Numerous studies on the antibacterial activity of chitosan and its oligomers have been carried out, but controversial evidences on a correlation between antibacterial activity and chitosan MW have been reported. It was claimed [12] that 10 kDa is the minimum MW required for microorganism inhibition; this difference may be explained by the higherpH (6.00) used by the authors, which appears to negatively affect the antimicrobial effect, where an increase in pH leads to a decrease of the antibacterial action, since fewer amino groups in chitosan molecules will be free, as they become ionized at pH below 6 [2, 18]. However, Zheng and Zhu [42] (using similar conditions to those described in this paper; e.g., pH,) reported that a mixture of 0.25% (w/v) chitooligosaccharides with MW <5 kDa yielded the highest inhibition over E. coli, whereas a 305-kDa fraction exhibited the highest effect against S. aureus at a similarconcentration. Antibacterial activity over E. coli brought about by oligomers (i.e., trimers to hexamers) was also reported, even at 0.01% (w/v) [11], whereas 2.2-kDa oligomers were claimed to cause little effect on microbial growth [36].
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