Understanding Antimicrobial Activity of Chitosan Microparticles against Escherichia coli O157:H7

Introduction: Antibiotics have been fundamental to support human and animal health. However, the essential role of antibiotics has been challenged by the occurrence of antimicrobial resistance (AR). The rise of AR is a tremendous concern, and problems of AR are growing in importance mainly because new antibiotic development is not keeping pace with the emergence of AR. Recently, it has been shown that oral feeding of chitosan microparticles (CMs) reduces Escherichia coli O157:H7 in cattle, suggesting potential use of this agent as an alternative antimicrobial agent, but the mode of action was not clearly understood.

Purpose: The purpose of this study was to understand the underlying mechanisms of antimicrobial activity of CMs against E. coli O157:H7.

Methods: Antimicrobial assay was conducted using CMs at various concentrations at different pH. Live/dead assay was performed to test if bacterial membranes were disrupted by CMs. Gene deletion was performed to identify CMs binding targets using λ-Red mutagenesis system. In vitro CMs binding assay was used to determine the binding activity of CMs to E. coli O157:H7. 

Results: Binding assays and genetic studies with an ompA mutant strain demonstrated that outer membrane protein OmpA of E. coli O157:H7 is critical for CMs binding, and this binding activity is coupled with a bactericidal effect of CMs at neutral pH. In addition, we found that LPS is another binding target of CMs that enhances antimicrobial activity at acidic pH. Furthermore, CMs showed remarkable antimicrobial activity against Gram-negative and Gram-positive pathogens such as E. coli, Salmonella, Vibrio, and Streptococcus, exerting a broad spectrum of activity.

Significance: These data demonstrate how CMs interacts with pathogens to exert antimicrobial activity and we propose that CMs hold potential use as a natural antibiotic to reduce the risk of foodborne pathogens.