MspA provides the main hydrophilic pathway through the cell wall of Mycobacterium smegmatis

Abstract

While analysing the uptake of solutes by porin mutants of Mycobacterium smegmatis we recognized that the permeability coefficients of the wild-type strain and of the ΔmspA mutant for glucose as published in Stahl et al. (2001) are not correct. The concentration of glucose in all uptake experiments was 100-fold lower than described in the article. Thus, the following corrections, which are marked in bold, must be made: On page 457, Fig. 7B: ‘The glucose concentration was 33 µM.’; The last paragraph on page 457 and the first paragraph on page 458 should read: To analyse whether mspA is important for uptake of nutrients by M. smegmatis we examined the accumulation of 14C-labelled glucose by intact cells. Uptake kinetics with 33 µM glucose showed a rapid saturation after about 5 min (Fig. 7B). . . . A series of uptake experiments with glucose concentrations ranging from 20 µM to 0.5 µM was done to determine apparent vmax and Km values of both strains. The effect of mspA deletion on glucose uptake was more pronounced at lower concentrations in agreement with findings for Gram-negative bacteria (Ferenci, 1996). For example, at 0.5 µM glucose, uptake was about 6.7-fold slower in the ΔmspA mutant compared with the wild type (data not shown). Data analysis using the Michaelis–Menten equation yielded vmax and Km values of 13.1 nmol mg−1 min−1 and 29.8 µM, and 3.9 nmol mg−1 min−1 and 47.9 µM for M. smegmatis wild type and the ΔmspA mutant, respectively. Assuming that glucose penetrates into the cell by first diffusing passively through the cell wall and then being actively transported through the cytoplasmic membrane (Jarlier and Nikaido, 1990) one can calculate minimal values for the permeability coefficient P yielding 2.8 × 10−5 cm s−1 and 5.1 × 10−6 cm s−1 for wild-type M. smegmatis and the ΔmspA mutant, respectively. Thus, deletion of mspA reduced the permeability of M. smegmatis for glucose fivefold to a value, which is 3000-fold lower than that determined for E. coli (Bavoil et al. 1977). These data demonstrate that diffusion through MspA is rate-limiting for glucose uptake by M. smegmatis. The permeability coefficient of wild-type M. smegmatis for glucose is higher than 3.7 × 10−6 cm s−1 as calculated from published data using Eq. 1 (Bai et al., 1978). It is obvious that in our study glucose uptake by M. smegmatis increased linearly with time only up to about 2 min and reached much earlier the saturation level. . . . Our results show that uptake of both glucose and cephaloridine by M. smegmatis is faster than that measured for M. chelonae (Jarlier and Nikaido, 1990). On page 463, ‘Glucose uptake measurements’ in Experimental procedures: ‘. . . final concentrations ranging from 0.5 µM to 33 µM.’ On page 451, Abstract: The following sentence should be deleted: ‘The minimal permeability coefficient of the ΔmspA mutant for glucose was 7.2 × 10−8 cm s−1 which is the lowest value reported so far for bacteria.’ We want to underline that these corrections do not affect the conclusions of the article which are based on relative permeability differences between wild-type M. smegmatis and the mspA mutant. A repetition of the glucose uptake experiments for both strains yielded permeability coefficients identical to those corrected above. We sincerely apologize for this error.