Mycobacteria tolerate carbon monoxide by remodelling their respiratory chain

Abstract

18Carbon monoxide (CO) is a gas infamous for its acute toxicity. The toxicity of CO 19 predominantly stems from its tendency to form carbonyl complexes with transition 20 metals, thus inhibiting the heme-prosthetic groups of proteins, including the terminal 21 oxidases of the respiratory chain. While CO has been proposed as an antibacterial 22 agent, the evidence supporting its toxicity towards bacteria is equivocal, and its 23 cellular targets remain poorly defined. In this work, we investigate the physiological 24 response of mycobacteria to CO. We show that Mycobacterium smegmatis is highly 25 resistant to the toxic effects of CO, exhibiting normal growth parameters when 26 cultured in its presence. We profiled the proteome of M. smegmatis during growth in 27 CO, identifying strong induction of cytochrome bd oxidase and members of the dos 28 regulon, but relatively few other changes. We show that the activity of cytochrome bd 29 oxidase is resistant to CO, whereas cytochrome bcc-aa 3 oxidase is strongly inhibited 30 by this gas. Consistent with these findings, growth analysis shows that M. smegmatis 31 lacking cytochrome bd oxidase displays a significant growth defect in the presence 32 of CO, while induction of the dos regulon appears to be unimportant for adaption to 33 CO. Altogether, our findings suggest that M. smegmatis has considerable resistance 34 to CO and benefits from respiratory flexibility to withstand its inhibitory effects. 35 36 Importance 37 Carbon monoxide has an infamous reputation as a toxic gas and it has been 38 suggested that it has potential as an antibacterial agent. Despite this, the means by 39 which bacteria resist its toxic effects are not well understood. In this study we 40 determine the physiological response of Mycobacterium smegmatis to growth in CO. 41 We show for the first time that the cytochrome bd oxidase is inherently resistant to 42 CO and is deployed by M. smegmatis to tolerate the presence of this gas. Further, 43we show that aside from this remodelling of its respiratory chain, M. smegmatis 44 makes few other functional changes to its proteome, suggesting it has a high level of 45 inherent resistance to CO. 46 47