How does hop resistance work in lactic acid bacteria?

How does hop resistance work? 

Ordinarily, most lactic acid bacteria are very sensitive to hops. They are sensitive to the iso-alpha acids resulting from boiling hops, as well as some other soluble hop compounds. Some brewing-relevant bacteria including Lactobacillus and Pediococcus species can develop resistance to hops, becoming quite resistant over time. 

The primary method of hop resistance in lactic acid bacteria is with hop resistance genes. The most well-known one is called HorA (there is also a HorB, and HorC). Bacteria can pass plasmids, which are little pieces of circular DNA, between each other much more effectively than yeast. This means that one lactic acid bacterial cell can "share" a hop resistance gene with its neighbours. 

In lactic acid bacteria, iso-alpha-acids in hops disrupt the pH gradient between the inside and outside of the cell, making it much harder for the bacteria to take up nutrients and regulate their pH. The end result is eventual cell death. Hop resistance genes such as HorA circumvent this by allowing the bacteria to excrete the hop compounds. The tradeoff is that this has an energy burden for the bacteria, meaning they have to work harder to excrete the hop compounds. 

Note that other hop compounds such as beta acids can also inhibit lactic acid bacteria. As a result, late kettle additions and dry hopping can still inhibit bacteria despite contributing less iso-alpha-acid to the beer. 

Image from Sakamoto & Konings (2003). 

Which Escarpment Labs bacteria strains have hop resistance? 

In our own lab, the following species have been shown to obtain hop-resistance and contain HorA/B/C: L. brevis, L. paracasei, P. damnosus, P. pentosaceus. 

Hop resistance of individual strains or blends depends on many factors including how long the bacteria have been forced to adapt to hops. We find that hop-resistance lactic acid bacteria can develop greater hop resistance over time such as successive generations of mixed culture beer ferments. 

We have not observed hop resistance in any strains of L. plantarum or L. rhamnosus. As such, our Lactobacillus Blend 2.0 is completely hop-sensitive and to our knowledge does not pose a risk for achieving hop resistance. This makes it an excellent choice for quick-souring in your brewery without excessive risk of cross-contamination. 

* Selected L. brevis strain does not have HorABC but this species is capable of obtaining hop resistance. 

If I don't use hop-resistant Lacto in my beer, should I be worried about wild bacteria? 

In short, yes. Lactobacillus and Pediococcus can grow favourably in beer environments, and these bacteria are ubiquitous in the environment, living on fruit, soil, and humans. Previous studies have shown the presence of HorABC hop resistance genes in many areas in the brewery: 

From Bokulich et al. (2015)

Since lactic acid bacteria are ubiquitous and can trade genetic material, there is always a risk of hop-resistant lactic acid bacteria contamination in the brewery environment, which makes it important to check beers proactively for bacterial contamination using a suitable agar medium (such as MRS) and/or using a PCR detection method for HorABC (such as Midwest Microbio). 

The less hops used in the beer, the greater the risk. While highly hopped IPAs are fairly low risk, light lagers and hefeweizen for example pose a very high risk for contamination by hop-resistant lactic acid bacteria if brewery microbiology is not tightly monitored and/or beer is not pasteurized. 

Further reading: 

Sakamoto, K., & Konings, W. N. (2003). Beer spoilage bacteria and hop resistance. International Journal of Food Microbiology, 89, 105–124. https://doi.org/10.1016/S0168-1605(03)00153-3

Bokulich, N. A., Bergsveinson, J., Ziola, B., & Mills, D. A. (2015). Mapping microbial ecosystems and spoilage-gene flow in breweries highlights patterns of contamination and resistance. ELife, 2015(4), e04634. https://doi.org/10.7554/eLife.04634

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