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==Antimicrobial Properties==
Hops are known to have antimicrobial properties against Gram-positive bacteria. This includes bacteria that can be present in beer both as spoilage organisms and as intentionally added in sour and mixed fermentation beer such as ''[[Lactobacillus]]'' and ''[[Pediococcus]]''. Certain other bacteria found in beer such as ''Acetobacteraciae'' are Gram-negative and are not susceptible to the antimicrobial properties of hops <ref name="Hough_1957">[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1957.tb06267.x J. S. Hough, B.Sc, Ph.D., G. A. Howard, M.Sc., Ph.D., and C. A. Slater, Ph.D. 1957.]</ref>. Certain Gram-positive bacteria strains that have adapted to the brewing environment are known to be more resistant to the antimicrobial effects of hops. The antimicrobial effect is characterized as inhibiting the growth and lactic acid production of lactic acid bacteria, however, this does not always also include cell death as ''Lactobacillus'' that has been inhibited by hops can later be revived <ref>[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.1964.tb02001.x SIGNIFICANCE OF THE USE OF HOPS IN REGARD TO THE BIOLOGICAL STABILITY OF BEER: I. REVIEW AND PRELIMINARY STUDIES. R. M. Macrae. 1964.]</ref>. Multiple mechanisms have been proposed to explain why hops are antimicrobially active.
One mechanism of the antimicrobial activity of hops is due to the role of alpha acids and similar hop acids (such beta acids and iso-α-acids) as ionophores, or compounds which can transport ions across cell membranes <ref name="Fernandez and Simpson, 1993"> [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.1993.tb02782.x/full Fernandez and Simpson (1993)] </ref><ref name="Sakamoto and Konings, 2003"> [http://www.sciencedirect.com/science/article/pii/S0168160503001533 Sakamoto and Konings (2003)]</ref>. 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. Protonated iso-α-acids act against bacteria by crossing into the cell and dissociating (releasing H+ ions from the iso-α-acid), therefore disrupting the cellular proton gradient which is necessary for cells to function, before binding an equal charge in metal ions and crossing back out of the cell. Cells with resistance to hop bitter acids are better able to eject disassociated iso-α-acids from the cell and therefore preserve their proton gradients. 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"/>. Hop resistant bacteria cultured in the absence of hop acids can lose their resistance if grown in an environment without antibacterial hop compounds<ref name="Fernandez and Simpson, 1993"/> and some hop resistant microbes need to be acclimated to hop acids by growth in sub-limiting levels of antibacterial acids before they are able to resist higher levels <ref name="Sakamoto and Konings, 2003"/>.