Changes

[http://www.sciencedirect.com/science/article/pii/S0168160503001533 Sakamoto and Konings (2003)] attributed the antimicrobial effects the iso-α-acids acting as ionophores to cause leaking of ions across cell membranes. The protonated iso-α-acid (the form of the acid with an associated H+ ion, an H+ ion is a proton) is the antimicrobially active form. This means that for a beer with a given iso-α-acid concentration, the antimicrobial effects will be stronger at lower pH values because a greater percentage of the acid will be protonated. The protonated iso-α-acids are antimicrobially active by crossing into the cell and dissociating (releasing the H+ ion from the iso-α-acid), therefore disrupting the cellular proton gradient which is necessary for cells to function, before crossing back out of the cell. Cells with a resistance to hop bitter acids are better able to eject undissociated iso-α-acids from the cell and therefore preserve their proton gradients. This resistance to hop antimicrobial activities through iso-α-acids appears to be stable within bacteria with the trait, and the mechanism to expel iso-α-acids appears to be specific toward this type of compound rather than by a more general antimicrobial resistance mechanism such as multi-drug resistant bacteria possess<ref name="Sakamoto and Konings, 2003"> [http://www.sciencedirect.com/science/article/pii/S0168160503001533 Sakamoto and Konings (2003)] </ref>.