Importance of host factors in human salmonellosis caused by multiresistant strains of Salmonella

Chemical and pathogen-induced inflammation disrupt the murine intestinal microbiome

BACKGROUND

Salmonella is one of the most significant food-borne pathogens to affect humans and agriculture. While it is well documented that Salmonella infection triggers host inflammation, the impacts on the gut environment are largely unknown. A CBA/J mouse model was used to evaluate intestinal responses to Salmonella-induced inflammation. In parallel, we evaluated chemically induced inflammation by dextran sodium sulfate (DSS) and a non-inflammation control. We profiled gut microbial diversity by sequencing 16S ribosomal ribonucleic acid (rRNA) genes from fecal and cecal samples. These data were correlated to the inflammation marker lipocalin-2 and short-chain fatty acid concentrations.

RESULTS

We demonstrated that inflammation, chemically or biologically induced, restructures the chemical and microbial environment of the gut over a 16-day period. We observed that the ten mice within the Salmonella treatment group had a variable Salmonella relative abundance, with three high responding mice dominated by >46% Salmonella at later time points and the remaining seven mice denoted as low responders. These low- and high-responding Salmonella groups, along with the chemical DSS treatment, established an inflammation gradient with chemical and low levels of Salmonella having at least 3 log-fold lower lipocalin-2 concentration than the high-responding Salmonella mice. Total short-chain fatty acid and individual butyrate concentrations each negatively correlated with inflammation levels. Microbial communities were also structured along this inflammation gradient. Low levels of inflammation, regardless of chemical or biological induction, enriched for Akkermansia spp. in the Verrucomicrobiaceae and members of the Bacteroidetes family S24-7. Relative to the control or low inflammation groups, high levels of Salmonella drastically decreased the overall microbial diversity, specifically driven by the reduction of Alistipes and Lachnospiraceae in the Bacteroidetes and Firmicutes phyla, respectively. Conversely, members of the Enterobacteriaceae and Lactobacillus were positively correlated to high levels of Salmonella-induced inflammation.

CONCLUSIONS

Our results show that enteropathogenic infection and intestinal inflammation are interrelated factors modulating gut homeostasis. These findings may prove informative with regard to prophylactic or therapeutic strategies to prevent disruption of microbial communities, or promote their restoration.

Helicobacter and salmonella persistent infection strategies

Some host-adapted bacterial pathogens are capable of causing persistent infections in humans. For example, Helicobacter pylori inhabits the human gastric mucosa and persistence can be lifelong. Salmonella enterica serovar Typhi causes systemic infections that involve colonization of the reticuloendothelial system and some individuals become lifelong carriers. In this review, I compare and contrast the different lifestyles of Helicobacter and Salmonella within the host and the strategies they have evolved to persist in mammalian hosts. Persistently infected carriers serve as the reservoirs for these pathogens, and the carrier state is an essential feature that is required for survival of the bacteria within a restricted host population. Therefore, investigating the chronic carrier state should provide insight into bacterial survival strategies, as well as new therapeutic approaches for treatments.

Antimicrobial resistance among invasive nontyphoidal Salmonella enterica isolates in the United States: National Antimicrobial Resistance Monitoring System, 1996 to 2007

Nontyphoidal salmonellae (NTS) are important causes of community-acquired bloodstream infection. We describe patterns of antimicrobial resistance among invasive NTS in the United States. We compared bloodstream NTS isolates with those from stool submitted to the National Antimicrobial Resistance Monitoring System (NARMS) from 1996 to 2007. We describe antimicrobial resistance among invasive strains by serogroup and serotype. Of the 19,302 NTS isolates, 17,804 (92.2%) were from stool or blood. Of these, 1,050 (5.9%) were bloodstream isolates. The median ages (ranges) of patients with and without bacteremia were 36 (<1 to 97) years and 20 (<1 to 105) years, respectively (P < 0.001). Males (odds ratio [OR], 1.21; 95% confidence interval [CI], 1.06 to 1.38) and those ≥65 years of age were at greater risk for invasive disease. Salmonella enterica serotypes Enteritidis, Typhimurium, and Heidelberg were the most common serotypes isolated from blood; S. enterica serotypes Dublin, Sandiego, and Schwarzengrund were associated with the greatest risk for bloodstream isolation. Of invasive isolates, 208 (19.8%) were resistant to ampicillin, 117 (11.1%) to chloramphenicol, and 26 (2.5%) to trimethoprim-sulfamethoxazole; 28 (2.7%) isolates were resistant to nalidixic acid and 26 (2.5%) to ceftriaxone. Antimicrobial resistance to traditional agents is common. However, the occurrence of nalidixic acid and ceftriaxone resistance among invasive NTS is cause for clinical and public health vigilance.

Molecular Epidemiology

Molecular epidemiology is now an established discipline in epidemiology.(1) It is the contemporary stage in the evolution of laboratory-based epidemiology that may have begun with the discovery in the late 1800s of ways to differentiate bacterial organisms by pure culture in artificial media.(2) Molecular epidemiology uses new molecular biology tools to address questions difficult or not possible to address by old laboratory tools. Just as statistical tools have become indispensable in epidemiological investigations and interpretations of epidemiologic data, molecular biology tools today have come to elucidate epidemiologic features of diseases that cannot be easily characterized by conventional techniques. Applied to infectious diseases, molecular biology methods have also come to challenge our traditional notions about the epidemiology of these diseases and have engendered novel opportunities for their prevention and control. This chapter will (1) review definitions commonly used in molecular epidemiology, (2) present an overview of molecular biology methods used to study infectious disease epidemiology, and (3) describe examples of the types of epidemiologic problems that can be addressed by molecular biology techniques, highlighting new concepts that emerged in the process of applying this approach to study bacterial infectious diseases.

Host transmission of Salmonella enterica serovar Typhimurium is controlled by virulence factors and indigenous intestinal microbiota

Transmission is an essential stage of a pathogen's life cycle and remains poorly understood. We describe here a model in which persistently infected 129X1/SvJ mice provide a natural model of Salmonella enterica serovar Typhimurium transmission. In this model only a subset of the infected mice, termed supershedders, shed high levels (>10(8) CFU/g) of Salmonella serovar Typhimurium in their feces and, as a result, rapidly transmit infection. While most Salmonella serovar Typhimurium-infected mice show signs of intestinal inflammation, only supershedder mice develop colitis. Development of the supershedder phenotype depends on the virulence determinants Salmonella pathogenicity islands 1 and 2, and it is characterized by mucosal invasion and, importantly, high luminal abundance of Salmonella serovar Typhimurium within the colon. Immunosuppression of infected mice does not induce the supershedder phenotype, demonstrating that the immune response is not the main determinant of Salmonella serovar Typhimurium levels within the colon. In contrast, treatment of mice with antibiotics that alter the health-associated indigenous intestinal microbiota rapidly induces the supershedder phenotype in infected mice and predisposes uninfected mice to the supershedder phenotype for several days. These results demonstrate that the intestinal microbiota plays a critical role in controlling Salmonella serovar Typhimurium infection, disease, and transmissibility. This novel model should facilitate the study of host, pathogen, and intestinal microbiota factors that contribute to infectious disease transmission.

Epidemiology and risk factors for endemic typhoid fever in Uzbekistan

BACKGROUND

To investigate the risk factors for infection with endemic typhoid fever in the Samarkand region of Uzbekistan.

METHODS

Case-control study of culture-confirmed bloodstream infection with Salmonella Typhi. Patients were compared to age-matched community controls. Salmonella Typhi isolates were tested for antimicrobial susceptibility.

RESULTS

We enrolled 97 patients and 192 controls. The median age of patients was 19 years. In a conditional regression model, consumption of unboiled surface water outside the home [adjusted odds ratio (aOR)=3.0, 95% confidence interval (CI)=1.1-8.2], use of antimicrobials in the 2 weeks preceding onset of symptoms (aOR=12.2, 95% CI 4.0-37.0), and being a student (aOR=4.0, 95% CI 1.4-11.3) were independently associated with typhoid fever. Routinely washing vegetables (aOR 0.06, 95% CI 0.02-0.2) and dining at a tea-house (aOR 0.4, 95% CI 0.2-1.0) were associated with protection against illness. Salmonella Typhi resistant to ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole was identified in 6 (15%) of 41 isolates tested.

CONCLUSIONS

Endemic typhoid fever in Uzbekistan is transmitted by contaminated water. Recent use of antimicrobials also increased risk of infection. Targeted efforts at improving drinking water quality, especially for students and young adults, are likely to decrease transmission of typhoid fever. Measures to decrease the unnecessary use of antimicrobials would be expected to reduce the risk of typhoid fever and decrease the spread of multiple drug-resistant Salmonella Typhi.

Human health consequences of antimicrobial drug-resistant Salmonella and other foodborne pathogens

There are several clinical and public health consequences associated with antimicrobial drug resistance in non-Typhi Salmonella species and other foodborne pathogens. If bacteria acquire resistance to clinically important antimicrobial drugs, early empirical treatment may fail, and there will be limitations in the choices of treatment after the establishment of microbial diagnosis. Drug-resistant gastrointestinal pathogens preferentially cause illness in persons receiving antimicrobial drugs for any medical condition. Consequently, emerging resistance in foodborne pathogens may result in increased burdens of illness and outbreaks in settings where patients are treated with antimicrobial drugs. Finally, resistance may be associated with increased virulence, and several epidemiological studies have demonstrated that infections with drug-resistant non-Typhi Salmonella serotypes and Campylobacter species are associated with excess mortality and morbidity. Mitigation of drug resistance in foodborne bacteria is likely to be of benefit for human health.

Hospitalization and antimicrobial resistance in Salmonella outbreaks, 1984-2002

Few studies have evaluated the health consequences of antimicrobial-resistant Salmonella strains associated with outbreaks. Among 32 outbreaks occurring in the United States from 1984 to 2002, 22% of 13,286 persons in 10 Salmonella-resistant outbreaks were hospitalized, compared with 8% of 2,194 persons in 22 outbreaks caused by pansusceptible Salmonella strains (p<0.01).

Antimicrobial use in agriculture: controlling the transfer of antimicrobial resistance to humans

Salmonella and Campylobacter infections occur commonly in children. Some of these infections are severe, requiring treatment with antimicrobial agents. Many classes of antimicrobial agents that are used in humans also are used in food animals for growth promotion, disease prevention, and therapy. The use of such antimicrobial agents in food animals increases the likelihood that human bacterial pathogens that have food animal reservoirs, such as Salmonella or Campylobacter, will develop cross-resistance to drugs approved for use in human medicine. Resistance determinants also may be transmitted from food animals to humans through the food supply with bacteria that usually are commensal, such as Escherichia coli and enterococci. Clinicians should be aware that antimicrobial resistance is increasing in food-borne pathogens and that patients who are taking antimicrobial agents for any reason are at increased risk for acquiring antimicrobial-resistant food-borne infections. Several European countries have demonstrated that restricting the use of antimicrobial agents in food animals can be followed by a decrease in antimicrobial resistance in humans without compromising animal health or significantly increasing the cost of production. Appropriate use of antimicrobial agents in humans and food animals is an important factor in maintaining their effectiveness.

A nosocomial outbreak of Salmonella enteritidis associated with lyophilized enteral nutrition

Outbreaks of Salmonella spp. gastro-enteritis in hospitals are of concern because of the increased susceptibility of patients and associated high morbidity. This study is a report of a nosocomial outbreak of Salmonella enteritidis associated with enteral nutrition. In December 1999, one sample of enteral feed tested positive for S. enteritidis. During the subsequent 6 weeks, eight cases of nosocomial salmonellosis occurred. Patients involved in the outbreak were aged 19-79 years (median = 36.5), and salmonella was isolated from the blood of two patients. All patients were receiving enteral nutrition at the time and all had diarrhoea. Three patients died. All 13 employees of the Nutrition Department were asymptomatic and their stool samples were negative. Environmental and water samples were also negative. The diet, however, contained lyophilized egg albumin. Molecular typing showed that the isolates of seven patients were indistinguishable from the one obtained from the enteral diet. It was thought that the nosocomial salmonellosis probably occurred due to the use of a commercial lyophilized diet. Another method of processing diets may be necessary to ensure patient safety.

Public Health Consequences of Use of Antimicrobial Agents in Food Animals in the United States

The use of antimicrobial agents in food animals has caused concern regarding the impact these uses have on human health. Use of antimicrobial agents in animals and humans results in the emergence and dissemination of resistant bacteria. Resistant bacteria from food animals may be passed through the food chain to humans resulting in resistant infections. Increasing resistance to antimicrobial agents that are important in the treatment of human diseases, such as fluoroquinolones and third-generation cephalosporins for the treatment of Salmonella and Campylobacter infections, has significant public health implications. Efforts to mitigate the effects of increasing resistance require collaboration by several partners, including the farming, veterinary, medical, and public health communities.

Risk factors for mortality caused by nontyphoidal Salmonella sp. in children

OBJECTIVE

To identify the risk factors for mortality in extraintestinal nontyphoidal Salmonella (NTS) infections in infants and children.

METHODS

We performed a retrospective analysis of 107 patients with at least one nonfecal culture for NTS seen from January 1988 to December 1995.

RESULTS

The median age was 12 (range 1-216) months. Malnutrition was found in 55 patients (51%), and 22 (20%) displayed severe features (weight loss >40%). Seventy-two patients (67%) had previously been hospitalized, and 59 (55%) had received antibiotics during the month before admission. Fever (85%) and diarrhea (56%) were the most frequent clinical manifestations. Nineteen children (18%) had leukopenia. Forty-nine patients (46%) had only bacteremia, 33 (31%) bacteremia with focal infections, and 25 (23%) focal infections with negative blood cultures. Forty-seven strains (44%) were multiresistant, and 40 of them were nosocomially acquired. Eight patients (7%) had received inappropriate antibiotic treatment, and two of them died. Thirteen (12%) children died. Age, underlying disease, previous admission, previous antibiotic therapy, type of infection, susceptibility of the strains and inappropriate antibiotic treatment were not statistically significant risk factors for mortality. A logistic regression analysis selected the following variables as independently influencing outcome: malnutrition (P<0.01), leukopenia (P<0.002) and presence of diarrhea (P<0.02).

CONCLUSIONS

Children with extraintestinal infections by NTS with leukopenia, malnutrition and presence of diarrhea have a higher risk of death.

Excess infections due to antimicrobial resistance: the "Attributable Fraction"

Antimicrobial use causes a transient decrease in an individual's resistance to colonization by noncommensal bacteria ("competitive effect") and increases the likelihood of infection upon exposure to a foodborne pathogen. The additional "selective effect" of antimicrobial resistance results in a >3-fold increase in vulnerability to infection by an antimicrobial-resistant pathogen among individuals receiving antimicrobial therapy for unrelated reasons. Combining the increase in vulnerability to infection with the prevalence of taking an antimicrobial agent, it is possible to estimate the attributable fraction, or the number of excess infections that occurred as a result of the unrelated use of an antimicrobial agent to which the pathogen was resistant. Calculations based on estimates of the annual infection rates and attributable fractions of infections with nontyphoidal Salmonella and Campylobacter jejuni suggest that resistance to antimicrobial agents results annually in an additional 29,379 nontyphoidal Salmonella infections, leading to 342 hospitalizations and 12 deaths, and an additional 17,668 C. jejuni infections, leading to 95 hospitalizations.

Prevalence and antibiotic susceptibility of Salmonella isolated from beef animal hides and carcasses

This study determined the prevalence of Salmonella on beef animal hides and carcasses and antimicrobial susceptibility profiles against a panel of 13 antibiotics. In each of the eight commercial packing facilities, of which five processed primarily heifers and steers and the remaining three processed primarily cows and bulls, hide and carcass sponge swab samples were obtained immediately before hide removal and before carcass chilling, respectively. Overall, prevalence of Salmonella on external surfaces (hides) of cattle was 15.4% (49 of 319), whereas prevalence after dehiding and other slaughtering/dressing processes, including the application of decontamination treatments, was, as expected, reduced (P < 0.05) to 1.3% (4 of 320) on carcass surfaces. From 53 total Salmonella-positive hide and carcass samples, 526 biochemically confirmed isolates were obtained to determine antimicrobial susceptibility profiles. Of 53 Salmonella-positive samples, individually, 24 (45.3%), 17 (32.1%), 17 (32.1%), 11 (20.8%), 8 (15.1%), 8 (15.1%), 8 (15.1%), 4 (7.5%), and 2 (3.8%) samples yielded at least one isolate resistant to amoxicillin/clavulanic acid, tetracycline, streptomycin, sulfonamides, ampicillin, ampicillin/sulbactam, chloramphenicol, gentamicin, and trimethoprim/sulfamethoxazole, respectively. None of the Salmonella-positive samples yielded an isolate resistant to ceftriaxone, ciprofloxacin, enrofloxacin, or levofloxacin. Although none of the samples yielded an isolate simultaneously resistant to three or four antimicrobials, a total of eight samples yielded at least one isolate resistant to five or more antimicrobials tested. Included among the 18 group B-positive samples were three samples that, individually, yielded at least one Salmonella Typhimurium var. Copenhagen DT104 isolate resistant to at least six antimicrobials tested. Results from this study support current prudent therapeutic and subtherapeutic antimicrobial use recommendations.

Human health hazards associated with the administration of antimicrobials to slaughter animals. Part II. An assessment of the risks of resistant bacteria in pigs and pork

Risks for the consumer regarding the acquisition of resistant bacteria and/or resistance genes via the consumption of pork are discussed. In general, Salmonella spp. and Escherichia coli that originate from animals do not easily transfer their resistance genes to the resident intestinal flora of humans. The prevalence of resistant E. coli in humans seems more associated with being a vegetarian (odds ratio (OR) 1.89) than with the consumption of meat and meat products. Other risk factors are treatment with antimicrobials (OR 2-5), becoming hospitalized (OR 5.93), or working in a health setting (OR 4.38). In the Netherlands, annually an estimated 45,000 people (0-150,000) become a carrier of resistant E. coli and/or resistance genes that ori ginate from pigs, while an estimated 345,000 persons (175,000-600,000) become a carrier of resistant E. coli and/or resistance genes that originate from hospitals, e.g. other patients. Any problems with resistant Salmonella spp. that stem from pigs are, in fact, an integral part of the total problem of food-borne salmonellosis. Sometimes there are outbreaks of a specific multi-resistant clone of S. typhimurium that causes problems in both farm animals and humans. The probability that in the next 30 years there is no or maximally one outbreak of a specific clone that originates from pig herds is estimated at about 75%. Antimicrobials used as a growth promoter can have a measurable influence on the prevalence of resistant bacteria. The likely chain of events regarding avoparcin and the selection and dissemination of resistance against vancomycin in the enterococci gives the impression that the impact of the use of antimicrobials in animals on the prevalence of resistance in humans is largely determined by whether resistance genes are, or become, located on a self-transferable transposon. Furthermore, consumer health risks of antimicrobials used in slaughter pigs are mainly determined by the selection and dissemination of bacterial resistance and much less by the toxicological properties of any residues in pork. It is also concluded that most of the problems with resistant bacteria in humans are associated with the medical use of antimicrobials, and that the impact of particularly the veterinary use of antimicrobials is limited. However, the impact of antimicrobials used as a feed additive appears to be much greater than that of antimicrobials used for strictly veterinary purposes. The use of antimicrobials as a feed additive should therefore be seriously reconsidered.

Origins and consequences of antimicrobial-resistant nontyphoidal Salmonella: implications for the use of fluoroquinolones in food animals

Human Salmonella infections are common; most infections are self-limiting, however severe disease may occur. Antimicrobial agents, while not essential for the treatment of Salmonella gastroenteritis, are essential for the treatment of thousands of patients each year with invasive infections. Fluoroquinolones and third-generation cephalosporins are the drugs-of-choice for invasive Salmonella infections in humans; alternative antimicrobial choices are limited by increasing antimicrobial resistance, limited efficacy, and less desirable pharmacodynamic properties. Antimicrobial-resistant Salmonella results from the use of antimicrobial agents in food animals, and these antimicrobial resistant Salmonella are subsequently transmitted to humans, usually through the food supply. The antimicrobial resistance patterns of isolates collected from persons with Salmonella infections show more resistance to antimicrobial agents used in agriculture than to antimicrobial agents used for the treatment of Salmonella infections in humans. Because of the adverse health consequences in humans and animals associated with the increasing prevalence of antimicrobial-resistant Salmonella, there is an urgent need to emphasize non-antimicrobial infection control strategies, such as improved sanitation and hygiene, to develop guidelines for the prudent usage of antimicrobial agents, and establishment of adequate public health safeguards to minimize the development and dissemination of antimicrobial resistance and dissemination of Salmonella resistant to these agents.

Microbial risk assessment: dose-response relations and risk characterization

Characterizations of the risks associated with foodborne pathogens are dependent on the availability of information on the population's exposure to the biological agents. However, by itself, exposure data are insufficient to assess the public health impact of pathogenic microorganisms. This requires the availability of effective dose-response models. Successful development of models that describe dose-response relations for enteric pathogens is dependent on a sound understanding of the mechanisms of pathogenicity associated with individual pathogens. This includes knowledge of how the various pathogen, host, and food matrix factors influence pathogenicity. Currently, a group of sigmoidal mathematical equations are used to empirically describe dose-response relations. While these have proven to be highly useful, advances in microbial food safety risk assessment will likely require the development of mechanistic models that more effectively consider the range of factors that influence the frequency and severity of foodborne infections in a population.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

Impact on human health of Salmonella spp. on pork in The Netherlands and the anticipated effects of some currently proposed control strategies

The impact on human health of Salmonella spp. on pork in The Netherlands is described. Subsequently, the effects of some currently proposed control strategies in the Dutch pork production chain are evaluated and quantified with the aid of a simple mathematical model. The estimated average incidence of cases of salmonellosis in the Netherlands is about 450 cases per 100,000 person years at risk (pyar). Some special risk groups for which the risks could be quantified are (1) persons with underlying diseases, such as neoplasms or diabetes mellitus (1200 cases/100,000 pyar); (2) persons with achlorhydria or who excessively use antacids (1100 cases/100,000 pyar); (3) persons who have recently been treated with antibiotics that disturb the normal gut flora (1700 cases/100,000 pyar); (4) nurses (900 cases/100,000 pyar); (5) caterers (900 cases/100,000 pyar); (6) slaughterline personnel (1800 cases/100,000 pyar). Furthermore, it is estimated that 15% (5-25%) of all cases of salmonellosis in The Netherlands are associated with the consumption of pork. Currently, proposed control measures regarding Salmonella in pigs and on pork in The Netherlands are codes of good manufacturing practices (GMP) that, in fact, formalize recommendations that can be found in many handbooks about pig breeding and pig slaughtering. When evaluated by a mathematical model constructed for this purpose, the proposed GMP codes from farm to cutting/retail could, at best, reduce the current levels of Salmonella-positive pigs and pork by 50-60%. If pigs were bred according to the rather costly specific pathogen-free concept (SPF), the prevalence of contaminated carcasses and pork could in total be reduced by 95% or more. However, implementing GMP codes from the transport phase up to the cutting/retail phase coupled with a decontamination step at the end of the slaughterline would be just as effective as GMP in combination with breeding using the SPF-concept. It is therefore concluded that the most efficient and cost-effective way of reducing the 'Salmonella problem' entailed by the consumption of pork would be to decontaminate carcasses under the precondition that the entire production chain strictly adheres to GMP principles. Therefore, the EU should also allow for more possibilities regarding the decontamination of carcasses than is currently the case. It is also concluded that current EU regulations relying on hazard analysis of critical control points (HACCP)-inspired production in cutting plants will not be effective in reducing the prevalence of Salmonella spp. on pork. This is mainly because (1) there is currently an almost steady stream of Salmonella-positive carcasses that enter the cutting process; (2) when contaminated carcasses are being processed, further cross-contamination during working hours is unavoidable; (3) no steps in the cutting process are intentionally designed to effectively reduce the risks or consequences of cross contamination of cuts and retail-ready products.

Emergence of multidrug-resistant Salmonella enterica serotype typhimurium DT104 infections in the United States

BACKGROUND

Strains of salmonella that are resistant to antimicrobial agents have become a worldwide health problem. A distinct strain of Salmonella enterica serotype typhimurium, known as definitive type 104 (DT104), is resistant to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline and has become a major cause of illness in humans and animals in Europe, especially the United Kingdom.

METHODS

To characterize typhimurium DT104 infections in the United States, we analyzed data collected by local and state health departments and public health laboratories between 1979 and 1996 in national surveys of the antimicrobial-drug resistance of salmonella. Selected typhimurium isolates with the five-drug pattern of resistance were phage typed.

RESULTS

The prevalence of typhimurium isolates with the five-drug pattern of resistance increased from 0.6 percent in 1979-1980 to 34 percent in 1996. In 1994-1995, such isolates were identified in samples from 36 of the 46 surveillance sites (78 percent). Thirty-nine of 43 typhimurium isolates with the five-drug pattern of resistance identified in 1994-1995 and 1996 were phage type DT104 or a closely related phage type.

CONCLUSIONS

Multidrug-resistant typhimurium DT104 has become a widespread pathogen in the United States. More prudent use of antimicrobial agents in farm animals and more effective disease prevention on farms are necessary to reduce the dissemination of multidrug-resistant typhimurium DT104 and to slow the emergence of resistance to additional agents in this and other strains of salmonella.

Bacterial infections

Non-opportunistic bacterial infections are an important cause of morbidity and mortality for HIV-infected adults and children. Factors associated with increased risk of these include altered B- and T-cell function; altered phagocytic cell function; skin and mucous membrane defects; and use of indwelling vascular catheters, antibiotics, or cytotoxic agents. The pathogens encountered most frequently are S. aureus, S. pneumoniae, H. influenzae, Salmonella sp., and Pseudomonas aeruginosa. Less commonly encountered organisms include Rhodococcus equi, Listeria monocytogenes, Shigella sp., and Nocardia asteroides, Strategies for prevention as well as diagnosis and treatment of these are discussed.

Impact of age and drug resistance on mortality in typhoid fever

The risk factors for mortality were analysed in a consecutive group of 1158 children presenting to the Aga Khan University Medical Center, Karachi, with multidrug resistant typhoid fever that had been proved on culture. There were 19 deaths, representing an overall case fatality rate of 1.6%. Multidrug resistant typhoid was associated with a more severe clinical illness and higher rates of toxicity, hepatomegaly, hypotensive shock, and death. Irrespective of drug resistance status, typhoid fever was found to be a more severe illness in young infants with significantly higher rates of diarrhoea, hypotensive shock, and mortality. Univariate analysis of admission characteristics associated with increased risk for mortality revealed significant association with younger age (p < 0.05), hypotensive shock or hypothermia (p < 0.001), obtundation (p < 0.001), seizures (p < 0.05), anaemia at admission (p < 0.005), and leucocytosis (p < 0.001). Logistic regression analysis of risk factors for mortality showed persistent association of hypothermia, toxicity, and anaemia with mortality. The data provides evidence that multidrug resistant typhoid in childhood is associated with increased risk of mortality, especially in infancy and closer attention to several risk factors for increased morbidity and case fatality rates may lead to improved outcome of treatment.

A case-control study of Salmonella gastrointestinal infection in Italian children

A case-control study of 85 cases with non-typhoid Salmonella gastroenteritis, 85 outpatient controls and 79 inpatient controls was conducted among children in Monfalcone, north-east Italy, between June 1989 and June 1994. Logistic regression was used to evaluate the effect of demographic and socio-economic characteristics, duration of breastfeeding, history of intestinal illnesses and household diarrhoea, and the recent use of antimicrobials. Breastfeeding was the single most important factor associated with a 5-fold decreased risk of Salmonella infection. In addition, children who were treated with antimicrobials before onset of gastroenteritis had a 3-fold increased risk. Low social class and history of other chronic non-infectious intestinal diseases were also directly associated with illness.

Identification and quantification of risk factors in animal management and transport regarding Salmonella spp. in pigs

This article discusses the main elements of a descriptive epidemiological model for Salmonella spp. in the pre-harvest stages of pork production, and the subsequent quantification of risk factors. About two thirds of all Dutch pig farms are more or less permanently infected. At infected finishing farms, the current probability that Salmonella-free pigs will become infected is about 85%. In the case that a certain pen is infected, the current probability that further pen-to-pen transmission will occur is about 90% and that (human) vectors will also spread the infection 60%. Between 5-30% of the animals may still excrete Salmonella spp. at the end of the finishing period, and this percentage can double during transport and lairage. In infected pigs at slaughter weight, the Salmonella spp. are foremost located in the digestive tract, its contents and the closely associated lymph nodes. Under the current circumstances, the lack of farm hygiene (odds ratio (OR) 39.7), (re)contaminated feed (OR 1.6), the use of broad spectrum antibiotics (OR 5.6), a positive Salmonella-status of animals before transport (roughly estimated OR 4.0), the lack of transport hygiene (roughly estimated OR 1.1) and transport stress (OR 1.9) are the most important risk factors regarding infections with Salmonella spp. Currently the role of on-farm contamination cycles with endemic ('house flora') Salmonella spp. is so important, that the role of other factors is difficult to ascertain. It is also concluded that the farm-phase forms the core of all current problems, and that better prevention and control can be achieved by, (i) very strict and consistent farm hygiene in combination with promotion of the colonization resistance of animals kept together with a prudent use of broad spectrum antibiotics; (ii) simultaneous execution of control programmes at breeding farms, multiplying farms and finishing farms; (iii) separate transport, lairage and slaughter of the animals thus produced.

Epidemiology of drug resistance: implications for a post-antimicrobial era

In the last several years, the frequency and spectrum of antimicrobial-resistant infections have increased in both the hospital and the community. Certain infections that are essentially untreatable have begun to occur as epidemics both in the developing world and in institutional settings in the United States. The increasing frequency of drug resistance has been attributed to combinations of microbial characteristics, selective pressures of antimicrobial use, and societal and technologic changes that enhance the transmission of drug-resistant organisms. Antimicrobial resistance is resulting in increased morbidity, mortality, and health-care costs. Prevention and control of these infections will require new antimicrobial agents, prudent use of existing agents, new vaccines, and enhanced public health efforts to reduce transmission.

Comparison of plasmid profiles, phage types, and antimicrobial resistance patterns of Salmonella enteritidis isolates in the United States

To evaluate the laboratory techniques for subtyping isolates of Salmonella enteritidis, we compared the plasmid profiles (PP), phage types (PT), and antimicrobial susceptibility patterns (AS) of two nationally representative samples of sporadic human S. enteritidis isolates from 1979 (n = 28) and 1984 (n = 37), 43 isolates from 20 outbreaks of S. enteritidis infections between 1983 and 1987, and 46 animal isolates selected from the U.S. Department of Agriculture Veterinary Services Laboratory in 1986 and 1987. Sporadic and outbreak isolates from humans showed similar rates of resistance to at least one of a panel of antimicrobial drugs (23 and 14%, respectively), PT (91 and 98%, respectively), and PP (97 and 100%, respectively). Sixteen different PP were identified in sporadic, outbreak, and animal isolates; two PP accounted for 76% of sporadic and outbreak isolates. Sporadic human isolates were of PT 8 (42%), of PT 13a (37%), nontypeable (9%), of PT 14b (8%), of PT 9a (3%), and of PT 13 (2%). Outbreak human isolates had similar distributions of PT. PT 8 was associated with poultry: 58% (7 of 12) of the poultry isolates but only 24% (8 of 34) of the isolates from other animals were of PT 8 (P less than 0.04). Although antimicrobial susceptibility patterns do not appear as useful as an epidemiologic marker, PP and PT effectively subtyped S. enteritidis.

Antibiotic use among children in an urban Brazilian slum: a risk factor for diarrhea?

Among a cohort of children in a poor urban setting in Brazil, the relative risk for the occurrence of a new episode of diarrhea in the two weeks following antibiotic use vs all other weeks was 1.44 (95% confidence interval (CI) = 1.33, 2.45). Among children ever [corrected] exposed to antibiotics, the odds ratio was 1.34 (95% CI = 0.84, 2.16) after stratifying by individual child and controlling for previous diarrhea. Further research is needed to confirm whether antibiotics are a risk factor for diarrhea in such settings.

Antibiotic availability and use: consequences to man and his environment

Antibiotics are the cornerstone of therapy for infectious diseases affecting people living worldwide. The ability of these agents to cure infections, with little if any harm to the host, has helped to propagate their designation as "miracle drugs". This idea has continued throughout the past four decades and into the present one. To a large extent, this concept has led to the inappropriate use of these agents. The consequence of this misuse affects not only the individuals taking antibiotics, but indirectly others who may subsequently need antibiotics, but find that the infecting pathogen is resistant. This phenomenon results from the strong selective environmental effect of these drugs for resistant bacteria. Resistant pathogens have emerged and spread worldwide. In response to this global health problem, the Alliance for the Prudent Use of Antibiotics was established. This international network of concerned prescribers and users aims to improve antibiotic use and eliminate inappropriate use worldwide. Prudent use of antibiotics will not only curtail health care costs and the potential side effects to the individual taking these drugs, but also diminish the wide ecologic effects leading to selection of antibiotic resistant forms of common disease-causing agents.

Investigation of a food-borne outbreak of salmonellosis among hospital employees

An explosive outbreak of Salmonella enterocolitis developed in 27 hospital employees in an acute-care community hospital in Rhode Island in 1987. Salmonella typhimurium was isolated from the stools of 19 employees during the outbreak. In each patient the implicated organism had an identical antibiotic susceptibility pattern, biotype, plasmid profile, and restriction endonuclease digestion pattern. The outbreak was limited to health care workers and other hospital employees; there were no cases in hospitalized patients. Of the afflicted employees 96% ate in the hospital cafeteria on July 11 or 12, 1987. Food-specific attack rates, based on the dietary histories of ill employees and 50 healthy employees who ate in the cafeteria that weekend, indicated an association between the ingestion of salads and illness (p less than 0.01). One food service employee, in whom symptoms of abdominal cramping and diarrhea had developed 6 days earlier, had prepared the implicated foods. S. typhimurium with the identical characteristics of the outbreak strain was isolated from the stools of this food service employee. Environmental cultures and cultures of meat, poultry, and dairy sources for the cafeteria all showed negative results. Food service employees need to be counseled against working during any symptomatic enteric illness and require thorough instruction on hygienic food handling.

Plasmid profile analysis of salmonellae in a large-animal hospital

The plasmid profiles of salmonellae isolated from the patients and environment of the Purdue Large Animal Hospital were used as markers to identify strains and to assess the impact of improvements in hospital operation on nosocomial salmonellosis. Plasmid profile analysis proved to be more sensitive than either serotyping or antimicrobial susceptibility testing in identifying Salmonella isolates. During June and July 1983, 29 of 34 salmonellae isolated were one of three strains with distinct plasmid profiles: one S. typhimurium var. Copenhagen, and two S. muenchen. Each of these strains was isolated from at least one patient and two environmental sites, suggesting the possibility that infections were hospital-acquired. Patient and environmental sampling was repeated in June and July 1984, after improvements had been made in hospital traffic flow and sanitation. In contrast to 1983, only seven isolates, representing six strains not seen previously, were obtained in 1984. None of these strains was isolated from both patient and environmental sources. The results indicate that the high incidence of clinical salmonellosis in 1983 was largely due to nosocomial infections. The decrease in the incidence of salmonellosis and the absence of the 1983 strains from samples taken in 1984 were presumed to be due to improvements made in hospital operation. This study demonstrates the value of plasmid analysis in monitoring nosocomial salmonellosis in a veterinary hospital.

Chloramphenicol-resistant Salmonella newport traced through hamburger to dairy farms. A major persisting source of human salmonellosis in California

Animal-to-human transmission of drug-resistant salmonella and the role of antimicrobial use in food animals in the emergence of these bacteria are controversial subjects. Investigation of a 4.9-fold increase in Salmonella newport isolations from Californians in 1985 showed that 87 percent of the isolates had an unusual antimicrobial-resistance pattern (including chloramphenicol resistance) and a single, identical plasmid. Interviews of 45 patients and 89 matched controls in Los Angeles County showed that illness was associated with penicillin or tetracycline use during the month before onset (P less than 0.001) and with eating ground beef during the week before onset (P = 0.052). The epidemic strain was isolated from hamburger products eaten by cases, abattoirs where the animals from which the meat came were slaughtered, dairies that sent cows for slaughter on days when culture-positive products were processed, and ill dairy cows. Isolation of salmonella from beef carcasses in abattoirs correlated with the proportion of dead or moribund animals received for slaughter (r = 0.60, P less than 0.05). Isolation of chloramphenicol-resistant salmonella from dairy farms was associated with the use of chloramphenicol at those dairies. We conclude that food animals are a major source of antimicrobial-resistant salmonella infections in humans and that these infections are associated with antimicrobial use on farms.

Drug-resistant Salmonella in the United States: an epidemiologic perspective

Salmonellosis poses a health problem of large proportions in the United States. Annually, it accounts for more than 40,000 reported cases, 500 deaths, and financial costs well in excess of $50 million. Antimicrobial resistance is increasing in Salmonella strains, a finding that has important public health implications. Although the chain of transmission of the bacteria is often complex, combined epidemiologic and laboratory studies with the use of new methods in molecular biology make it possible to trace antimicrobial-resistant salmonellae to their primary source--foods of animal origin. These studies suggest that the antimicrobial drugs to which food animals are exposed provide selective pressure that leads to the appearance and persistence of resistant strains.

Selective antimicrobial modulation of the intestinal tract by norfloxacin in human volunteers and in gnotobiotic mice associated with a human fecal flora

Intestinal endogenous members of the family Enterobacteriaceae were eliminated in 12 human volunteers treated with 400 or 800 mg of oral norfloxacin per day for 5 days. No clones resistant to quinolone derivatives were isolated. Counts of aerotolerant streptococci were affected to various degrees, depending on their susceptibility to norfloxacin. During treatment, counts of anaerobes remained above 9.8 log10 CFU/g of feces. A total of 932 anaerobic isolates from the predominant flora (over 10(9) CFU/g) in fecal samples obtained before or during norfloxacin treatment were classified by a simple morphological and physiological scheme. The composition of this flora was fairly stable from one sample to another before treatment and was not substantially modified by norfloxacin. Intestinal resistance to colonization by exogenous microorganisms was studied in gnotobiotic mice associated with a human fecal flora. The composition of the fecal flora of the human donor and the fecal concentrations of norfloxacin in the volunteers were reproduced in the intestine of the mice. Resistance to colonization by exogenous microorganisms was reduced by norfloxacin for only 2 of 14 (14%) of the strains tested. These results suggest that norfloxacin is a good candidate for selective antimicrobial modulation of the intestinal tract in humans.

The use of subtherapeutic antibiotics in animal feed and its implications on human health

Antibiotics were approved for use as animal feed additives in 1950 after it was discovered that their use increased growth rate, improved feed utilization, and reduced mortality and morbidity from clinical and subclinical infections in animals. Subtherapeutic use of antibiotics in animal feed grew extensively and concern has arisen that this "indiscriminate" use of antibiotics could lead to increased numbers of antimicrobial-resistant bacteria and ultimately compromise treatment of human bacterial infections. Three recently published studies seem to indicate that animal-to-man transmission of antimicrobial-resistant bacteria is possible. Increased debate on the continuing allowance of antibiotic feed additives has ensued. The National Resources Defense Council has recently petitioned the FDA to disallow penicillin and tetracycline use in animal feed, and legislation has been introduced in the U.S. House of Representatives that would limit the subtherapeutic use of antibiotics in animal feed. Experts have predicted that meat prices would increase and meat quality would decrease if antibiotics are disallowed as a feed additive. It is the opinion of the authors that there is no conclusive evidence at this time to support the premise that subtherapeutic use of antibiotics in animal feed poses a greater threat to human health than if antibiotic feed additives were banned.

Drug-resistant Salmonella from animals fed antimicrobials

It has been difficult to document the postulated sequence of events that begins with the selection of drug-resistant organisms in animals fed subtherapeutic amounts of antimicrobials and ends with clinically important infections in human beings. In early 1983 we identified 18 persons in four Midwestern states who were infected with Salmonella newport that was resistant to ampicillin, carbenicillin, and tetracycline and characterized by a 38-kilobase R plasmid. Twelve of these patients had been taking penicillin derivatives for medical problems other than diarrhea in the 24 to 48 hours before the onset of salmonellosis. Eleven patients were hospitalized for salmonellosis for an average of eight days, and one had a fatal nosocomial infection. We compared plasmid profiles of all human (six-state area) and animal (United States) S. newport isolates over an 18-month period and examined selected records of meat distribution. The results indicated that the patients had been infected before they took antimicrobials, by eating hamburger originating from South Dakota beef cattle fed subtherapeutic chlortetracycline for growth promotion. This study demonstrates that antimicrobial-resistant organisms of animal origin cause serious human illness, and emphasizes the need for more prudent use of antimicrobials in both human beings and animals.