Molecular species of two major subclasses of mycolic acids from Mycobacterium phlei, α-mycolic acids (M₁) and dicarboxy mycolic acids (M₃), were separated gas-chromatographically and identified mass-spectrometrically. The mycolic acid compositions of extractable and cell wall bound lipids were markedly influenced by growth temperature. Increasing growth temperature from 20°C to 50°C resulted in an increase in longer chain species of both mycolic acid subclasses with a concomitant decrease in shorter chain homologues. The most abundant molecular species were C₇₆ and C₅₈ of M₁ and M₃ in the 20°C grown cells, while the 50°C grown cells contained C₈₀ in M₁ and C₆₂ in M₃, most abundantly. Changes in mycolic acid composition occurred rapidly after growth temperature was raised from 20°C to 50°C with an increase in C₆₂ and a concomitant decrease in C₅₈. Mass fragmentographic analysis revealed that an increase in total carbon numbers of mycolic acids was caused by the elongation of straight chain alkyl unit, without any changes in α-branch. Changes in the molecular species composition of secondary alcohols presumably derived from the ester mycolic acids were also observed and an increase in longer species (C₂₀-ol-2) with a concomitant decrease in shorter ones (C₁₈-ol-2) was noted as the temperature rose. An increase in the growth temperature also resulted in a decrease in unsaturated fatty acids in extractable lipids.
These observations suggest that mycobacteria alter the molecular species composition of mycolic acid subclasses and phospholipids, in response to growth temperature, to maintain a suitable membrane function.