| Summary: | 碩士 === 國立宜蘭大學 === 食品科學系碩士班 === 99 === Low molecular weight chitosan (LMWC) and nisin are generally known to possess antibacterial activity and have great potential to be used in food preservation. These two cationic antibacterial agents (CAAs) can inhibit bacterial growth by interacting with the bacterial, anionically charged cell wall. Three strains of S. aureus tested in this study demonstrated an inverse relationship with anionic surface charge and resistance against nisin and LMWC. S. aureus resistances against nisin and LMWC_6 were related negatively and positively, respectively, to their anionic surface charges, as determined by zeta potential (ZP) measurements. Alanine uptake significantly reduced the anionic cell surface charge amount and thus reduced or enhanced resistance against LMWC and nisin, respectively. The hurdle effect was observed when nisin was applied prior to LMWC_6 treatment, where nisin MIC reduced by four fold in the presence of LMWC_6 at a concentration of 250 μg/ml. Besides, uptake of alanine could gradually reduce S. aureus anionic cell surface charge along the sequential batch of culture and significantly enhance their aggregation. The RT-PCR results revealed that iraA was possibly involved in the transportation of alanine into the cell, which could activate dlt operon to
increase the D-alanyl degree of teichoic acid and thus changed the physiological characteristics. The differential expression of cell membrane proteins induced by D-alanine uptake could alter the sensitivity of BCRC 10451 to LMWC and nisin.
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