| Summary: | Chitosan is a polysaccharide biopolymer with excellent biocompatibility, biodegradability, and low toxicity, which allows for potential wide applications. Recently, antimicrobial activities of chitosan against foodborne pathogens have been studied; many used disk diffusion to determine the activity. However, this method is unable to obtain minimal inhibitory concentrations (MICs), i.e., not quantitative. The objective of this study was to compare disk diffusion with agar dilution and broth microdilution, two quantitative methods used routinely in clinical laboratories, to determine MICs of chitosan against foodborne pathogens. Five chitosan compounds with molecular weights ranging between 43 and 1,100 kDa were tested against 36 representative foodborne pathogens using the three methods. A water-soluble chitosan (43 kDa) was found to be the most effective one against Escherichia coli O157:H7 and Salmonella enterica, especially using the agar dilution method. The overall agreement of MICs (within 2-fold dilution) between agar dilution and broth microdilution was only 14.6% and MICs determined by broth microdilution were generally lower than those obtained by agar dilution. Among all strains tested, Vibrio spp. strains were most susceptible to chitosan whereas Salmonella serovars were least susceptible. The MIC values by either agar dilution or broth microdilution for Vibrio spp. were at least one dilution level lower than those for other bacteria. The effectiveness of chitosan against Vibrio spp. demonstrated in this study may prompt future applications of chitosan to control Vibrio spp. in foods, particularly raw oysters. The variability shown when different susceptibility testing methods were used suggests the need to apply multiple methods when conducting in vitro antimicrobial susceptibility testing of chitosans.
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