Hop resistance Antimicrobial effect of hops
Beer and the brewery are stressful environments in toto, though hops are certainly considered the defining stress for microorganism growth. When hops are added to wort and boiled, alpha acids are extracted and transformed into iso-alpha-acids, the principal bitter components in beer (Steenackers et al., 2015). These bitter compounds have a demonstrated antimicrobial effect on Gram-positive bacteria (e.g. LAB), which was originally recognized to be through increasing the permeability of the bacterial cell wall, causing leakage of the cytoplasmic membrane, inhibiting normal cellular processes, and causing changes in the cellular proton ionophore activity and uptake of leucine (Simpson, 1993a,b). Ultimately, bitter compounds act as ionophores, which sequester protons within the cell and dissipate the pH gradient across the cellular membrane, thereby reducing the proton motive force (PMF) and all PMF-dependent cellular activities, such as nutrient uptake (Sakamoto and Konings, 2003; Simpson, 1993a). Further, the strength of the inhibitory effect of hops was found to be dependent on pH and mediated by a cation (K+, Mn2+) exchange across the bacterial membrane (Simpson, 1993b). Importantly, these same mechanisms were not found to be active against Gram-negative isolates, probably as a result of the protection afforded by their outer membrane. Further investigation found that a transmembrane redox reaction of hop compounds occurred at low pH (such as in beer) and in the presence of Mn2+, and that this redox activity causes cellular oxidative damage (Behr and Vogel, 2010). Therefore, hop resistance is a multifactorial process where at least two distinct levels of resistance mediate the stress of hops, namely proton ionophore-induced and oxidative stress mechanisms (Behr and Vogel, 2010).