Candidate targets for new antivirulence drugs: selected cases of bacterial adhesion and biofilm formation

Hyperimmune egg yolk antibodies developed against Clostridium perfringens antigens protect against necrotic enteritis

Necrotic enteritis (NE) is a widespread infectious disease caused by Clostridium perfringens that inflicts major economic losses on the global poultry industry. Due to regulations on antibiotic use in poultry production, there is an urgent need for alternative strategies to mitigate the negative effects of NE. This paper presents a passive immunization technology that utilizes hyperimmune egg yolk immunoglobulin Y (IgY) specific to the major immunodominant antigens of C. perfringens. Egg yolk IgYs were generated by immunizing hens with 4 different recombinant C. perfringens antigens, and their protective effects against NE were evaluated in commercial broilers. Six different spray-dried egg powders were produced using recombinant C. perfringens antigens: α-toxin, NE B-like toxin (NetB; EB), elongation factor-Tu (ET), pyruvate:ferredoxin oxidoreductase, a mixture of 4 antigens (EM-1), and a nonimmunized control (EC). The challenged groups were either provided with different egg powders at a 1% level or no egg powders (EN). The NE challenge model based on Eimeria maxima and C. perfringens dual infection was used. In Experiments 1 and 2, the EB and ET groups exhibited increased body weight gain (BWG; P < 0.01), decreased NE lesion scores (P < 0.001), and reduced serum NetB levels (P < 0.01) compared to the EN and EC groups. IgY against NetB significantly reduced Leghorn male hepatocellular cytotoxicity in an in vitro test (P < 0.01). In Experiment 3, the protective effect of the IgYs mixture (EM-2) against C. perfringens antigens (NetB and EFTu) and Eimeria antigens (elongation factor-1-alpha: EF1α and Eimeria profilin: 3-1E) was tested. The EM-2 group showed similar body weight, BWG, and feed intake from d 7 to 22 compared to the NC group (P < 0.05). On d 20, the EM-2 group showed comparable intestinal permeability, NE lesion scores, and jejunal NetB and collagen adhesion protein levels to the NC group (P < 0.05). In conclusion, dietary mixture containing antibodies to NetB and EFTu provides protection against experimental NE in chickens through passive immunization.

A SAR-based mechanistic study on the combined toxicities of sulfonamides and quorum sensing inhibitors on Escherichia coli

Quorum sensing inhibitors (QSIs) are promising alternatives to antibiotics, but they are discharged into the environment after their use cycle. This poses joint effects on the organisms in the environment. Therefore, it is of great importance to study the combined toxicities of QSIs and antibiotics. In this study, we investigated the single and combined toxicities of four potential QSIs and 11 sulfonamides (SAs) on Escherichia coli. The results revealed that the single toxicities of SAs were greater than those of QSIs, and the toxicities were found positively related to the binding energies (E_bind) with their target proteins, for both antibiotics and QSIs. The combined toxicities of the binary mixtures were observed to be either antagonism or addition. The antagonism could be explained by the phenomenon that QSIs changed SAs molecules into ionic forms, preventing the SA molecules entering the bacteria. Furthermore, it was found that the ratios of the effective concentration (the actual concentration involved in the interaction with the proteins) in the antagonistic cases were higher than those in the additive cases. This study would benefit both rational use of the drug combination and ecological risk assessment of antibiotics and QSIs in the real environment.

The adherent abilities of Clostridium perfringens strains are critical for the pathogenesis of avian necrotic enteritis

Necrotic enteritis of poultry is an emerging disease of substantial economic importance, but aspects of the pathogenesis of this multi-factorial disease are still unclear. We recently demonstrated that the ability of avian strains of the causative bacterium, Clostridium perfringens, to bind to specific collagen types correlated strongly with their virulence and we postulated that binding of the pathogen to collagen types IV and V and gelatin may involve the putative adhesin-encoding gene cnaA, which is found in the VR-10B locus. In this study we have used site-directed mutagenesis to demonstrate that disruption of the cnaA gene leads to a reduction in the expression of the three genes immediately downstream of cnaA and reduced adherence to collagen types IV and V and gelatin. In addition, a cnaA mutant of strain EHE-NE18 was no longer capable of causing necrotic enteritis in a chicken disease induction model and had a significantly reduced ability to colonise the chicken intestinal mucosa. These results were confirmed by generating and analysing a similar mutant in an independent necrotic enteritis causing C. perfringens strain. This study expands our understanding of the mechanisms involved in necrotic enteritis pathogenesis by demonstrating the importance of C. perfringens adherence to extracellular matrix proteins.

Targeted Treatment for Bacterial Infections: Prospects for Pathogen-Specific Antibiotics Coupled with Rapid Diagnostics

Antibiotics are a cornerstone of modern medicine and have significantly reduced the burden of infectious diseases. However, commonly used broad-spectrum antibiotics can cause major collateral damage to the human microbiome, causing complications ranging from antibiotic-associated colitis to the rapid spread of resistance. Employing narrower spectrum antibiotics targeting specific pathogens may alleviate this predicament as well as provide additional tools to expand an antibiotic repertoire threatened by the inevitability of resistance. Improvements in clinical diagnosis will be required to effectively utilize pathogen-specific antibiotics and new molecular diagnostics are poised to fulfill this need. Here we review recent trends and the future prospects of deploying narrower spectrum antibiotics coupled with rapid diagnostics. Further, we discuss the theoretical advantages and limitations of this emerging approach to controlling bacterial infectious diseases.

Inhibitory effects of rosemary (Rosemarinus officinalis L.) essential oil on pathogenicity of irradiated and non-irradiated Pseudomonas aeruginosa

Pseudomonas aeruginosa is considered one of the most important opportunistic pathogens, which is responsible for nosocomial infections. The complex pathophysiology of infections associated with P. aeruginosa is due to its ability to grow in a biofilm mass and to produce a large number of virulence factors. This study aimed to investigate the antipseudomonal efficacy of rosemary essential oil (REO) against 25 clinical isolates control and gamma irradiated (at 24.4Gy). Susceptibility testing towards 15 standard antibiotics and REO was carried out using disc diffusion method. The potency of REO on motility, biofilm and pyocyanin production was evaluated. Total protein profile was estimated by SDS polyacrylamide gel electroporesis. REO has showed a broad spectrum of antimicrobial activity compared to the standard antibiotics. According to their high sensitivity to REO, six isolates were chosen for MIC90 with minimum inhibitory concentration (MIC90) values ranged from 5 to 25μl/ml. REO exerted marked effects on irradiated and non-irradiated tested isolates. The results displayed high significant reduction in biofilm and pyocyanin production. Significant reduction in cell-surface hydrophobicity was observed for some isolates. As for motility, REO altered twitching, swarming and swimming ability of the tested isolates. The ultra structure of bacterial cell results in ruptures of the bacterial cell, leakage of the cytoplasmic components and consequently cell death. Total protein analysis of control, irradiated and irradiated with treatment of REO revealed some changes in protein profile as a result of treatment.

CONCLUSION

This study may hasten the application of REO in the treatment and prevention of pseudomonas resistant isolates in nosocomial infections.

Chemoinformatics-assisted development of new anti-biofilm compounds

Bacterial biofilms are associated with a large number of infections. Biofilm-dwelling bacteria are particularly resistant to antibiotics, making it hard to eradicate biofilm-associated infections. Here, we use a novel cross-disciplinary approach combining microbiology and chemoinformatics to identify new and efficient anti-biofilm drugs. We found that ellagic acid (present in green tea) significantly inhibited biofilm formation of Streptococcus dysgalactiae. Based on ellagic acid, we performed in silico screening of the Chinese Natural Product Database to predict a 2nd-generation list of compounds with similar characteristics. One of these, esculetin, proved to be more efficient in preventing biofilm formation by Staphylococcus aureus. From esculetin a 3rd-generation list of compounds was predicted. One of them, fisetin, was even better to abolish biofilm formation than the two parent compounds. Fisetin dramatically inhibited biofilm formation of both S. aureus and S. dysgalactiae. The compounds did not affect planktonic growth in concentrations where they affected biofilm formation and appeared to be specific antagonists of biofilms. Arguably, since all three compounds are natural ingredients of dietary plants, they should be well-tolerated by humans. Our results indicate that such small plant components, with bacterial lifestyle altering properties are promising candidates for novel generations of antimicrobial drugs. The study underlines the potential in combining chemoinformatics and biofilm research.

5-Fluorouracil reduces biofilm formation in Escherichia coli K-12 through global regulator AriR as an antivirulence compound

The uracil analog, 5-fluorouracil (5-FU), reduces virulence and biofilm formation for Pseudomonas aeruginosa PA14 without affecting its growth. As 5-FU is an approved anticancer drug, its antivirulence attributes in P. aeruginosa prompted us to examine the effect of this compound on three different Escherichia coli K-12 strains and its effect on virulence genes in E. coli O157:H7 (EHEC); the mechanism by which it functions was also examined. 5-FU decreased biofilm formation in a dose-dependent manner in E. coli K-12 and repressed the expression of virulence genes in EHEC. Five other uracil analogs were also tested for their effects on biofilm formation, and none of these compounds affected the biofilm formation in E. coli K-12. Whole-transcriptome analysis revealed that 5-FU induced the expression of 157 genes and repressed the expression of 19 genes. Biofilm formation with the addition of 5-FU was checked in 21 isogenic knockout mutants whose gene expression was induced in the microarray data; we found that 5-FU does not decrease biofilm formation of the cells that lack AriR, a global DNA regulator that controls acid resistance in E. coli. Hence, 5-FU represses biofilm formation of E. coli K-12 through AriR and is a novel antivirulence compound for this strain.

A conserved C-terminal 13-amino-acid motif of Gap1 is required for Gap1 function and necessary for the biogenesis of a serine-rich glycoprotein of Streptococcus parasanguinis

Adhesion of Streptococcus parasanguinis to saliva-coated hydroxyapatite (SHA), an in vitro tooth model, is mediated by long peritrichous fimbriae. Fap1, a fimbria-associated serine-rich glycoprotein, is required for fimbrial assembly. Biogenesis of Fap1 is controlled by an 11-gene cluster that contains gly, nss, galT1 and -2, secY2, gap1 to -3, secA2, and gtf1 and -2. We had previously isolated a collection of nine nonadherent mutants using random chemical mutagenesis approaches. These mutants fail to adhere to the in vitro tooth model and to form fimbriae. In this report, we further characterized these randomly selected nonadherent mutants and classified them into three distinct groups. Two groups of genes were previously implicated in Fap1 biogenesis. One group has a mutation in a glycosyltransferase gene, gtf1, that is essential for the first step of Fap1 glycosylation, whereas the other group has defects in the fap1 structural gene. The third group mutant produces an incompletely glycosylated Fap1 and exhibits a mutant phenotype similar to that of a glycosylation-associated protein 1 (Gap1) mutant. Analysis of this new mutant revealed that a conserved C-terminal 13-amino-acid motif was missing in Gap1. Site-directed mutagenesis of a highly conserved amino acid tryptophan within this motif recapitulated the deletion phenotype, demonstrating the importance of the Gap1 C-terminal motif for Fap1 biogenesis. Furthermore, the C-terminal mutation does not affect Gap1-Gap3 protein-protein interaction, which has been shown to mediate Fap1 glycosylation, suggesting the C-terminal motif has a distinct function related to Fap1 biogenesis.

Inactivation of efflux pumps abolishes bacterial biofilm formation

Bacterial biofilms cause numerous problems in health care and industry; notably, biofilms are associated with a large number of infections. Biofilm-dwelling bacteria are particularly resistant to antibiotics, making it hard to eradicate biofilm-associated infections. Bacteria rely on efflux pumps to get rid of toxic substances. We discovered that efflux pumps are highly active in bacterial biofilms, thus making efflux pumps attractive targets for antibiofilm measures. A number of efflux pump inhibitors (EPIs) are known. EPIs were shown to reduce biofilm formation, and in combination they could abolish biofilm formation completely. Also, EPIs were able to block the antibiotic tolerance of biofilms. The results of this feasibility study might pave the way for new treatments for biofilm-related infections and may be exploited for prevention of biofilms in general.