A newly identified group of P-like (PL) fimbriae from extra-intestinal pathogenic Escherichia coli (ExPEC) encode distinct adhesin subunits and mediate adherence to host cells

Fimbrial adhesins play a critical role for bacterial adherence and biofilm formation. Sequencing of avian pathogenic Escherichia coli (APEC) strain QT598 identified a fimbrial gene cluster belonging to the π group that wenamed PL (P-like) fimbriae, since genetic organization and sequence are similar to Pap and related fimbriae. Screening of genomic databases indicated that genes encoding PL fimbriae located on IncF plasmids are present in a diversity of E. coli isolates from poultry, human systemic infections, and other sources. As with P fimbriae, PL fimbriae exhibit sequence divergence in adhesin encoding genes, and strains could be divided into 5 classes based on differences in sequences of the PlfG adhesin protein. The plf genes from two predominant PlfG adhesin classes, PlfG-I and PlfG-II were cloned. PL fimbriae were visualized by electron microscopy, promoted biofilm formation, demonstrated distinct hemagglutination profiles and promoted adherence to human bladder and kidney epithelial cell lines. Hybrid fimbriae comprised of genes from plfQT598 wherein plfG was replaced by papG encoding adhesin genes were also shown to be functional and mediate adherence to epithelial cells, further indicating similarity and functional compatibility between these two types of fimbriae. Although deletion of plf genes did not significantly reduce colonization of the mouse urinary tract, plf gene expression was increased over 40-fold in the bladder compared to during in vitro culture. Overall, PL fimbriae represent a new group of fimbriae demonstrating both functional differences and similarities to P fimbriae and may contribute to adherence to cells and colonization of host tissues.

Toggle switch residues control allosteric transitions in bacterial adhesins by participating in a concerted repacking of the protein core

Critical molecular events that control conformational transitions in most allosteric proteins are ill-defined. The mannose-specific FimH protein of Escherichia coli is a prototypic bacterial adhesin that switches from an ‘inactive’ low-affinity state (LAS) to an ‘active’ high-affinity state (HAS) conformation allosterically upon mannose binding and mediates shear-dependent catch bond adhesion. Here we identify a novel type of antibody that acts as a kinetic trap and prevents the transition between conformations in both directions. Disruption of the allosteric transitions significantly slows FimH’s ability to associate with mannose and blocks bacterial adhesion under dynamic conditions. FimH residues critical for antibody binding form a compact epitope that is located away from the mannose-binding pocket and is structurally conserved in both states. A larger antibody-FimH contact area is identified by NMR and contains residues Leu-34 and Val-35 that move between core-buried and surface-exposed orientations in opposing directions during the transition. Replacement of Leu-34 with a charged glutamic acid stabilizes FimH in the LAS conformation and replacement of Val-35 with glutamic acid traps FimH in the HAS conformation. The antibody is unable to trap the conformations if Leu-34 and Val-35 are replaced with a less bulky alanine. We propose that these residues act as molecular toggle switches and that the bound antibody imposes a steric block to their reorientation in either direction, thereby restricting concerted repacking of side chains that must occur to enable the conformational transition. Residues homologous to the FimH toggle switches are highly conserved across a diverse family of fimbrial adhesins. Replacement of predicted switch residues reveals that another E. coli adhesin, galactose-specific FmlH, is allosteric and can shift from an inactive to an active state. Our study shows that allosteric transitions in bacterial adhesins depend on toggle switch residues and that an antibody that blocks the switch effectively disables adhesive protein function.

Enhanced Virulence of Salmonella enterica serovar Enteritidis ATCC13076 under Acid Stress by Global Transcriptomics

Salmonella enterica serovar Enteritidis is a primary pathogen causing foodborne diseases and intestinal inflammatory responses. Acid tolerance response (ATR), as a strategy of adaption and resistance to acid stress, may contribute to enhanced virulence. In this study, there was a moderately acid adaption (pH 5.0) for S. Enteritidis cells prior to treatment with acid stress (pH 3.0). To figure out whether S. Enteritidis up-regulated the virulence or not, a global transcriptomic analysis was carried out by high-throughout RNA-sEq. The results showed 74 differentially expressed genes (DEGs) involved in virulence were identified after acid stress, among which, 62 DEGs were up-regulated and 12 DEGs were down-regulated. Afterwards, those virulence-linked DEGs were discussed and classified into four aspects based on the steps of infection, including flagellar functions, fimbrial adhesins, T3SS-mediated invasion and other virulent determinants. In conclusion, S. Enteritidis seemed to exhibit a trend of virulent genes towards high-expression under acid stress, revealing risks of Salmonella in acid-containing food. To our knowledge, there were few studies on comprehensively analyzing virulent genes expression changes of Salmonella, but it’s novel to put forward pathogenicity as the highest priority under acid environment.

FimH and Anti-Adhesive Therapeutics: A Disarming Strategy Against Uropathogens

Chaperone-usher fimbrial adhesins are powerful weapons against the uropathogens that allow the establishment of urinary tract infections (UTIs). As the antibiotic therapeutic strategy has become less effective in the treatment of uropathogen-related UTIs, the anti-adhesive molecules active against fimbrial adhesins, key determinants of urovirulence, are attractive alternatives. The best-characterized bacterial adhesin is FimH, produced by uropathogenic Escherichia coli (UPEC). Hence, a number of high-affinity mono- and polyvalent mannose-based FimH antagonists, characterized by different bioavailabilities, have been reported. Given that antagonist affinities are firmly associated with the functional heterogeneities of different FimH variants, several FimH inhibitors have been developed using ligand-drug discovery strategies to generate high-affinity molecules for successful anti-adhesion therapy. As clinical trials have shown d-mannose’s efficacy in UTIs prevention, it is supposed that mannosides could be a first-in-class strategy not only for UTIs, but also to combat other Gram-negative bacterial infections. Therefore, the current review discusses valuable and effective FimH anti-adhesive molecules active against UTIs, from design and synthesis to in vitro and in vivo evaluations.

Structures of two fimbrial adhesins, AtfE and UcaD, from the uropathogenProteus mirabilis

The important uropathogenProteus mirabilisencodes a record number of chaperone/usher-pathway adhesive fimbriae. Such fimbriae, which are used for adhesion to cell surfaces/tissues and for biofilm formation, are typically important virulence factors in bacterial pathogenesis. Here, the structures of the receptor-binding domains of the tip-located two-domain adhesins UcaD (1.5 Å resolution) and AtfE (1.58 Å resolution) from twoP. mirabilisfimbriae (UCA/NAF and ATF) are presented. The structures of UcaD and AtfE are both similar to the F17G type of tip-located fimbrial receptor-binding domains, and the structures are very similar despite having only limited sequence similarity. These structures represent an important step towards a molecular-level understanding ofP. mirabilisfimbrial adhesins and their roles in the complex pathogenesis of urinary-tract infections.

Virulence factors of Escherichia coli in relation to the importance of vaccination in pigs

ABSTRACT: Enterotoxigenic Escherichia coli (ETEC) is the major cause of diarrhea in newborn and weaned pigs. Bacteria adhesion to the host cell is considered a specific phenomenon among fimbrial and non-fimbrial adhesins with their respective receptors on enterocytes. Enteric disorders are related with the fimbriae F4 (K88), F5 (K99), F6 (987P), F41, and F18. In addition to ETEC, another category of E. coli , porcine pathogenic E. coli (PEPEC),can cause diarrhea in pigs; it produces the porcine attaching and effacing-associated (Paa) adhesin in, which is capable to cause a typical lesion known as an attaching and effacing (A/E) lesion. Immunization of sows with adhesin is important to stimulate the production of antibodies and their subsequent transfer to piglets through colostrum. The aim of this paper is to illustrate the main impacts of enteric diseases caused by E. coli in swine production and to highlight the importance of continuing research on this bacterium to improve disease prevention through vaccination.