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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 Gram-negative 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><ref name="Macrae_1964">[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>. Certain Gram-positive bacteria strains that have adapted to the brewing environment, such as some strains of ''Lactobacillus brevis'', 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 name="Macrae_1964" />. The effectiveness of hops to inhibit Gram-positive bacteria is also dependent on pH; at a lower pH, hops have a greater effect on inhibiting bacteria <ref name="Almaguer_2015" />. 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 iso-alpha alpha acids and possibly similar hop acids (such beta acids and oxidized hop acids) as ionophores, or compounds which can transport ions across cell membranes. While their antimicrobial properties are strong, alpha and beta acids in beer and wort and their effects on brewing are generally disregarded because they do not solubilize <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 and decreasing the pH within the cell <ref name="zhao_1027">[https://www.frontiersin.org/articles/10.3389/fmicb.2017.00239/full#B28 Heterogeneity between and within Strains of Lactobacillus brevis Exposed to Beer Compounds. Yu Zhao, Susanne Knøchel and Henrik Siegumfeldt. 2017. DOI: https://doi.org/10.3389/fmicb.2017.00239.]</ref>), 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"/>. The anti-microbial power of iso-α-acids is pH dependent. At a higher pH (5.6) iso-α-acids begin to lose their anti-microbial properties, but at a typical beer pH (4.3) iso-α-acids inhibited a sample of 6 strains of ''L. brevis'' that exhibited a range of general hop tolerance in one study <ref name="zhao_1027" />. 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"/>.