PERMEABILITY BARRIER TO HYDROPHILIC SOLUTES IN MYCOBACTERIUM-CHELONEI

In order to define the permeability barrier to hydrophilic molecules in mycobacteria, we used as a model a smooth, β-lactamase-producing strain of Mycobacterium chelonei. The rates of hydrolysis of eight cephalosporins by intact and sonicated cells were measured, and the permeability coefficient (P) was calculated from these rates by the method of Zimmermann and Rosselet (W. Zimmermann and A. Rosselet, Antimicrob. Agents Chemother. 12: 368-372, 1977). P ranged from (0.9 ± 0.3) x 10-8 (benzothienylcephalosporin) to (10 ± 3.3) x 10-8 cm/s (cephaloridine); i.e., the P values were lower than those reported for Pseudomonas aeruginosa and Escherichia coli by 1 and 3 orders of magnitude, respectively. The permeability barrier was shown to reduce drastically the stream of drug molecules entering the cell, allowing the rather low level of β-lactamase (0.1 U/mg of protein with penicillin G) to decrease radically the concentration of the drug at the target; this explains the poor in vitro activities of the β-lactams against M. chelonei. We also estimated P for small, hydrophilic molecules (glucose, glycerol, glycine, leucine), by studying their uptake kinetics. The values found, ranging from 15 x 10-8 to 490 x 10-8 cm/s, were consistent again with a very low permeability of M. chelonei cell wall. The permeation of cephalosporins was not very dependent on the hydrophobicity of the molecules or on the temperature, suggesting a hydrophilic pathway of penetration for the molecules.