The Peptidoglycan of Stationary-Phase Mycobacterium tuberculosis Predominantly Contains Cross-Links Generated by l,d-Transpeptidation▿

Our understanding of the mechanisms used by Mycobacterium tuberculosis to persist in a “dormant” state is essential to the development of therapies effective in sterilizing tissues. Gene expression profiling in model systems has revealed a complex adaptive response thought to endow M. tuberculosis with the capacity to survive several months of combinatorial antibiotic treatment. We show here that this adaptive response may involve remodeling of the peptidoglycan network by substitution of 4→3 cross-links generated by the d , d -transpeptidase activity of penicillin-binding proteins by 3→3 cross-links generated by a transpeptidase of l,d specificity. A candidate gene, previously shown to be upregulated upon nutrient starvation, was found to encode an l,d -transpeptidase active in the formation of 3→3 cross-links. The enzyme, Ldt Mt1 , was inactivated by carbapenems, a class of β-lactam antibiotics that are poorly hydrolyzed by the M. tuberculosis β-lactamases. Ldt Mt1 and carbapenems may therefore represent a target and a drug family relevant to the eradication of persistent M. tuberculosis .