Growth of Mycobacterium tuberculosis in a Defined Medium Is Very Restricted by Acid pH and Mg(2+) Levels

Mycobacterium tuberculosis grows within the phagocytic vacuoles of macrophages, where it encounters a moderately acidic and possibly nutrient-restricted environment. Other mycobacterial species encounter acidic conditions in soil and aquatic environments. We have evaluated the influence of pH and divalent cation levels on the growth of M. tuberculosis and seven other mycobacterial species. In a defined medium, the growth of M. tuberculosis was very restricted by acidic pH. Higher levels of Mg(2+) were required for growth of M. tuberculosis in mildly acidic media (pH 6.0 to 6.5) compared to pH 7.0 medium. The divalent cations Ca(2+), Zn(2+), or Mn(2+) could not replace Mg(2+) during growth at pH 6.25, but Ca(2+) could at least partially substitute for Mg(2+) during growth at pH 7.0. Among eight species of mycobacteria tested, there was a diversity of growth rates in media with acidic pH and low Mg(2+) levels. M. tuberculosis was the most restricted in growth at pH 6.0, and all of this growth required elevated levels of Mg(2+). M. kansasii and M. smegmatis also grew very poorly in acidic media with limiting Mg(2+). M. fortuitum, M. marinum, M. scrofulaceum, M. avium, and M. chelonae grew at pH 6.0 in an unrestricted manner. These results demonstrate that M. tuberculosis is unique among the mycobacteria in its extreme sensitivity to acid and indicate that M. tuberculosis must acquire sufficient Mg(2+) in order to grow in a mildly acidic environment such as within the phagosome of macrophages.

结核分枝杆菌（Mycobacterium tuberculosis）可在巨噬细胞的吞噬泡内增殖，在此微环境中面临中等酸性且可能存在营养限制的生长条件。其余分枝杆菌属物种则会在土壤与水生环境中遭遇酸性条件。本研究评估了pH值与二价阳离子水平对结核分枝杆菌及另外7种分枝杆菌属物种生长的影响。在限定化学成分培养基中，酸性pH会显著抑制结核分枝杆菌的增殖。相较于pH 7.0的培养基，结核分枝杆菌在弱酸性培养基（pH 6.0至6.5）中生长所需的Mg²+浓度更高。在pH 6.25的环境下增殖时，Ca²+、Zn²+或Mn²+无法替代Mg²+的作用，但在pH 7.0的环境中，Ca²+可至少部分替代Mg²+的功能。在所测试的8种分枝杆菌中，在酸性pH且低Mg²+浓度的培养基内，各物种的生长速率存在显著差异。结核分枝杆菌在pH 6.0的环境下生长受抑制程度最为显著，且其增殖过程需要升高浓度的Mg²+。堪萨斯分枝杆菌（M. kansasii）与耻垢分枝杆菌（M. smegmatis）在Mg²+有限的酸性培养基中同样生长状况极差。偶发分枝杆菌（M. fortuitum）、海分枝杆菌（M. marinum）、瘰疬分枝杆菌（M. scrofulaceum）、鸟分枝杆菌（M. avium）以及龟分枝杆菌（M. chelonae）可在pH 6.0的培养基中不受限制地增殖。上述结果表明，结核分枝杆菌在分枝杆菌属中具有独特性，其对酸性环境极度敏感；同时研究显示，结核分枝杆菌必须摄取足够量的Mg²+，才能在巨噬细胞吞噬泡这类弱酸性环境中完成增殖。