Evaluation of the Antimicrobial Efficacy of N-Acetyl-l-Cysteine, Rhamnolipids, and Usnic Acid—Novel Approaches to Fight Food-Borne Pathogens

Abstract

In the food industry, the increasing antimicrobial resistance of food-borne pathogens to conventional sanitizers poses the risk of food contamination and a decrease in product quality and safety. Therefore, we explored alternative antimicrobials N-acetyl-L-cysteine (NAC), rhamnolipids (RLs), and usnic acid (UA) as a novel approach to prevent biofilm formation and reduce existing biofilms formed by important food-borne pathogens (three strains of Salmonella enterica and two strains of Escherichia coli, Listeria monocytogenes, Staphylococcus aureus). Their effectiveness was evaluated by determining minimum inhibitory concentrations needed for inhibition of bacterial growth, biofilm formation, metabolic activity, and biofilm reduction. Transmission electron microscopy and confocal scanning laser microscopy followed by image analysis were used to visualize and quantify the impact of tested substances on both planktonic and biofilm-associated cells. The in vitro cytotoxicity of the substances was determined as a half-maximal inhibitory concentration in five different cell lines. The results indicate relatively low cytotoxic effects of NAC in comparison to RLs and UA. In addition, NAC inhibited bacterial growth for all strains, while RLs showed overall lower inhibition and UA inhibited only the growth of Gram-positive bacteria. Even though tested substances did not remove the biofilms, NAC represents a promising tool in biofilm prevention.

In Vitro Antibiofilm Effect of N-Acetyl-L-cysteine/Dry Propolis Extract Combination on Bacterial Pathogens Isolated from Upper Respiratory Tract Infections

“…Although Gram-positive bacteria had lower MIC (<5 mg/mL) compared to Gram-negative bacteria (≥7.5 mg/mL), no statistically significant difference was found between these two groups of bacteria. Chlumsky et al also concluded that the antimicrobial and antibiofilm effects of NAC are not related to cell wall structure, as NAC inhibits both Gram-positive and Gram-negative bacteria [28].…”

Successfully treating biofilms of extensively drug resistant Klebsiella pneumoniae isolates from hospital patients with N-Acetyl Cysteine

“…Recently use of NAC as an antibacterial and antibiofilm agent has gained tremendous interest. Some studies have successfully used NAC to demonstrate its in vitro efficacy against planktonically grown as well as biofilms of laboratory strains of bacteria (Li et al, 2020; Aiyer et al, 2021; Chlumsky et al, 2021; Tenório et al, 2021). The antibiofilm activity of NAC is attributed to different aspects such as inhibition of EPS production, bacterial adhesion inhibition, competitive inhibition of cysteine in bacteria, interaction with bacterial proteins via thiol groups, and liquification of biofilm matrix (Zhao et al, 2010; Costa et al,2017; Manoharan et al,2020; Guerini et al,2020; Guerini et al, 2022).…”