Differential proteomic analysis of outer membrane enriched extracts of Bacteroides fragilis grown under bile salts stress

Carbapenem resistance in Bacteroides fragilis: A review of molecular mechanisms

Carbapenems are an applicable subclass of β-lactam drugs in the antibiotic therapy of anaerobic infections, especially for poly-microbial cases, due to their broad antimicrobial spectrum on aerobic and anaerobic bacteria. Bacteroides fragilis is the most commonly recovered anaerobic bacteria in the clinical laboratories from mono- and poly-microbial infections. B. fragilis is relatively non-susceptible to different antibiotics, including β-lactams, tetracyclines, fluoroquinolones, and macrolides. Carbapenems are among the most effective drugs against B. fragilis strains with high-level resistance to different antibiotics. Increased antibiotic resistance of B. fragilis strains has been reported following the overuse of an antimicrobial agent. Earlier contact with carbapenems is linked with increased resistance to them that limits the options for treatment of B. fragilis caused infections, especially in cases caused by multidrug-resistant strains. Several molecular mechanisms of resistance to carbapenems have been described for different carbapenem-resistant bacteria. Understanding the mechanisms of resistance to antimicrobial agents is necessary for selecting alternative antimicrobial agents and the application of control strategies. In the present study, we reviewed the mechanisms contributing to resistance to carbapenems in B. fragilis strains.

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.

Good Gone Bad: One Toxin Away From Disease for Bacteroides fragilis

The human gut is colonized by hundreds of trillions of microorganisms whose acquisition begins during early infancy. Species from the Bacteroides genus are ubiquitous commensals, comprising about thirty percent of the human gut microbiota. Bacteroides fragilis is one of the least abundant Bacteroides species, yet is the most common anaerobe isolated from extraintestinal infections in humans. A subset of B. fragilis strains carry a genetic element that encodes a metalloprotease enterotoxin named Bacteroides fragilis toxin, or BFT. Toxin-bearing strains, or Enterotoxigenic B. fragilis (ETBF) cause acute and chronic intestinal disease in children and adults. Despite this association with disease, around twenty percent of the human population appear to be asymptomatic carriers of ETBF. BFT damages the colonic epithelial barrier by inducing cleavage of the zonula adherens protein E-cadherin and initiating a cell signaling response characterized by inflammation and c-Myc-dependent pro-oncogenic hyperproliferation. As a consequence, mice harboring genetic mutations that predispose to colonic inflammation or tumor formation are uniquely susceptible to toxin-mediated injury. The recent observation of ETBF-bearing biofilms in colon biopsies from humans with colon cancer susceptibility loci strongly suggests that ETBF is a driver of colorectal cancer. This article will address ETBF biology from a host-pathobiont perspective, including clinical data, analysis of molecular mechanisms of disease, and the complex ecological context of the human gut.

Importance of microbial defence systems to bile salts and mechanisms of serum cholesterol reduction

An important feature of the intestinal microbiota, particularly in the case of administered probiotic microorganisms, is their resistance to conditions in the gastrointestinal tract, particularly tolerance to and growth in the presence of bile salts. Bacteria can use several defence mechanisms against bile, including special transport mechanisms, the synthesis of various types of surface proteins and fatty acids or the production of exopolysaccharides. The ability to enzymatically hydrolyse bile salts occurs in a variety of bacteria. Choloylglycine hydrolase (EC 3.5.1.24), a bile salt hydrolase, is a constitutive intracellular enzyme responsible for the hydrolysis of an amide bond between glycine or taurine and the steroid nucleus of bile acids. Its presence was demonstrated in specific microorganisms from several bacterial genera (Lactobacillus spp., Bifidobacterium spp., Clostridium spp., Bacteroides spp.). Occurrence and gene arrangement encoding this enzyme are highly variable in probiotic microorganisms. Bile salt hydrolase activity may provide the possibility to use the released amino acids by bacteria as sources of carbon and nitrogen, to facilitate detoxification of bile or to support the incorporation of cholesterol into the cell wall. Deconjugation of bile salts may be directly related to a lowering of serum cholesterol levels, from which conjugated bile salts are synthesized de novo. Furthermore, the ability of microorganisms to assimilate or to bind ingested cholesterol to the cell wall or to eliminate it by co-precipitation with released cholic acid was also documented. Some intestinal microflora produce cholesterol reductase that catalyses the conversion of cholesterol to insoluble coprostanol, which is subsequently excreted in faeces, thereby also reducing the amount of exogenous cholesterol.

Inactivation of MarR gene homologs increases susceptibility to antimicrobials in Bacteroides fragilis

Bacteroides fragilis is the strict anaerobic bacteria most commonly found in human infections, and has a high mortality rate. Among other virulence factors, the remarkable ability to acquire resistance to a variety of antimicrobial agents and to tolerate nanomolar concentrations of oxygen explains in part their success in causing infection and colonizing the mucosa. Much attention has been given to genes related to multiple drug resistance derived from plasmids, integrons or transposon, but such genes are also detected in chromosomal systems, like the mar (multiple antibiotic resistance) locus, that confer resistance to a range of drugs. Regulators like MarR, that control expression of the locus mar, also regulate resistance to organic solvents, disinfectants and oxygen reactive species are important players in these events. Strains derived from the parental strain 638R, with mutations in the genes hereby known as marRI (BF638R_3159) and marRII (BF638R_3706) were constructed by gene disruption using a suicide plasmid. Phenotypic response of the mutant strains to hydrogen peroxide, cell survival assay against exposure to oxygen, biofilm formation, resistance to bile salts and resistance to antibiotics was evaluated. The results showed that the mutant strains exhibit statistically significant differences in their response to oxygen stress, but no changes were observed in survival when exposed to bile salts. Biofilm formation was not affected by either gene disruption. Both mutant strains however, became more sensitive to multiple antimicrobial drugs tested. This indicates that as observed in other bacterial species, MarR are an important resistance mechanism in B. fragilis.