Growth ofMycobacterium tuberculosisin a Defined Medium Is Very Restricted by Acid pH and Mg2+Levels

Abstract

Mycobacterium tuberculosisgrows 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 ofM. tuberculosisand seven other mycobacterial species. In a defined medium, the growth ofM. tuberculosiswas very restricted by acidic pH. Higher levels of Mg²⁺were required for growth ofM. tuberculosisin mildly acidic media (pH 6.0 to 6.5) compared to pH 7.0 medium. The divalent cations Ca²⁺, Zn²⁺, or Mn²⁺could not replace Mg²⁺during growth at pH 6.25, but Ca²⁺could at least partially substitute for Mg²⁺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 Mg²⁺levels.M. tuberculosiswas the most restricted in growth at pH 6.0, and all of this growth required elevated levels of Mg²⁺.M. kansasiiandM. smegmatisalso grew very poorly in acidic media with limiting Mg²⁺.M. fortuitum,M. marinum,M. scrofulaceum,M. avium, andM. chelonaegrew at pH 6.0 in an unrestricted manner. These results demonstrate thatM. tuberculosisis unique among the mycobacteria in its extreme sensitivity to acid and indicate thatM. tuberculosismust acquire sufficient Mg²⁺in order to grow in a mildly acidic environment such as within the phagosome of macrophages.