title: 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 (pHex) from pHex 6 to a range of concentrations down to pHex 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 (pHi) to a minimum pHi of 4.0. Changes in pHi 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 pHi of 5.5-6.1 at pHex 4 and 5 via H+ extrusion. In contrast, L. delbrueckii subsp. bulgaricus progressively lowered pHi towards pHex over the entire pHex range examined. The type of acidulant used influenced pH regulation with both pHi 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 pHi, L. delbrueckii subsp. bulgaricus allows proton entry after acidic treatment so that pHi follows pHex.
authors.affiliation: Australian Food Safety Centre of Excellence, School of Agricultural Science, University of Tasmania, Hobart, Tas 7001, Australia. L.Schabala@utas.edu.au
authors.forename: Lana
authors.initials: L
authors.lastname: Shabala
authors.forename: Tom
authors.initials: T
authors.lastname: McMeekin
authors.forename: Birgitte Bjørn
authors.initials: BB
authors.lastname: Budde
authors.forename: Henrik
authors.initials: H
authors.lastname: Siegumfeldt
mesh_terms: D000222:Adaptation, Physiological; D005516:Food Microbiology; D006706:Homeostasis; D006851:Hydrochloric Acid / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000494:pharmacology*; D048668:Lactobacillus delbrueckii / Q000187:drug effects* / Q000254:growth & development / Q000502:physiology; D008087:Listeria / Q000187:drug effects* / Q000254:growth & development / Q000502:physiology; D008856:Microscopy, Fluorescence / Q000379:methods; D008954:Models, Biological
chemical_list: D019344:Lactic Acid; D006851:Hydrochloric Acid