Evaluation of diffusion and dilution methods to determine the antimicrobial activity of water-soluble chitosan derivatives

Abstract

Aims: Chitosan has gained wide applications in the food industry and biomedical field owing to its biodegradability, biocompatibility, nontoxicity and its antimicrobial activity against a wide spectrum of micro-organisms. However, the methods used to investigate antimicrobial effects of chitosan vary considerably among studies, making comparisons difficult. Methods and Results: One diffusion (disc diffusion) and two dilution (agar dilution and broth microdilution) methods commonly used in clinical laboratories to assess microbial susceptibility/resistance to antimicrobial agents were comparatively used to determine the antimicrobial activity of two watersoluble chitosan derivatives (molecular weights of 43 and 67 kDa) against 31 representative foodborne pathogens. When tested at 1Á6% for the 43-kDa chitosan and 3Á2% for the 67-kDa chitosan, by disc diffusion, approximately 10-to 11-mm-diameter inhibition zones were observed for all of the bacterial groups, except for Salmonella tested for the 67-kDa chitosan where no inhibition zone was observed. By agar dilution and broth microdilution, the minimal inhibitory concentration (MIC) values varied largely dependent upon the molecular weight of chitosan, bacterial genus/species and the testing method. The agreement between MIC values obtained by the two methods was poor, with broth microdilution generally having lower MIC values than agar dilution. Regardless of the testing method, Salmonella strains were the least susceptible among Gram-negative strains for both chitosans, followed by Escherichia coli and Vibrio. Conclusions: Besides chitosan's molecular weight and bacterial genus/species, the antimicrobial activity of chitosan was also influenced largely by the susceptibility testing method used. Significance and Impact of the Study: This is the first study that comparatively evaluated these diffusion and dilution methods, particularly two quantitative methods (agar dilution and broth microdilution), to assess the antimicrobial activity of two water-soluble chitosans against a large number of foodborne pathogens. The study highlights the need for standardized methods to be used in evaluating chitosan's antimicrobial properties in future studies.

Evaluation of dietary chitosan effects on growth performance, immunity, body composition and histopathology of Nile tilapia (Oreochromis niloticus) as well as the resistance toStreptococcus agalactiaeinfection

“…It was found that chitosan was more effective than some traditional antibiotics used as amoxicillin, chloramphenicol and sulfamethoxazole, and those ofAsan-Ozusaglam et al (2016) supported this result. There were strong variations in chitosan antimicrobial activity results which is supported by many findings revealing that the antimicrobial activity of chitosan is dependent upon the physicochemical nature of chitosan, the conditions of the chitosan production process, the testing method and bacterial genus/species tested(Jiang et al, 2012;Younes et al, 2014).As it was noticed, chitosan exhibited very similar activities against fish pathogenic bacteria in a range of 13.02-13.88 mm. There were strong variations in chitosan antimicrobial activity results which is supported by many findings revealing that the antimicrobial activity of chitosan is dependent upon the physicochemical nature of chitosan, the conditions of the chitosan production process, the testing method and bacterial genus/species tested(Jiang et al, 2012;Younes et al, 2014).As it was noticed, chitosan exhibited very similar activities against fish pathogenic bacteria in a range of 13.02-13.88 mm.…”

Physicomechanical and antibacterial properties of experimental resin‐based dental sealants modified with nylon‐6 and chitosan nanofibers

“…However, the reports of its antimicrobial activity vary, sometimes with contradictory findings. The type of chitosan (e.g., molecular weight, degree of deacetylation) and other external factors (e.g., target organism, test methodology) may explain the unpredictability of its antimicrobial activity. As indicated in our results, none of the experimental resin‐based sealants and controls indicated halos of inhibition surrounding the discs or inhibition by contact against S. mutans .…”

Antimicrobial activities of high molecular weight water‐soluble chitosans against selected gram‐negative and gram‐positive foodborne pathogens

“…For instance, Jiang et al . () reported no inhibition zone determined by a disc diffusion method for Salmonella at 24‐h exposure to 67 kDa chitosan. Rodríguez‐Núñez et al .…”

“…Inconsistent antimicrobial activities of chitosans against S. Typhimurium have been reported. For instance, Jiang et al (2013) reported no inhibition zone determined by a disc diffusion method for Salmonella at 24-h exposure to 67 kDa chitosan. Rodr ıguez-N uñez et al (2012) observed a 2 Log CFU mL À1 (from 6.4 to 4.0) reduction of S. Typhimurium subjected to 0.1% chitosan with approximately 135 kDa.…”