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Like other single celled organisms, bacteria are uniquely sensitive to changes in their physical and chemical environment. With only the minimal protection offered by their cell envelope, fluctuations in nutrient availability, pH, osmolarity, and temperature have an immediate impact on diverse aspects of cell physiology as bacteria struggle to adapt to the new condition. On a more extended time scale, nutrient availability is one of the major determinants of bacterial cell morphology. Escherichia coli cells are three times larger when cultured at steady state in nutrient rich conditions than in nutrient poor ones, due to the actions of nutrient-dependent division inhibitors, accelerated lipid synthesis and a concomitant increase in plasma membrane capacity, and other yet to be identified factors. Little is known, however, about the effect of other environmental conditions on bacterial cell morphology. Focusing on one environmental variable, I will discuss how modest changes in pH—too small to affect growth rate—alter the activity of the essential proteins that construct E. coli’s peptidoglycan cell wall, increase resistance to clinically relevant antibiotics, and reduce cell length by as much as 20%. Together this work identifies pH as a significant environmental determinant of bacterial physiology and morphogenesis whose impact is mediated primarily through changes in the cell envelope.

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