**1. Introduction**

88 Understanding Tuberculosis – Deciphering the Secret Life of the Bacilli

Yesilkaya, H.; Dale, J. W.; Strachan, N. J. & Forbes, K. J. (2005). Natural transposon

1137.

6732, ISSN 0021-9193.

*Journal of Clinical Microbiology*. Vol. 36, No. 4, (April 1998) pp. 1003-1007, ISSN 0095-

mutagenesis of clinical isolates of *Mycobacterium tuberculosis*: how many genes does a pathogen need? *Journal of Bacteriology*, Vol. 187, No. 19, (October 2005), pp. 6726-

> Bacteria must be able to respond to a host of environmental stresses. The ability is vital if bacteria are to withstand insults both of an external location as well as during infection of an animal host. Many investigators study these responses individually in order to better understand pathogenic processes. Desiccation, response to UV light, heat, and cold stress can be encountered in the external environment. Low oxygen tension, heat stress, oxidative stress, nitrosidative stress, and acidic stress are all environments that can be encountered upon infection *in vivo* of a host. This review focuses on acidity as a stress which can be important in mycobacterial pathogenesis. Acidity can be found in such environments as acidic water and soil and can affect mycobacteria in an animal host. Mycobacteria encounter acidic stress at sites of inflammation and within phagosomes of macrophages. Exposure to acidic stress in the external environment may prime mycobacteria to upregulate genes involved in pathogenesis. Upregulation in response to acidic stress may prime mycobacteria to be more resistant to other stresses and to be more able to persist *in vivo*.
