Discrepancies between Cyclic and Linear Antimicrobial Peptide Actions on the Spectrochemical and Nanomechanical Fingerprints of a Young Biofilm

Quantitative Imaging of the Action of vCPP2319, an Antimicrobial Peptide from a Viral Scaffold, against Staphylococcus aureus Biofilms of a Clinical Isolate

The formation of biofilms is a common virulence factor that makes bacterial infections difficult to treat and a major human health problem. Biofilms are bacterial communities embedded in a self-produced matrix of extracellular polymeric substances (EPS). In this work, we show that vCPP2319, a polycationic peptide derived from the capsid protein of Torque teno douroucouli virus, is active against preformed Staphylococcus aureus biofilms produced by both a reference strain and a clinical strain isolated from a diabetic foot infection, mainly by the killing of biofilm-embedded bacteria. The direct effect of vCPP2319 on bacterial cells was imaged using atomic force and confocal laser scanning microscopy, showing that the peptide induces morphological changes in bacterial cells and membrane disruption. Importantly, vCPP2319 exhibits low toxicity toward human cells and high stability in human serum. Since vCPP2319 has a limited effect on the biofilm EPS matrix itself, we explored a combined effect with α-amylase (EC 3.2.1.1), an EPS matrix-degrading enzyme. In fact, α-amylase decreases the density of S. aureus biofilms by 2.5-fold. Nonetheless, quantitative analysis of bioimaging data shows that vCPP2319 partially restores biofilm compactness after digestion of the polysaccharides, probably due to electrostatic cross-bridging of the matrix nucleic acids, which explains why α-amylase fails to improve the antibacterial action of the peptide.

The structure and matrix dynamics of bacterial biofilms as revealed by antimicrobial peptides' diffusion

From the biological point of view, bacterial biofilms are communities of bacteria embedded in a self-produced gel matrix composed of polysaccharides, DNA, and proteins. Considering the biophysical point of view, the biofilm matrix is a highly dense, crowded medium that imposes constraints to solute diffusion, depending on the size, conformational dynamics, and net charge. From the pharmacological point of view, biofilms are additional difficulties to drug development as heterogeneity in oxygen and nutrient distribution, and consequently, heterogeneity in bacterial metabolic status leads to recalcitrance. For peptide scientists, biofilms are both a challenge and an opportunity. Biofilms can be intruded by peptides, revealing important biological, biophysical, and pharmacological insights. Peptides can be engineered for different sizes, flexibilities, and net charges, unravelling the determinants of diffusion; they kill bacteria by lysis, overcoming the hurdles of metabolic status heterogeneity, and they are able to kill bacteria in the biofilm core, leaving the matrix intact, that is, without causing bacterial biofilm dispersion as side effect. This concise review addresses the knowledge reached while interrogating bacterial biofilms with peptides and other reporter molecules, and the advances therefrom in biology, biophysics, and drug development.

A designed cyclic analogue of gomesin has potent activity against Staphylococcus aureus biofilms

BACKGROUND

Infections caused by bacterial biofilms are very difficult to treat. The use of currently approved antibiotics even at high dosages often fails, making the treatment of these infections very challenging. Novel antimicrobial agents that use distinct mechanisms of action are urgently needed.

OBJECTIVES

To explore the use of [G1K,K8R]cGm, a designed cyclic analogue of the antimicrobial peptide gomesin, as an alternative approach to treat biofilm infections.

METHODS

We studied the activity of [G1K,K8R]cGm against biofilms of Staphylococcus aureus, a pathogen associated with several biofilm-related infections. A combination of atomic force and real-time confocal laser scanning microscopies was used to study the mechanism of action of the peptide.

RESULTS

The peptide demonstrated potent activity against 24 h-preformed biofilms through a concentration-dependent ability to kill biofilm-embedded cells. Mechanistic studies showed that [G1K,K8R]cGm causes morphological changes on bacterial cells and permeabilizes their membranes across the biofilm with a half-time of 65 min. We also tested an analogue of [G1K,K8R]cGm without disulphide bonds, and a linear unfolded analogue, and found both to be inactive.

CONCLUSIONS

The results suggest that the 3D structure of [G1K,K8R]cGm and its stabilization by disulphide bonds are essential for its antibacterial and antibiofilm activities. Moreover, our findings support the potential application of this stable cyclic antimicrobial peptide to fight bacterial biofilms.

Harvesting of Antimicrobial Peptides from Insect (Hermetia illucens) and Its Applications in the Food Packaging

About one-third of the total food produced is wasted, rising the concern to adopt proper management. Simultaneously with the increase in population, demand for food is increasing which may lead to scarcity. Adequate packaging is one of the ways to avoid deterioration of food and prevent wastage. In recent years, active packaging has attained interest due to its commendable results in food preservation. Several studies proved that the embodiment of antimicrobial components into the packaging material has the ability to prevent microbial contamination. Antimicrobial peptides (AMP) are newly discovered antimicrobial agents for impregnation into packaging material. Among various sources for AMP, insects have shown great resistivity against a wide spectrum of microorganisms. Insects feed on substances consisting of a varying range of contaminations, which often results in infections. Insects synthesise AMPs to fight such infections and survive in that atmosphere. The disease-causing agents in humans are the same as those found in insects. Hence, AMPs extracted from insects have the potential to fight the microorganisms that act as hazards to human health. This review highlights the harvesting and synthesis of AMPs from Hermetia illucens, which is a promising source for AMP and its applications in the food packaging industry.

Extracellular Polymeric Substance Protects Some Cells in an Escherichia coli Biofilm from the Biomechanical Consequences of Treatment with Magainin 2

Bacterial biofilms have long been recognized as a source of persistent infections and industrial contamination with their intransigence generally attributed to their protective layer of extracellular polymeric substances (EPS). EPS, consisting of secreted nucleic acids, proteins, and polysaccharides, make it difficult to fully eliminate biofilms by conventional chemical or physical means. Since most bacteria are capable of forming biofilms, understanding how biofilms respond to new antibiotic compounds and components of the immune system has important ramifications. Antimicrobial peptides (AMPs) are both potential novel antibiotic compounds and part of the immune response in many different organisms. Here, we use atomic force microscopy to investigate the biomechanical changes that occur in individual cells when a biofilm is exposed to the AMP magainin 2 (MAG2), which acts by permeabilizing bacterial membranes. While MAG2 is able to prevent biofilm initiation, cells in an established biofilm can withstand exposure to high concentrations of MAG2. Treated cells in the biofilm are classified into two distinct populations after treatment: one population of cells is indistinguishable from untreated cells, maintaining cellular turgor pressure and a smooth outer surface, and the second population of cells are softer than untreated cells and have a rough outer surface after treatment. Notably, the latter population is similar to planktonic cells treated with MAG2. The EPS likely reduces the local MAG2 concentration around the stiffer cells since once the EPS was enzymatically removed, all cells became softer and had rough outer surfaces. Thus, while MAG2 appears to have the same mechanism of action in biofilm cells as in planktonic ones, MAG2 cannot eradicate a biofilm unless coupled with the removal of the EPS.

Ultrastructural changes in methicillin-resistant Staphylococcus aureus (MRSA) induced by a novel cyclic peptide ASP-1 from Bacillus subtilis: A scanning electron microscopy (SEM) study

Increasing antimicrobial resistance among Staphylococcus aureus necessitates a new antimicrobial with a different site of action. We have isolated a novel cyclic peptide-1 (ASP-1) from Bacillussubtilis with potent activity against methicillin-resistant S. aureus (MRSA) at a minimum inhibitory concentration (MIC) of 8-64μg/ml. Scanning electron micrographs demonstrated drastic changes in the cellular architecture of ASP-1 treated cells of S. aureus ATCC 29213 and an MRSA clinical isolate at MICs, with damages to the cell wall, membrane lysis and probable leakage of cytoplasmic contents at minimum bactericidal concentrations. The ultrastructure alterations induced by ASP-1 have also been compared with those of oxacillin-treated MRSA cells at its MIC using scanning electron microscopy.

Parietal Structures of Escherichia coli Can Impact the D-Cateslytin Antibacterial Activity

Bacterial resistance to conventional antibiotics is of major concern. Antimicrobial peptides (AMPs) are considered excellent alternatives. Among them, D-cateslytin (D-Ctl, derivative of a host defense peptide) has shown high efficiency against a broad spectrum of bacteria. The first target of AMPs is the outer membrane of the bacterium. However, the role of bacterial cell-wall structures on D-Ctl's mechanism of action has not yet been understood. In this study, we investigated the activity of D-Ctl on two isogenic strains of E. coli: one is devoid of any parietal structures; the other constitutively overexpresses only type 1 fimbriae. We studied the damage caused by D-Ctl at several initial concentrations of bacteria and D-Ctl, and exposure times to D-Ctl were examined using a combination of epifluorescence microscopy, atomic force microscopy (AFM), and Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR). The analysis of nanomechanical and spectrochemical properties related to the antibacterial mechanism showed a concentration dependent activity. Whereas the membrane permeabilization was evidenced for all concentrations of D-Ctl and both mutants, no pore formation was observed. The bacterial stiffness is modified dramatically concomitantly to major membrane damage and changes in the spectral fingerprints of the bacteria. In the case of the occurrence of type 1 fimbriae only, an intracellular activity was additionally detected. Our results evidenced that D-Ctl activity is highly impacted by the cell-wall external structures and surface properties of the bacteria.

Chemical Functionalization of the Zinc Selenide Surface and Its Impact on Lactobacillus rhamnosus GG Biofilms

Bacteria grow on surfaces and form communities called biofilms. Bacterial adhesion and properties of the derived biofilms depend on, among others, the nature of the supporting substrate. Here, we report how the surface properties of the substrate affect the biofilm growth of probiotic Lactobacillus rhamnosus GG (LGG). Hydrophilic (OH), hydrophobic (CH₃), and positively charged (NH₃⁺) surfaces were obtained by the functionalization of a ZnSe crystal with alkanethiol self-assembled monolayers (SAM). The self-assembly of alkanethiols onto ZnSe was studied in situ using infrared spectroscopy in attenuated total reflection mode (ATR-FTIR). The organization of grafted SAMs was analyzed based on the results of ATR-FTIR, high-energy elastic backscattering spectrometry, and contact angle measurements. The kinetics and adhesion strength of LGG initial attachment as well as its physiological state on surfaces terminated by the different functional groups were assessed by the combination of ATR-FTIR, force measurements based on atomic force microscopy, and fluorescent staining of bacteria. The strength of interactions between LGG and the surface was strongly affected by the terminal group of the alkanethiol chain. The -NH₃⁺ groups displayed the highest affinity with LGG at the first stage of interaction. The surface properties also played an important role when LGG biofilms were further grown in a nutritive medium for 24 h under flow conditions. Notably, the analysis of the infrared spectra recorded during the biofilm cultivation revealed differences in the kinetics of growth and in the polysaccharide features of the biofilm depending on the substrate functionality. LGG biofilm was stable only on the positively charged surface upon rinsing. Findings of this work clearly show that the adhesion features and the growth of LGG biofilms are substrate-dependent.

In situ spectroscopic analysis of Lactobacillus rhamnosus GG flow on an abiotic surface reveals a role for nutrients in biofilm development

In this work, infrared spectroscopy was used to monitor the changes in the biochemical composition of biofilms of the probiotic bacterium Lactobacillus rhamnosus GG (LGG) in three nutritive media (10-fold diluted MRS, AOAC, and mTSB), in situ and under flow conditions. Epifluorescence microscopy was used to observe the shape of LGG cells and their distribution on the surface. Spectroscopic fingerprints recorded as a function of time revealed a medium-dependent content of nucleic acids, phospholipids and polysaccharides in the biofilms. In addition, time-dependent synthesis of lactic acid was observed in MRS/10 and AOAC/10. Polysaccharides were produced to the highest extent in mTSB/10, and the biofilms obtained were the densest in this medium. The rod shape of the cells was preserved in MRS/10, whereas acidic stress induced in AOAC/10 and the nutritional quality of mTSB/10 led to strong morphological changes. These alterations due to the nutritive environment are important to consider in research and use of LGG biofilms.

Insect Antimicrobial Peptides, a Mini Review

Antimicrobial peptides (AMPs) are crucial effectors of the innate immune system. They provide the first line of defense against a variety of pathogens. AMPs display synergistic effects with conventional antibiotics, and thus present the potential for combined therapies. Insects are extremely resistant to bacterial infections. Insect AMPs are cationic and comprise less than 100 amino acids. These insect peptides exhibit an antimicrobial effect by disrupting the microbial membrane and do not easily allow microbes to develop drug resistance. Currently, membrane mechanisms underlying the antimicrobial effects of AMPs are proposed by different modes: the barrel-stave mode, toroidal-pore, carpet, and disordered toroidal-pore are the typical modes. Positive charge quantity, hydrophobic property and the secondary structure of the peptide are important for the antibacterial activity of AMPs. At present, several structural families of AMPs from insects are known (defensins, cecropins, drosocins, attacins, diptericins, ponericins, metchnikowins, and melittin), but new AMPs are frequently discovered. We reviewed the biological effects of the major insect AMPs. This review will provide further information that facilitates the study of insect AMPs and shed some light on novel microbicides.

The inhibitory effect of the combination of two new peptides on biofilm formation by Acinetobacter baumannii

The emergence of extensively drug-resistant (XDR) Acinetobacter baumannii strains and the limited number of efficacious antibiotics demonstrate an urgent need to develop novel agents to treat infections caused by this dangerous pathogen. To find antimicrobial peptides against A. baumannii growing either in planktonic or in biofilm mode, biopanning was carried out with a peptide library on five XDR A. baumannii strains grown in the medium containing human blood (blood biopanning) and biofilms formed by these strains (biofilm biopanning). Two groups of peptides were identified, among which two peptides N10 (from blood biopanning) and NB2 (from biofilm biopanning) were selected and synthesized for more assessments. The selected peptides showed significant binding to A. baumannii rather than to the human cell line Caco-2. Both peptides were effective against A. baumannii and showed antibacterial activities (minimum inhibitory concentration (MIC) 500 μg/ml). In the biofilm inhibition assay, NB2 reduced biofilm more efficiently (75%) than N10 (50%). The combination of the two peptides could function better than each peptide alone to prevent biofilm formation by A. baumannii. Supplementation of conventional therapy with a mixture of peptides targeting A. baumannii or using peptides to deliver antibiotics specifically to the site of infection may be promising to control A. baumannii-related diseases.