Acid stress
Bacteria often encounter a variety of potentially lethal acid stress conditions both in nature and during pathogenicity in a host against which they have abilities to sense and respond to pH stress as a protection mechanism. Acid stress is described as the combined biological effect of H + ion (that is, pH) and weak acids (organic) in the environment as a result of fermentation or when added as preservatives in foods (Zhao and others 1993; Garlant Miller and Kasper 1994). The mechanisms of crobial inactivation by inorganic (that is, HC) and organic acids are different. However, both result in intracellular acidification to levels that damage or disrupt key biochemical processes. Under severe acidic pH (that is, pH 3), proton leakage is faster than the cell's ability to maintain homeostasis. Organic acids penetrate the cell membrane and after dissociation inside the cell, the released proton acidifies intracellular pH. The lower the exterior pH, the greater the influx of organic acids. The membrane-impermeable ionized form of the organic acid accumulates and the constant influx of protons will eventually deplete cellular energy, causing cell death in enterobacteriaceae (Bearson and others 1997).