| Lactobacillus acidophilus is one of probiotics perched in the human gut, when the number of the livebacteria reaches a certain amount, they can play to promote the host health. However, the acidicenvironment food and gastrointestinal tract is a very severe challenges for their survival, acid stress canaffect the survival of Lactobacillus acidophilus and further influence their physiological health functions. Alarge nμmber of studies showed that Lactobacillus acidophilus could tolerate low pH environment, but theacid tolerance mechanism is inconclusive now. This paper studied on the acid tolerance mechanism of twoLactobacillus acidophilus strains isolated by our laboratory, the main results were as follows:The viable cell’s number of Lactobacillus acidophilus in the low pH condition can directly react itsacid resistant capability. The viable count methods were adopted to observe the living bacteria number ofLactobacillus acidophilus Ind-1and Lakcid, the results showed that the viable bacteria counts ofLactobacillus acidophilus were still more then104cfu/mL when cultured at low pH1.5after2h. Althoughthe majority of Lactobacillus acidophilus died when cultured at low pH1.5after6h, some Lactobacillusacidophilus were still alive, the viable bacteria counts of Ind-1was1.55×103cfu/mL and Lakcid was4.23×103cfu/mL. This indicated that Lactobacillus acidophilus Ind-1and Lakcid had strong acid-resistantability.The Change of cell morphology is a common response of Lactobacillus acidophilus to acid stressenvironment, the results of Scanning electron microscope showed that the mycelia morphology ofLactobacillus acidophilus was short rod with rounded ends and thickset thallus at pH values of1.5, but themycelia morphology was long rod with slender thallus at pH values of6.2. Through comparison, it wasfound that the mycelia morphology of Lactobacillus acidophilus changed from long rod to short rod, and the thallus changed from slender to thickset followed with the decrease of pH values.The cell membrane was one of the primary targets attacked by the physicochemical pressure inexternal environment, and the fatty acid composition and proportion changes of cell membrane were thecommon reaction. Eight kinds of special fatty acids were detected in Lactobacillus acidophilus Ind-1andLakcid by GC-MS. In different conditions, the results showed that the total monounsaturated fatty acid(C16:1，C18:1and C18:2) increased respectively from67.9%to70.69%and61.85%to65.06%, and thetotal long chain fatty acid (C18:0，C19:0and C20:0)of Lactobacillus acidophilus Ind-1and Lakcidincreased respectively from6.35%to7.78%and11.9%to13.06%with the decreasement of pH values,while the content of short chain(C14:0and C16:0) fatty acids declined from25.70%to21.53%and26.25%to21.88%. This indicated that Lactobacillus acidophilus could decreased the permeability of H+toprevent the rapid decrease of intracellular pH by increasing the content of monounsaturated fatty acidsand long chain fatty acid and reducing the content of short chain fatty acids, in order to maintain the normalphysiological function of cells.The alkali production enzymes can catalyze arginine and urea to produce ammonia which canneutralize the intracellular H+, in order to maintain the relative stability of intracellular pH values.The study on mechanism of alkali production enzymes suggested that the intracellular argininedecarboxylase activity of Lactobacillus acidophilus increased with the decrease of pH values, comparedwith the most suitable pH6.2, the arginine decarboxylase activity of Ind-1increased by14times and Lakcidincreased nearly7times at pH1.5value. the determination of urase activity suggested that the activity ofurase was lower and changed little at pH1.5value compared with the most suitable pH6.2, and what’s more,it couldn’t improve the living bacteria number of Lactobacillus acidophilus when cultured in in themediμm supplemented with urea, this indicated that the role of urease was not obvious in the process of Lactobacillus acidophilus acid adaptation.The proton pump of Lactic acid bacteria membrane has the capacity for pumping the proton toregulate intracellular pH value. The study on mechanism of proton pμmp suggested that the H+-ATPaseactivity of Ind-1increased with the decrease of pH values, the H+-ATPase activity increased by2timespH1.5value compared with the most suitable pH6.2; however, the H+-ATPase activity of Lakcid didn’tchange significantly, the H+-ATPase activity was highest at pH4.5value and the H+-ATPase activity waslower at pH1.5value and the most suitable pH6.2value. The activity of glutamic acid decarboxylase ofLactobacillus acidophilus Ind-1and Lakcid increased with the decrease of pH values, the activity ofglutamic acid decarboxylase of Lactobacillus acidophilus Ind-1increased by6times and Lakcid increasedby14times at pH1.5value compared with the most suitable pH6.2value, this indicated that Lactobacillusacidophilus could activate the activity of glutamic acid decarboxylase to improve the acid-resistant ability. |
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