The adaptative acid tolerance of Shigella flexneri strain 307

The acid stress responses of the enteric pathogen S. flexneri strain 307, a clinical isolate from Malaysia was studied. Cells were cultured to various phases of growth, harvested and challenged in nutrient broth acidified to pH 2.5, a pH that was known to kill common bacteria. This strain exhibits a growth phase-dependent acid resistance mechanism with the onset of acid resistance system beginning at the late exponential growth phase that persists throughout the stationary phase of growth. The degree of resistance further increased as the cells mature into stationary phase and peaks at the late stationary phase (24^th-hour). Another acid stress mechanism was identified for this strain, known as acid tolerance response. This tolerance response was studied by pre-adapting mid exponential cells in a mild acidic environment (pH 4.5) for 90 minutes and further challenging in a more extreme acidic environment (pH 3.0) for at least two hours. It was also found that cells require de novo protein synthesis for this adaptive tolerance since the impedance of protein production during the adaptation period renders the cells sensitive to subsequent acid challenge.