Analyzing mechanisms of action of antimicrobial peptides on bacterial membranes requires multiple complimentary assays and different bacterial strains

Thiolated chitosan nanoparticles encapsulated nisin and selenium: antimicrobial/antibiofilm/anti-attachment/immunomodulatory multi-functional agent

BACKGROUND

The increase in the resistance of bacterial strains to antibiotics has led to research into the bactericidal potential of non-antibiotic compounds. This study aimed to evaluate in vitro antibacterial/ antibiofilm properties of nisin and selenium encapsulated in thiolated chitosan nanoparticles (N/Se@TCsNPs) against prevalent enteric pathogens including standard isolates of Vibrio (V.) cholerae O1 El Tor ATCC 14,035, Campylobacter (C.) jejuni ATCC 29,428, Salmonella (S.) enterica subsp. enterica ATCC 19,430, Shigella (S.) dysenteriae PTCC 1188, Escherichia (E.) coli O157:H7 ATCC 25,922, Listeria (L.) monocytogenes ATCC 19,115, and Staphylococcus (S.) aureus ATCC 29,733.

METHODS

The synthesis and comprehensive analysis of N/Se@TCsNPs have been completed. Antibacterial and antibiofilm capabilities of N/Se@TCsNPs were evaluated through broth microdilution and crystal violet assays. Furthermore, the study included examining the cytotoxic effects on Caco-2 cells and exploring the immunomodulatory effects of N/Se@TCsNPs. This included assessing the levels of both pro-inflammatory (IL-6 and TNFα) and anti-inflammatory (IL-10 and TGFβ) cytokines and determining the gene expression of TLR2 and TLR4.

RESULTS

The N/Se@TCsNPs showed an average diameter of 136.26 ± 43.17 nm and a zeta potential of 0.27 ± 0.07 mV. FTIR spectroscopy validated the structural features of N/Se@TCsNPs. Scanning electron microscopy (SEM) images confirmed their spherical shape and uniform distribution. Thermogravimetric Analysis (TGA)/Differential Scanning Calorimetry (DSC) tests demonstrated the thermal stability of N/Se@TCsNPs, showing minimal weight loss of 0.03%±0.06 up to 80 °C. The prepared N/Se@TCsNPs showed a thiol content of 512.66 ± 7.33 µmol/g (p < 0.05), an encapsulation efficiency (EE) of 69.83%±0.04 (p ≤ 0.001), and a drug release rate of 74.32%±3.45 at pH = 7.2 (p ≤ 0.004). The synthesized nanostructure demonstrated potent antibacterial activity against various isolates, with effective concentrations ranging from 1.5 ± 0.08 to 25 ± 4.04 mg/mL. The ability of N/Se@TCsNPs to reduce bacterial adhesion and internalization in Caco-2 cells underscored their antibiofilm properties (p ≤ 0.0001). Immunological studies indicated that treatment with N/Se@TCsNPs led to decreased levels of inflammatory cytokines IL-6 (14.33 ± 2.33 pg/mL) and TNFα (25 ± 0.5 pg/mL) (p ≤ 0.0001), alongside increased levels of anti-inflammatory cytokines IL-10 (46.00 ± 0.57 pg/mL) and TGFβ (42.58 ± 2.10 pg/mL) in infected Caco-2 cells (p ≤ 0.0001). Moreover, N/Se@TCsNPs significantly reduced the expression of TLR2 (0.22 ± 0.09) and TLR4 (0.16 ± 0.05) (p < 0.0001).

CONCLUSION

In conclusion, N/Se@TCsNPs exhibited significant antibacterial/antibiofilm/anti-attachment/immunomodulatory effectiveness against selected Gram-positive and Gram-negative enteric pathogens. However, additional ex-vivo and in-vivo investigations are needed to fully assess the performance of nanostructured N/Se@TCsNPs.

The enigmatic mode of action of the lantibiotic epilancin 15X

Epilancin 15X is a lantibiotic that has an antimicrobial activity in the nanomolar concentration range towards Staphylococcus simulans. Such low MICs usually imply that these peptides employ a mechanism of action (MoA) involving high affinity targets. Here we studied this MoA by using epilancin 15X's ability to dissipate the membrane potential of intact S. simulans cells. These membrane depolarization assays showed that treatment of the bacteria by antibiotics known to affect the bacterial cell wall synthesis pathway decreased the membrane depolarization effects of epilancin 15X. Disruption of the Lipid II cycle in intact bacteria using several methods led to a decrease in the activity of epilancin 15X. Antagonism-based experiments on 96-well plate and agar diffusion plate pointed towards a possible interaction between epilancin 15X and Lipid II and this was confirmed by Circular Dichroism (CD) based experiments. However, this interaction did not lead to a detectable effect on either carboxyfluorescein (CF) leakage or proton permeability. All experiments point to the involvement of a phosphodiester-containing target within a polyisoprene-based biosynthesis pathway, yet the exact identity of the target remains obscure so far.

Activity of Gut-Derived Nisin-like Lantibiotics against Human Gut Pathogens and Commensals

Recent advances in sequencing techniques unveiled the vast potential of ribosomally synthesized and post-translationally modified peptides (RiPPs) encoded in microbiomes. Class I lantibiotics such as nisin A, widely used as a food preservative, have been investigated for their efficacy in killing pathogens. However, the impact of nisin and nisin-like class I lantibiotics on commensal bacteria residing in the human gut remains unclear. Here, we report six gut-derived class I lantibiotics that are close homologues of nisin, four of which are novel. We applied an improved lantibiotic expression platform to produce and purify these lantibiotics for antimicrobial assays. We determined their minimal inhibitory concentration (MIC) against both Gram-positive human pathogens and gut commensals and profiled the lantibiotic resistance genes in these pathogens and commensals. Structure-activity relationship (SAR) studies with analogs revealed key regions and residues that impact their antimicrobial properties. Our characterization and SAR studies of nisin-like lantibiotics against both pathogens and human gut commensals could shed light on the future development of lantibiotic-based therapeutics and food preservatives.

Redefining Peptide 14D: Substitutional Analysis for Accelerated TB Diagnosis and Enhanced Activity against Mycobacterium tuberculosis

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a predominant cause of mortality, especially in low- and middle-income nations. Recently, antimicrobial peptides have been discovered that at low concentrations could stimulate the growth of M. tuberculosis (hormetic response). In this study, such a peptide was used to investigate the effects on the time to positivity (TTP). A systematic substitution analysis of peptide 14D was synthesized using Spot synthesis technology, resulting in 171 novel peptides. Our findings revealed a spectrum of interactions, with some peptides accelerating M. tuberculosis growth, potentially aiding in faster diagnostics, while others exhibited inhibitory effects. Notably, peptide NH2-wkivfiwrr-CONH2 significantly reduced the TTP by 25 h compared to the wild-type peptide 14D, highlighting its potential in improving TB diagnostics by culture. Several peptides demonstrated potent antimycobacterial activity, with a minimum inhibitory concentration (MIC) of 20 µg/mL against H37Rv and a multidrug-resistant M. tuberculosis strain. Additionally, for two peptides, a strongly diminished formation of cord-like structures was observed, which is indicative of reduced virulence and transmission potential. This study underscores the multifaceted roles of antimicrobial peptides in TB management, from enhancing diagnostic efficiency to offering therapeutic avenues against M. tuberculosis.

Staphylococcus epidermidis bacteriocin A37 kills natural competitors with a unique mechanism of action

Many bacteria produce antimicrobial compounds such as lantibiotics to gain advantage in the competitive natural environments of microbiomes. Epilancins constitute an until now underexplored family of lantibiotics with an unknown ecological role and unresolved mode of action. We discovered production of an epilancin in the nasal isolate Staphylococcus epidermidis A37. Using bioinformatic tools, we found that epilancins are frequently encoded within staphylococcal genomes, highlighting their ecological relevance. We demonstrate that production of epilancin A37 contributes to Staphylococcus epidermidis competition specifically against natural corynebacterial competitors. Combining microbiological approaches with quantitative in vivo and in vitro fluorescence microscopy and cryo-electron tomography, we show that A37 enters the corynebacterial cytoplasm through a partially transmembrane-potential-driven uptake without impairing the cell membrane function. Upon intracellular aggregation, A37 induces the formation of intracellular membrane vesicles, which are heavily loaded with the compound and are essential for the antibacterial activity of the epilancin. Our work sheds light on the ecological role of epilancins for staphylococci mediated by a mode of action previously unknown for lantibiotics.

Investigation into the mechanism of action of the antimicrobial peptide epilancin 15X

Addressing the current antibiotic-resistance challenge would be aided by the identification of compounds with novel mechanisms of action. Epilancin 15X, a lantibiotic produced by Staphylococcus epidermidis 15 × 154, displays antimicrobial activity in the submicromolar range against a subset of pathogenic Gram-positive bacteria. S. epidermidis is a common member of the human skin or mucosal microbiota. We here investigated the mechanism of action of epilancin 15X. The compound is bactericidal against Staphylococcus carnosus as well as Bacillus subtilis and appears to kill these bacteria by membrane disruption. Structure-activity relationship studies using engineered analogs show that its conserved positively charged residues and dehydroamino acids are important for bioactivity, but the N-terminal lactyl group is tolerant of changes. Epilancin 15X treatment negatively affects fatty acid synthesis, RNA translation, and DNA replication and transcription without affecting cell wall biosynthesis. The compound appears localized to the surface of bacteria and is most potent in disrupting the membranes of liposomes composed of negatively charged membrane lipids in a lipid II independent manner. Epilancin 15X does not elicit a LiaRS response in B. subtilis but did upregulate VraRS in S. carnosus. Treatment of S. carnosus or B. subtilis with epilancin 15X resulted in an aggregation phenotype in microscopy experiments. Collectively these studies provide new information on epilancin 15X activity.