Fermentation of lactose by Bacteroides polypragmatus

Lactose fermentation by the mesophilic anaerobic bacterium Bacteroides polypragmatus was studied in 100-mL batch cultures using a mineral salts – vitamin medium containing 0.4% (w/v) yeast extract and cysteine-Na2S reducing agent. Bacteroides polypragmatus could be cultivated in the presence of initial lactose concentrations as high as 20.0% (w/v). However, at these levels, the lag period was approximately 12 h compared with less than 6 h in media containing up to 5.0% lactose. Optimum growth and substrate conversion to H2 was obtained in medium supplemented with 1.5–2.0% lactose. Under these growth conditions, the metabolic end products (moles per mole of lactose) after 48 h of incubation were 3.33 H2, 4.20 CO2, 1.82 ethanol, 1.46 acetic acid, and 0.06 butyric acid. The growth rate in approximately 2.0% (w/v) glucose or galactose medium (μ = 0.23 h−1) was higher than that (μ = 0.14 h−1) in lactose. The maximum rate of glucose and lactose utilization was 6.3 and 3.8 g∙L−1∙24 h−1, respectively. The maximum lactase in lactose-grown culture was about twice that of galactose-cultivated cells, and the activity in glucose culture was negligible. The results indicate that B. polypragmatus may be of potential use in the disposal of lactose, since besides H2, the cell biomass (approximately 35% cell protein) may also prove to be a useful by-product.