within the microbiota , individual 
 bacteria containing important genes may benefit the host in different ways . as 
 one considers the vast community of commensal microbes , subsets of these 
 organisms may have important physiologic benefits for the host in the context 
 of human nutrition and host : microbe interactions . probiotics may stimulate 
 immunity , regulate immune signaling pathways , produce antipathogenic factors , 
 or induce the host to produce antipathogenic factors . probiotics may produce 
 secreted factors that stimulate or suppress cytokines and cell - mediated 
 immunity . these factors may also interfere with key immune signaling pathways 
 such as the nf-b and map kinase cascades . probiotics may produce factors that 
 inhibit pathogens and other commensal bacteria , effectively enabling these 
 microbes to compete effectively for nutrients in complex communities . microbes 
 that produce antipathogenic factors may represent sources of novel classes of 
 antimicrobial compounds , and these factors may be regulated by master 
 regulatory genes in particular classes of bacteria . microbes can also regulate 
 signaling pathways in immune cells that result in the production of 
 antimicrobial factors by mammalian cells , effectively resulting in remodeling 
 of intestinal communities and prevention or treatment of infections . studies 
 conducted in 2006 found that , globally , severe diarrhea and dehydration are 
 responsible each year for the death of 1,575,000 children under the age of 
 five . this represents 15% of the 10.5 million deaths per year of children in 
 this age group . according to recent 
 estimates , acute gastroenteritis causes as many as 770,000 hospitalizations per 
 year in the united states . enteric pathogens include 
 viruses ( rotaviruses , noroviruses ) and bacteria such as different strains of 
 pathogenic escherichia coli , 
 toxigenic clostridium difficile , campylobacter jejuni , and vibrio cholerae . these 
 pathogens produce different types of toxins that can cause severe or 
 life - threatening dehydration and diarrhea . despite medical advances in 
 diagnosis and treatment , the percent and number of hospitalized pediatric 
 patients less than 5 years of age with severe rotavirus infection significantly 
 increased when a recent time period ( 20012003 ) was compared to an earlier 
 time period ( 19931995 ) . in addition to the typical 
 pattern of acute gastroenteritis , infectious agents such as enteropathogenic e. coli ( epec ) may cause persistent , 
 chronic diarrhea in children lasting longer than 1 week . such persistent infections may increase the risk of dehydration 
 and long - term morbidities . importantly , the relative contributions of epec and 
 other bacterial pathogens to disease remains controversial to some extent . a recent 
 study highlighted that increased relative risk of gastrointestinal disease in 
 children was only demonstrable for enteric viruses . early childhood diarrhea 
 predisposes children to lasting disabilities , including impaired fitness , 
 stunted growth , and impaired cognition and school performance . along with this data , new research on maternal and child undernutrition 
 reported in the lancet in january 
 2008 links poor nutrition with an increased risk for enteric infections in 
 children . furthermore , irritable bowel syndrome ( ibs ) , a costly and difficult 
 to treat condition that affects 20% of the united states population , has medical costs of up to 
 $ 30 billion per year , excluding prescription and over - the - counter drug costs . ibs is precipitated by an 
 episode of acute gastroenteritis in up to 30% of all cases in prior studies . therefore , preventing or 
 treating acute gastroenteritis before long - term sequelae develop would 
 drastically reduce hospitalizations , disability - adjusted life years , and both 
 direct and indirect medical costs . accurate diagnosis of acute 
 gastroenteritis is an ongoing challenge even in sophisticated academic medical 
 centers . in a pediatric patient population exceeding 4,700 children , less than 
 50% of stool samples that underwent complete microbiologic evaluation yielded a 
 specific diagnosis . enteric viruses represented the predominant etiologic agents in acute gastroenteritis 
 in children less than 3 years of age , and bacteria caused the majority of cases 
 of acute gastroenteritis in children older than 3 years of age . the diagnostic challenges 
 with enteric viruses include the relative paucity of stool - based molecular or 
 viral antigen tests and the inability to readily culture most enteric viruses . bacterial pathogens may be difficult to identify ( such as most strains of 
 disease - causing e. coli ) because of the lack of specific assays 
 for these infections . the relative insensitivity of stool - based toxin assays 
 for the detection of toxigenic c. 
 difficile precludes accurate diagnosis . in a children 's hospital setting , 
 combination toxin antigen testing yielded sensitivity below 40% in pediatric 
 patients ( j. versalovic , unpublished data ) . the introduction of new molecular 
 assays for real - time pcr detection of toxin genes directly in stool has 
 markedly improved the ability to diagnose antimicrobial - associated diarrhea and 
 colitis due to toxigenic c. difficile . in addition , 
 approximately 1525% of cases of antimicrobial - associated diarrhea are caused 
 by c. difficile . the prevalence of 
 antimicrobial - associated diarrhea and gastrointestinal disease highlights the 
 importance of alternatives to antibiotic strategies for treatment . furthermore , 
 antibiotics have limited utility for the treatment of gastroenteritis in 
 general . antimicrobial agents are not generally recommended as prevention 
 strategies because of the problems of antibiotic resistance and 
 antimicrobial - associated disease . thus , instead of suppressing bacterial 
 populations with antibiotics , can probiotics be used to remodel or shift microbial 
 communities to a healthy state ? the use 
 of probiotics to prevent and treat a wide variety of conditions has gained 
 favor in the past decade . this is in part due to a need to find alternatives to 
 traditional therapies such as antibiotics as well as the lack of good 
 treatments for gi ailments . while there are increasing reports of the efficacy 
 of probiotics in the treatment of diseases such as pouchitis [ 13 , 14 ] , 
 diarrhea [ 1517 ] , 
 and irritable bowel syndrome , the scientific basis for the 
 use of probiotics is just beginning to be understood . we will focus on the 
 potential applications for probiotics in the treatment of diarrheal disease . several 
 examples will highlight how probiotics may be selected for and utilized against 
 pathogens causing gastroenteritis . the concept of using probiotic 
 microorganisms to prevent and treat a variety of human ailments has been around 
 for more than 100 years . 
 with the rise in the number of multidrug resistant pathogens and the 
 recognition of the role that the human microbiota plays in health and disease , 
 a recent expansion in the interest in probiotics has been generated . this 
 phenomenon is apparent in both the numbers of probiotic products being marketed 
 to consumers as well as the increased amount of scientific research occurring 
 in probiotics . although many of the mechanisms by which probiotics benefit 
 human beings remain unclear , probiotic bacteria are being utilized more 
 commonly to treat specific diseases . , we use the definition derived in 2001 by the food and agricultural 
 organization ( fao ) and the world health organization ( who)probiotics are 
 live microorganisms which when administered in adequate amount confer a health 
 benefit on the host .    . this definition is the currently accepted definition by the international 
 scientific association for probiotics and prebiotics ( isapp ) 
 ( http://www.isapp.net/ ) . perhaps 
 the most important scientific question regarding the use of probiotics in 
 medicine is the identification of mechanisms by which probiotics impact human 
 health . several mechanisms have been implicated but most have not been 
 experimentally proven ( figure 1 ) . here , we discuss possible mechanisms that are 
 relevant for the treatment of diarrheal diseases . many probiotics have been shown to 
 produce antipathogenic compounds ranging from small molecules to bioactive 
 antimicrobial peptides . most of these studies have focused on the in vitro 
 susceptibility of pathogens to products secreted by probiotic bacteria . in most 
 cases , the ability of an antimicrobial compound secreted by a probiotic 
 organism to inhibit the growth of a pathogen in vivo has not been demonstrated . conceptually , an antimicrobial compound produced by an organism would need to 
 be produced at a high enough level and in the right location in the intestinal 
 tract to exert a strong effect on a pathogen in vivo . an elegant proof of principle for 
 direct action of a probiotic - produced antimicrobial against a pathogen was 
 recently reported by corr   et al . who demonstrated that production of the bacteriocin 
 abp118 by lactobacillus salivarius was 
 sufficient to protect mice from disease by infection with listeria monocytogenes . to prove the action of the bacteriocin was directly responsible for the 
 protection of the mice , they generated a l. 
 salivarius strain that was unable to produce abp118 and showed that this 
 mutant was incapable of protecting against l. 
 monocytogenes infection . notably , they were able to express a gene that 
 confers immunity to the abp118 bacteriocin within l. monocytogenes and showed that this strain was now resistant to 
 the probiotic effect of l. salivarius within 
 the mouse . this study provided clear evidence that a probiotic - derived 
 bacteriocin could function directly on a pathogen in vivo . in addition to 
 producing antimicrobial compounds that act directly on pathogens , probiotics 
 may stimulate host antimicrobial defense pathways . the intestinal tract has a 
 number of mechanisms for resisting the effects of pathogens including the 
 production of defensins . defensins are cationic antimicrobial peptides that are produced in a number of 
 cell types including paneth cells in the crypts of the small intestine and 
 intestinal epithelial cells . a deficiency in alpha - defensin production has been 
 correlated with ileal crohn 's disease [ 23 , 24 ] . tissue 
 samples from patients with crohn 's disease showed a lower level of 
 alpha - defensin production and extracts from these samples exhibited a reduced 
 ability to inhibit bacterial growth in vitro . moreover , some pathogenic 
 bacteria have evolved mechanisms to inhibit the production or mechanism of 
 action of defensins ( reviewed in ) . this has been demonstrated for human beta 
 defensin 2 ( hbd-2 ) , whose expression is upregulated by the presence of several 
 probiotic bacteria via the transcription factor nf-b [ 26 , 27 ] . the 
 implication is that probiotic strains with this capability would strengthen 
 intestinal defenses by increasing defensin levels . this effect is also observed 
 with certain pathogenic bacteria and thus is not a specific property of 
 probiotic bacteria . second , many defensins are produced in a propeptide form 
 that must be activated via the action of proteases . one well - characterized example 
 is the activation of the murine defensin cryptdin ( an alpha - defensin that is 
 produced by paneth cells ) by the action of matrix metalloprotease 7 ( mmp-7 ) . thus , 
 one mechanism in which probiotics could participate in activating defensins is 
 by stimulating the production of mmps in the intestinal tract . alternatively , 
 probiotics could produce proteases that themselves activate defensins in the 
 intestinal lumen . although there is no evidence yet to support this mechanism , a 
 subset of lactobacilli and streptococci encode mmp - like proteins in their 
 genomes ( r. britton , unpublished observation ) . these mmps are not found in any 
 other bacteria and thus it will be interesting to determine what effect they 
 have on host cell function . rather than   directly inhibiting the growth or 
 viability of the pathogen , probiotics may compete for an ecological niche or , 
 otherwise , create conditions that are unfavorable for the pathogen to take hold 
 in the intestinal tract . first , several probiotics have been demonstrated to 
 alter the ability of pathogens to adhere to or invade colonic epithelial cells 
 in vitro , for example , see [ 30 , 31 ] . second , probiotics could sequester essential 
 nutrients from invading pathogens and impair their colonization ability . third , 
 probiotics may alter the gene expression program of pathogens in such a way as 
 to inhibit the expression of virulence functions . lastly , probiotics may create an unfavorable environment for pathogen 
 colonization by altering ph , the mucus layer , and other factors in the local 
 surroundings . it is important to note that although many of these possible 
 effects have been demonstrated in 
 vitro , the ability of probiotics to exclude pathogens in vivo remains to 
 be proven . different 
 strains representing different lactobacillus species demonstrated contrasting effects with respect to proinflammatory 
 cytokine production by murine bone marrow - derived dendritic cells . specific probiotic strains counteracted the immunostimulatory effects of other 
 strains so that probiotics have the potential to yield additive or antagonistic 
 results . interestingly , in this study , the anti - inflammatory cytokine il-10 was 
 maintained at similar levels . different probiotic lactobacillus strains 
 of the same species may also yield contrasting effects with respect to 
 immunomodulation . human breast milk - derived lactobacillus 
 reuteri strains either stimulated the key proinflammatory cytokine , human 
 tumor necrosis factor ( tnf ) , or suppressed its production by human myeloid 
 cells . the mechanisms of action may be due , not surprisingly , to contrasting effects 
 on key signaling pathways in mammalian cells . probiotic strains such as lactobacillus rhamnosus gg ( lgg ) may 
 activate nf-b and the signal transducer and activator of transcription ( stat ) 
 signaling pathways in human macrophages . in 
 contrast , probiotic lactobacillus strains may suppress nf-b 
 signaling [ 36 , 37 ] or map kinase-/c - jun - mediated signaling . stimulation of key signaling pathways and enhancement of proinflammatory 
 cytokine production may be important to  prime  the immune system for defense 
 against gastrointestinal infections . conversely , suppression of immune 
 signaling may be an important mechanism to promote homeostasis and tolerance to 
 microbial communities with many potential antigens , and these immunosuppressive 
 functions may promote healing or resolution of infections . the disruption of 
 epithelial barrier function and loss of tight junction formation in the 
 intestinal epithelium may contribute to pathophysiology and diarrheal symptoms 
 observed during infection with certain pathogens [ 38 , 39 ] . loss of 
 tight junctions can lead to increased paracellular transport that can result in 
 fluid loss and pathogen invasion of the submucosa . pathogens may secrete 
 factors such as enterotoxins that may promote excessive apoptosis or necrosis 
 of intestinal epithelial cells , thereby disrupting the intestinal barrier . enteric pathogens may also cause effacing lesions at the mucosal surface due to 
 direct adherence with intestinal epithelial cells ( e.g. , epec ) . in contrast , 
 probiotics have been reported to promote tight junction formation and 
 intestinal barrier function [ 40 , 41 ] . although 
 the mechanisms of promoting barrier integrity are not well understood , 
 probiotics may counteract the disruption of the intestinal epithelial barrier 
 despite the presence of pathogens . probiotics may also suppress toxin 
 production or interfere with the abilities of specific pathogens to adhere 
 directly to the intestinal surface . as a result , pathogens may have a 
 diminished ability to disrupt intestinal barrier function . an important 
 challenge in the field of probiotics is the identification of genes and 
 mechanisms responsible for the beneficial functions exerted by these microbes . 
 successful identification of mechanistic details for how probiotics function 
 will have at least three important benefits . first , understanding mechanisms of 
 action will provide a scientific basis for the beneficial effects provided by 
 specific microbes . these breakthrough investigations will help move probiotics 
 from the status of dietary supplements to therapeutics . second , understanding 
 mechanisms of probiosis and the gene products produced by probiotics will allow 
 for the identification of more potent probiotics or the development of 
 bioengineered therapeutics . as an example , the anti - inflammatory cytokine il-10 
 was postulated to be a potential therapeutic for the treatment of inflammatory 
 bowel disease . to test this hypothesis , a strain of lactococcus lactis engineered to produce and secrete il-10 was 
 constructed and demonstrated to reduce colitis in a murine model . early clinical trials in patients with inflammatory bowel disease indicate some 
 relief from symptoms when treated with the il-10 overproducing strain . third , 
 the identification of gene products that are responsible for ameliorating 
 disease will allow researchers , industry , and clinicians to follow the production 
 of these products as important biomarkers during probiotic preparation . as 
 discussed below , the physiological state of microbes can be crucial to the 
 functions of probiotics . thus , it will be important to be able to follow the 
 production of important bioactive molecules when culturing and processing 
 probiotics for applications in animals and humans . unlike 
 small molecules that are stable entities , probiotics are dynamic microorganisms 
 and will change gene expression patterns when exposed to different 
 environmental conditions . this reality has two important implications for those 
 who choose to use these organisms to combat human or animal diseases . first , 
 probiosis is a strain - specific phenomenon . as defining a bacterial species is 
 challenging in this age of full genome sequencing , it is clear that probiotic 
 effects observed in vitro and in vivo are 
 strain specific . for example , modulation of tnf production by strains of lactobacillus reuteri identified strains 
 that were immunostimulatory , immunoneutral , and immunosuppressive for tnf 
 production [ 34 , 43 ] . these findings highlight the strain - specific nature of probiotic effects 
 exerted by bacteria . thus , it is important for research groups and industry to 
 be cautious with strain handling and tracking so that inclusion of correct 
 strains is verified prior to administration in clinical trials . the second key point is that the 
 physiology of the probiotic strain is an important consideration . being live 
 microorganisms , the proteins and secondary metabolites that are being produced 
 will change depending on growth phase . this feature raises a number of 
 important issues for the stability and efficacy of probiotic strains . first , 
 probiotics are subjected to numerous environmental stresses during production 
 and after ingestion by the host . most notably , probiotics used to treat 
 intestinal ailments or whose mode of action is thought to be exerted in the 
 intestinal tract must be able to survive both acid and bile stress during 
 transit through the gut . the physiological state of the microbe is an important 
 characteristic that determines whether cells will be susceptible to different 
 types of environmental stress [ 44 , 45 ] . for 
 example , exponentially growing cells of l. 
 reuteri are much more susceptible to killing by bile salts than cells in 
 stationary phase . thus , it is important to consider the physiological state of the cells in terms 
 of stress adaptation not only for survival in the host but also during 
 production . second , the expression of bioactive molecules , which are most often 
 responsible for the health benefits exerted by probiotics , is often growth phase - dependent . for example , our groups have been investigating the production of 
 immunomodulatory compounds and antimicrobial agents by strains of l. reuteri . in both cases , these 
 compounds are more highly expressed in the entry into and during stationary 
 phase ( unpublished observation ) . commensal - derived probiotic 
 bacteria have been implicated as therapy for a range of digestive diseases , 
 including antibiotic - associated colitis , helicobacter 
 pylori gastritis , and traveler 's diarrhea . l. reuteri is indigenous to the 
 human gastrointestinal tract , is widely present in mammals , and has never been 
 shown to cause disease . in 
 human trials , probiotic treatment with l. 
 reuteri in small children with rotaviral gastroenteritis reduced the 
 duration of disease and facilitated patient recovery [ 15 , 16 ] , while in another study , it prevented 
 diarrhea in infants . despite the promising data from clinical trials , the primary molecular 
 mechanisms underlying the antipathogenic properties of l. reuteri remain unknown . probiotics may be effective for the 
 prevention or treatment of infectious gastroenteritis . in the context of 
 disease prevention , several studies with different probiotic strains have 
 documented that these bacteria may reduce the incidence of acute diarrhea by 1575% 
 depending on the study [ 17 , 4750 ] . although 
 the relative impacts on disease incidence vary depending on the specific 
 probiotic strain and patient population , consistent benefits for disease 
 prevention have been demonstrated in multiple clinical studies . in one disease 
 prevention study , supplementation with bifidobacterium 
 lactis significantly reduced the incidence of acute diarrhea and rotavirus 
 shedding in infants . studies that examined potential benefits of probiotics for 
 preventing antimicrobial - associated diarrhea have yielded mixed results [ 5154 ] . one 
 prevention study reported a reduction in incidence of antimicrobial - associated 
 diarrhea in infants by 48% . several meta - analyses of 
 numerous clinical trials with different probiotics documented reductions in 
 disease course of gastroenteritis that ranged from 17 to 30 hours [ 49 , 50 , 55 ] . examined 
 another way , meta - analyses of probiotics used in clinical trials of 
 gastroenteritis noted significant reductions of incidence of diarrhea lasting 
 longer than 3 days ( prolonged diarrhea ) . the incidence of prolonged diarrhea 
 was diminished by 30% or 60% , respectively , depending on the study [ 50 , 56 ] ( summarized 
 in ) . the probiotic agent , lgg , contributed 
 to a significant reduction in rotavirus diarrhea by 3 days of treatment when 
 administered to children as part of oral rehydration therapy . recent data compilations of a large series of probiotics trials by the cochrane 
 database of systematic reviews ( http://www.cochrane.org/ ) have yielded promising conclusions . as of 2008 , probiotics appear to be effective for preventing acute gastroenteritis in 
 children and may reduce duration of acute disease . additionally , probiotics are 
 promising agents for preventing and treating antimicrobial - associated diarrhea , 
 although intention - to - treat analyses have not demonstrated benefits . in what follows , we 
 highlight some possible mechanisms by which probiotics can be used to 
 ameliorate gastroenteritis . because a number of infectious agents cause 
 diarrhea , colitis , and gastroenteritis , we will only focus on a few examples 
 with the idea that many of the mechanisms discussed can be extended to other 
 bacterial or viral causes of diarrhea . an 
 estimated 500,0003,000,000 cases of clostridium 
 difficile - associated diarrhea ( cdad ) occur annually with related health 
 care costs exceeding $ 1 billion per year [ 5860 ] . cdad occurs primarily in patients that have undergone 
 antibiotic therapy in a health care setting , indicating that alterations in the 
 intestinal microbiota are important for the initiation of cdad . in a small but 
 increasing number of cases moreover , the emergence of metronidazole - resistant 
 strains of c. difficile has 
 diminished the efficacy of metronidazole , and vancomycin- and 
 metronidazole - induced cecitis reinforces the need for new therapies for the 
 treatment and prevention of cdad [ 61 , 62 ] . approximately 
 1040% of patients treated for an initial bout of cdad will show recurrent 
 disease , often with multiple episodes . patients with recurrent cdad had a 
 marked decrease in the diversity of organisms in their fecal microbiota while 
 patients that were free of recurrent disease had a normal microbiota . thus , therapies that restore a normal microbiota or 
 suppress c. difficile growth while 
 allowing the repopulation of the intestine with a favorable microbiota may be 
 important to resolve infections and maintain intestinal health . probiotic organisms have been used to treat recurrent c. difficile in the past and in a few 
 cases have showed a modest effect in ameliorating recurrent disease . this application has been somewhat controversial and at 
 this time the use of probiotics in ameliorating cdad is not recommended . however , the organisms tested were not specifically 
 isolated for the treatment of cdad and , therefore , may have not been the 
 appropriate strains to be used to prevent recurrent cdad . in what follows , we 
 outline potential mechanisms in which carefully selected or engineered 
 probiotics could be used in the treatment of c. difficile and the eradication of this pathogen . cdad is currently treated by the use 
 of antimicrobial agents that are effective against c. difficile , most often vancomycin or metranidazole . because these 
 drugs are broad - spectrum antibiotics , they likely play a role in recurrent 
 disease by suppressing the normal intestinal microbiota . using antimicrobial 
 compounds that target c. difficile while 
 allowing restoration of resident organisms would be one possible mechanism to 
 prevent recurrent cdad . as mentioned above , cdad is usually an 
 infection that is acquired in the hospital or other health care setting . therefore , a 
 probiotic that could competitively exclude c. 
 difficile could be administered prior to entry into the hospital . once this information is 
 known , strategies for blocking colonization with probiotics can be developed . nonetheless , a promising probiotic 
 approach using non - toxigenic c. difficile has been described . using a hamster model of c. difficile infection , gerding et al . demonstrated a protective 
 effect of populating the hamster with strains of c. difficile that are unable to produce toxin prior to challenge 
 with a virulent toxin - producing strain . colonization of the intestinal tract by the nontoxigenic strain appeared to be 
 required for protection . currently , this probiotic approach is under 
 investigation for use in humans ( http://www.viropharma.com/ ) . a likely 
 contributor to the difficulty in eradicating c. difficile from the intestine is the ability of the organism to develop 
 stress - resistant spores . the identification of probiotic strains that can 
 prevent either spore formation or the germination of spores in the intestinal 
 tract provides a promising avenue to combat cdad . recent work on spore 
 germination has provided in vitro assays in which inhibitory activities 
 of probiotics can be tested . germination of spores in the laboratory requires the presence of 
 bile acids , with taurocholate and cholate demonstrating the best activity . thus , bile acids could play a role in signaling to c. difficile that spores are in the correct location of the gut to 
 germinate . sorg and sonenshein have recently proposed a mechanism by which the 
 reduction in the intestinal microbiota could lead to efficient spore 
 germination and overgrowth of c. 
 difficile . they found that the bile acid deoxycholate ( doc ) was able to induce spore 
 germination but that subsequent growth was inhibited due to toxic effects of 
 doc on vegetative c. difficile . their 
 work suggests a model in which a reduction in the concentration of doc in the 
 intestine , due to the disruption of the normal microbiota , removes this key 
 inhibitor of c. difficile growth . doc 
 is a secondary bile acid produced from dehydroxylation of cholate by the enzyme 
 7-dehydroxylase , an activity that is produced by members of the intestinal 
 microbiota . while it is unclear whether or not antibiotic therapy reduces the 
 level of doc in the intestine , it is tempting to speculate that providing 
 probiotic bacteria capable of producing 7-dehydroxylase may prevent intestinal 
 overgrowth by c. difficile while the 
 normal microbiota is being reestablished . enterohemorrhagic e. coli ( ehec ) infections cause sporadic 
 outbreaks of hemorrhagic colitis throughout the world ( 100,000 cases per year 
 in the united states ) . most infections result in the development of bloody diarrhea but a subset 
 ( 510% ) of ehec patients ( mostly children ) will develop the life - threatening 
 condition hemolytic uremic syndrome ( hus ) [ 69 , 70 ] . ehec , which likely 
 evolved from an epec strain , 
 also produces attaching and effacing lesions on host epithelial cells and 
 reduces intestinal epithelial barrier function . in addition , ehec strains are 
 characterized by the expression of shiga toxin ( stx ) genes , and thus they can 
 be labeled as shiga - toxin - producing e. 
 coli ( stec ) . currently , only supportive therapy for ehec infection is 
 available since antibiotic therapy may increase the risk of developing hus , 
 and therefore , novel therapies must be developed . one promising alternative 
 therapeutic may be the use of probiotics to treat ehec infections . shiga toxins are 
 ribosome - inactivating proteins that inhibit protein synthesis by removing a 
 specific adenine residue from the 28s rrna of the large ribosomal subunit . shiga toxin is required for the development of hus and recent work has 
 indicated that ehec strains mutated for shiga toxin production fail to cause 
 disease in a germfree mouse model . 
 indeed , injection of shiga toxin with lps directly into mice is sufficient to 
 generate a hus - like disease in the kidneys of mice . therefore , shiga toxin is an important mediator of hus and therapies aimed at 
 neutralizing its activity are expected to reduce or eliminate this life - threatening 
 complication although current attempts at shiga toxin neutralization have been 
 unsuccessful . as a possible mechanism for treating 
 ehec disease and reducing the incidence of hus cases , paton et al . have 
 generated  designer probiotics  in which the oligosaccharide receptor ( gb3 ) for stx is expressed on the cell surface of 
 an e. coli strain [ 7678 ] . this 
 probiotic strain was shown to be capable of neutralizing stx in vitro . as a 
 proof - of - concept , mice that were challenged with a stec strain were protected 
 by administration of the probiotic expressing the gb3   receptor   . the protective effect was observed even when the strains were formalin - killed 
 prior to use , supporting the hypothesis that toxin sequestration and removal 
 was the mechanism by which the mice were protected . similar results have been 
 obtained using bacteria - expressing receptors for toxins produced by other 
 diarrheal pathogens including enterotoxigenic e. coli ( most common cause of traveler 's diarrhea ) and vibrio cholerae . stx genes are carried on lambdoid prophages and are usually 
 located in a late transcribed region of the virus , near the lytic genes . since no mechanism for toxin secretion has been identified , the location of stx 
 near the lytic genes suggests 
 that phage activation and cell lysis are responsible for stx production and 
 release . this genetic juxtaposition suggests that therapeutics that suppress 
 the lytic decision of lambda in vivo would greatly reduce or eliminate complications 
 caused by systemic release of stx . a key 
 interaction of ehec , as well as epec , with the intestinal epithelium is the 
 formation of attaching and effacing lesions on the surface of the epithelium . this interaction is brought about by factors secreted directly from the bacterium 
 into the host cell , where a redistribution of the actin cytoskeleton occurs . ehec and epec infection also induces a loss of tight junction formation and 
 reduction of the intestinal epithelial barrier by inducing the rearrangement of 
 key tight junction proteins including occludin [ 82 , 83 ] . therapies that would either disrupt this interaction of ehec / epec with the 
 intestinal epithelium or inhibit the loss of barrier function should ameliorate 
 disease . probiotics have shown some success 
 inhibiting adhesion , a / e lesion formation and enhancing barrier function in 
 response to ehec infection in vitro . johnson - henry et al . tested the ability of lactobacillus 
 rhamnosus gg to prevent loss of barrier integrity and formation of a / e 
 lesions induced by ehec infection of cell culture in vitro . they found that pretreatment of intestinal epithelial cells in vitro with lgg 
 was sufficient to reduce the number of a / e lesions and to prevent loss of 
 barrier function as measured by transepithelial resistance , localization of 
 tight junction proteins , and barrier permeability assays . importantly , live lgg 
 was required for these effects as heat - killed bacteria were not effective in 
 preventing ehec effects on epithelial cells . enteric viruses including noroviruses 
 and rotavirus represent major causes of gastroenteritis , especially in young 
 children . rotavirus infection results in acute gastroenteritis with 
 accompanying dehydration and vomiting mainly in children 324 months of age . human rotavirus primarily infects intestinal epithelial cells of the distal small 
 intestine , resulting in enterotoxin - mediated damage to intestinal barrier 
 function . recent studies indicate that probiotics may reduce the duration and 
 ameliorate disease due to rotavirus infection (; probiotics promoted intestinal 
 immunoglobulin production and appeared to reduce the severity of intestinal 
 lesions due to rotavirus infection in a mouse model . these findings and related 
 investigations suggest that probiotics may diminish the severity and duration 
 of gastrointestinal infections by mechanisms independent of direct pathogen 
 antagonism . probiotics may also promote healing and homeostasis by modulating 
 cytokine production and facilitating intestinal barrier function . probiotics may provide an important 
 strategy for the prevention and treatment of gastrointestinal infections . specific bacteria derived from human microbial communities may have key 
 features that establish these microbes as primary candidates for probiotic 
 therapies . these beneficial microbes may have different effects within the host 
 such as prevention of pathogen proliferation and function . probiotics may also 
 stimulate the host 's immune function and mucosal barrier integrity . by working 
 via different mechanisms of probiosis probiotics may also suppress or diminish 
 severity or duration of disease in the context of treatment . as our knowledge 
 of the human microbiome advances , rational selection of probiotics based on 
 known mechanisms of action and mechanisms of disease will facilitate 
 optimization of strategies in therapeutic microbiology . ultimately , we expect that 
 probiotics will help to promote stable , diverse , and beneficial microbial 
 communities that enhance human health and prevent disease .
gastrointestinal infections are a major cause of morbidity and mortality 
                   worldwide , particularly in developing countries . the use of probiotics to prevent and 
                   treat a variety of diarrheal diseases has gained favor in recent years . 
 examples where 
                   probiotics have positively impacted gastroenteritis will be highlighted . 
 however , the overall 
                   efficacy of these treatments and the mechanisms by which probiotics ameliorate gastrointestinal 
                   infections are mostly unknown 
 . we will discuss possible mechanisms by which probiotics could have a 
                   beneficial impact by enhancing the prevention or 
                   treatment of diarrheal diseases .