Enterotoxigenic Escherichia coli (ETEC) is a significant source of morbidity and mortality worldwide. One major virulence factor released by ETEC is the heat-labile enterotoxin LT, which is structurally and functionally similar to cholera toxin. LT consists of five B subunits carrying a single catalytically active A subunit. LTB binds the monosialoganglioside GM1, the toxin’s host receptor, but interactions with A-type blood sugars and E. coli lipopolysaccharide have also been identified within the past decade. Here, we review the regulation, assembly, and binding properties of the LT B-subunit pentamer and discuss the possible roles of its numerous molecular interactions.

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of traveler's diarrhea worldwide. One major virulence factor released by this pathogen is the heat-labile enterotoxin LT, which upsets the balance of electrolytes in the intestine. After export, LT binds to lipopolysaccharide (LPS) on the bacterial surface. Although the residues responsible for LT's binding to its host receptor are known, the portion of the toxin which mediates LPS binding has not been defined previously. Here, we describe mutations in LT that impair the binding of the toxin to the external surface of E. coli without altering holotoxin assembly. One mutation in particular, T47A, nearly abrogates surface binding without adversely affecting expression or secretion in ETEC. Interestingly, T47A is able to bind mutant E. coli expressing highly truncated forms of LPS, indicating that LT binding to wild-type LPS may be due primarily to association with an outer core sugar. Consequently, we have identified a region of LT distinct from the pocket involved in eukaryotic receptor binding that is responsible for binding to the surface of E. coli.Enterotoxigenic Escherichia coli (ETEC), a common etiologic agent behind traveler's diarrhea, is also a significant cause of mortality worldwide (38). Many strains of ETEC elaborate a virulence factor called heat-labile enterotoxin or LT (34). LT is an AB 5 toxin, consisting of a single A subunit, LTA, and a ring of five B subunits, LTB (33). LTB mediates the toxin's binding properties, and LTA ADP ribosylates host G proteins, increasing levels of cyclic AMP and causing the efflux of electrolytes and water into the intestinal lumen (27,35). Each subunit of LT is translated separately from a bicistronic message and then transported to the periplasm, where holotoxin assembly spontaneously occurs (16). Subsequent export into the extracellular milieu is carried out by the main terminal branch of the general secretory pathway (31,36).LT binds eukaryotic cells via an interaction between LTB and host gangliosides, primarily the monosialoganglioside GM 1 (35). The binding site for GM 1 , situated at the interface of two B subunits, has been identified by crystallography (26). GM 1 binding can be strongly impaired by a point mutation in LTB that converts Gly-33 to an aspartic acid residue (37). LT is highly homologous to cholera toxin (CT), both in sequence and structure (7, 35), contributing to ETEC's potentially cholera-like symptoms (39).Previous work in our lab has demonstrated that LT possesses an additional binding capacity beyond its affinity for host glycolipids: the ability to associate with lipopolysaccharide (LPS) on the surface of E. coli (20). LPS, the major component of the outer leaflet of the gram-negative outer membrane, consists of a characteristic lipid moiety, lipid A, covalently linked to a chain of sugar residues (30). In bacteria like E. coli, this sugar chain can be further divided into an inner core oligosaccharide of around five sugars, an outer core of four to six additional sugars, and in ...

BackgroundThe second messenger cyclic diguanylate (c-di-GMP) plays a central role in bacterial adaptation to extracellular stimuli, controlling processes such as motility, biofilm development, cell development and, in some pathogens, virulence. The intracellular level of c-di-GMP is controlled by the complementary activities of diguanylate cyclases containing a GGDEF domain and two classes of c-di-GMP phosphodiesterases containing an EAL or HD-GYP hydrolytic domain. Compared to the GGDEF and EAL domains, the functions of HD-GYP domain family proteins are poorly characterized. The human diarrheal pathogen Vibrio cholerae encodes nine putative HD-GYP domain proteins. To determine the contributions of HD-GYP domain proteins to c-di-GMP signaling in V. cholerae, we systematically analyzed the enzymatic functionality of each protein and their involvement in processes known to be regulated by c-di-GMP: motility, biofilm development and virulence.ResultsComplementary in vitro and in vivo experiments showed that four HD-GYP domain proteins are active c-di-GMP phosphodiesterases: VC1295, VC1348, VCA0210 and VCA0681. Mutation of individual HD-GYP domain genes, as well as combinatorial mutations of multiple HD-GYP domain genes, had no effect on motility or biofilm formation of V. cholerae under the conditions tested. Furthermore, no single HD-GYP domain gene affected intestinal colonization by V. cholerae in an infant mouse model. However, inactivation of multiple HD-GYP domain genes, including the four encoding functional phosphodiesterases, significantly attenuated colonization.ConclusionsThese results indicate that the HD-GYP family of c-di-GMP phosphodiesterases impacts signaling by this second messenger during infection. Altogether, this work greatly furthers the understanding of this important family of c-di-GMP metabolic enzymes and demonstrates a role for HD-GYP domain proteins in the virulence of V. cholerae.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-014-0272-9) contains supplementary material, which is available to authorized users.

Syphilis, cholera and TB have re-emerged and now affect the health of countless humans globally. In this article, we review current information concerning the biology and epidemiology of these bacterial diseases with the goal of developing a better understanding of factors that have led to their resurgence and that threaten to compromise their control. The impact of microbial and environmental change notwithstanding, the main factors common to the re-emergence of syphilis, cholera and TB are human demographics and behavior. This information is critical to developing targeted strategies aimed at preventing and controlling these potentially deadly infectious diseases.