades, the quinolone antimicrobial agents, particularly the fluoroquinolones, have become the drugs of choice. Recently, resistance to nalidixic acid has emerged among both Typhi and non-Typhi Salmonella serotypes. Such Salmonella isolates typically also have decreased susceptibility to fluoroquinolones, although minimum inhibitory concentrations of the fluoroquinolones usually are within the susceptible range of the interpretive criteria of the NCCLS. A growing body of clinical and microbiological evidence indicates that such nalidixic acid-resistant S. enterica infections also exhibit a decreased clinical response to fluoroquinolones. In this article, we recommend that laboratories test extraintestinal Salmonella isolates for nalidixic acid resistance, we recommend that short-course fluoroquinolone therapy be avoided for infection with nalidixic acid-resistant extraintestinal salmonellae, and we summarize existing data and data needs that would contribute to reevaluation of the current NCCLS fluoroquinolone breakpoints for salmonellae.

The AcrB multidrug transporter plays a major role in high-level fluoroquinolone resistance in Salmonella enterica serovar typhimurium phage type DT204

Salmonella enterica serovar Typhimurium phage type DT204 strains isolated from cattle and animal feed in Belgium were characterized for high-level fluoroquinolone resistance mechanisms [MICs to enrofloxacin (Enr) and ciprofloxacin (Cip), 64 and 32 microg/ml, respectively]. These strains isolated during the periods 1991-1994, and in 2000 were clonally related as shown by pulsed-field gel electrophoresis (PFGE). Selected strains studied carried several mutations in the quinolone target genes, i.e., a double mutation in the quinolone resistance-determining region (QRDR) of gyrA leading to amino acid changes Ser83Ala and Asp87Asn, a single mutation in the QRDR of gyrB leading to amino acid change Ser464Phe, and a single mutation in the QRDR of parC leading to amino acid change Ser80Ile. Moreover, Western blot analysis showed overproduction of the AcrA periplasmic protein belonging to the AcrAB-ToIC efflux system. This suggested active efflux as additional resistance mechanism resulting in a multiple antibiotic resistance (MAR) phenotype, which was measurable by an increased level of resistance to the structurally unrelated antibiotic florfenicol in the absence of the specific floR resistance gene. The importance of the AcrAB-TolC efflux system in high-level fluoroquinolone resistance was further confirmed by inactivating the acrB gene coding for the multidrug transporter. This resulted in a 32-fold reduction of resistance level to Enr (MIC = 2 microg/ml) and actually in a susceptible phenotype according to clinical breakpoints. Thus, AcrB plays a major role in high-level fluoroquinolone resistance, even when multiple target gene mutations are present. The same effect was obtained using the recently identified efflux pump inhibitor (EPI) Phe-Arg-naphthylamide also termed MC207,110. Among several fluoroquinolones tested in combination with EPI, the MIC of Enr was reduced most significantly. Thus, using EPI together with fluoroquinolones such as Enr may be promising in combination therapy against high-level fluoroquinolone-resistant S. enterica serovar Typhimurium.

Phenotypic and genotypic characterization of food animal isolates of Salmonella with reduced sensitivity to ciprofloxacin

Reports of nontyphoidal Salmonella enterica subsp. enterica showing reduced sensitivity to ciprofloxacin (RSC) have increased rapidly during the past decade. Infection in humans with Salmonella possessing RSC may compromise the effectiveness of ciprofloxacin therapy. Nineteen among 4,357 Salmonella strains isolated from food animals in Canada from 1998 to 1999 showed RSC; 17 were from turkeys and 2 from chickens. All were resistant to nalidixic acid and sulfisoxazole and possessed RSC at a level of 0.125-0.5 microg/ml. PCR-RFLP of the gyrA quinolone resistance-determining region (QRDR) with Hinfl revealed that S. Bredeney and S. Heidelberg isolates possessed a mutation in this region. Single-strand conformational polymorphism (SSCP) analysis showed that S. Schwarzengrund and S. Senftenberg isolates also possessed a point mutation in the QRDR. DNA sequencing confirmed the findings and showed that all isolates possessed a base substitution in the gyrA QRDR. Sequencing revealed no mutations in the gyrB and silent wobble mutations in the parC QRDR. Reserpine, a known efflux pump inhibitor, did not effect the MICs for ciprofloxacin, nalidixic acid, and tetracycline. The mar operon could be induced in all isolates at 37 degrees C and in 18 of 19 at 30 degrees C; induction resulted in a two- to four-fold increase in the MIC of ciprofloxacin. In 14 of the 19 isolates, the mutation rate was two-fold or higher than in a ciprofloxacin sensitive S. Bredeney and S. Typhimurium LT2 control strain. Examination of clonal relatedness using pulsed-field gel electrophoresis (PFGE) and plasmid profiles indicated that some degree of clonal dispersion may have occurred, but the majority of isolates may have arisen from de novo mutations.

Resistance to quinolones in Campylobacter jejuni and Campylobacter coli from Danish broilers at farm level

AIMS

To investigate the prevalence of quinolone resistance among Campylobacter jejuni and Camp. coli isolates from Danish poultry at the farm level, as well as for the whole country.

METHODS AND RESULTS

Data and isolates were collected from a national surveillance of Campylobacter in poultry. Quinolone resistance was investigated by determination of minimum inhibitory concentration (MIC) to nalidixic acid and enrofloxacin. Among Camp. jejuni and Camp. coli combined, 7.5% were resistant to nalidixic acid. Quinolone resistance varied considerably from farm to farm, with 0% on some farms and almost 100% on others, but the resistance was evenly distributed geographically. With respect to isolates from farms where resistance was detected, quinolone resistance was higher among Camp. coli (28.7%) than among Camp. jejuni (11.3%). PFGE typing of quinolone-resistant and quinolone-susceptible isolates from four farms indicated that certain resistant isolates belonged to specific clones that were able to persist on the farms during several rotations, even in the absence of selective pressure. Some clones were present and repeatedly isolated in both a quinolone-susceptible and quinolone-resistant variant.

CONCLUSIONS

Overall, quinolone resistance among Campylobacter isolates from Danish broilers was 7.5% in 1998 and 1999; it was higher among Camp. coli than Camp. jejuni. Genetic diversity among resistant isolates was lower than among susceptible isolates, and certain clones existed in both a resistant and a susceptible variant. Some resistant clones appeared to persist on the farms and were repeatedly isolated from poultry flocks.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study is important for the understanding of persistence and dynamics of Campylobacter in broiler houses. It also highlights the extent, farm-to-farm variation and persistence of quinolone-resistant Campylobacter in broiler houses.

Multiple genetic typing of Salmonella enterica serotype typhimurium isolates of different phage types (DT104, U302, DT204b, and DT49) from animals and humans in England, Wales, and Northern Ireland

Salmonella enterica serotype Typhimurium is a common cause of salmonellosis among humans and animals in England, Wales, and Northern Ireland. Phage types DT104 and U302 were the most prevalent types in both livestock and humans in 2001. In addition, Salmonella serotype Typhimurium DT204b was responsible for a recent international outbreak involving England. A total of 119 isolates from humans (n = 28) and animals or their environment (n = 91), belonging to DT104 (n = 66), U302 (n = 33), DT204b (n = 12), and DT49 (n = 8), were fingerprinted by a combination of well-established genetic methods (pulsed-field gel electrophoresis [PFGE], PstI/SphI [PS] ribotyping, and plasmid profiling). The different techniques identified different degrees of polymorphism (from greatest to least, plasmid profiling [40 types], PS ribotyping [34 types], and PFGE [23 types]). It seems clear that a prevalent genomic clone, as well as a variety of less frequent clones, is present for each of the phage types. In most cases, the prevalent clones appeared within isolates from several animal species and from several geographical locations. We did not find clear evidence of a higher degree of diversity for any of the animal species included, or of any link between isolates from particular animal species and humans. The data presented show the inaccuracy of drawing epidemiological conclusions based on a single fingerprinting method. Strains that share one of the markers do not necessarily belong to the same clone, and a multiple typing approach is required to enable enough discrimination to track strains for epidemiological investigations.

Serovars of Salmonella isolated from Danish turkeys between 1995 and 2000 and their antimicrobial resistance

The prevalence of Salmonella serovars and their antimicrobial resistance patterns were investigated among Danish turkeys between 1995 and 2000, by sampling the flocks approximately 14 days before they were slaughtered. Within the flocks, the prevalence of salmonella varied from 7.1 per cent to 25 per cent, and 24 different serovars were detected. The five most prevalent, which accounted for 58.5 per cent of the isolates were Salmonella Heidelberg (16.2 per cent of the isolates), Salmonella Agona (15.8 per cent), Salmonella Derby (12.4 per cent), Salmonella Muenster (7.3 per cent) and Salmonella Anatum (6.8 per cent). In addition, a few rough isolates and isolates belonging to the antigenically incomplete formulae 6,7:-:- and 4,12:b:- were found. The level of antimicrobial resistance was low; the highest resistance was recorded to ampicillin (13.7 per cent) and streptomycin (9.0 per cent) followed by tetracycline (8.5 per cent), sulphonamides (7.7 per cent) and spectinomycin (4.7 per cent). Resistance to quinolones was very low: four isolates were resistant to nalidixic acid, and only one was resistant to enrofloxacin. No resistance was recorded to colistin, apramycin, ceftiofur, florfenicol, or amoxycillin with clavulanic acid. Only 24 isolates were resistant to two or more compounds in various combinations of up to six compounds; one Salmonella Havana isolate was resistant to six compounds. Six isolates were serovar Typhimurium, but none of them belonged to phage type DT104.

Comparison of gyrA mutations, cyclohexane resistance, and the presence of class I integrons in Salmonella enterica from farm animals in England and Wales

This study is focused on real-time detection of gyrA mutations and of the presence of class I integrons in a panel of 100 veterinary isolates of Salmonella enterica from farm animals. The isolates were selected on the basis of resistance to nalidixic acid, representing a variety of the most prevalent serotypes in England and Wales. In addition, organic solvent (cyclohexane) resistance in these isolates was investigated in an attempt to elucidate the presence of efflux pump mechanisms. The most prevalent mutation among the isolates studied was Asp87-Asn (n = 42), followed by Ser83-Phe (n = 38), Ser83-Tyr (n = 12), Asp87-Tyr (n = 4), and Asp87-Gly (n = 3). Two distinct subpopulations were identified, separated at the 1-mg/liter breakpoint for ciprofloxacin: 86% of isolates with mutations in codon 83 showed MICs of >or=1 mg/liter, while 89.8% of isolates with mutations in codon 87 presented MICs of <or=0.5 mg/liter. Cyclohexane resistance was more prevalent among Ser83 mutants than among Asp87 mutants (34.7 and 4%, respectively), and in 79% of isolates that presented both gyrA mutations and cyclohexane resistance, the level of ciprofloxacin resistance was >or=2.0 mg/liter. Thirty-four isolates contained class I integrons, with 71% of the S. enterica serovar Typhimurium isolates and 6.9% of isolates belonging to other serotypes containing such elements. The methods used represent sensitive ways of investigating the presence of gyrA mutations and of detecting class-I integrons in Salmonella isolates. The results can be obtained in less than 1 h from single colonies without the need for purifying DNA.

Fluoroquinolone resistance in Salmonella serovars isolated from humans and food animals

Quinolone-resistant Salmonella enterica usually contain a mutation in gyrA within the region encoding the quinolone resistance determining region of the A subunit of DNA gyrase. These mutations confer substitutions analogous to Escherichia coli Ser83-->Phe and Asp87-->Gly or Tyr, or a novel mutation resulting in Ala119-->Glu or Val. Mutations in gyrB are rare, and no mutations in parC or parE have been described. Quinolone-resistant Salmonella can also be cross-resistant to other agents including chloramphenicol and tetracycline. Increased efflux has been demonstrated and for some strains this has been associated with increased expression of acrB. Mutation in soxR has also been shown for one isolate. Detection of low level resistance (minimum inhibitory concentrations <0.5 microg ml(-1)) to fluoroquinolones is proving an increasing problem in the treatment of invasive Salmonella infections.

Drug resistance of Salmonella strains isolated from community infections in Ankara, Turkey, 1993-99

160 Salmonella strains were isolated from children at the paediatrics department of Ankara University. 48.1% of the isolates were Salmonella enteritidis, 41.9% Salmonella typhimurium and 10% other serotypes. For the analysis of data, the study period was divided into 2 periods: 1993-95 and 1996-99. A decline in the isolation rate of S. typhimurium (from 63.1% to 30.1%) and rapid rise in S. enteritidis (from 31.6% to 57.3) was observed during the review period. However, for S. typhimurium isolates, the 5-drug (ampicillin, chloramphenicol, streptomycin, tetracycline and sulfonamides) pattern of resistance was increased from 13.5% to 38.7% in the second period. Since S. enteritidis and 5-drug-resistant S. typhimurium have also increased in other countries, their pandemic spread in humans indicates the continuing importation and exportation of these pathogens.

Why is anti-microbial resistance a veterinary problem as well?

Resistance to anti-microbial agents has become one of the main issues in public health strategies world-wide. Much attention has been paid to the emergence of pathogenic micro-organisms such as enterococci or Salmonella that have developed resistance mechanisms that render them almost untreatable with current antibiotics. One of the alleged reasons for such an emergence is the non-medical use of antibiotics, especially in animals. However, only recently have veterinary forums and journals begun to discuss this topic. On the other hand, anti-microbial resistance has also become a problem in veterinary medicine and the number of reports indicating high rates of resistance among animal-originated micro-organisms is considerable. The present review deals with the mechanisms of resistance known for antibiotics in common veterinary use, the problem of anti-microbial resistance in veterinary medicine and the links between the use of antibiotics in animals and the emergence of anti-microbial resistance in humans.

Quinolone resistance among Salmonella enterica from cattle, broilers, and swine in Denmark

This study was conducted to determine the susceptibility to nalidixic acid and fluoroquinolones of Salmonella Dublin, S. Enteritidis, and S. Typhimurium isolates from cattle, broilers, and pigs over time in Denmark and to characterise the gyrA, gyrB, and parC genes in quinolone-resistant isolates. A total of 584 S. Typhimiurium and 573 S. Dublin isolates from cattle during 1984 through 1999, and 241 S. Enteritidis and 131 S. Typhimurium from broilers and 452 S. Typhimurium from pigs isolated during 1997-1999 were tested. All isolates from cattle from the period 1984 through 1992 were susceptible to quinolones. A single (1.1%) S. Typhimurium isolate from 1995 and three (5.9%) from 1998 were resistant to nalidixic acid. Six (9.0%) S. Dublin isolates from 1996, four (4.2%) from 1997, and one (1.7%) from 1998 were resistant to nalidixic acid. Resistance was not observed among isolates from cattle in 1999. All broiler isolates from 1997 except for one were susceptible to nalidixic acid, whereas seven (6.2%) S. Enteritidis and two (6.3%) of the S. Typhimurium isolates from 1998 and 9 S. Enteritidis (26.5%) from 1999 were resistant. Among isolates from pigs, four isolates from 1997, three from 1998, and one from 1999 were resistant to nalidixic acid. All the nalidixic acid-resistant isolates had reduced susceptibility to fluoroquinolones. Sequence analysis of the gyrA gene in 37 nalidixic-resistant isolates identified two different base substitutions at codon serine-83 and two at aspartate-87. The base substitutions in serine-83 were TCC (Ser)-->TAC (Tyr), and TCC (Ser)-->TTC (Phe). The base substitutions in aspartic-87 were GAC (Asp)-->AAC (Asn), and GAC (Asp)-->GGC (Gly). Sequence analysis of the gyrB and parC genes revealed no mutations in 27 selected isolates. This study showed that quinolone-resistant isolates have emerged in recent years among food-producing animals, especially among S. Enteritidis from broilers in Denmark, and that the resistance mainly is associated with mutations in gyrA.

Evidence for active efflux as the primary mechanism of resistance to ciprofloxacin in Salmonella enterica serovar typhimurium

The occurrence of active efflux and cell wall modifications were studied in Salmonella enterica serovar Typhimurium mutants that were selected with enrofloxacin and whose phenotypes of resistance to fluoroquinolones could not be explained only by mutations in the genes coding for gyrase or topoisomerase IV. Mutant BN18/21 exhibited a decreased susceptibility to ciprofloxacin (MIC = 0.125 microg/ml) but did not have a mutation in the gyrA gene. Mutants BN18/41 and BN18/71 had the same substitution, Gly81Cys in GyrA, but exhibited different levels of resistance to ciprofloxacin (MICs = 2 and 8 microg/ml, respectively). None of the mutants had mutations in the parC gene. Evidence for active efflux was provided by a classical fluorimetric method, which revealed a three- to fourfold decrease in ciprofloxacin accumulation in the three mutants compared to that in the parent strain, which was annulled by addition of the efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone. In mutant BN18/71, a second fluorimetric method also showed a 50% reduction in the level of accumulation of ethidium bromide, a known efflux pump substrate. Immunoblotting and enzyme-linked immunosorbent assay experiments with an anti-AcrA antibody revealed that the resistance phenotype was strongly correlated with the expression level of the AcrAB efflux pump and suggested that decreased susceptibility to ciprofloxacin due to active efflux probably related to overproduction of this pump could occur before that due to gyrA mutations. Alterations were also found in the outer membrane protein and lipopolysaccharide profiles of the mutants, and these alterations were possibly responsible for the decrease in the permeability of the outer membrane that was observed in the mutants and that could act synergistically with active efflux to decrease the level of ciprofloxacin accumulation.

Harmonization of antibiotic susceptibility testing for Salmonella: results of a study by 18 national reference laboratories within the European Union-funded Enter-net group

For the effective surveillance of antimicrobial drug resistance within Salmonella organisms from humans, harmonization of methods used for sensitivity testing by laboratories responsible for the typing of such organisms is essential. A study of resistance or sensitivity to a panel of 11 antimicrobials by the Enter-net international surveillance network was therefore undertaken. There are 18 national Salmonella reference laboratories within this European Union-funded network. Forty-eight strains of Salmonella enterica were distributed to each laboratory for testing for resistance or sensitivity to ampicillin, cefotaxime, chloramphenicol, gentamicin, kanamycin, streptomycin, sulfonamides, tetracyclines, trimethoprim, nalidixic acid, and ciprofloxacin. Over 8,500 tests were assessed involving disk diffusion (DD), agar breakpoint (BP), or full minimum inhibitory concentration (MIC). Results indicated that whichever method was used, there was a high degree of concordance for the detection of resistance to most antimicrobials; only for decreased sensitivity to ciprofloxacin was there substantial nonconcordance. Because all isolates with decreased sensitivity to ciprofloxacin were resistant to nalidixic acid, it is suggested that, if required, MICs to ciprofloxacin could be determined for isolates resistant to nalidixic acid. For the detection of sensitivity, the main area of nonconcordance was in the detection of sensitivity to streptomycin. With the exception of decreased sensitivity to ciprofloxacin, we are confident that a database of antimicrobial susceptibilities can now be established and harmonized antibiogram data for Salmonella can now be exchanged for national Salmonella reference laboratories within the European Union.

Molecular characterization of Irish Salmonella enterica serotype typhimurium: detection of class I integrons and assessment of genetic relationships by DNA amplification fingerprinting

Salmonella enterica is among the principal etiological agents of food-borne illness in humans. Increasing antimicrobial resistance in S. enterica is a cause for worldwide concern. There is concern at present in relation to the increasing incidence of human infection with antimicrobial agent-resistant strains of S. enterica serotype Typhimurium, in particular of phage type DT104. Integrons appear to play an important role in the dissemination of antimicrobial resistance genes in many Enterobacteriaceae including S. enterica. In this study the antimicrobial susceptibilities and phage types of 74 randomly collected strains of S. enterica serotype Typhimurium from the Cork region of southern Ireland, obtained from human, animal (clinical), and food sources, were determined. Each strain was examined for integrons and typed by DNA amplification fingerprinting (DAF). Phage type DT104 predominated (n = 48). Phage types DT104b (n = 3), -193 (n = 9), -195 (n = 6), -208 (n = 3), -204a (n = 2), PT U302 (n = 1), and two nontypeable strains accounted for the remainder. All S. enterica serotype Typhimurium DT104 strains were resistant to ampicillin, chloramphenicol, streptomycin, Sulfonamide Duplex, and tetracycline, and one strain was additionally resistant to trimethoprim. All DT104 strains but one were of a uniform DAF type (designated DAF-I) and showed a uniform pattern of integrons (designated IP-I). The DT104b and PT U302 strains also exhibited the same resistance phenotype, and both had the DAF-I and IP-I patterns. The DAF-I pattern was also observed in a single DT193 strain in which no integrons were detectable. Greater diversity of antibiograms and DAF and IP patterns among non-DT104 phage types was observed. These data indicate a remarkable degree of homogeneity at a molecular level among contemporary isolates of S. enterica serotype Typhimurium DT104 from animal, human, and food sources in this region.