Characterization of fimbriae from Bacteroides fragilis

Pathogenic Bacteroides fragilis strains can emerge from gut-resident commensals

Bacteroides fragilis is a prominent member of the human gut microbiota, playing crucial roles in maintaining gut homeostasis and host health. Although it primarily functions as a beneficial commensal, B. fragilis can become pathogenic. To determine the genetic basis of its duality, we conducted a comparative genomic analysis of 813 B. fragilis strains, representing both commensal and pathogenic origins. Our findings reveal that pathogenic strains emerge across diverse phylogenetic lineages, due in part to rapid gene exchange and the adaptability of the accessory genome. We identified 16 phylogenetic groups, differentiated by genes associated with capsule composition, interspecies competition, and host interactions. A microbial genome-wide association study identified 44 genes linked to extra-intestinal survival and pathogenicity. These findings reveal how genomic diversity within commensal species can lead to the emergence of pathogenic traits, broadening our understanding of microbial evolution in the gut.

Gut biofilms: Bacteroides as model symbionts to study biofilm formation by intestinal anaerobes

Bacterial biofilms are communities of adhering bacteria that express distinct properties compared to their free-living counterparts, including increased antibiotic tolerance and original metabolic capabilities. Despite the potential impact of the biofilm lifestyle on the stability and function of the dense community of micro-organisms constituting the mammalian gut microbiota, the overwhelming majority of studies performed on biofilm formation by gut bacteria focused either on minor and often aerobic members of the community or on pathogenic bacteria. In this review, we discuss the reported evidence for biofilm-like structures formed by gut bacteria, the importance of considering the anaerobic nature of gut biofilms and we present the most recent advances on biofilm formation by Bacteroides, one of the most abundant genera of the human gut microbiota. Bacteroides species can be found attached to food particles and colonizing the mucus layer and we propose that Bacteroides symbionts are relevant models to probe the physiology of gut microbiota biofilms.

To resist and persist: Important factors in the pathogenesis of Bacteroides fragilis

Bacteroides fragilis is a most frequent anaerobic pathogen isolated from human infections, particularly found in the abdominal cavity. Different factors contribute to the pathogenesis and persistence of B. fragilis at infection sites. The knowledge of the virulence factors can provide applicable information for finding alternative options for the antibiotic therapy and treatment of B. fragilis caused infections. Herein, a comprehensive review of the important B. fragilis virulence factors was prepared. In addition to B. fragilis toxin (BFT) and its potential role in the diarrhea and cancer development, some other important virulence factors and characteristics of B. fragilis are described including capsular polysaccharides, iron acquisition, resistance to antimicrobial agents, and survival during the prolonged oxidative stress, quorum sensing, and secretion systems.

A New Pillar in Pilus Assembly

Pilus assembly in bacteria typically occurs by one of four pathways. In the study by Xu et al., the structures of 20 pilin subunits of human oral and gut Bacteroidales are elucidated, revealing a new pilin superfamily, assembled into pili by a distinct fifth pathway.

A Distinct Type of Pilus from the Human Microbiome

Pili are proteinaceous polymers of linked pilins that protrude from the cell surface of many bacteria and often mediate adherence and virulence. We investigated a set of 20 Bacteroidia pilins from the human microbiome whose structures and mechanism of assembly were unknown. Crystal structures and biochemical data revealed a diverse protein superfamily with a common Greek-key β sandwich fold with two transthyretin-like repeats that polymerize into a pilus through a strand-exchange mechanism. The assembly mechanism of the central, structural pilins involves proteinase-assisted removal of their N-terminal β strand, creating an extended hydrophobic groove that binds the C-terminal donor strands of the incoming pilin. Accessory pilins at the tip and base have unique structural features specific to their location, allowing initiation or termination of the assembly. The Bacteroidia pilus, therefore, has a biogenesis mechanism that is distinct from other known pili and likely represents a different type of bacterial pilus.

Virulence factors in anaerobic bacteri

Various surface structures can be expressed in Bacteroides fragilis, but little is known about capsular structures in other non-spore-forming anaerobes. Fimbriae have been isolated from Bacteroides fragilis and Porphyromonas gingivalis. The importance of iron-repressible outer membrane proteins as virulence factors in Bacteroides fragilis is under study. The low endotoxic activity of Bacteroides fragilis lipopolysaccharide can be attributed to the chemical composition of this organism's lipid A. A tissue culture system for the demonstration of Bacteroides fragilis enterotoxin has recently been described. The toxins A and B of Clostridium difficile are immunologically distinct. The importance of IgA proteases and other enzymes as virulence factors in anaerobic bacteria remains unclear.

Sialic acid as receptor of Bacteroides fragilis lectin-like adhesin

It was observed that sialic acid and macromolecules rich in this sugar were able to inhibit the hemagglutination activity (HA) of Bacteroides fragilis strains in low concentrations. Reversion of the HA and also of the adsorption to beads of Sepharose coupled to bovine submaxillary mucin, by this sugar residue corroborated the recognition capacity of the bacterial lectin-like adhesin. However, when erythrocytes were treated with clostridial neuraminidase, an increase in the HA of some strains was observed. Protease treatment of erythrocytes abolished the HA, indicating that cell receptors of B. fragilis are probably a glycoprotein moiety.

Characterization and detection of the 40 kDa fimbrial subunit of Bacteroides fragilis BE1

The amino acid composition and amino-terminal amino acid sequence of the 40 kDa fimbrial subunit of Bacteroides fragilis were determined. No similarity with other known fimbrial subunits or protein sequences was found. The isoelectric point of the fimbriae was determined to be 3.8. The acidic nature of these fimbriae is in agreement with the amino acid composition of the fimbrial subunit. An IgM monoclonal antibody, raised against the denaturated subunit of strain BE1 fimbriae, reacted with native BE1 fimbriae and appeared to be strain specific. Bacteroides fragilis strain BE38 expressed fimbriae which did not react with the polyclonal or monoclonal antibodies directed against strain BE1 fimbriae.

Diarrheal disease caused by enterotoxigenic Bacteroides fragilis in infant rabbits

Enterotoxigenic Bacteroides fragilis caused severe, nonhemorrhagic, watery diarrhea when 10(9) CFU of a porcine or human isolate was administered orogastrically to 3-day-old rabbits. The bacterium colonized the intestinal tract with a predilection for the large intestine (10(9) CFU/g of cecal contents). Diarrhea occurred at an average of 4.6 days postinoculation, and 84% of rabbits were dead or moribund at an average of 8.8 days postinoculation. The disease was characterized by watery diarrhea and dehydration. Severe histologic lesions including inflammation, exfoliation of epithelial cells, and crypt hyperplasia were observed throughout the colon. There was no indication of bacteremia or of bacterial adherence to or invasion of intestinal epithelial cells. Rabbits inoculated with nonenterotoxigenic B. fragilis were colonized with B. fragilis but did not develop clinical disease or intestinal lesions. While the pathogenesis of this disease is undefined, clinical signs of disease and histologic changes were consistent with a mechanism of net secretion of fluid into the small intestine and decreased absorption of fluid from the large intestine. Enteric disease caused by enterotoxigenic B. fragilis in infant rabbits was similar to naturally occurring enteric disease associated with the bacterium in humans and livestock. This study established that enterotoxigenic B. fragilis is enteropathogenic in intact infant rabbits.

Purification of fimbriae from Pasteurella haemolytica A-1

Pasteurella haemolytica A-1 produces large, rigid fimbriae with a diameter of 12 nm and a denisty of 1.32. Their subunit molecular weight was 35,000 as determined by SDS-polyacrylamide gel electrophoresis and immunoblotting with monoclonal antibodies raised against native fimbriae. The isoelectric point of the purified fimbriae was 4.8. While Pasteurella haemolytica whole cells plus a crude shear fraction were capable of agglutinating bovine erythrocytes, purified fimbriae exhibited no such activity.

Outer membrane proteins of Bacteroides fragilis and Bacteroides vulgatus in relation to iron uptake and virulence

A virulent strain B. fragilis BE1 and an avirulent strain B. vulgatus BE20 were grown in a culture medium with and without the addition of a synthetic chelator (Bipyridyl) to induce iron limitation. Cells grew more slowly under iron stress, although the growth rate of the B. vulgatus strain was more affected under these conditions than the strain of B. fragilis. The outer membrane protein profile of these strains was studied in relation to the iron concentration in the growth medium by means of SDS-polyacrylamide gel electrophoresis. Four proteins, with the apparent molecular weights of 89, 49, 44 and 23.5 kDa, were consistently present in the outer membrane of B. fragilis BE1 grown under iron restricted conditions. In B. vulgatus BE20 cells a 44 and a 23.5 kDa protein were absent and only the expression of an 89 kDa protein was clearly seen under these conditions. The iron regulated proteins, particularly the 44 kDa protein, could be involved to an iron uptake mechanism in B. fragilis. So the presence of these proteins might play an important role in the virulence of this anaerobic bacterium.