AEM Accepts, published online ahead of print on 30 October 2009

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Google Scholar Articles by Behr, J. Articles by Vogel, R. F. PubMed PubMed Citation Articles by Behr, J. Articles by Vogel, R. F. Agricola Articles by Behr, J. Articles by Vogel, R. F.

 Previous Article  |  Next Article 

Appl. Environ. Microbiol. doi:10.1128/AEM.01693-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Mechanisms of hop inhibition - The transmembrane redox reaction

Jürgen Behr and Rudi F. Vogel^*

Technische Mikrobiologie, Technische Universität München, D-85350 Freising, Germany

^* To whom correspondence should be addressed. Email: rudi.vogel{at}wzw.tum.de.

In this work, a novel mechanistic model of hop inhibition beyond the proton ionophore action towards (beer spoiling) bacteria was developed. Investigations were performed in model systems using cyclic voltammetry for determination of redox processes/conditions in connection with growth challenges with hop sensitive and resistant Lactobacillus (L.) brevis strains in the presence of oxidants. Cyclic voltammetry identified a transmembrane redox reaction of hop compounds at low pH (as common in beer) and in the presence of manganese (present in millimolar levels in lactic acid bacteria). The antibacterial action of hop compounds could be extended from the described proton ionophore activity, lowering the intracellular pH to pronounced redox reactivity, causing cellular oxidative damage. Accordingly, a correlation of a sole oxidant resistance in L. brevis strains to hop resistance could not be expected and was not detected. However in connection with our recent study concerning the hop ionophore properties and the resistance of hop sensitive and tolerant L. brevis strains towards proton ionophores (Behr and Vogel, 2009), we suggest, that both ionophore- as well as oxidant resistance is required for survival under hop stress conditions and confirmed this correlation, according to the novel mechanistic model. In consequence, the expression of several published hop resistance mechanisms involved in manganese binding/transport and intracellular redox balance, as well as proteins of oxidative stress under "highly reducing" conditions (c.f. anaerobic cultivation and "antioxidative" hop compounds in the growth medium) is comprehensible, now. Accordingly, hop resistance as a multifactorial dynamic property, at least implies distinct resistance levels against two different mechanism of hop inhibition, namely proton ionophore induced- and oxidative stress. Beyond this specific model of hop inhibition, these investigations provide general insight on the role of electrophysiology and ion homeostasis in bacterial stress responses to membrane active drugs.