Responses of Listeria monocytogenes to acid stress and glucose availability monitored by measurements of intracellular pH and viable counts

Physiological aspects of the response of Listeria monocytogenes to acidic conditions and effect of glucose availability were studied by fluorescence ratio-imaging microscopy (FRIM) as compared with traditional viable counts. Three types of experiments were conducted: (i) static with measurements of intracellular pH (pHi) at extracellular pH (pHo) values ranging from pH 3.0 to 6.0 at 0.5 pH unit intervals; (ii) kinetic with monitoring of bacterial responses to changes in the pHo from the value of 6.0 to 4.0 or 3.0; (iii) survival experiments studying bacterial recovery in response to a shift to favourable conditions after a treatment at low pH. All the experiments were performed at three levels of glucose in the medium (0, 1, and 10 mM). Both survival and pHi were greatly affected by pHo and glucose availability with the highest values for CFU and pHi at highest glucose concentration and pHo values in the medium in all trials. A high correlation (R2=0.995) between pHi and CFU counts was observed. The pH gradient started to collapse at pHo 4 and below for trials with glucose in the medium and at pHo 5.5 and below without glucose. A recovery step was proposed after the apparently lethal treatment to assess cell viability by FRIM.