Abstract
The increased resistance of bacteria against conventional pharmaceutical solutions, the antibiotics, has raised serious health concerns. This has stimulated interest in the development of bio-based therapeutics with limited resistance, namely, essential oils (EOs) or antimicrobial peptides (AMPs). This study envisaged the evaluation of the antimicrobial efficacy of selected biomolecules, namely LL37, pexiganan, tea tree oil (TTO), cinnamon leaf oil (CLO) and niaouli oil (NO), against four bacteria commonly associated to nosocomial infections: Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa. The antibiotic vancomycin and silver nanoparticles (AgNPs) were used as control compounds for comparison purposes. The biomolecules were initially screened for their antibacterial efficacy using the agar-diffusion test, followed by the determination of minimal inhibitory concentrations (MICs), kill-time kinetics and the evaluation of the cell morphology upon 24 h exposure. All agents were effective against the selected bacteria. Interestingly, the AgNPs required a higher concentration (4000–1250 μg/mL) to induce the same effects as the AMPs (500–7.8 μg/mL) or EOs (365.2–19.7 μg/mL). Pexiganan and CLO were the most effective biomolecules, requiring lower concentrations to kill both Gram-positive and Gram-negative bacteria (62.5–7.8 μg/mL and 39.3–19.7 μg/mL, respectively), within a short period of time (averaging 2 h 15 min for all bacteria). Most biomolecules apparently disrupted the bacteria membrane stability due to the observed cell morphology deformation and by effecting on the intracellular space. AMPs were observed to induce morphological deformations and cellular content release, while EOs were seen to split and completely envelope bacteria. Data unraveled more of the potential of these new biomolecules as replacements for the conventional antibiotics and allowed us to take a step forward in the understanding of their mechanisms of action against infection-related bacteria.
Highlights
Bacterial growth can be inhibited by antimicrobial agents, causing disruption of vital cellular functions resulting in rapid cell death
In Gram-positive bacteria, cytosol is enveloped by one bilayer membrane, the cytoplasmic membrane, attached to a thick layer of peptidoglycans, formed of linear polysaccharide chains cross-linked by short peptides that generate a three-dimensional (3D) rigid structure
Initial screening of the antimicrobial activity of the investigated agents was studied against the four microorganisms using the agar-well diffusion test, which showed the presence and absence of zones of inhibition (ZoI) (Table 1)
Summary
Bacterial growth can be inhibited by antimicrobial agents, causing disruption of vital cellular functions resulting in rapid cell death. The architecture and molecular components of the cell peripheral wall differ between Gram-positive and Gram-negative bacteria, in what concerns membrane and cell wall structure and disposition [1,2] The latter is more complex, containing two distinct lipid membranes, the cytoplasmic cell membrane and the outer membrane, with a thin. In Gram-positive bacteria, cytosol is enveloped by one bilayer membrane, the cytoplasmic membrane, attached to a thick layer of peptidoglycans, formed of linear polysaccharide chains cross-linked by short peptides that generate a three-dimensional (3D) rigid structure. In this case, lipoteichoic acids are adhered to the peptidoglycan layer. These components provide the bacterial membrane with an amphiphilic and anionic character [2,3,4]
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