Physiology and development of Pectinatus cerevisiiphilus and Pectinatus frisingensis, two strict anaerobic beer spoilage bacteria

The genus Pectinatus was isolated recently and the deposited strains were classified as beer spoilage bacteria producing propionate as a major fermentation product. A recent investigation of this genus demonstrated the existence of two species: Pectinatus cerevisiiphilus, the type strain and Pectinatus frisingensis, a new species with a different pattern of growth substrates. Different culture media tested for both species demonstrated a higher specific growth rate for P. cerevisiiphilus. However, final biomass production was in every case around 20% higher in P. frisingensis. A 400% decrease of final biomass production was measured when the species were cultivated on poor culture medium; this decrease was found to be broadly proportional to amounts of acetate excreted in the medium. Both species produced CO2 from glucose; however, no significant modifications of biomass and volatile fatty acid production were demonstrated when varying head space composition with regard to CO2 levels. Growth experiments on glucose with increasing amounts of ethanol added in the culture, revealed a higher sensitivity of P. cerevisiiphilus to ethanol inhibition. Ethanol concentrations over 1.7 M resulted in a complete inhibition of growth for both Pectinatus species. Combined effects of culture medium pH, lactate, and glucose concentrations, demonstrated the prevailing role of glucose in the development of the bacteria. However, these three parameters had a different influence on growth characteristics of both Pectinatus species. P cerevisiiphilus grew very poorly with a glucose concentration of 5 mM for pH values below 4.1. This species had optimal pH for growth between 6 and 6.2 and it excreted increasing amounts of acetate with increase of pH, glucose or lactate in the culture medium. P. frisingensis showed a wide range of pH allowing a good growth. For glucose concentrations below 20 mM, highest final biomass productions were measured in the culture for pH values around 4.9; this also corresponded to a minimum in acetate excretion. The above results pointed at P. frisingensis as the prevailing species of Pectinatus in beer spoilage.

Effects of culture conditions on Pectinatus cerevisiiphilus and Pectinatus frisingensis metabolism: a physiological and statistical approach

The genus Pectinatus has been often reported in beer spoilage with off-flavours. The bacteria are strictly anaerobic, Gram-negative rods. Propionate and acetate are the main fermentation products from glucose in the two species belonging to the genus, P. cerevisiiphilus and P. frisingensis. Amino acids routinely present at a high level in beer were not growth substrates for both species, and a significant accumulation of succinate was observed with lactate as growth substrate. Both Pectinatus ssp. showed almost identical fermentation balances on glucose. Growth kinetics of both glucose-grown species were unchanged under a N2, H2 or 20% CO2-containing atmosphere. Combinations of culture medium pH values from pH 3 x 9 to pH 7 x 2, of glucose levels between 5 and 55 mmol l-1, and of lactate concentrations varied from 4 to 40 mmol l-1 demonstrated that biomass and volatile fatty acids production were proportional to glucose concentration for both Pectinatus species. A significant increase of volatile fatty acid production was measured for both species at the lowest pH values with a lactate or a glucose concentration increase. The maximum biomass production was observed at pH 6 x 2 for P. cerevisiiphilus, and between pH 4 x 5 and pH 4 x 9 for P. frisingensis. Glucose and lactate or pH value were dependent with regard to propionate and acetate production in P. frisingensis. On the other hand, the variations of these three parameters were independent with regard to biomass production for both strains, and to volatile fatty acids production for P. cerevisiiphilus. Addition of ethanol to glucose-grown cultures completely inhibited growth at 1 x 3 mol l-1 ethanol for P. cerevisiiphilus, and at 1 x 8 mol l-1 for P. frisingensis.