Commensal Bacteria: Not Just Innocent Bystanders

Cyclization increases bactericidal activity of arginine-rich cationic cell-penetrating peptide for Neisseria gonorrhoeae

We previously reported that a linear cationic 12-amino acid cell-penetrating peptide (CPP) was bactericidal for Neisseria gonorrhoeae. In this study, our objectives were to determine the effect of cyclization of the linear CPP on its antibacterial activity for N. gonorrhoeae and cytotoxicity for human cells. We compared the bactericidal effect of 4-hour treatment with the linear CPP to that of CPPs cyclized by a thioether or a disulfide bond on human challenge and multi-drug resistant (MDR) strains of N. gonorrhoeae grown in cell culture media with 10% fetal bovine serum (FBS). The effect of lipooligosaccharide (LOS) sialylation on bactericidal activity was analyzed. We determined the ability of the CPPs to treat human cells infected in vitro with N. gonorrhoeae, to reduce the inflammatory response of human monocytic cells to gonococci, to kill strains of three commensal Neisseria species, and to inhibit gonococcal biofilms. The cyclized CPPs killed 100% of gonococci from all strains at 100 µM and >90% at 20 µM and were more potent than the linear form. The thioether-linked but not the disulfide-linked CPP was less cytotoxic for human cervical cells compared to the linear CPP. LOS sialylation had minimal effect on bactericidal activity. In treating infected human cells, the thioether-linked CPP at 20 µM killed >60% of extra- and intracellular bacteria and reduced TNF-α expression by THP-1 cells. The potency of the CPPs for the pathogenic and the commensal Neisseria was similar. The thioether-linked CPP partially eradicated gonococcal biofilms. Future studies will focus on determining efficacy in the female mouse model of gonorrhea.IMPORTANCENeisseria gonorrhoeae remains a major cause of sexually transmitted infections with 82 million cases worldwide in 2020, and 710,151 confirmed cases in the US in 2021, up 25% from 2017. N. gonorrhoeae can infect multiple tissues including the urethra, cervix, rectum, pharynx, and conjunctiva. The most serious sequelae are suffered by infected women as gonococci ascend to the upper reproductive tract and cause pelvic inflammatory disease, chronic pelvic pain, and infertility in 10%-20% of women. Control of gonococcal infection is widely recognized as increasingly challenging due to the lack of any vaccine. N. gonorrhoeae has quickly developed resistance to all but one class of antibiotics and the emergence of multidrug-resistant strains could result in untreatable infections. As such, gonorrhea is classified by the Center for Disease Control (CDC) as an urgent public health threat. The research presented herein on new therapeutics for gonorrhea has identified a cyclic cell-penetrating peptide (CPP) as a potent molecule targeting N. gonorrhoeae.

Direct visualization of sequence-specific DNA binding by gonococcal type IV pili

Neisseria gonorrhoeae, the causative agent of gonorrhoea, is a major burden on global healthcare systems, with an estimated ~80-90 million new global cases annually. This burden is exacerbated by increasing levels of antimicrobial resistance, which has greatly limited viable antimicrobial therapies. Decreasing gonococcal drug susceptibility has been driven largely by accumulation of chromosomal resistance determinants, which can be acquired through natural transformation, whereby DNA in the extracellular milieu is imported into cells and incorporated into the genome by homologous recombination. N. gonorrhoeae possesses a specialized system for DNA uptake, which strongly biases transformation in favour of DNA from closely related bacteria by recognizing a 10-12 bp DNA uptake sequence (DUS) motif, which is highly overrepresented in their chromosomal DNA. This process relies on numerous proteins, including the DUS-specific receptor ComP, which assemble retractile protein filaments termed type IV pili (T4P) extending from the cell surface, and one model for neisserial DNA uptake proposes that these filaments bind DNA in a DUS-dependent manner before retracting to transport DNA into the periplasm. However, conflicting evidence indicates that elongated pilus filaments may not have such a direct role in DNA binding uptake as this model suggests. Here, we quantitatively measured DNA binding to gonococcal T4P fibres by directly visualizing binding complexes with confocal fluorescence microscopy in order to confirm the sequence-specific, comP-dependent DNA binding capacity of elongated T4P fibres. This supports the idea that pilus filaments could be responsible for initially capturing DNA in the first step of sequence-specific DNA uptake.

Antimicrobial Resistance Profiles of Human Commensal Neisseria Species

Pathogenic Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea. N. gonorrhoeae has evolved high levels of antimicrobial resistance (AR) leading to therapeutic failures even in dual-therapy treatment with azithromycin and ceftriaxone. AR mechanisms can be acquired by genetic transfer from closely related species, such as naturally competent commensal Neisseria species. At present, little is known about the antimicrobial resistance profiles of commensal Neisseria. Here, we characterized the phenotypic resistance profile of four commensal Neisseria species (N. lactamica, N. cinerea, N. mucosa, and N. elongata) against 10 commonly used antibiotics, and compared their profiles to 4 N. gonorrhoeae strains, using disk diffusion and minimal inhibitory concentration assays. Overall, we observed that 3 of the 4 commensals were more resistant to several antibiotics than pathogenic N. gonorrhoeae strains. Next, we compared publicly available protein sequences of known AR genes, including penicillin-binding-protein 2 (PBP2) from commensals and N. gonorrhoeae strains. We found mutations in PBP2 known to confer resistance in N. gonorrhoeae also present in commensal Neisseria sequences. Our results suggest that commensal Neisseria have unexplored antibiotic resistance gene pools that may be exchanged with pathogenic N. gonorrhoeae, possibly impairing drug development and clinical treatment.