Propionispira (formerly Zymophilus) and Selenomonas
When the three species for these two genera were first described, all isolates were from pitching yeast or undefined brewery waste; none were from contaminated beer (Schleifer et al., 1990). In the intervening years, no definitive information has been forthcoming which associates P paucivorans, P raffinosivorans, or S. lacticifix with growth in packaged product. Of the three bacteria, S. lacticifix appears to be the most likely to be a beer spoiler, since laboratory modelling work has shown this bacterium can grow in beer with a pH of 4.3 to 4.6. In contrast, P. paucivorans and P raffinosivorans require a higher pH of 6.0 or 5.0, respectively, for growth in beer (Seidel-Rtifer, 1990). As such, these bacteria are considered to be indicator microbes, whose detected presence within the brewery is indicative of a breakdown in brewery hygiene.
Accumulating evidence points to plants as the environmental origin for the nine currently defined anaerobic GNB species found in breweries. The initial indication that these bacteria are plant- associated came from analyses showing that their lipopolysaccharides contain unusual carbohydrate structures showing similarity with what is found in plant-associated Rhizobium spp. (Helander et al., 2004). The first direct evidence for such plant association was provided by the finding of P. cer- evisiiphilus in samples of mangrove sediment in Thailand (Saimmai et al., 2012). About the same time, two new, albeit non-brewing-related Pectinatus species, P brassicae (Zhang et al., 2012) and P sotticetonis (Caldwell et al., 2013) were isolated from pickle waste water and spoiled pickles, respectively, Finally, a recent metagenomics analysis of a root sample from a sedge plant (Carex spp.) collected in a wetland at high altitude in the Tien Shan Mountains in the Issyk Kul region of Kyrgyzstan revealed that Propionispira spp. comprised some 25% of the bacterial population present (M. Haakensen and V. Friesen, Contango Strategies Ltd; personal communication). As sequencing the V3-V4 region of the 16S rRNA gene that was done on the sample does not allow P paucivorans and P raffinosivorans to be differentiated, identification to the species level could not be done.