Models of parasite virulence

Several evolutionary processes influence virulence, the amount of damage a parasite causes to its host. For example, parasites are favored to exploit their hosts prudently to prolong infection and avoid killing the host. Parasites also need to use some host resources to reproduce and transmit infections to new hosts. Thus parasites face a tradeoff between prudent exploitation and rapid reproduction-a life history tradeoff between longevity and fecundity. Other tradeoffs among components of parasite fitness also influence virulence. For example, competition among parasite genotypes favors rapid growth to achieve greater relative success within the host. Rapid growth may, however, lower the total productivity of the local group by overexploiting the host, which is a potentially renewable food supply. This is a problem of kin selection and group selection. I summarize models of parasite virulence with the theoretical tools of life history analysis, kin selection, and epidemiology. I then apply the theory to recent empirical studies and models of virulence. These applications, to nematodes, to the extreme virulence of hospital epidemics, and to bacterial meningitis, show the power of simple life history theory to highlight interesting questions and to provide a rich array of hypotheses. These examples also show the kinds of conceptual mistakes that commonly arise when only a few components of parasite fitness are analysed in isolation. The last part of the article connects standard models of parasite virulence to diverse topics, such as the virulence of bacterial plasmids, the evolution of genomes, and the processes that influenced conflict and cooperation among the earliest replicators near the origin of life.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Enterohaemorrhagic Escherichia coli: emerging issues on virulence and modes of transmission

Enterohaemorrhagic Escherichia coli (EHEC) constitute a subset of serotypes (E. coli O157 and some other serogroups) of Shiga toxin (Stx)-producing E. coli (STEC) firmly associated with severe human illnesses like bloody diarrhoea and haemolytic uraemic syndrome. Stx production is essential but not sufficient for EHEC virulence. Most strains are capable of colonising the intestinal mucosa of the host with the "attaching and effacing" mechanism, genetically governed by a large pathogenicity island (PAI) defined as the Locus of Enterocyte Effacement. Other virulence factors carried by mobile genetic elements like PAI and plasmids have been recently described, and their role in the pathogenic process has not been fully elucidated. EHEC are zoonotic pathogens. They rarely cause disease in animals, and ruminants are recognised as their main natural reservoir. Cattle are considered to be the most important source of human infections with EHEC O157, and the ecology of the organism in cattle farming has been extensively studied. The organism has also been reported in sheep, goats, water buffalos, and deer. Pigs and poultry are not considered to be a source of EHEC and the sporadic reports may derive from accidental exposure to ruminant dejections. The epidemiology of EHEC infections has remarkably changed during the past ten years and an increasing number of unusual food vehicles have been associated with human infections. New routes of transmission have emerged, like contact with animals during farm visits and a wide variety of environment-related exposures. As for other zoonotic agents, having animals and raw products that are free from EHEC is not possible in practice. However, their occurrence can be minimised by applying high standards of hygiene in all the steps of the food production chain.

Emerging foodborne pathogens: Escherichia coli O157:H7 as a model of entry of a new pathogen into the food supply of the developed world

There would appear to be little argument that the large outbreaks of E. coli O157:H7 which have occurred since the early 1980s represent a distinct, new phenomenon. The number of reported cases have increased dramatically, starting from zero in 1981; however, it is also clear that this increase in reported cases is in part an artifact of improved surveillance and reporting. Available data suggest that E. coli O157:H7 infections were present prior to 1982, although numbers appear to have been small. At a molecular level, the organism shows evidence of clonal origin, but there is not the striking clonality, with virtually identical pulsed-field gel electrophoresis and ribotyping patterns, which has been seen in situations such as the emergence of Vibrio cholerae O139 Bengal in the Indian subcontinent in 1992 or the introduction of V. cholerae O1 into naïve populations in South America in 1991 (127-129). Findings are more consistent with the image of an organism which arose from a common ancestor, but which has had time to become distributed geographically and to show some evidence of genetic divergence. While this is an "emerging" infection, at least in terms of its distribution and public recognition, it is unlikely that it will be possible to identify the "first" O157:H7 case or to track the clonal spread of the organism through cattle or human populations.

A severe outbreak of Escherichia coli O157:H7--associated hemorrhagic colitis in a nursing home

In September 1985, an outbreak of Escherichia coli O157:H7 enteritis affected 55 of 169 residents and 18 of 137 staff members at a nursing home. The outbreak was characterized by two phases: a primary wave whose source was probably a contaminated sandwich meal and a secondary wave compatible with person-to-person transmission of infection. Among the elderly residents, the incubation period was 4 to 9 days (mean, 5.7 +/- 1.2). Older age and previous gastrectomy increased the risk of acquiring the infection (P = 0.01 and 0.03, respectively). Antibiotic therapy during exposure was associated with acquiring a secondary infection (P = 0.001). Hemolytic uremic syndrome developed in 12 affected residents (22 percent), 11 of whom died. Overall, 19 (35 percent) of the affected residents died, 17 (31 percent) from causes attributable to their infection. Antibiotic therapy after the onset of symptoms was associated with a higher case fatality rate in the more severe cases, possibly because patients with more severe disease tended to be treated with antibiotics. There were no complications or deaths among the affected members of the staff. Evidence of infection by verotoxin-producing E. coli O157:H7 was detected in 30 of 70 cases on the basis of isolation of this organism or demonstration of free verotoxin in stools. All isolates belonged to the same phage type. The high morbidity and mortality associated with this condition emphasize the need for proper food hygiene, rapid identification of outbreaks of disease, and prompt institution of infection-control techniques among the institutionalized elderly.

Predominant pathogens in hospital infections

To determine the distribution of pathogens causing nosocomial infections in United States hospitals, we analysed data from the National Nosocomial Infections Surveillance (NNIS) System. From October 1986 to December 1990, amongst hospitals conducting hospital-wide surveillance, the five most commonly reported pathogens were Escherichia coli (13.7%), Staphylococcus aureus (11.2%), enterococci (10.7%), Pseudomonas aeruginosa (10.1%), and coagulase-negative staphylococci (9.7%). The commonest pathogens reported by site included, bloodstream: coagulase-negative staphylococci, S. aureus, enterococci, E. coli, and Candida spp.; lower respiratory tract infection: S. aureus, P. aeruginosa and Enterobacter spp.; surgical wound infection: S. aureus, enterococci and coagulase-negative staphylococci; and urinary tract infection: E. coli, enterococci, and P. aeruginosa. Among hospitals conducting intensive care unit (ICU) surveillance, the commonest pathogens were P. aeruginosa (12.4%), S. aureus (12.3%), coagulase-negative staphylococci (10.2%), Candida spp. (10.1%), Enterobacter spp. and enterococci (8.6% each). In the ICUs, the commonest pathogens found in the bloodstream were coagulase-negative staphylococci, S. aureus, and enterococci; in lower respiratory tract infections P. aeruginosa, S. aureus, and enterococci; in surgical wound infections enterococci, coagulase-negative staphylococci, and Enterobacter spp. and in urinary tract infections Candida spp., E. coli, enterococci, P. aeruginosa, and Enterobacter spp. These data show that S. aureus, E. coli and P. aeruginosa remain important nosocomial pathogens, that coagulase-negative staphylococci, enterococci and C. albicans are pathogens of increasing importance, and that the distribution of pathogens differs by site and hospital location.

Fate of enterohemorrhagic Escherichia coli O157:H7 in bovine feces

Dairy cattle have been identified as a principal reservoir of Escherichia coli O157:H7. The fate of this pathogen in bovine feces at 5, 22, and 37 degrees C was determined. Two levels of inocula (10(3) and 10(5) CFU/g) of a mixture of five nalidixic acid-resistant E. coli O157:H7 strains were used. E. coli O157:H7 survived at 37 degrees C for 42 and 49 days with low and high inocula, respectively, and at 22 degrees C for 49 and 56 days with low and high inocula, respectively. Fecal samples at both temperatures had low moisture contents (about 10%) and water activities ( < 0.5) near the end of the study. E. coli O157:H7 at 5 degrees C survived for 63 to 70 days, with the moisture content (74%) of feces remaining high through the study. Chromosomal DNA fingerprinting of E. coli O157:H7 isolates surviving near the completion of the study revealed that the human isolate strain 932 was the only surviving strain at 22 or 37 degrees C. All five strains were isolated near the end of incubation from feces held at 5 degrees C. Isolates at each temperature were still capable of producing both verotoxin 1 and verotoxin 2. Results indicate that E. coli O157:H7 can survive in feces for a long period of time and retain its ability to produce verotoxins. Hence, bovine feces are a potential vehicle for transmitting E. coli O157:H7 to cattle, food, and the environment. Appropriate handling of bovine feces is important to control the spread of this pathogen.

A new route of transmission for Escherichia coli: infection from dry fermented salami

OBJECTIVES

This study evaluated the production of dry fermented salami associated with an outbreak of Escherichia coli O157.H7 infection in Washington State and California.

METHODS

Facility inspections, review of plant monitoring data, food handler interviews, and microbiological testing of salami products were conducted.

RESULTS

Production methods complied with federal requirements and industry-developed good manufacturing practices. No evidence suggested that postprocessing contamination occurred. Calculations suggested that the infectious dose was smaller than 50 E. coli O157:H7 bacteria.

CONCLUSIONS

Dry fermented salami can serve as a vehicle of transmission for O157:H7 strains. Our investigation and prior laboratory studies suggest that E. coli O157:H7 can survive currently accepted processing methods.

Super-shedding and the link between human infection and livestock carriage of Escherichia coli O157

Cattle that excrete more Escherichia coli O157 than others are known as super-shedders. Super-shedding has important consequences for the epidemiology of E. coli O157 in cattle--its main reservoir--and for the risk of human infection, particularly owing to environmental exposure. Ultimately, control measures targeted at super-shedders may prove to be highly effective. We currently have only a limited understanding of both the nature and the determinants of super-shedding. However, super-shedding has been observed to be associated with colonization at the terminal rectum and might also occur more often with certain pathogen phage types. More generally, epidemiological evidence suggests that super-shedding might be important in other bacterial and viral infections.

Escherichia coli O157

Escherichia coli O157 is an uncommon but serious cause of gastroenteritis. This bacterium is noteworthy because a few, but significant, number of infected people develop the haemolytic uraemic syndrome, which is the most frequent cause of acute renal failure in children in the Americas and Europe. Many infections of E coli O157 could be prevented by the more effective application of evidence-based methods, which is especially important because once an infection has been established, no therapeutic interventions are available to lessen the risk of the development of the haemolytic uraemic syndrome. This Review takes into account the evolution and geographical distibution of E coli O157 (and its close pathogenic relatives); the many and varied routes of transmission from its major natural hosts, ruminant farm animals; and other aspects of its epidemiology, its virulence factors, the diagnosis and management of infection and their complications, the repercussions of infection including costs, and prevention.

Cattle as a possible source of verocytotoxin-producing Escherichia coli O157 infections in man

In May-June 1992 cases of infection with verocytotoxin-producing (VT+) Escherichia coli O157 in South Yorkshire could have been associated with prior consumption of beef from a local abattoir. During investigation of the abattoir, bovine rectal swabs and samples of meat and surface swabs from beef carcasses were examined for E. coli O157, isolates of which were tested for toxigenicity, plasmid content and phage type. E. coli O157 was isolated from 84 (4%) of 2103 bovine rectal swabs; of these 84, 78 (93%) were VT+, the most common phage types being 2 and 8, the types implicated in the cluster of human cases. Positive cattle were from diverse sources within England. E. coli O157 was isolated from 7 (30%) of 23 carcasses of rectal swab-positive cattle and from 2 (8%) of 25 carcasses of rectal swab-negative cattle. The study has shown that cattle may be a reservoir of VT+ E. coli O157, and that contamination of carcasses during slaughter and processing may be how beef and beef products become contaminated and thereby transmit the organism to man.

Extraintestinal pathogenic Escherichia coli: "the other bad E coli"

Extraintestinal pathogenic Escherichia coli (ExPEC), the specialized strains of E coli that cause most extraintestinal E coli infections, represent a major but little-appreciated health threat. Although the reasons for their evolution remain mysterious, by virtue of their numerous virulence traits ExPEC clearly possess a unique ability to cause disease outside the host intestinal tract. Broader appreciation of the existence and importance of ExPEC and better understandings of their distinctive virulence mechanisms, reservoirs, and transmission pathways may lead to effective preventive interventions against the morbid and costly infections ExPEC cause.

Experimentally induced intrauterine infection causes fetal brain white matter lesions in rabbits

OBJECTIVE

Periventricular leukomalacia, a common brain white matter lesion in preterm neonates, is a major risk factor for cerebral palsy. Epidemiologic studies have demonstrated an association between infection and periventricular leukomalacia. The purpose of this study was to determine whether ascending intrauterine infection could cause brain white matter lesions in the fetal rabbit.

STUDY DESIGN

Rabbits with timed pregnancies underwent hysteroscopy at 20 to 21 days of gestation (70%). Animals were allocated in a ratio of 2:1 for inoculation with either Escherichia coli (0.2 ml containing 10(3) to 10(4) colony-forming units) or sterile saline solution. Both groups were treated with ampicillin-sulbactam (Unasyn, 100 mg/kg per day; Pfizer, Seoul) every 8 hours until they were killed 5 to 6 days after hysteroscopy. Histologic examination of the placentas and fetal brains was conducted.

RESULTS

Forty-five animals underwent hysteroscopy; 31 were inoculated with E. coli and 14 with sterile saline solution. At the time the animals were killed, the rate of intrauterine infection was higher and there were fewer live fetuses in the E. coli-inoculated animals than in the saline solution group. Histologic evidence of brain white matter damage was identified in 12 fetuses born to 10 E. coli-inoculated rabbits but none in the saline solution group (p < 0.05). All rabbits with brain white matter lesions had evidence of intrauterine infection. Evidence of white matter damage included increased karyorrhexis, rarefaction, and disorganization of white matter. Apoptosis was demonstrated in areas of white matter damage by immunohistochemical studies.

CONCLUSION

Experimental ascending intrauterine infection can cause fetal brain white matter lesions.

Enteropathogenic Escherichia coli of classic serotypes associated with infant diarrhea: epidemiology and pathogenesis

A series of O:H serotypes of E. coli were incriminated by epidemiologic studies in the period 1945-1960 as a cause of epidemic diarrhea in infant nurseries as well as a major cause of sporadic infant diarrhea in the community. The term enteropathogenic E. coli was coined to refer to these infant diarrhea-associated serotypes. In the early 1970s, with the advent of laboratory tests to assess heat-labile and heat-stable enterotoxin production and enteroinvasiveness of E. coli, the classic serotype enteropathogenic E. coli strains were found to lack those particular properties. These observations led some to question their pathogenicity. However, since 1978, enteropathogenic E. coli have come to be appreciated anew as a separate class of diarrheagenic E. coli that cause diarrhea by distinct pathogenic mechanisms. The pathogenesis of these strains, which have been shown to cause diarrhea in volunteers, appears to involve both an enteroadhesiveness step and production of a toxin identical to Shigella toxin. A 55- to 65-Mdalton plasmid is involved in the attachment of enteropathogenic E. coli to intestinal mucosa which results in a pathognomonic histopathologic lesion visualized by electron microscopy. The lesion involves dissolution of enterocyte microvilli by the bacteria, effacement of the enterocyte outer membrane, and formation of a pedestal around the bacterium at point of contact with the outer membrane of the enterocyte. Case-control epidemiologic studies carried out since 1975 document that enteropathogenic E. coli remain an important cause of sporadic infant diarrhea in the community with up to 30 per cent of cases of acute diarrhea in young infants in Brazil and South Africa being attributed to these pathogens. Although nursery epidemics of enteropathogenic E. coli diarrhea have virtually disappeared from industrialized countries, some sporadic enteropathogenic E. coli diarrhea in infants in the community continues to occur. The relative importance of enteropathogenic E. coli as a cause of sporadic diarrhea in both industrialized and developing countries needs to be reassessed. New diagnostic techniques are awaited to simplify this task.

Heterogeneous shedding of Escherichia coli O157 in cattle and its implications for control

Identification of the relative importance of within- and between-host variability in infectiousness and the impact of these heterogeneities on the transmission dynamics of infectious agents can enable efficient targeting of control measures. Cattle, a major reservoir host for the zoonotic pathogen Escherichia coli O157, are known to exhibit a high degree of heterogeneity in bacterial shedding densities. By relating bacterial count to infectiousness and fitting dynamic epidemiological models to prevalence data from a cross-sectional survey of cattle farms in Scotland, we identify a robust pattern: approximately 80% of the transmission arises from the 20% most infectious individuals. We examine potential control options under a range of assumptions about within- and between-host variability in infection dynamics. Our results show that the within-herd basic reproduction ratio, R(0), could be reduced to <1 with targeted measures aimed at preventing infection in the 5% of individuals with the highest overall infectiousness. Alternatively, interventions such as vaccination or the use of probiotics that aim to reduce bacterial carriage could produce dramatic reductions in R(0) by preventing carriage at concentrations corresponding to the top few percent of the observed range of counts. We conclude that a greater understanding of the cause of the heterogeneity in bacterial carriage could lead to highly efficient control measures to reduce the prevalence of E. coli O157.

A fifteen month study of Escherichia coli O157:H7 in a dairy herd

A dairy herd associated with Escherichia coli O157 infection in humans was studied for the 15 months following the outbreak to examine seasonal, age and management factors affecting faecal excretion of the organism and to determine the mode and frequency of milk contamination with the organism. Between May 1993 and July 1994, 28 visits were made to the farm to collect a total of 3593 rectal swabs from cows, heifers and calves and 329 milk samples. E. coli O157:H7 was isolated from 153 (4.3%) of 3593 bovine rectal swabs. The maximum prevalence at any one visit was 14% in lactating cows, 40% in non-lactating cows, 56% in calves and 68% in heifers. The prevalence in lactating cows, which was significantly lower than in the other groups, peaked during May-July 1993 and again briefly after the cattle were housed during November 1993 and then again during May 1994. Excretion rates of E. coli O157:H7 in lactating cows were highest during the first month after calving, falling during lactation and rising to another peak at 7 months postpartum. Between November 1993 and May 1994 there was no evidence of excretion in any group. Eighty-seven (74%) of the animals which excreted E. coli O157:H7 did so on only one occasion but 23 (32%) of 73 cows and heifers and 7 (16%) of 44 calves which excreted the organism did so on more than one occasion. E. coli O157:H7 was not isolated from milk taken from the bulk tank but it was isolated from individual milk samples (one milk jar and one fore-milk) from two animals previously shown to be faecal excretors of the organism. All isolates of E. coli O157:H7 obtained were of the same phage type, toxin genotype and plasmid profile.

Effect of mechanical ventilation strategy on dissemination of intratracheally instilled Escherichia coli in dogs

OBJECTIVE

To test the effect of different mechanical ventilation strategies on dissemination of intratracheally instilled Escherichia coli in dogs and to determine the extent and distribution of lung damage.

DESIGN

Prospective, randomized study.

SETTING

Experimental animal laboratory.

SUBJECTS

Eighteen anesthetized and paralyzed dogs.

INTERVENTIONS

We studied the effect of three ventilatory strategies based on two variables: transpulmonary pressure and positive end-expiratory pressure (PEEP). Group 1 animals (n = 6) were ventilated with a PEEP of 3 cm H2O and a tidal volume of 15 mL/kg, which generated an end-inspiratory transpulmonary pressure of < or = 15 cm H2O. In group 2(n = 6), tidal volume was adjusted to generate a transpulmonary pressure of 35 cm H2O and PEEP was set to 3 cm H2O. In group 3(n = 6), tidal volume was also adjusted to yield a transpulmonary pressure of 35 cm H2O but PEEP was set to 10 cm H2O. In each group, we instilled approximately 10(8) colony-forming units of E. coli into the trachea of the dogs and ventilated them with the chosen tidal volume and PEEP for 6 hrs afterward.

MEASUREMENTS AND MAIN RESULTS

We measured the pressure-volume relationship (pressure-volume curve) of the respiratory system before and 6 hrs after bacterial instillation. We obtained blood cultures before and 0.5, 1,2,3,4,5, and 6 hrs after bacterial instillation. After 6 hrs, the lungs were removed for histologic (histologic score) and gravimetric (wet-to-dry weight ratio, WW/DW) analysis. During the experiment 0, 5, and 1 dogs developed positive blood cultures in groups 1, 2, and 3, respectively. The number of dogs that developed bacteremia in group 2 was significantly greater than in the other two groups (p < .05). In group 1, pressure-volume curves demonstrated a lower inflection point which was greater than the end-inspiratory transpulmonary pressure suggesting that low transpulmonary pressure/low PEEP strategy ventilated aerated regions without expanding atelectatic areas. In group 2, pressure-volume curves demonstrated both a lower inflection point and an upper deflection point which were spanned by the tidal volume, suggesting that high transpulmonary pressure/low PEEP strategy might have caused both overdistention and cyclic closure and reopening. In group 3, pressure-volume curves demonstrated only a upper deflection point which was less than the maximal alveolar tidal pressure. At the end of the experimental protocol, group 2 manifested the most lung injury as assessed by gravimetric and histologic indices of lung injury. WW/DW of group 2(13.1 +/- 1.0 (SD); p < .05) was greater than groups 1 and 3(7.5 +/- 1.2 and 8.6 +/- 1.0, respectively). Similarly, the overall weighted histologic injury score for group 2 (1.19 +/- 0.26; p < .02) was greater than for groups 1 and 3 (0.82 +/- 0.20 and 0.88 +/- 0.22, respectively). For groups 2 and 3, the overall weighted histologic injury scores of the dependent regions were greater than the nondependent regions (p < .004).

CONCLUSIONS

We conclude that the ventilatory strategy most likely to overdistend the lungs while allowing repetitive opening and closure of alveoli (group 2) facilitated bacterial translocation from the alveoli to the bloodstream and increased lung injury, as determined by histologic and gravimetric analysis. PEEP ameliorated these effects, despite lung overdistention, but increased histologic and gravimetric indices of lung injury in dependent as compared with the nondependent regions.

Escherichia coli O157:H7 diarrhea in a nursing home: clinical, epidemiological, and pathological findings

In September 1984, an outbreak of Escherichia coli O157:H7 infection occurred in a nursing home. It was the first recognized outbreak of this organism in the United States since 1982, when two outbreaks led to its recognition as a pathogen. Thirty-four of 101 residents developed a diarrheal illness; 14 were hospitalized with a severe illness characterized by crampy abdominal pain, marked abdominal distention, and grossly bloody diarrhea, and four died. The spectrum of illness associated with the infection was broad and included the following: asymptomatic infection, nonbloody diarrhea, hemorrhagic colitis, hemolytic uremic syndrome, and death. Clinical, radiographic, and postmortem pathological findings suggested involvement of the cecum and right colon. No evidence of response to antimicrobial agents could be documented, and antidiarrheal agents may have aggravated the disease. This investigation implicated hamburger as the vehicle transmission. Seventeen of 19 residents with hemorrhagic colitis, but only 28 of 67 healthy residents, had eaten hamburger on 13 September (P less than .001, Fisher's two-tailed exact test; relative risk [RR] = 7.7). Infection with E. coli O157:H7 can cause a wide range of manifestations. In the elderly these can be particularly severe and may resemble ischemic colitis.

Extraintestinal pathogenic Escherichia coli

Extraintestinal pathogenic Escherichia coli (ExPEC) possesses virulence traits that allow it to invade, colonize, and induce disease in bodily sites outside of the gastrointestinal tract. Human diseases caused by ExPEC include urinary tract infections, neonatal meningitis, sepsis, pneumonia, surgical site infections, as well as infections in other extraintestinal locations. ExPEC-induced diseases represent a large burden in terms of medical costs and productivity losses. In addition to human illnesses, ExPEC strains also cause extraintestinal infections in domestic animals and pets. A commonality of virulence factors has been demonstrated between human and animal ExPEC, suggesting that the organisms are zoonotic pathogens. ExPEC strains have been isolated from food products, in particular from raw meats and poultry, indicating that these organisms potentially represent a new class of foodborne pathogens. This review discusses various aspects of ExPEC, including its presence in food products, in animals used for food or as companion pets; the diseases ExPEC can cause; and the virulence factors and virulence mechanisms that cause disease.

Antimicrobial‐Resistant and Extraintestinal PathogenicEscherichia coliin Retail Foods

BACKGROUND

Extraintestinal Escherichia coli infections are associated with specialized extraintestinal pathogenic E. coli (ExPEC) strains and, increasingly, with antimicrobial resistance. The food supply may disseminate ExPEC and antimicrobial-resistant E. coli.

METHODS

In a prospective survey of 1648 diverse food items from 10 retail markets in the Minneapolis-St. Paul area during 2001-2003, selective cultures and disk-diffusion assays for the isolation and characterization of antimicrobial-resistant E. coli and polymerase chain reaction-based assays and O serotyping to define ExPEC-associated traits were performed.

RESULTS

E. coli contamination exhibited a prevalence gradient from miscellaneous foods (9%), through beef or pork (69%), to poultry (92%; P<.001). Among E. coli-positive samples, similar prevalence gradients were detected for antimicrobial resistance (27%, 85%, and 94% of samples, respectively; P<.001) and ExPEC contamination (4%, 19%, and 46%, respectively; P<.001). By multivariate analysis, beef or pork and poultry from natural-food stores exhibited reduced risks of E. coli contamination and antimicrobial resistance. Indirect evidence suggested on-farm selection of resistance. Four food-source ExPEC isolates (from pea pods, turkey parts, ground pork, and vegetable dip) closely resembled selected human clinical isolates by O antigen and genomic profile.

CONCLUSIONS

Retail foods may be an important vehicle for community-wide dissemination of antimicrobial-resistant E. coli and ExPEC, which may represent a newly recognized group of medically significant foodborne pathogens.

Super-shedding cattle and the transmission dynamics of Escherichia coli O157

The prevalence of Escherichia coli O157 displays striking variability across the Scottish cattle population. On 78% of farms, in a cross-sectional survey of 952, no shedding of E. coli O157 was detected, but on a small proportion, approximately 2%, very high prevalences of infection were found (with 90-100% of pats sampled being positive). We ask whether this variation arises from the inherent stochasticity in transmission dynamics or whether it is a signature of underlying heterogeneities in the cattle population. A novel approach is taken whereby the cross-sectional data are viewed as providing independent snapshots of a dynamic process. Using maximum-likelihood methods to fit time-dependent epidemiological models to the data we obtain estimates for the rates of immigration and transmission of E. coli O157 infection - parameters which have not been previously quantified in the literature. A comparison of alternative model fits reveals that the variation in the prevalence data is best explained when a proportion of the cattle are assumed to transmit infection at much higher levels than the rest - the so-called super-shedders. Analysis of a second dataset, comprising samples taken from 32 farms at monthly intervals over a period of 1 year, additionally yields an estimate for the rate of recovery from infection. The pattern of prevalence displayed in the second dataset also strongly supports the super-shedder hypothesis.

Antimicrobial drug-resistant Escherichia coli from humans and poultry products, Minnesota and Wisconsin, 2002-2004

Similarities were found between drug-resistant E. coli from humans and poultry products.

The food supply, including poultry products, may transmit antimicrobial drug–resistant Escherichia coli to humans. To assess this hypothesis, 931 geographically and temporally matched E. coli isolates from human volunteers (hospital inpatients and healthy vegetarians) and commercial poultry products (conventionally raised or raised without antimicrobial drugs) were tested by PCR for phylogenetic group (A, B1, B2, D) and 60 virulence genes associated with extraintestinal pathogenic E. coli. Isolates resistant to trimethoprim-sulfamethoxazole, quinolones, and extended-spectrum cephalosporins (n = 331) were compared with drug-susceptible isolates (n = 600) stratified by source. Phylogenetic and virulence markers of drug-susceptible human isolates differed considerably from those of human and poultry isolates. In contrast, drug-resistant human isolates were similar to poultry isolates, and drug-susceptible and drug-resistant poultry isolates were largely indistinguishable. Many drug-resistant human fecal E. coli isolates may originate from poultry, whereas drug-resistant poultry-source E. coli isolates likely originate from susceptible poultry-source precursors.

Management practices associated with the incidence rate of clinical mastitis

Risk factors for the incidence rate of clinical mastitis were studied in 274 Dutch dairy herds. Variables that were associated with resistance to disease were the feeding, housing, and milking machine factors. Variables that were associated with exposure were grazing, combined housing of dry cows and heifers, and calving area hygiene. Postmilking teat disinfection in herds with a low bulk milk somatic cell count and years of practicing dry cow therapy were positively associated with the incidence rate of clinical mastitis. Herds with a low bulk milk somatic cell count and in which postmilking teat disinfection was not used had lower incidence rates of clinical mastitis than did other herds. The incidence rate of clinical mastitis caused by Escherichia coli was mostly related to housing conditions, hygiene, and machine milking. The incidence rate of clinical mastitis caused by Staphylococcus aureus was mostly related to factors associated with bulk milk somatic cell count and factors that might be due to cause and effect reversal. A strong positive correlation existed between the incidence rate of clinical mastitis caused by Streptococcus dysgalactiae and the incidence rate of clinical mastitis caused by Staph. aureus. The incidence rate of clinical mastitis caused by Streptococcus dysgalactiae was related to nutrition, milking technique, and machine milking. The incidence rate of clinical mastitis caused by Streptococcus uberis was associated with factors related to housing, nutrition, and machine milking.