RelQ-mediated alarmone signaling regulates growth, sporulation, and stress-induced biofilm formation in Clostridioides difficile.

The bacterial stringent response (SR) is a conserved transcriptional reprogramming pathway mediated by the nucleotide signaling alarmones, (pp)pGpp. The SR has been implicated in antibiotic survival in Clostridioides difficile, a biofilm- and spore-forming pathogen that causes resilient, highly recurrent C. difficile infections. The role of the SR in other processes and the effectors by which it regulates C. difficile physiology are unknown. C. difficile RelQ is a clostridial alarmone synthetase. Deletion of relQ dysregulates C. difficile growth in unstressed conditions, affects susceptibility to antibiotic and oxidative stressors, and drastically reduces biofilm formation. While wild-type C. difficile displays increased biofilm formation in the presence of sub-lethal stress, the ΔrelQ strain cannot upregulate biofilm production in response to stress. Deletion of relQ slows spore accumulation in planktonic cultures but accelerates it in biofilms. This work establishes biofilm formation and sporulation as alarmone-mediated processes in C. difficile and reveals the importance of RelQ in stress-induced biofilm regulation.

Host-defense piscidin peptides as antibiotic adjuvants against Clostridioides difficile.

The spore-forming intestinal pathogen Clostridioides difficile causes multidrug resistant infection with a high rate of recurrence after treatment. Piscidins 1 (p1) and 3 (p3), cationic host defense peptides with micromolar cytotoxicity against C. difficile, sensitize C. difficile to clinically relevant antibiotics tested at sublethal concentrations. Both peptides bind to Cu2+ using an amino terminal copper and nickel binding motif. Here, we investigate the two peptides in the apo and holo states as antibiotic adjuvants against an epidemic strain of C. difficile. We find that the presence of the peptides leads to lower doses of metronidazole, vancomycin, and fidaxomicin to kill C. difficile. The activity of metronidazole, which targets DNA, is enhanced by a factor of 32 when combined with p3, previously shown to bind and condense DNA. Conversely, the activity of vancomycin, which acts at bacterial cell walls, is enhanced 64-fold when combined with membrane-active p1-Cu2+. As shown through microscopy monitoring the permeabilization of membranes of C. difficile cells and vesicle mimics of their membranes, the adjuvant effect of p1 and p3 in the apo and holo states is consistent with a mechanism of action where the peptides enable greater antibiotic penetration through the cell membrane to increase their bioavailability. The variations in effects obtained with the different forms of the peptides reveal that while all piscidins generally sensitize C. difficile to antibiotics, co-treatments can be optimized in accordance with the underlying mechanism of action of the peptides and antibiotics. Overall, this study highlights the potential of antimicrobial peptides as antibiotic adjuvants to increase the lethality of currently approved antibiotic dosages, reducing the risk of incomplete treatments and ensuing drug resistance.

Nanosecond pulsed electric fields increase antibiotic susceptibility in methicillin-resistant Staphylococcus aureus.

IMPORTANCE: We have found that treatment with short electric pulses potentiates the effects of multiple antibiotics against methicillin-resistant Staphylococcus aureus. By reducing the dose of antibiotic necessary to be effective, co-treatment with electric pulses could amplify the effects of standard antibiotic dosing to treat S. aureus infections such as skin and soft-tissue infections (SSTIs). SSTIs are accessible to physical intervention and are good candidates for electric pulse co-treatment, which could be adopted as a step-in wound and abscess debridement.

Sound the (Smaller) Alarm: The Triphosphate Magic Spot Nucleotide pGpp.

It has recently become evident that the bacterial stringent response is regulated by a triphosphate alarmone (pGpp) as well as the canonical tetra- and pentaphosphate alarmones ppGpp and pppGpp [together, (p)ppGpp]. Often dismissed in the past as an artifact or degradation product, pGpp has been confirmed as a deliberate endpoint of multiple synthetic pathways utilizing GMP, (p)ppGpp, or GDP/GTP as precursors. Some early studies concluded that pGpp functionally mimics (p)ppGpp and that its biological role is to make alarmone metabolism less dependent on the guanine energy charge of the cell by allowing GMP-dependent synthesis to continue when GDP/GTP has been depleted. However, recent reports that pGpp binds unique potential protein receptors and is the only alarmone synthesized by the intestinal pathogen Clostridioides difficile indicate that pGpp is more than a stand-in for the longer alarmones and plays a distinct biological role beyond its functional overlap (p)ppGpp.