Deletion of the histone-like protein (Hlp) from Mycobacterium smegmatis results in increased sensitivity to UV exposure, freezing and isoniazid.

Adaptation to environmental stress is an important survival characteristic of any bacterial species. As a soil-dwelling saprophyte, Mycobacterium smegmatis is exposed to factors such as UV light and rounds of freezing and thawing that occur in temperate climates. Numerous studies in Escherichia coli have linked histone-like proteins to stress resistance and adaptation. We hypothesized that the 'histone-like' protein Hlp might likewise be involved in the stress response of M. smegmatis. The hlp gene was inactivated and the M. smegmatis Δhlp strain was found to be more susceptible to UV light and to the stress created by repeated cycles of freezing and thawing. In addition, loss of Hlp altered the colony morphology and allowed the organism to grow dispersed in the absence of a detergent, suggesting changes in the cell wall composition. As cell wall changes could affect permeability to certain antibiotics, the susceptibility of M. smegmatis Δhlp to kanamycin, rifamipicin, ethambutol and isoniazid (INH) was tested. M. smegmatis Δhlp was more susceptible to INH, but loss of Hlp did not affect susceptibility to the other antibiotics tested. This suggests that the increased sensitivity of M. smegmatis Δhlp to INH was unlikely to be the result of alterations in cell permeability.