Acid Tolerance Response in Streptococcus mutans Biofilms: Role of Membrane Lipid Adaptations and ATPase Activity
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Abstract
Streptococcus mutans is one of the main etiological factors of dental caries since it has an exceptional capacity for surviving and growing in acidic conditions in the mouth. Nevertheless, its cellular mechanisms of acid tolerance are not fully comprehended. This paper investigated the functions of membrane lipid remodeling and proton ATPase activity in the acid tolerance response (ATR) of S. mutans biofilms. The biofilms were cultivated in a flow-cell system and subjected to lethal (pH 3.5) or sub-lethal (pH 5.5) conditions after 3 hours, with neutral pH (7.5) as a control. Viable counts on blood agar were performed over a 2-hour exposure to determine cell survival. Pre-adaptation to pH 5.5 significantly improved survival at pH 3.5, with 66% survival recorded versus 1% in non-adapted biofilms. The fluorescence microscopy showed an increase in biofilm structural integrity after adaptation to acid. Lipid analysis of the membranes showed that there were significant changes in the fatty acid composition, with increases in the percentages of monounsaturated and long-chain fatty acids under sub-lethal acidic stress. Simultaneously, membrane-bound proton ATPase activity increased, facilitating cytoplasmic pH homeostasis by increasing proton extrusion. A combination of these adaptive responses will ensure the survival of bacteria in recurrent acidic challenges by safeguarding acid-sensitive intracellular elements. The results enhance the knowledge of S. mutans virulence and resistance.
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