The transmission of enteric pathogens to human populations by the consumption of contaminated food and water has become a worldwide concern. Surveillance data compiled by the World Health Organization estimate that gastrointestinal infections and their sequelae result in approximately 4 million to 6 million deaths annually. More than 80% of these cases are among children under the age of five with mortality reaching 4 million. The majority of these deaths are in children less than 2 years of age. In the United States, diarrhea is the second most common infectious illness, accounting for one out of every six infectious diseases. In some developing countries, children have more than 12 episodes of diarrhea per year and diarrheal diseases account for 15 to 34 percent of all deaths.
In the United States food/waterborne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5000 deaths each year. More than 90% of the foodborne illnesses of known causes are of microbial origin. Costs associated with medical expenses and losses in productivity associated with microbial agents are estimated to be between $5.6 and $9.4 billion dollars annually. The most commonly recognized food/borne pathogens contributing to gastrointestinal infections have been shown to be bacteria (e.g., Salmonella spp., Escherichia coli, Shigella spp., and Vibrio spp.).
The virulence and pathogenesis of enteric pathogens involves both host and pathogen specific factors. Many pathogen-specific virulence determinants contribute to the pathogenesis of these bacteria. The bacterial virulence of these bacteria is the result of many different attributes, which often contribute to different steps in the complicated series of events we recognize as an infection. Infection occurs primarily by the consumption of contaminated water, food or by direct person to person contact. Once ingested the stages of infection common to enteric pathogens can include attachment, colonization, proliferation, tissue damage, invasion and dissemination. Less frequently, enteric pathogens can produce a bacteremic condition inducing reactive arthritis, kidney failure, Guillian-Barre, Reiter syndrome and other extra-intestinal symptoms.
The first host barrier that enteric pathogens must overcome is the mucosal surface. A single epithelial cell layer separates the host from the lumen of the gastrointestinal tract. This barrier and a plethora of other host antimicrobial mechanisms deter commensal, opportunistic and pathogenic microorganisms from establishing infection. Enteric pathogens have evolved some elaborate pathogenic strategies to attach, invade and translocate across the gut epithelium to cause infection. Adherence to mucosal surfaces is a prerequisite of most enteric pathogens to establish infection. In its simplest form adherence or attachment requires two factors: a receptor and an adhesin. A number of specialized structures (adhesins) have been identified in enteric pathogens that enhance intestinal colonization of the organism. These specialized structures (e.g., pili or fimbriae) act as ligands to bind the bacterial cell to specific complex carbohydrate receptors on the epithelial cell surface of the intestine. Once colonization is established enteric pathogens have a multitude of virulence factors that enhance the ability of the pathogen to invade its host. One of the more pronounced clinical manifestations of intestinal colonization is diarrhea. This clinical syndrome is typically induced by the synthesis and excretion of a variety of enterotoxins, (e.g., heat-labile toxin (LT), heat-stable toxin (ST) cholera toxin (CT) and shiga toxin (Stx)) that cause a net secretion of fluid and electrolytes (diarrhea). Many other specific virulence factors of enteric pathogens have been described that affect a wide range of eukaryotic cell processes in the host, to including invasion of specific cell types, cell to cell interactions and signal transduction by integrins, attaching and effacing with destruction of the epithelial surface, elaboration of exotoxins, and actin polymerization enhancing cell to cell spread, etc.
The diversity of enteric pathogens and virulence factors has complicated the development of new and improved vaccines with long lasting protection. The search for a better vaccine is prompted by the results of epidemiological and challenge studies showing that the recovery from natural infection is often followed by long lasting immunity while providing cross-protection against multiple strains and/or serotypes.
Current vaccines under development for such enteric pathogens as Vibrio cholera, Escherichia coli, Salmonella, and Shigella are based on parenteral and oral vaccines. Moderately effective vaccines have been tested and implemented for controlling cholera. The oral vaccines currently under development include two types: killed Vibrio cholera bacteria that are combined with purified cholera B subunit toxin, and live-attenuated strains of V. cholera with known genetic deletions (Butterton et al., Infect. Immun. 65: 2127-2135). Field trials sponsored by The World Health Organization using an oral vaccine consisting of a whole-cell B subunit reported levels of only 50% protection in human populations in underdeveloped countries. The vaccine required multiple doses over a four month period; unfortunately, young children were not well protected (Sack et al. Infect. Immun. 66:1968-1972 (1998); Sanchez et al, Lancet. 349: 1825-1830 (1997); and Trach et al. Lancet. 349: 231-235 (1997)). A whole-cell vaccine containing four common isolates of V. cholera not containing B subunit toxin has also been tested in human subjects that showed a protective efficacy of 65% (Taylor et al. Infect. Immun. 65: 3852-3856 (1997)). A whole-cell vaccine containing four common isolates of cholera not containing B subunit toxin has also been tested in human subjects (Taylor et al. Infect. Immun. 65: 3852-3856 (1997)). The vaccine required two administrations 7-14 days apart and induced a protective index of approximately 65%. However, the vaccine was not well tolerated due to its reactive nature upon injection. Several live-attenuated vaccine candidates have been tested in large scale efficacy trials involving more than 60,000 human subjects. Unfortunately, the results of this pivotal trial did not demonstrate the effectiveness of the vaccine in preventing cholera. Further development in live attenuated gene deleted vaccines has recently shown promise against the 01 and 0139 serotypes in human volunteers. However, efficacy of the vaccine in large populations and protection against multiple serotypes have yet to be demonstrated.
There are five categories of diarrheagenic Escherichia coli that cause foodborne and waterborne diseases in humans: the enteropathogenic (EPEC), enterohemorrhagic (EHEC), enterotoxigenic (ETEC), enteroinvasive (EIEC) and enteroaggregative (EAEC) strains. The mechanism of disease associated with these pathogens depends on specific characteristics which involve attaching and effacing adherence of the organism to intestinal epithelial cells and damage to the intestinal microvilli. Of particular interest has been the emergence of the Shiga toxin-producing E. coli, also referred to as EHEC, primarily of the O157:H7 serotype. This strain of E. coli has been shown to synthesize either one or both of the Shiga toxins (Stx-1 and/or Stx-2). This strain has been associated with gastrointestinal infections that begin with diarrhea that can exasperate into hemorrhagic colitis, followed by hemolytic-uremic syndrome (HUS) and/or encephalopathy, particularly in the young, immunocompromized, and elderly adults. The Shiga toxin (Stx) produced by this isolate is believed to be important in the pathogenesis of this organism. Current efforts at vaccine development are primarily focused on animals known to asymptomatically carry these organisms and shed them in their feces. Research has focused on a number of strategies for controlling this organism, which revolve around the concept of preventing colonization by targeting the colonization factor intimin, and immunization of animals with genetically modified non-toxin producing versions of the parent isolate. The intimin protein has been shown to be responsible for the attaching and effacing lesions also characteristic of both Shigella dysenteriae (STEC) and the enteropathogenic (EPEC) strains of E. coli. In addition, researchers have been investigating the expression of intimin in animal feed products such as canola and alfalfa for use as an edible animal vaccine. If any of these strategies work in animals it could find its way to human usage (Acheson et al. Infect. Immun. 64: 355-357 (1996); Bokete et al. J. Infect. Dis. 175: 1382-1389 (1997); Bosworth et al. Infect. Immun. 64:55-60 (1996) and Konadu et al. Infect. Immun. 62: 5048-5054 (1964)).
The National Institute of Child Health and Human development have proposed the use of conjugate vaccines using the B-subunit of Stx-1 in conjunction with a whole cell as developed for V. cholerae, which has shown promising results in experimental animal models as well as toxoids and immunotherapeutics using antitoxin antibodies as well as human monoclonal antibodies to neutralize the Stx-1 and Stx-2 toxin. Such prophylactic and immunotherapeutic strategies could protect against STEC infection as well as infections caused by closely related organisms such as EPEC and EHEC strains of E. coli. 
Enterotoxigenic (ETEC) strains of E. coli are an important cause of diarrhea in infants in less developed countries. It is estimated that ETEC causes more than 650 million cases of diarrhea per year and more than 800,000 deaths in children less than 5 years of age. ETEC is also the major cause of traveler's diarrhea, which affects at least 8 million United States citizens who travel to endemic regions of the world each year. Virulence factors associated with these strains of E. coli include primarily adhesins and enterotoxins such as LT1, STa and STb. In volunteer studies infection with ETEC generates protective immunity against rechallenge with the same strain. The vaccine candidate currently being developed consists of a mixture of five formalin-inactivated ETEC strains, which together express the required adhesins, combined with a recombinant Cholera toxin B subunit, which generates antibody that cross-reacts with the ETEC-LT toxin. Clinical studies have shown that the vaccine is immunogenic and safe in human volunteers.
Shigella spp. such as S. sonnei, S. flexneri, S. boydii and S. dysenteriae are causative agents of shigellosis or bacillary dysentery. In the United States approximately 13,000 cases of shigellosis were reported in 2002, a 22% increase from 2001 (CDC, Shigella Annual Summary 2002). Nearly 30% of the reported cases occurred in children under the age of five. The mechanism of disease associated with these pathogens is characterized by specific attaching and effacing lesions involving microvilli destruction, and the production of potent exotoxins (Shiga toxin) that frequently results in hemolytic uremic syndrome. A virulence plasmid present in all invasive Shigella strains has been identified that encode a number of outer membrane proteins that mediate attachment to the epithelial cell. Several of the plasmid-encoded proteins initiate parasite-induced phagocytosis which in turn breaks down the membrane of the phagocytic vacuole, allowing bacteria to multiply within the cytoplasm.
Vaccine strategies created to control shigellosis have focused on attenuated strains with known genetic deletions. A deletion mutant of S. flexneri has shown excellent protection after a single oral dose. This vaccine candidate provides protection against severe shigellosis in volunteers challenged with S. flexneri. Other vaccine strategies include the development of auxotrophic mutants and recent studies have shown protection using 0-specific polysaccharides conjugates from S. sonnei and S. flexneri. As with many of these diseases a comprehensive vaccine approach to controlling shigellosis must include various bacterial components to protect against the multiple serotypes of Shigella that are responsible for endemic outbreaks of dysentery (Ashkenazi et al., J. Infect. Immun. 179: 1565-1568 (1999); Cohen et al., Lancet. 349: 155-159 (1997); Coster et al., Infect. Immun. 67: 3437-3437 (1999); Kotloff et al., infect Immun. 64: 4542-4548 (1996) and Sansonetti et al., Res. Immunol. 147:595-602 (1996)).
Salmonella infections are the leading cause of bacterial foodborne diseases worldwide and are one of the most common enteric diseases in the United States. There are approximately 2,213 different Salmonella strains currently identified which can be classified according to their adaptation to human and animal hosts. For instance, S. typhi and S. paratyphi causes enteric or typhoid fever only in humans and globally infect 20-30 million people annually and cause 600,000 deaths. In the United States, more than 41,000 cases were reported in 1993 with the highest incidence being in children 5 to 19 years of age. Non-typhoidal Salmonella enterica is one of the most common causes of food poisoning in the United States, responsible for an estimated 1.4 million cases of salmonellosis annually (Mead et al. Emerg. Infect. Dis. 5:607-625 (1999)). The cost of human salmonellosis in the U.S. is estimated to be several billion dollars annually based on healthcare costs and lost productivity.
There has been a number of virulence factors associated with disease caused by Salmonella. Briefly, the pathogenesis of the organism begins with the colonization of the host followed by localized degeneration of the epithelial surface resulting in penetration of the epithelial barrier and proliferation in the lamina propria, multiplication, and stimulation of an inflammatory response. Diarrhea associated with salmonellosis is associated primarily with the inflammatory response, which stimulates the release of prostaglandins and production of cAMP, which increase the secretion of fluid and electrolytes into the lumen of the bowel (diarrhea).
A number of parenteral whole-cell vaccines for typhoid fever have been developed but have been found to be only marginally effective because of severe adverse reactions in vaccinates. Currently the National Institute of Child Health and Human Development has developed and tested a vaccine consisting of the Vi antigen. Clinical trials have demonstrated an efficacy of 72-80% with a single injection. A number of gene deleted mutants have been developed for controlling S. typhi with varying degrees of success (Germanier et al. J. Infect. Dis 131:553-558 (1975); Hohmann et al. J. Infect. Dis. 173:1408-1414 (1996); Nardelli-Haefliger et al. Infect. Immun. 64:5219-5224 (1996); Stocker et al. Vaccine. 6:141-145 (1988); Szu et al. Infect. Immun. 62: 4440-4444 (1964); Tacket et al. Infect Immun. 60: 536-541 (1992); and Tacket et al. Vaccine. 10: 443-446 (1992)).
The remaining Salmonella strains commonly referred to as nontyphoidal are primarily transmitted from animals to humans (Calnek et al., Diseases of Poultry-9th ed., pp. 99-130, Iowa State University, Ames Iowa (1991)). In the United States, the most common serotypes of S. enterica isolated from humans are serotypes Typhimurium, Enteritidis, and Newport (CDC Salmonella Annual Summary, 2002). These three serotypes accounted for 51% of human Salmonella isolates in 2002. Notably, the serotypes Typhimurium and Newport are frequently resistant to multiple antibiotics. In a 2001 annual survey, 53% of Typhimurium isolates were resistant to at least one antibiotic and 30% were resistant to five antibiotics in a manner characteristic of the DT104 phage type (CDC National Antimicrobial Resistance Monitoring System: Enteric Bacteria, available at www.cdc.gov/narms/). In addition, 26% of Newport isolates were resistant to at least nine antibiotics in the 2001 annual survey. The Typhimurium and Newport serotypes are primarily associated with the consumption of a variety of different types of animal products that become contaminated during processing or handling. In contrast, Salmonella serotype Enteritidis is almost exclusively associated with the consumption of contaminated chicken eggs. This serotype has a propensity to colonize poultry ovarian tissues for extended periods of time (Okamura et al., Avian Dis., 45: 61-69 (2001) and Okamura et al., Avian Dis., 45: 962-971 (2001)), and can gain entry to the egg environment by vertical transmission during egg formation (Gast et al., Avian Dis. 44: 706-710 (2000) and Humphrey et al., Int. J. Food Microbiol. 21: 31-40 (1994)). A recent risk assessment estimated that 2.3 million eggs are contaminated in the United States annually, resulting in approximately 660,000 human infections (Hope et al., Risk Anal., 22:203-218 (2002)). Additional serotypes that have been associated with human salmonellosis derived from poultry and other animals include S. enterica Heidelberg, Hadar, Infantis, Agona, Montevideo, Thompson, and Braenderup.
Research for controlling nontyphoidal Salmonella has been primarily limited to the bacterins, which consist of killed Salmonella cells, and the live attenuated strains of Salmonella. Bacterins typically stimulate antibody responses in vaccinated animals but may be limited in their ability to promote cell-mediated immunity (Babu et al., Vet. Immunol. Immunopathol. 91:39-44 (2003) and Okamura et al., Comp. Immunol. Microbiol. Infect. Dis. 27:255-272 (2004)), an important host response for effective clearance of Salmonella (Lalmanach and Lantier. Microbes Infect. 1:719-726 (1999) and Naiki et al., J. Immunol. 163:2057-2063 (1999)). In addition, bacterins have generally produced inconsistent protection against fecal shedding of Salmonella (House et al., Am. J. Vet. Res. 12: 1897-1902 (2001) and Davison et al., Avian Dis. 43:664-669 (1999)). Other disadvantages of bacterins include injection-site granulomas, weight loss, and serotype-specific protection. The live attenuated Salmonella vaccines are generally considered to provide better cross-protection than observed with the bacterins (Hassan and Curtiss, III. Infect. Immun. 62:5519-5527. (1994)), and additionally stimulate both humoral and cell-mediated immune responses (Curtiss, III et al., Vet Microbiol. 37:397-405 (1993) and Villarreal-Ramos et al., Vaccine 16: 45-54 (1998)). However, there are significant obstacles regarding the safety of introducing these organisms into commercial animals; specifically, there is concern that genetic reversion will occur and render the vaccine strain virulent. A second potential problem with using modified live vaccines is that antibodies generated to the somatic antigen of the vaccination strains can interfere with national and state Salmonella monitoring programs by generating false positive reactions. In addition, antibiotics are often administered in commercial flocks to control infection rates which can eliminate the attenuated vaccine strain; hence, repeated immunizations of live Salmonella vaccines are often required. There have been relatively few attempts to formulate subcellular vaccines for controlling Salmonella in agricultural animals. A few key studies in poultry species utilized crude cell extracts in their vaccinations, showing S. Enteritidis-specific mucosal and/or circulating antibody responses (Fukutome et al., Dev. Comp. Immunol. 25:475-484 (2001) and Ochoa-Reparaz et al., Vet. Res. 35:291-298 (2004)). In other studies, purified outer membrane protein compositions were demonstrated to promote heightened antibody responses and reduced intestinal colonization or fecal shedding following challenge with S. Enteritidis (Charles et al., Am. J. Vet. Res. 55:636-642 (1994), Khan et al., J. Appl. Microbiol. 95:142-145 (2003), and Meenakshi et al., Vet. Res. Commun. 23:81-90 (1999)).
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