BACILLUS SUBTILIS CELL ENVELOPE STRESS RESPONSES INDUCED BY ANTIBIOTICS AND GENETIC PERTURBATIONS OF MEMBRANE COMPOSITION
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Bacteria thrive in many different habitats, and therefore will encounter a multitude of environmental stresses. Survival requires the development of sophisticated response mechanisms that can identify potential threats and initiate regulatory cascades orchestrating the expression of defensive components. The cell envelope is the first layer of the cell to come in contact with any environmental agent. In Gram-positive bacteria this envelope is comprised of a cytoplasmic membrane surrounded by a thick multilayered cell wall. The cytoplasmic membrane forms an essential permeability barrier and is made of different types of complex lipids which vary not only in the length and modifications of the acylated fatty acid groups but also in the composition of their headgroups. The cell wall is a complex matrix consisting predominantly of equal amounts of peptidoglycan and teichoic acids with the addition of surface attached proteins. Cell wall synthesis and maturation requires both incorporation of new peptidoglycan by penicillin-binding proteins, as well as cleavage of the existing peptidoglycan by autolysins. The work presented here describes the use of external stimuli or genetic manipulations to elucidate cell envelope stress responses. First, I have identified a possible novel post-translational modifier of autolysin activity, which is induced by cell wall-targeting antibiotics. This modifier is proposed to inhibit autolytic enzyme activity under conditions of impaired cell wall synthesis, thereby reducing the rate of antibiotic-dependent cell death. Second, I studied a transcriptional regulator controlling the expression of efflux pumps proposed to be involved in resistance to cell-wall targeting antibiotics. Finally, by genetic perturbation I created a series of B. subtilis strains with altered cytoplasmic membrane composition, and characterized them for growth, antibiotic resistance, morphology, and alterations in global gene expression patterns. B. subtilis retains viability and even rapid growth when the membrane is comprised predominantly, if not exclusively, of one type of complex lipid, suggesting that the cell can tolerate even large changes in membrane composition.