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Applied and Environmental Microbiology, September 2002, p. 4399-4406, Vol. 68, No. 9
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.9.4399-4406.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

In Situ Determination of the Intracellular pH of Lactococcus lactis and Lactobacillus plantarum during Pressure Treatment

Adriana Molina-Gutierrez,1 Volker Stippl,2 Antonio Delgado,2 Michael G. Gänzle,1* and Rudi F. Vogel1

Lehrstuhl für Technische Mikrobiologie,1 Lehrstuhl für Fluidmechanik und Prozessautomation, TU München, 85350 Freising, Germany2

Received 28 January 2002/ Accepted 20 June 2002

Hydrostatic pressure may affect the intracellular pH of microorganisms by (i) enhancing the dissociation of weak organic acids and (ii) increasing the permeability of the cytoplasmic membrane and inactivation of enzymes required for pH homeostasis. The internal pHs of Lactococcus lactis and Lactobacillus plantarum during and after pressure treatment at 200 and 300 MPa and at pH values ranging from 4.0 to 6.5 were determined. Pressure treatment at 200 MPa for up to 20 min did not reduce the viability of either strain at pH 6.5. Pressure treatment at pH 6.5 and 300 MPa reduced viable cell counts of Lactococcus lactis and Lactobacillus plantarum by 5 log after 20 and 120 min, respectively. Pressure inactivation was faster at pH 5 or 4. At ambient pressure, both strains maintained a transmembrane pH gradient of 1 pH unit at neutral pH and about 2 pH units at pH 4.0. During pressure treatment at 200 and 300 MPa, the internal pH of L. lactis was decreased to the value of the extracellular pH during compression. The same result was observed during treatment of Lactobacillus plantarum at 300 MPa. Lactobacillus plantarum was unable to restore the internal pH after a compression-decompression cycle at 300 MPa and pH 6.5. Lactococcus lactis lost the ability to restore its internal pH after 20 and 4 min of pressure treatment at 200 and 300 MPa, respectively. As a consequence, pressure-mediated stress reactions and cell death may be considered secondary effects promoted by pH and other environmental conditions.

* Corresponding author. Mailing address: TU München, Lehrstuhl für Technische Mikrobiologie, Weihenstephaner Steig 16, 85350 Freising, Germany. Phone: 49 8161 713959. Fax: 49 8161 713327. E-mail: michael.gaenzle{at}blm.tu-muenchen.de.

Applied and Environmental Microbiology, September 2002, p. 4399-4406, Vol. 68, No. 9
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.9.4399-4406.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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