Giles Hartman and colleagues at the University of Bristol (U.K.) believe that they may have identified a target and devised an assay with which to screen potential drugs. They presented their findings at the 40th Interscience Conference on Antimicrobial Agents and Chemotherapy in Toronto.

No virulence factors or exotoxins have ever been identified in mycobacteria, baffling researchers as to how the organism gains such a stranglehold on the host. But growing evidence points to two heat-shock proteins, GroEL and GroES, believed to be critical to the survival of the bacteria within host macrophages. These chaperonins, which function to ensure correct protein folding, may play a pivotal role. “Mycobacterium tuberculosis is predominantly intracellular—it doesn’t exist for a great deal of time outside of cells,” says Hartman. “One of the most striking things is that [these] heat shock proteins are consistently upregulated because their intracellular milieu is stressful—they must evade all the intracellular defense mechanisms of the macrophage.”

The Bristol group is investigating a compound called guanidine hydrochloride (HCl) that can inhibit the activity of these chaperonins. Their spectrophotometric assay uses malonate dehydrogenase (MDH) and either guanidine HCl or guanidinium-containing moxonidine and relies on changes in optical density at 340 nm to measure the extent to which the chaperonin complex restores MDH to its correct structure. In the absence of moxonidine, the Gro chaperonins triggered the refolding of denatured MDH. But when either moxonidine or guanidine HCl was present, the chaperonins could not function. In theory, knocking out these chaperonins should fatally cripple M. tuberculosis as it tries to establish infection in the lungs.

The researchers plan to use the assay as a drug screen to identify inhibitors specific to M. tuberculosis that are nontoxic to humans. Since many drugs contain guanidinium moieties, Hartman doesn’t believe toxicity will be an obstacle. “Only those organisms that are highly stressed or intracellular should be affected by a chaperonin inhibitor,” he says.