Changes

":''Editor's note: the following are comments by Bryan Heit of [http://suigenerisbrewing.blogspot.com/ Sui Generis Brewing blog]'' There is a fair bit of research into hop tolerance out there; its not a simple topic as a number of factors come into play to produce hop tolerance. To make things even more complicated, hop tolerance is an inducable trait in many ''Lacto'' species - meaning that a seemingly susceptible strain can become resistant by culturing in ever-increasing doses, and a seemingly resistant strain can become susceptible after a generation or four in a hop-free media.

There are some other optionoptions; I've purified (but didn't keep - doh) some pretty resistant strains from grain by by making plates where you half-fill a plate, on an angle, with a high-IBU wort, and then overlay that with a no-IBU wort. This gives you a gradient plate, with low-IBUs on the end where the hopped-wort layer is thinnest and high IBUs where it is thickest. Some of those strains were resistant to over 30IBU, but being early in my yeast farming days I didn't bother keeping those." - Bryan Heit of <ref>[httphttps://suigenerisbrewingwww.blogspotfacebook.com/ Sui Generis Brewing bloggroups/MilkTheFunk/permalink/1002795743081973/?comment_id=1003625646332316&offset=0&total_comments=16&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation 1 with Bryan Heit on Milk The Funk regarding Lactobacillus hop tolerance. 01/19/2015.] </ref>. Hops contain multiple compounds which are bacteriostatic. Alpha acids are the best understood, but other compounds such as beta acids, a number of polyphenols (e.g. xanthohumol), and even some of the aromatic oils (e.g. humulene) have been found to have some inhibitory effects on Lactobacillus ''lactobacilli''. The later compounds (especially the beta acids) are why aged hops retain inhibitory characteristics, despite being nearly devoid of alpha acids. In all cases these compounds appear to inhibit the bacteria in the same way - all of these compounds contain fairly large, flat-ish, hydrophobic regions. These regions do not "like" to be in water, and thus will be driven into the hydrophobic core of the bacterial plasma membrane. This opens minute holes in the membrane which prevents the bacteria from maintaining ion (in particular, proton) gradients, leading to suppression of growth and even death of the bacteria. Hop Tolerance resistance is generally due to the induced expression of "multi-drug transport" (MDT) genes, which are "pumps" that recognize the general chemical signature of membrane-disruptive compounds, and then pump them out of the cell. Other mechanisms may also be involved - a few papers have identified changes in the lipid make-up of the plasma membrane, which may increase stability. This change also occurs in response to alcohol (to improve stability), so its not clear if that particular change has anything to do with hop resistance. ''Lactobacilli'' usually don't have these MDT genes 'on', which is why a lot of strains won't do well with hops in the first batch of beer, but over time become more and more tolerant as they increase expression of the MDT's. The overall MDT expression level, in theory, determines the maximum resistance of the bacteria. In the case of my experiments, I'm looking for mutants whose MDT's are permanently stuck 'on' for the resistant strain and 'off' for the sensitive strain <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/10027957430819731143165635711649/?comment_id=10036256463323161155715074456705&offset=0&total_comments=1650&comment_tracking=%7B%22tn%22%3A%22R22R0%22%7D Conversation 2 with Bryan Heit on Milk The Funkregarding Lactobacillus hop tolerance. 0109/1928/2015.]</ref>.