Antimicrobial use and resistance in animals

Antibiotics--past, present, and future

The drug therapy for diseases other than those caused by microbial agents involves treating the host. In infectious diseases therapy, the goal is to rid the host of the pathogen. Hence, drug therapy is aimed at the pathogen. Because a second living agent is involved in the triangle, drug therapy is affected by the pathogen's nature, its tissue specificity, and, most importantly, the changes it undergoes to survive. The history of antimicrobial therapy has clearly demonstrated that the drugs that are used to treat infections are also responsible for making them more difficult to treat in future. The only way to keep antimicrobial agents useful is to use them appropriately and judiciously.

Flavophospholipol use in animals: Positive implications for antimicrobial resistance based on its microbiologic properties

Bambermycin (flavophospholipol) is a phosphoglycolipid antimicrobial produced by various strains of Streptomyces. It is active primarily against Gram-positive bacteria because of inhibition of transglycosylase and thus of cell wall synthesis. Bambermycin is used as a feed additive growth promoter in cattle, pigs, chickens, and turkeys, but has no therapeutic use in humans or animals. Flavophospholipol is known to suppress certain microorganisms (e.g., Staphylococcus spp. and Enterococcus faecalis) and thus contributes to an improved equilibrium of the gut microflora providing a barrier to colonization with pathogenic bacteria and resultant improved weight gain and feed conversion. Flavophospholipol has also been shown to decrease the frequency of transferable drug resistance among Gram-negative enteropathogens and to reduce the shedding of pathogenic bacteria such as Salmonella in pigs, calves, and chickens. Plasmid-mediated resistance to bambermycin has not been described. Likewise, cross-resistance among bacteria between bambermycin and penicillin, tetracycline, streptomycin, erythromycin, or oleandromycin has not been observed. This brief review summarizes the antimicrobial properties of bambermycin, in particular, its potentially favorable role in decreasing antimicrobial resistance.

Molecular epidemiology of antimicrobial resistance among Salmonella enterica serovar Infantis from poultry in Kagoshima, Japan

Antimicrobial susceptibility and resistance genes of 135 strains of Salmonella enterica serovar Infantis isolated from poultry in Kagoshima were examined. One strain (0.7%) was resistant to ampicillin (A), 97% to streptomycin (S), 95.6% to sulphamethoxazole (Su), 96.3% to oxytetracycline (T), 11.1% to kanamycin (Km) and 36.3% to ofloxacin (O). Multiresistant phenotypes identified were ASSuT-Km, SSuT-Km, SSuT-O and SSuT. Class 1 integrons were detected in 94.8% of isolates. Approximately 89% of oxytetracycline-resistant strains carried the tetA gene and all of the 131 streptomycin-resistant isolates carried the aadA1a gene. Forty-percent of kanamycin-resistant isolates carried the aphA1-Iab gene. All isolates were susceptible to chloramphenicol. Recognition of TEM-type beta-lactamase in a S. Infantis strain from chickens is a recent rare finding in Japan.

Occurrence of antibiotics in hospital, residential, and dairy effluent, municipal wastewater, and the Rio Grande in New Mexico

This study had three objectives: 1) determine occurrence of antibiotics in effluent from hospitals, residential facilities, and dairies, and in municipal wastewater 2) determine antibiotic removal at a large wastewater treatment plant (WWTP) in Albuquerque, NM, and 3) determine concentrations of antibiotics in the Rio Grande, which receives wastewater from the Albuquerque WWTP. Twenty-three samples of wastewater and 3 samples of Rio Grande water were analyzed for the presence of 11 antibiotics. Fifty-eight percent of samples had at least one antibiotic present while 25% had three or more. Hospital effluent had detections of sulfamethoxazole, trimethoprim, ciprofloxacin, ofloxacin, lincomycin, and penicillin G, with 4 of 5 hospital samples having at least one antibiotic detected and 3 having four or more. At the residential sampling sites, ofloxacin was found in effluent from assisted living and retirement facilities, while the student dormitory had no detects. Only lincomycin was detected in dairy effluent (in 2 of 8 samples, at 700 and 6600 ng/L). Municipal wastewater had detections of sulfamethoxazole, trimethoprim, ciprofloxacin, and ofloxacin, with 4 of 6 samples having at least one antibiotic present and 3 having 3 or more. The relatively high concentrations (up to 35,500 ng/L) of ofloxacin found in hospital and residential effluent may be of concern due to potential genotoxic effects and development of antibiotic resistance. At the Albuquerque WWTP, both raw wastewater and treated effluent had detections of sulfamethoxazole, trimethoprim, and ofloxacin, at concentrations ranging from 110 to 470 ng/L. However, concentrations in treated effluent were reduced by 20% to 77%. No antibiotics were detected in the Rio Grande upstream of the Albuquerque WWTP discharge, and only one antibiotic, sulfamethoxazole, was detected in the Rio Grande (300 ng/L) below the WWTP.

Effect of exposure to fluoroquinolones and beta-lactams on the in vitro activity of other groups of antibiotics in Salmonella spp

After designing in vitro models of repeated exposure of Salmonella spp. to various beta-lactams and fluoroquinolones we studied the decrease in susceptibility to other antibiotic families of the mutants generated. There was a decrease in the susceptibility of all the mutants to tetracycline, cotrimoxazole and chloramphenicol. Mutants generated following exposure to fluoroquinolones showed reduced susceptibility to amoxicillin, amoxicillin/clavulanic acid, cefoxitin and cefuroxime, whereas mutants generated following beta-lactam exposure showed reduced susceptibility to nalidixic acid and ciprofloxacin. We observed that the efflux pump systems are activated in the mutants generated and this may therefore be the cause of the decrease in susceptibility. In many cases the decrease is small and is not detected if the CLSI criteria are applied. Nevertheless, more detailed studies should be done to evaluate the importance of this phenomenon and rationalize the use of antibiotics in both humans and animals so as to control the increase in the number of multiresistant strains.

Effect of conventional and organic production practices on the prevalence and antimicrobial resistance of Campylobacter spp. in poultry

Intestinal tracts of broilers and turkeys from 10 conventional broiler farms and 10 conventional turkey farms, where antimicrobials were routinely used, and from 5 organic broiler farms and 5 organic turkey farms, where antimicrobials had never been used, were collected and cultured for Campylobacter species. A total of 694 Campylobacter isolates from the conventional and organic poultry operations were tested for antimicrobial resistance to nine antimicrobial agents by the agar dilution method. Although Campylobacter species were highly prevalent in both the conventional and organic poultry operations, the antimicrobial resistance rates were significantly different between the organic operations and the conventional operations. Less than 2% of Campylobacter strains isolated from organically raised poultry were resistant to fluoroquinolones, while 46% and 67% of Campylobacter isolates from conventionally raised broilers and conventionally raised turkeys, respectively, were resistant to these antimicrobials. In addition, a high frequency of resistance to erythromycin (80%), clindamycin (64%), kanamycin (76%), and ampicillin (31%) was observed among Campylobacter isolates from conventionally raised turkeys. None of the Campylobacter isolates obtained in this study was resistant to gentamicin, while a large number of the isolates from both conventional and organic poultry operations were resistant to tetracycline. Multidrug resistance was observed mainly among Campylobacter strains isolated from the conventional turkey operation (81%). Findings from this study clearly indicate the influence of conventional and organic poultry production practices on antimicrobial resistance of Campylobacter on poultry farms.

Antimicrobial susceptibility and characterization of Salmonella isolates from processed bison carcasses

Seventeen Salmonella enterica serovar Hadar isolates recovered from bison were found to possess a range of virulence genes and resistance to tetracycline, gentamicin, sulfamethoxazole, and streptomycin simultaneously. A 1-kb class 1 integron containing the aadA1 gene was identified in all isolates. Pulsed-field gel electrophoresis found that all isolates were closely related, indicating the possibility of cross-contamination during processing.

Antimicrobial resistance in Enterococcus spp. isolated in inflow, effluent and sludge from municipal sewage water treatment plants

Antimicrobial resistance of enterococci was investigated in 42 samples of crude inflow, treated effluent and sludge collected in 14 municipal sewage treatment plants of Portugal. A total of 983 enterococci were recovered and tested, using the diffusion agar method, regarding their sensitivity to 10 different antimicrobial drugs. Multidrug resistance was present in 49.4% of the isolates. Only 3.3% and 0.6% of the investigated strains were resistant to ampicillin and vancomycin, respectively. Resistances found against rifampicin (51.5%), tetracycline (34.6%), erythromycin (24.8%) and nitrofurantoin (22.5%), are causes for substantial concern. Almost 14% of isolates were resistant to ciprofloxacin. Wastewater treatment resulted in enterococci decrease between 0.5 and 4log; nevertheless, more than 4.4 x 10(5)CFU/100ml were present in the outflow of the plants. Our data indicate that the use of antimicrobials had created a large pool of resistance genes and that sewage treatment processes are unable to avoid the dissemination of resistant enterococci into the environment.

Susceptibility of Escherichia coli from growing piglets receiving antimicrobial feed additives

Concerns regarding an apparent association between the use of antimicrobial feed additives (AFAs) in food animal production and a concomitant increase in antimicrobial drug resistance among zoonotic enteropathogens have provided the impetus to propose cessation of their use. While AFAs have been used in food animal production for nearly 50 years, the future use of AFAs will require an understanding of the effects of different classes of antimicrobials on the antimicrobial resistance of commensal flora. The present study examines the effect of three AFAs (apramycin, carbadox, and chlortetracycline) on the antimicrobial susceptibility of Escherichia coli in growing piglets and on animal performance. Three replicate trials were conducted using growing piglets fed standard diets with and without antimicrobial feed additives (AFAs). Fecal samples were cultured selectively for E. coli at regular intervals from all piglets from birth to market and antimicrobial susceptibility testing of E. coli isolates was performed using a replica-plate screening method and a broth microdilution method. While resistance to tetracycline in E. coli varied widely by sample, group, and trial, a significant increase in the percentage of resistant isolates was observed in piglets receiving AFAs when compared to controls (p < 0.0001). Resistance to apramycin increased in E. coli from piglets fed apramycin when compared to controls (p < 0.0001). However, upon removal of apramycin, resistance in E. coli declined to baseline levels by day 75. Piglets receiving AFAs demonstrated improved feed efficiency during phase 4 (p < 0.001), and higher average daily gains in phases 3 and 4 (p < 0.0001). This study suggests that antimicrobial resistance to AFAs in E. coli is drug-dependent and that some antimicrobials may be suitable for continued use in feeds during specified growth periods without concern for persistence of resistant E. coli populations.

Variability in Antimicrobial Resistance amongSalmonella entericaStrains from Fattening Pigs and Sows

The aim of this study was to determine the prevalence of antibiotic resistance and different resistance patterns for Salmonella isolates collected from Belgian sows and fattening pigs at different ages and at slaughter. The most frequently isolated serotypes were S. Typhimurium (42.3%), S. Derby (25.1%), S. Goldcoast (7.3%), and S. Infantis (4.8%). All 901 isolates were submitted to antimicrobial susceptibility testing for 14 compounds using the disc agar diffusion test. The highest percentage of resistance was found to oxytetracycline (34.2%), streptomycin (32.5%), sulfamethizole (27.6%), and ampicillin (24.9%). Three of the isolates showed resistance to cephalosporins and none to second-generation fluoroquinolones. Multiresistance (resistance to > or =2 antimicrobials) was observed in 33.2% of the strains. Differences in resistance patterns were observed between and within serotypes as well as genotypes. A significant lower proportion (p < 0.01) of resistant strains was recovered in fecal samples from sows (23/56) than from fattening pigs during the weaning (30/30), growing (79/85), and finishing periods (45/52). The proportion of resistant strains recovered from fecal samples taken at the herd (154/167) was higher than in the samples collected at the slaughterhouse (75/140). When designing antimicrobial resistance surveillance programs for Salmonella in pigs, it is important to take multiple samples within each herd from both sows and fattening pigs at different time points.

Trends in antimicrobial resistance phenotypes in non-typhoid Salmonellae from human and poultry origins in France

A total of 1873 strains from human origin and 4283 strains from non-human origin of Salmonella enterica serotypes Typhimurium, Enteritidis, Heidelberg, Hadar and Virchow, collected over three years 1993, 1997 and 2000, were examined in order to determine the rate of antimicrobial resistance to 12 antimicrobial drugs. The objective of the study was to describe and to compare the evolution of the main resistance types in human and non-human isolates, focusing on the poultry sector. The evolution and the rates of antimicrobial resistances for the five serotypes, with the exception of Virchow, were almost comparable in strains isolated from human and non-human sources over the period studied. The most striking result concerning single resistance was the spectacular increase of the resistance frequency to nalidixic acid for the strains belonging to serotypes Hadar and Virchow, especially in the poultry food sector (14% in 1993 vs. 72% in 2000 for Salmonella Virchow, 4% in 1993 vs. 70% in 2000 for Salmonella Hadar) and also in human isolates (24% in 1997 vs. 48% in 2000 for S. Virchow, 31% in 1997 vs. 78% in 2000 for S. Hadar). In addition to the classical resistance to ampicillin, streptomycin, sulphonamide, chloramphenicol and tetracycline (ASSuCT resistance type), which stabilized between 1997 and 2000, the emergence of a new resistance type was observed.

Public Health Advocate

With antimicrobial resistance mounting, an important public health goal is to preserve therapeutic effectiveness of remaining antimicrobials. To that end, fewer antimicrobials should be used in human medicine and in agriculture. Public health initiatives to reduce antimicrobial overuse could benefit from concurrent collection of agricultural usage data; they could aid scientific understanding of the resistance problem and confirm the efficacy of interventions. Data collection in this context should be a priority. However, usage data are nonessential to achieving the public health goal. U.S. regulation of agricultural antimicrobials today is very reliant on risk assessment. While more data can be useful for use in risk assessment, microbial risk assessment itself may not be well suited to the purpose of reducing antimicrobial overuse. Among other recognized shortcomings, current microbial risk assessment models typically fail to account for the essential ecological nature of antimicrobial resistance. This makes it inadequate for fully characterizing the human health or ecological risks of animal antimicrobials. European success at phasing out unnecessary antimicrobial usage in agriculture, on the other hand, has derived from decisions based on public health concerns and political will, and not on the collection of usage data or on the successful completion of a risk assessment.

Antimicrobial-resistant bacteria in the community setting

Over the past decade, antimicrobial resistance has emerged as a major public-health crisis. Common bacterial pathogens in the community such as Streptococcus pneumoniae have become progressively more resistant to traditional antibiotics. Salmonella strains are beginning to show resistance to crucial fluoroquinolone drugs. Community outbreaks caused by a resistant form of Staphylococcus aureus, known as community-associated meticillin (formerly methicillin)-resistant Staphylococcus aureus, have caused serious morbidity and even deaths in previously healthy children and adults. To decrease the spread of such antimicrobial-resistant pathogens in the community, a greater understanding of their means of emergence and survival is needed.

Relation between induction of the mar operon and cyclohexane tolerance and reduction in fluoroquinolone susceptibility in Salmonella spp

The relationship between cyclohexane tolerance and induction of the mar operon and a decrease in susceptibility to ciprofloxacin and moxifloxacin in isolates of Salmonella spp. from food and clinical isolates of Salmonella spp. was studied. We studied the influence of the mar operon using an inductor (acetylsalicylic acid) and we also studied the cyclohexane resistance. Induction was seen to produce an increase in the minimum inhibitory concentration (MIC) of these quinolones, which suggested that there was overexpression of the AcrAB type active efflux systems due to induction of the mar operon. Cyclohexane susceptibility was not shown to be a very sensitive method for studying this process, as only 3% (5/176) of the clinical isolates studied were cyclohexane-resistant; most of these belonged to the Hadar serotype. This study confirmed the participation of active efflux systems in the decrease in fluoroquinolone susceptibility in Salmonella spp. Furthermore, the study has indicated that these mechanisms (i.e., active efflux systems) are present in strains that are susceptible to the fluoroquinolone compounds, so their stimulation may be one of the mechanisms involved in the reduction in fluoroquinolone susceptibility. This suggests that the exposure of Salmonella spp. to antibiotics should be limited in order to prevent these active efflux systems from being activated. Consequently, the use of fluoroquinolones, both in the treatment of humans and in veterinary practice, should be controlled and rationalized in an attempt to curb the increase in the number of strains that are resistant to these compounds.

Antimicrobial resistance of Escherichia coli and therapeutic implications

Widespread antibiotic resistance has been recognized in Escherichia coli isolates from human, animal and environmental sources. Although prevalence rates for resistant E. coli strains are significantly distinct for various populations and environments, the impact of resistance to antimicrobial drugs is ubiquitous. This article provides information about the epidemiology, mechanisms and molecular principles of resistance, shows consequences for the antiinfective treatment of selected infections and describes measures to control the spread of antibiotic-resistant E. coli.

Activity of Telithromycin against Thirteen New Isolates of C. burnetii Including Three Resistant to Doxycycline

In this study we have evaluated the in vitro activity of antibiotics against 13 new isolates of Coxiella burnetii using a real-time quantitative PCR assay. MICs against doxycycline ranged from 1 to 8 microg/mL, telithromycin from 0.5 to 2 microg/mL, and all strains had MICs > or = 8 microg/mL for erythromycin. We report that strains resistant to doxycycline exist either in humans or animals.

Overview of issues pertaining to the manufacture, distribution, and use of antimicrobials in animals and other information relevant to animal antimicrobial use data collection in the United States

This paper provides condensed literature review and contextual information concerning antimicrobial use in animals and antimicrobial use data collection techniques. Information is summarized into the following topics: (I) antimicrobial use for animal growth promotion; (II) regulatory framework for approval of veterinary antimicrobials and feed additives in the United States; (III) animal health product manufacturing and distribution networks; (IV) existing estimates of animal antimicrobial use in the United States; (V) additional sources for animal use data in the United States; (VI) recommendations from international expert groups; (VII) animal antimicrobial use surveillance systems in other nations and (VIII) human use data collection systems in the United States.

Antibiotic uptake by plants from soil fertilized with animal manure

Antibiotics are commonly added to animal feed as supplements to promote growth of food animals. However, absorption of antibiotics in the animal gut is not complete and as a result substantial amounts of antibiotics are excreted in urine and feces that end up in manure. Manure is used worldwide not only as a source of plant nutrients but also as a source of organic matter to improve soil quality especially in organic and sustainable agriculture. Greenhouse studies were conducted to determine whether or not plants grown in manure-applied soil absorb antibiotics present in manure. The test crops were corn (Zea mays L.), green onion (Allium cepa L.), and cabbage (Brassica oleracea L. Capitata group). All three crops absorbed chlortetracycline but not tylosin. The concentrations of chlortetracycline in plant tissues were small (2-17 ng g(-1) fresh weight), but these concentrations increased with increasing amount of antibiotics present in the manure. This study points out the potential human health risks associated with consumption of fresh vegetables grown in soil amended with antibiotic laden manures. The risks may be higher for people who are allergic to antibiotics and there is also the possibility of enhanced antimicrobial resistance as a result of human consumption of these vegetables.

Molecular epidemiology of antimicrobial-resistant commensal Escherichia coli strains in a cohort of newborn calves

Pulsed-field gel electrophoresis (PFGE) was used to investigate the dissemination and diversity of ampicillin-resistant (Amp(r)) and nalidixic acid-resistant (Nal(r)) commensal Escherichia coli strains in a cohort of 48 newborn calves. Calves were sampled weekly from birth for up to 21 weeks and a single resistant isolate selected from positive samples for genotyping and further phenotypic characterization. The Amp(r) population showed the greatest diversity, with a total of 56 different genotype patterns identified, of which 5 predominated, while the Nal(r) population appeared to be largely clonal, with over 97% of isolates belonging to just two different PFGE patterns. Distinct temporal trends were identified in the distribution of several Amp(r) genotypes across the cohort, with certain patterns predominating at different points in the study. Cumulative recognition of new Amp(r) genotypes within the cohort was biphasic, with a turning point coinciding with the housing of the cohort midway through the study, suggesting that colonizing strains were from an environmental source on the farm. Multiply resistant isolates dominated the collection, with >95% of isolates showing resistance to at least two additional antimicrobials. Carriage of resistance to streptomycin, sulfamethoxazole, and tetracycline was the most common combination, found across several different genotypes, suggesting the possible spread of a common resistance element across multiple strains. The proportion of Amp(r) isolates carrying sulfamethoxazole resistance increased significantly over the study period (P < 0.05), coinciding with a decline in the most common genotype pattern. These data indicate that calves were colonized by a succession of multiply resistant strains, with a probable environmental source, that disseminated through the cohort over time.

Multidrug-resistant Klebsiella pneumoniae isolated from farm environments and retail products in Oklahoma

Multidrug-resistant enteric bacteria were isolated from turkey, cattle, and chicken farms and retail meat products in Oklahoma. Among the isolated species, multidrug-resistant Klebsiella pneumoniae was prevalently isolated from most of the collected samples. Therefore, a total of 132 isolates of K. pneumoniae were characterized to understand their potential roles in the dissemination of antibiotic-resistance genes in the food chains. Multidrug-resistant K. pneumoniae was most frequently recovered from a turkey farm and ground turkey products among the tested samples. All isolates were resistant to ampicillin, tetracycline, streptomycin, gentamycin, and kanamycin. Class 1 integrons located in plasmids were identified as a common carrier of the aadA1 gene, encoding resistance to streptomycin and spectinomycin. Production of beta-lactamase in the K. pneumoniae isolates played a major role in the resistance to beta-lactam agents. Most isolates (96%) possessed bla(SHV1). Five strains were able to express both SHV-11 (pI 6.2) and TEM-1 (pI 5.2) beta-lactamase. Transfer of these antibiotic-resistance genes to Escherichia coli was demonstrated by transconjugation. The bacterial genomic DNA restriction patterns by pulsed-field gel electrophoresis showed that the same clones of multidrug-resistant K. pneumoniae remained in feathers, feed, feces, and drinking water in turkey environments, indicating the possible dissemination of antibiotic-resistance genes in the ecosystem and cross-contamination of antibiotic-resistant bacteria during processing and distribution of products.

Prevalence of Antimicrobial Resistance Genes inListeria monocytogenesIsolated from Dairy Farms

Antimicrobial resistance of Listeria monocytogenes (n = 38) isolated from the four dairy farms to 15 antimicrobial agents was evaluated. All 38 L. monocytogenes isolates from the four farms evaluated were resistant to more than one antimicrobial in different combinations. All L. monocytogenes isolates evaluated were resistant to cephalosporin C (minimum inhibitory concentration [MIC] > or = 512 microg/mL), streptomycin (MIC > or = 32) and trimethoprim (MIC > or = 512). Most L. monocytogenes isolates were resistant to ampicillin (92%, MIC > or = 2), rifampicin (84%, MIC > or = 4), rifamycin (84%, MIC > or = 4), and florfenicol (66%, MIC > or = 32) and some were resistant to tetracycline (45%, MIC > or = 16), penicillin G (40%, MIC > or = 2) and chloramphenicol (32%, MIC > or = 32). All L. monocytogenes isolates were susceptible to amoxicillin, erythromycin, gentamicin, kanamycin and vancomycin. Susceptibility of L. monocytogenes to the antimicrobials evaluated was quite consistent among the dairy farms evaluated. However, some variability in antimicrobial susceptibility among dairy farms was noted. Nineteen of 38 L. monocytogenes isolates contained more than one antimicrobial resistance gene sequence. A high frequency of floR (66%) was found in L. monocytogenes followed by penA (37%), strA (34%), tetA (32%), and sulI (16%). Other tetracycline resistance genes (tetB, tetC, tetD, tetE, and tetG) and other antimicrobial resistance genes (cmlA, strB, aadA, sulI, vanA, vanB, ampC, ermB, ereA, and ereB) were not found in any of the L. monocytogenes isolates from the four dairy farms. Results of the present study demonstrated that L. monocytogenes isolated from the dairy farm environment were resistant to many antimicrobials and contained one or more antimicrobial resistance genes.

Correlation between ratio of serum doxycycline concentration to MIC and rapid decline of antibody levels during treatment of Q fever endocarditis

Endocarditis is the major clinical manifestation of chronic Q fever. Although doxycycline along with hydroxychloroquine remains the mainstay of medical therapy for Q fever endocarditis, there are wide variations in the rapidity of the patient's decline of antibody levels during such therapy. We undertook a retrospective examination of whether there was any correlation between the ratio of serum concentration to MIC of doxycycline and response to treatment in patients with Q fever endocarditis. Included herein are 16 patients from whom Coxiella burnetii was isolated from cardiac valve materials. Serology and measurement of doxycycline and hydroxychloroquine serum levels were performed and recorded after 1 year of treatment. The MIC of doxycycline for C. burnetii isolates was determined using the shell vial assay in a real-time quantitative PCR assay. At the completion of a year-long therapy with doxycycline-hydroxychloroquine, all those that showed a low decline of antibody levels (n = 6) (i.e., <2-fold decrease in antibody titer to phase I C. burnetii antigen) had a ratio of serum doxycycline concentration to MIC between 0.5 and 1. In contrast, those having a ratio of > or =1 showed a rapid decline of phase I antibody levels (n = 9; P < 0.05). The only patient who died had a serum doxycycline-to-MIC ratio of <0.5, and the isolate of C. burnetii cultured from this patient was resistant to doxycycline (MIC = 8 microg/ml). The ratio of serum doxycycline concentration to MIC should be monitored during the course of therapy in patients with Q fever endocarditis.

Antimicrobial resistance patterns of Campylobacter from feedlot cattle*

AIMS

This study examined 448 Campylobacter strains isolated in 1999 and 2000 from US feedlot cattle for resistance to 12 antimicrobials.

METHODS AND RESULTS

Isolates were tested for antimicrobial susceptibility using the E-test method. Approximately 60% (n = 267) were resistant to one or more antimicrobials, and 19.6% (n = 88) were resistant to two or more antimicrobials. Of the Campylobacter jejuni isolates, 49.1% (n = 187) were resistant to tetracycline, 10.2% (n = 39) were resistant to nalidixic acid, 8.4% were resistant to trimethoprim/sulfamethoxazole, and 1.8% (n = 7) were resistant to ciprofloxacin. Resistance to any of the other eight antimicrobials was 1.3% or less, but 14.4% (n = 55) were resistant to two or more antimicrobials. In the Campylobacter coli group, 65.7% (n = 44) were resistant to tetracycline, 52.2% (n = 35) were resistant to trimethoprim/sulfamethoxazole, 22.4% (n = 15) were resistant to nalidixic acid, and 9.0% (n = 6) were resistant to ciprofloxacin. Resistance to any of the remaining eight antimicrobials was 3.0% or less, although 49.3% (n = 33) were resistant to two or more antimicrobials.

CONCLUSIONS

Although antimicrobials are widely used in US feedlot cattle production, our results demonstrate generally low levels of resistance to a broad range of commonly used antimicrobials relative to other recent studies.

SIGNIFICANCE AND IMPACT OF THE STUDY

Resistance data on Campylobacter isolated from this major US livestock commodity is lacking. This overview enhances current knowledge and provides a basis for further studies.

Molecular characterization of multidrug-resistant Proteus mirabilis isolates from retail meat products

Sixty-four multidrug-resistant isolates of Proteus mirabilis were collected from retail meat products in Oklahoma. The isolates showed four different patterns of antibiotic resistance based on their resistant phenotype and genotypes. Most of these isolates were resistant to ampicillin, tetracycline, gentamycin, and kanamycin. Class 1 integrons were detected as a common carrier of the antibiotic-resistant genes, such as aadA1, aadB, and aadA2. A few isolates (9%) contained class 2 integrons with three gene cassettes included: dhfr1, sat1, and aadA1. These isolates were even resistant to nalidixic acid due to mutations in gyrA and parC. All ampicillin-resistant isolates contained blaTEM-1. Plasmids that contained class 1 or 2 integrons and blaTEM-1 were able to be transferred from P. mirabilis isolates into Escherichia coli by conjugation, indicating that conjugal transfer could contribute to the dissemination of antibiotic resistance genes between the Enterobacteriaceae species.

Comparison of ileum microflora of pigs fed corn-, wheat-, or barley-based diets by chaperonin-60 sequencing and quantitative PCR

We have combined the culture-independent methods of high-throughput sequencing of chaperonin-60 PCR product libraries and quantitative PCR to profile and quantify the small-intestinal microflora of pigs fed diets based on corn, wheat, or barley. A total of 2,751 chaperonin-60 PCR product clones produced from samples of ileum digesta were examined. The majority (81%) of these clones contained sequences independently recovered from all three libraries; 372 different nucleotide sequences were identified, but only 14% of the 372 different sequences were recovered from all three libraries. Taxonomic assignments of the library sequences were made by comparison to a reference database of chaperonin-60 sequences combined with phylogenetic analysis. The taxa identified are consistent with previous reports of pig ileum microflora. Frequencies of each sequence in each library were calculated to identify taxa that varied in frequency between the corn, barley, and wheat libraries. The chaperonin-60 sequence inventory was used as a basis for designing PCR primer sets for taxon-specific quantitative PCR. Results of quantitative PCR analysis of ileum digesta confirmed the relative abundances of targeted taxa identified with the library sequencing approach. The results of this study indicate that chaperonin-60 clone libraries can be valid profiles of complex microbial communities and can be used as the basis for producing quantitative PCR assays to measure the abundance of taxa of interest during experimentally induced or natural changes in a community.

Antimicrobial susceptibility of Escherichia coli isolates from dairy farms using organic versus conventional production methods

OBJECTIVE

To compare antimicrobial susceptibility patterns of Escherichia coli isolates cultured from fecal samples from cows and calves on dairy farms that used organic (ie, no or severely limited antimicrobial use) versus conventional production methods.

DESIGN

Cross-sectional study.

SAMPLE POPULATION

Fecal samples from 10 cows and 10 calves on each of 30 organic dairy farms and 30 neighboring conventional dairy farms in Wisconsin.

PROCEDURE

E. coli isolates obtained from the fecal samples were tested for susceptibility to 17 antimicrobials by means of a microbroth dilution test. Prevalence of antimicrobial resistance was compared between organic and conventional dairy farms.

RESULTS

E. coli was isolated from 1,121 (94%) fecal samples. Farm type (organic vs conventional) and animal age (cow vs calf) were significantly associated with odds that E. coli isolates would be resistant to various antimicrobials. After controlling for age, logistic regression analyses indicated that isolates from conventional dairy farms had significantly higher rates of resistance to ampicillin, streptomycin, kanamycin, gentamicin, chloramphenicol, tetracycline, and sulfamethoxazole than did isolates from organic dairy farms. However, no significant differences were detected for the 10 other antimicrobials that were tested.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that compared with isolates from conventional dairy farms, E. coli isolates from organic dairy herds have significantly lower prevalences of resistance to 7 antimicrobials; however, prevalence of resistance was not significantly different for 10 other antimicrobials. Resistance was more common for isolates from calves than for isolates from adult dairy cows.

Susceptibility of Campylobacter hyointestinalis subsp. hyointestinalis to antimicrobial agents and characterization of quinolone-resistant strains

OBJECTIVES

To study the susceptibility of Campylobacter hyointestinalis subsp. hyointestinalis to several antimicrobial agents and to investigate the mechanisms of nalidixic acid and ciprofloxacin resistance.

METHODS

The disc diffusion method was employed to study the susceptibility of 49 C. hyointestinalis subsp. hyointestinalis strains of reindeer and bovine origin to 12 different antimicrobial agents. In addition, the MICs of nalidixic acid and ciprofloxacin were determined. The nucleotide sequence of a 270 bp fragment of the gyrA gene was determined in ciprofloxacin-susceptible and -resistant strains. The effect of a multidrug efflux pump inhibitor Phe-Arg-beta-naphthylamide (PA beta N) on the MICs of ciprofloxacin and nalidixic acid was also studied.

RESULTS

The only decreased susceptibility for antimicrobial agents of this study was observed for sulphonamide compound and streptomycin (24% and 32% of the strains, respectively), and this phenomenon was observed exclusively in the bovine strains. In sequence studies, a Thr-86-->Ile change was found in strains with MICs of ciprofloxacin of > or = 64 mg/L, but this mutation was absent in strains with lower resistance levels. The use of PA beta N did not affect the MIC of ciprofloxacin but decreased the MIC of nalidixic acid 2-4-fold.

CONCLUSIONS

The Finnish C. hyointestinalis subsp. hyointestinalis strains are susceptible to a majority of the antimicrobials of veterinary importance. The mechanism of ciprofloxacin resistance at lower levels (< or = 32 mg/L) is not associated with a specific mutation in the quinolone resistance-determining region of the gyrA gene. Finally, there are distinct differences in the mechanisms of ciprofloxacin resistance compared with nalidixic acid resistance within the studied species.

A view on antimicrobial resistance in developing countries and responsible risk factors

Antimicrobial resistance is one of the biggest challenges facing global public health. Although antimicrobial drugs have saved many lives and eased the suffering of many millions, poverty, ignorance, poor sanitation, hunger and malnutrition, inadequate access to drugs, poor and inadequate health care systems, civil conflicts and bad governance in developing countries have tremendously limited the benefits of these drugs in controlling infectious diseases. The development of resistance in the responsible pathogens has worsened the situation often with very little resource to investigate and provide reliable susceptibility data on which rational treatments can be based as well as means to optimise the use of antimicrobial agents. The emergence of multi-drug-resistant isolates in tuberculosis, acute respiratory infections and diarrhoea, often referred to as diseases of poverty, has had its greatest toll in developing countries. The epidemic of HIV/AIDS, with over 30 million cases in developing countries, has greatly enlarged the population of immunocompromised patients. The disease has left these patients at great risk of numerous infections and even greater risks of acquiring highly resistant organisms during long periods of hospitalisation. This review discusses antimicrobial resistance in developing countries and the risk factors responsible.

Ascophyllum nodosum supplementation: a preharvest intervention for reducing Escherichia coli O157:H7 and Salmonella spp. in feedlot steers

Ascophyllum nodosum (Tasco-14) decreased the prevalence of enterohemorrhagic Escherichia coli (EHEC) O157:H7 in animals fed prior to harvest. Tasco-14 was supplemented on a 2% dry matter basis 14 days prior to harvest to determine its effects on EHEC and Salmonella spp. prevalence. Two hundred mixed crossbred steers and heifers (Bos indicus x Bos taurus), in a large commercial finishing facility, served as experimental units. Treatment (TRT, n = 100) animals received a steam-rolled corn-based diet containing 2% Tasco-14 on a dry matter basis, and control (CON, n = 100) animals received only the steam-rolled corn-based diet. Hide swabs and fecal samples were obtained for EHEC and Salmonella spp. evaluations. Animals were sampled 1 day prior to (d - 1) the feeding of Tasco-14 and immediately following exsanguinations. The prevalence of EHEC O157 on hide swabs and in fecal samples (P = 0.0001 and P < 0.0001, respectively) and the prevalence of EHEC O157:H7 on hide swabs and in fecal samples (P < 0.0001 and P < 0.0001, respectively) was reduced by 33 and 36% from d - 1 levels on TRT hide swabs and by 9 and 11% in TRT fecal samples. The prevalence of EHEC O157 and EHEC O157:H7 was reduced by 33 and 36% from d - 1 levels on TRT hide swabs and by 9 and 11% in TRT fecal samples. The prevalence of Salmonella spp. on hide swabs did not change for TRT animals (P = 0.64). CON animals showed an increase in Salmonella spp. prevalence (P < 0.0001) from d - 1 feeding levels on hide swabs. The prevalence of Salmonella spp. increased in both TRT and CON fecal samples when compared to d - 1 levels (P = 0.002). However, TRT samples exhibited a lower (P < 0.05) postfeeding prevalence of Salmonella spp. in fecal samples than did CON samples. Results from this study indicate that 2% Tasco-14 supplementation in feedlot cattle diets reduces EHEC O157 and EHEC O157:H7 prevalence on hide swabs and in fecal samples and may suppress increases in Salmonella spp.

Antimicrobial resistance risk assessment in food safety

Microbiological risk assessments generally focus on estimating adverse human health risks from exposures to human pathogenic microbes. The assessment of potential human health risks posed by pathogens that have acquired resistance to antimicrobial drugs is a new application of risk assessment that is closely related to microbiological risk assessment. Antimicrobial resistance risk assessment is a risk analyticalprocess that focuses on resistance determinants as hazardous agents that might lead to drug-resistant microbial infections in humans exposed to bacteria carrying the determinants. Antimicrobial-resistant infections could occur directly from actively inavading or opportunistic pathogens or indirectly from the transfer of resistance genes to other bacteria. Here, we discuss risk assessment models that might be employed to estimate risks from drug-resistant bacteria in the animal food pathway and the types of models and data that may be used for microbiological risk assessments or antimicrobial resistance risk assessments.

Antimicrobial resistance in nontyphoid Salmonella serotypes: a global challenge

Increasing antimicrobial resistance in nontyphoid Salmonella species has been a serious problem for public health worldwide. The high rate of resistance is hampering the use of conventional antibiotics, and growing resistance to newer antimicrobial agents is aggravating the situation. The circumstances of occurrence and spread of antimicrobial resistance are complex; however, a major cause is the widespread use of antimicrobial agents in food animals, particularly in animal feed. Genetic analysis has indicated that the source of resistance is frequently a transferable plasmid. Recent studies have revealed that some serotype-specific virulence plasmids form hybrid plasmids through recombination with resistance plasmids or acquire gene cassettes consisting of multiple resistance genes. Such evolutionary events provide a virulent strain the advantage of survival in an unfavorable drug environment. In view of the serious implications associated with drug-resistant Salmonella species, a more deliberate use of antibiotics in both human medicine and animal industry is warranted. Continued surveillance of antimicrobial resistance and use of antimicrobial agents in food animals is also indispensable.

Comparison of antimicrobial susceptibility of Staphylococcus aureus isolated from bulk tank milk in organic and conventional dairy herds in the midwestern United States and Denmark

An observational study was conducted to compare the antimicrobial susceptibility patterns of Staphylococcus aureus isolated from bulk tank milk in organic and conventional dairy farms in Wisconsin, United States, and southern Jutland, Denmark. Bulk tank milk samples and data regarding management and production were collected from 30 organic and 30 conventional dairy farms in Wisconsin and 20 organic and 20 conventional dairy farms in Denmark. S. aureus isolates were tested for resistance against 15 antimicrobial agents by semiautomatic microbroth dilution methods in each country. Of the 118 bulk tank milk samples in Wisconsin, 71 samples (60%) yielded at least one S. aureus isolate, and a total of 331 isolates were collected. Of the 40 bulk tank milk samples from Denmark, 27 samples (55%) yielded at least one S. aureus isolate, and a total of 152 isolates were collected. Significant differences between organic and conventional dairies were detected only to ciprofloxacin in Wisconsin and avilamycin in Denmark. Significant differences (P < 0.05) between the two countries were detected in nine antimicrobials. Denmark had a higher probability of having reduced susceptibility to ciprofloxacin and streptomycin (P = 0.015 and 0.003, respectively). Wisconsin isolates had a higher probability of having reduced susceptibility to seven other antimicrobial agents (bacitracin, gentamicin, kanamycin, penicillin, sulphamethoxazole, tetracycline, and trimethoprim). We found small differences between organic and conventional farm types in each country and larger differences between the two national agricultural systems.

Gram-positive bacteria are a major reservoir of Class 1 antibiotic resistance integrons in poultry litter

Reversing the spread of antibiotic multiresistant bacteria is hampered by ignorance of the natural history of resistance genes, the mobile elements carrying them, and the bacterial hosts harboring them. Using traditional cultivation and cultivation-independent molecular techniques, we quantified antibiotic resistance genes and mobile elements called integrons in poultry house litter from commercial poultry farms. Unexpectedly, the major reservoir for Class 1 integrons in poultry litter is not their previously identified hosts, Gram-negative Enterobacteriaceae such as Escherichia coli. Rather, integrons and associated resistance genes abound in several genera of Gram-positive bacteria that constitute >85% of the litter community compared with Enterobacteriaceae that comprise <2% of this ecosystem. This finding warrants reexamination of our assumptions about the persistence and spread of antibiotic resistance genes.

Antimicrobial resistance of commensal Escherichia coli from dairy cattle associated with recent multi-resistant salmonellosis outbreaks

The use of antimicrobial drugs in livestock is suspected to contribute to bacterial antimicrobial resistance (AR) development. Dairy farms experiencing recent outbreaks of salmonellosis involving multi-resistant (MR) Salmonella strains were compared to control farms with respect to AR among bovine commensal E. coli isolates. For most antimicrobials tested, the percentage of AR E. coli isolated from salmonellosis-affected farms was significantly higher than that from control farms. Calf E. coli from both case and control farms had greater levels of AR than cow isolates. Commensal E. coli isolates from case farms and calves tended to more frequently be MR. These data are consistent with the existence of higher antimicrobial selection pressure on farms with recent salmonellosis outbreaks, however, the directionality of the relationship remains to be elucidated. An improved understanding of the epidemiology of AR bacteria in livestock production, both at the herd and molecular level, is essential to mitigate risk to public health and food safety.

Intrinsic mechanism decreases susceptibility of Escherichia coli O157:H7 to multiple antibiotics

Chloramphenicol-resistant mutants of Escherichia coli O157:H7 (n = 52) demonstrated multiple resistance to tetracycline, nalidixic acid, and ciprofloxacin. Approximately half of the mutants approached the clinical level of resistance to the antibiotic tetracycline. Mutants demonstrated a wide range of sensitivity to each antibiotic; for tetracycline, some isolates demonstrated a twofold increase, whereas others demonstrated a 15-fold increase in resistance. Continuous exposure to chloramphenicol did not affect the growth of mutants, suggesting that mutation does not have a negative effect on cell survival. Complementation experiments with a functional marR restored antibiotic susceptibility of selected mutants to levels similar to wild-type strains, suggesting that mar mutation was responsible for the decrease in sensitivity. The multiple antibiotic resistance (mar) operon is a global regulator controlling intrinsic resistance toward structurally and functionally unrelated antibiotics and other noxious agents. Antimicrobial use in both human and agricultural practice should be administered in a manner to prevent selection of resistant mutants.

Class 1 and class 2 integrons in poultry carcasses from broiler house and poultry processing environments

Integrons have been identified as major genetic contributors to the dissemination of antimicrobial resistance in bacteria. The objective of this study was to examine the prevalence of integrons in poultry processing at the broiler house and in processing plants. Class 1 and class 2 integrons were found throughout the processing environment. Of the two classes of integrons, class 1 was the most prevalent in all processing areas. The levels of both classes of integrons decreased from the farm to the processing plant. Within the chiller tank in the processing plant, the persistence of these sequences appears to be related to the free chlorine concentration of the chiller tank water. The variable regions of the amplified integrons showed size diversity (from 680 to 2,000 bp), suggesting diversity in types of antibiotic-resistance-coding gene cassettes. The presence of the class 1 and class 2 integrons in the chlorinated chiller tank suggests that these sequences are capable of withstanding this critical step in the reduction of microbial loads on poultry carcasses. The persistence of the integron gene sequences on the farm and throughout processing highlights the stability of these transmissible antibiotic-resistance-coding nucleotide sequences and their potential role as reservoirs of antibiotic-resistance-coding genetic elements within the poultry rearing and processing environments.

Characterization of Escherichia coli O157:H7 from downer and healthy dairy cattle in the upper Midwest region of the United States

While cattle in general have been identified as a reservoir of Escherichia coli O157:H7, there are limited data regarding the prevalence and clonality of this pathogen in downer dairy cattle and the potential impact to human health that may occur following consumption of meat derived from downer dairy cattle. In the present study, conducted at two slaughter facilities in Wisconsin between May and October of 2001, we established a higher prevalence of E. coli O157:H7 in fecal and/or tissue samples obtained aseptically from intact colons of downer dairy cattle (10 of 203, 4.9%) than in those from healthy dairy cattle (3 of 201, 1.5%). Analyses of 57 isolates, representing these 13 positive samples (one to five isolates per sample), by pulsed-field gel electrophoresis, revealed 13 distinct XbaI restriction endonuclease digestion profiles (REDP). Typically, isolates from different animals displayed distinct REDP and isolates from the same fecal or colon sample displayed indistinguishable REDP. However, in one sample, two different, but highly related, REDP were displayed by the isolates recovered. Antimicrobial susceptibility testing indicated that 10 of the 57 isolates, recovered from 2 (1 downer and 1 healthy animal) of the 13 positive samples, were resistant to at least 1 of 18 antimicrobials tested. However, there was no appreciable difference in the frequency of resistance of isolates recovered from downer and healthy dairy cattle, and not all isolates with the same REDP displayed the same antimicrobial susceptibility profile. Lastly, it was not possible to distinguish between isolates recovered from downer and healthy cattle based on their XbaI REDP or antimicrobial susceptibility. These results indicate that downer cattle had a 3.3-fold-higher prevalence of E. coli O157:H7 than healthy cattle within the time frame and geographic scope of this study.

Antibacterial cleaning and hygiene products as an emerging risk factor for antibiotic resistance in the community

Antibiotic resistance within the community setting is an emerging public-health concern. Infection with antibiotic-resistant organisms in the community among people lacking traditional risk factors has been reported. In addition, prevalence studies have identified individuals carrying antibiotic-resistant organisms in the absence of known risk factors. These studies strongly suggest the presence of contributing factors that have yet to be identified. In this paper we review the burden of antibiotic resistance and known risk factors within the community setting, assess the potential role of antibacterial cleaning and hygiene products containing triclosan in the emergence of resistance, and recommend future research on the assessment of household cleaning and hygiene products containing triclosan.

Antimicrobial resistance markers of class 1 and class 2 integron-bearing Escherichia coli from irrigation water and sediments

Municipal and agricultural pollution affects the Rio Grande, a river that separates the United States from Mexico. Three hundred and twenty-two Escherichia coli isolates were examined for multiple antibiotic resistance phenotypes and the prevalence of class 1 and class 2 integron sequences. Thirty-two (10%) of the isolates were resistant to multiple antibiotics. Four (13%) of these isolates contained class 1-specific integron sequences; one isolate contained class 2 integron-specific sequences. Sequencing showed that the class 1 integron-bearing strain contained two distinct gene cassettes, sat-1 and aadA. Although three of the four class 1 integron-bearing strains harbored the aadA sequence, none of the strains was phenotypically resistant to streptomycin. These results suggest that integron-bearing E. coli strains can be present in contaminated irrigation canals and that these isolates may not express these resistance markers.

Models of antimicrobial resistance and foodborne illness: examining assumptions and practical applications

Antimicrobial resistance is an issue of increasing global concern. Several investigators have suggested that antibiotic use in food-producing animals is a major contributor to the increasing incidence of antimicrobial-resistant organisms causing illness in humans (F. J. Angulo, K. R. Johnson, R. V. Tauxe, and M. L. Cohen, Microb. Drug Res. 6:77-83, 2000; P. D. Fey, T. J. Safranek, M. E. Rupp, E. F. Dunne, R. Efrain, P. C. Iwen, P. A. Bradford, F. J. Angulo, and S. H. Hinrichs, N. Engl. J. Med. 342:1242-1249, 2000; S. A. McEwen and P. J. Fedorka-Cray, Commun. Infect. Dis. 34(Suppl. 3):S93-S106, 2002; D. L. Smith, A. D. Harris, J. A. Johnson, E. K. Silbergeld, and J. G. Morris, Jr., Proc. Natl. Acad. Sci. USA 99:6434-6439, 2002; D. G. White, S. Zhao, R. Sudler, S. Ayers, S. Friedman, S. Chen, P. F. McDermott, D. D. Wagner, and J. Meng, N. Engl. J. Med. 345:1147-1154, 2001; W. Witte, Science 279:996, 1998). In this paper, we discuss this and other assumptions relevant to a quantitative risk assessment model for salmonellosis in humans. We also discuss other important aspects of modeling food safety and food-associated antimicrobial resistance risk to humans. We suggest that the role of food-producing animals in the origin and transmission of antimicrobial resistance and "foodborne" pathogens has been overestimated and overemphasized in the scientific literature; consequently, nonfoodborne transmission, including pet-associated human cases, has been underemphasized. Much evidence exists for the potential contribution to infectious disease that may be of human or pet origin (that may contact humans through food but not be of a food origin). Risk analyses that do not acknowledge the potential for these sources of cross-contamination will understate the contribution that origin has in the realm of foodborne and food-associated diseases (e.g., Salmonella) and the resulting uncertainty levels in the food system, thus leading to biased inferences. We emphasize the importance of evaluating both the foodborne and nonfoodborne transmission risk for salmonellosis and outline the basics of an analytical modeling approach in food safety with examples to illustrate strengths and limitations in the modeling. Examples illustrate, on a simplistic level, how varying assumptions and other inputs can influence the output of food-associated quantitative risk models.

Antimicrobial resistance in livestock

Antimicrobial resistance may become a major problem in veterinary medicine as a consequence of the intensive use and misuse of antimicrobial drugs. Related problems are now arising in human medicine, such as the appearance of multi-resistant food-borne pathogens. Product characteristics, dose, treatment interval and duration of treatment influence the selection pressure for antimicrobial drug resistance. There are theoretical, experimental and clinical indications that the emergence of de novo resistance in a pathogenic population can be prevented by minimizing the time that suboptimal drug levels are present in the infected tissue compartment. Until recently, attention has been focused on target pathogens. However, it should be kept in mind that when antimicrobial drugs are used in an individual, resistance selection mainly affects the normal body flora. In the long term, this is at least equally important as resistance selection in the target pathogens, as the horizontal transfer of resistance genes converts almost all pathogenic bacteria into potential recipients for antimicrobial resistance. Other factors contributing to the epidemiology of antimicrobial resistance are the localization and size of the microbial population, and the age, immunity and contact intensity of the host. In livestock, dynamic herd-related resistance patterns have been observed in different animal species.

Salmonellosis in children in developing and developed countries and populations

PURPOSE OF REVIEW

The present review addresses recent developments that relate to the clinical management and prevention of childhood salmonellosis in developed and developing countries.

RECENT FINDINGS

Invasive disease due to serovar Typhi as well as nontyphoidal salmonellae (NTS) is common in children younger than 5 years old in developing countries, and multidrug resistance is an increasingly difficult problem to manage. A new conjugate vaccine was found to be very effective in preventing typhoid fever in young Vietnamese children and was well tolerated, showing great promise for the future. Antibiotic use in the food animal industry is an important source of disease with multidrug resistant NTS strains in the developed world. Efforts for prevention are aimed at immunization of animals, control of antibiotic use in the food animal industry and careful monitoring of food-borne outbreaks. On the other hand, although the burden of NTS disease in children is far greater in developing countries, especially in tropical Africa, knowledge of even basic epidemiology is lacking. Importantly, it may be that, as spp. acquire increasing resistance, they also acquire increasing virulence that will lead to even greater morbidity and mortality.

SUMMARY

Recent developments include a better knowledge of clinical aspects of invasive salmonellosis, an increasing response to the problem of multiple antibiotic resistance (including quinolones), and excellent results from the use of a recently developed conjugate vaccine for typhoid fever in children as young as 2 years old.

Molecular characterization of multidrug-resistant Salmonella enterica subsp. enterica serovar Typhimurium isolates from swine

As part of a longitudinal study of antimicrobial resistance among salmonellae isolated from swine, we studied 484 Salmonella enterica subsp. enterica serovar Typhimurium (including serovar Typhimurium var. Copenhagen) isolates. We found two common pentaresistant phenotypes. The first was resistance to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (the AmCmStSuTe phenotype; 36.2% of all isolates), mainly of the definitive type 104 (DT104) phage type (180 of 187 isolates). The second was resistance to ampicillin, kanamycin, streptomycin, sulfamethoxazole, and tetracycline (the AmKmStSuTe phenotype; 44.6% of all isolates), most commonly of the DT193 phage type (77 of 165 isolates), which represents an unusual resistance pattern for DT193 isolates. We analyzed 64 representative isolates by amplified fragment length polymorphism (AFLP) analysis, which revealed DNA fingerprint similarities that correlated with both resistance patterns and phage types. To investigate the genetic basis for resistance among DT193 isolates, we characterized three AmKmStSuTe pentaresistant strains and one hexaresistant strain, which also expressed resistance to gentamicin (Gm phenotype), all of which had similar DNA fingerprints and all of which were collected during the same sampling. We found that the genes encoding the pentaresistance pattern were different from those from isolates of the DT104 phage type. We also found that all strains encoded all of their resistance genes on plasmids, unlike the chromosomally encoded genes of DT104 isolates, which could be transferred to Escherichia coli via conjugation, but that the plasmid compositions varied among the isolates. Two strains (strains UT08 and UT12) had a single, identical plasmid carrying bla(TEM) (which encodes ampicillin resistance), aphA1-Iab (which encodes kanamycin resistance), strA and strB (which encode streptomycin resistance), class B tetA (which encodes tetracycline resistance), and an unidentified sulfamethoxazole resistance allele. The third pentaresistant strain (strain UT20) was capable of transferring by conjugation two distinct resistance patterns, AmKmStSuTe and KmStSuTe, but the genes were carried on plasmids with slightly different restriction patterns (differing by a single band of 15 kb). The hexaresistant strain (strain UT30) had the same plasmid as strains UT08 and UT12, but it also carried a second plasmid that conferred the AmKmStSuGm phenotype. The second plasmid harbored the gentamicin resistance methylase (grm), which has not previously been reported in food-borne pathogenic bacteria. It also carried the sul1 gene for sulfamethoxazole resistance and a 1-kb class I integron bearing aadA for streptomycin resistance. We also characterized isolates of the DT104 phage type. We found a number of isolates that expressed resistance only to streptomycin and sulfamethoxazole (the StSu phenotype; 8.3% of serovar Typhimurium var. Copenhagen strains) but that had AFLP DNA fingerprints similar or identical to those of strains with genes encoding the typical AmCmStSuTe pentaresistance phenotype of DT104. These atypical StSu DT104 isolates were predominantly cultured from environmental samples and were found to carry only one class I integron of 1.0 kb, in contrast to the typical two integrons (InC and InD) of 1.0 and 1.2 kb, respectively, of the pentaresistant DT104 isolates. Our findings show the widespread existence of multidrug-resistant Salmonella strains and the diversity of multidrug resistance among epidemiologically related strains. The presence of resistance genes on conjugative plasmids and duplicate genes on multiple plasmids could have implications for the spread of resistance factors and for the stability of multidrug resistance among Salmonella serovar Typhimurium isolates.