Listeria innocua and Lactobacillus delbrueckii subsp. bulgaricus employ different strategies to cope with acid stress

Abstract

Responses of Listeria innocua and Lactobacillus delbrueckii subsp. bulgaricus to a rapid change in extracellular pH (pH ex) from pH ex 6 to a range of concentrations down to pH ex 3.0 were examined, using HCl and lactic acid (LA) as acidulants. A new fluorescent probe 5-(and-6)-carboxy-2′, 7′-dichlorofluorescein diacetate succinimidyl ester (CDCFDA-SE) was employed that enabled reliable measurements of intracellular pH (pH i) to a minimum pH i of 4.0. Changes in pH i and H + fluxes from immobilised bacteria were measured using fluorescence ratio imaging microscopy (FRIM) and a non-invasive ion flux measuring technique (MIFE), respectively. L. innocua maintained a relatively constant pH i of 5.5–6.1 at pH ex 4 and 5 via H + extrusion. In contrast, L. delbrueckii subsp. bulgaricus progressively lowered pH i towards pH ex over the entire pH ex range examined. The type of acidulant used influenced pH regulation with both pH i and H +-fluxes being more severely affected by LA compared to HCl. Overall, our data demonstrated different adaptive strategies in these two bacteria. While L. innocua expels protons to maintain a constant pH i, L. delbrueckii subsp. bulgaricus allows proton entry after acidic treatment so that pH i follows pH ex.