Probing differences in gene essentiality between the human and animal adapted lineages of the Mycobacterium tuberculosis complex using TnSeq

Gibson, A.J. and Passmore, I.J. and Faulkner, V. and Xia, D. and Nobeli, Irene and Stiens, J. and Willcocks, S. and Clark, T.G. and Sobkowiak, B. and Werling, D. and Villarreal-Ramos, B. and Wren, B.W. and Kendall, S.L. (2021) Probing differences in gene essentiality between the human and animal adapted lineages of the Mycobacterium tuberculosis complex using TnSeq. Frontiers in Veterinary Science 8 , p. 760717. ISSN 2297-1769.

Abstract

Members of the <i>Mycobacterium tuberculosis</i> complex (MTBC) show distinct host adaptations, preferences and phenotypes despite being >99% identical at the nucleic acid level. Previous studies have explored gene expression changes between the members, however few studies have probed differences in gene essentiality. To better understand the functional impacts of the nucleic acid differences between <i>Mycobacterium bovis</i> and <i>Mycobacterium tuberculosis</i>, we used the Mycomar T7 phagemid delivery system to generate whole genome transposon libraries in laboratory strains of both species and compared the essentiality status of genes during growth under identical <i>in vitro</i> conditions. Libraries contained insertions in 54% of possible TA sites in <i>M. bovis</i> and 40% of those present in <i>M. tuberculosis</i>, achieving similar saturation levels to those previously reported for the MTBC. The distributions of essentiality across the functional categories were similar in both species. 527 genes were found to be essential in <i>M. bovis</i> whereas 477 genes were essential in <i>M. tuberculosis</i> and 370 essential genes were common in both species. CRISPRi was successfully utilised in both species to determine the impacts of silencing genes including <i>wag31</i>, a gene involved in peptidoglycan synthesis and <i>Rv2182c</i>/<i>Mb2204c</i>, a gene involved in glycerophospholipid metabolism. We observed species specific differences in the response to gene silencing, with the inhibition of expression of <i>Mb2204c</i> in <i>M. bovis</i> showing significantly less growth impact than silencing its orthologue (<i>Rv2182c</i>) in <i>M. tuberculosis</i>. Given that glycerophospholipid metabolism is a validated pathway for antimicrobials, our observations suggest that target vulnerability in the animal adapted lineages cannot be assumed to be the same as the human counterpart. This is of relevance for zoonotic tuberculosis as it implies that the development of antimicrobials targeting the human adapted lineage might not necessarily be effective against the animal adapted lineage. The generation of a transposon library and the first reported utilisation of CRISPRi in <i>M. bovis</i> will enable the use of these tools to further probe the genetic basis of survival under disease relevant conditions.

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