Growth of mixed cultures of bacteria on methanol.

Abstract

Summary: The bacterium Pseudomonas C was grown in a chemostat on methanol as sole source of carbon and energy. At a dilution rate of 0.1 h−1, other methanol-utilizing bacteria (Pseudomonas 1 and Pseudomonas 135), when added separately at a steady state, became dominant in the fermenter and Pseudomonas C was excluded. At a dilution rate of 0.3 h−1, however, Pseudomonas C dominated and the other bacteria were excluded. When various bacteria unable to utilize methanol were added to the chemostat during a steady state growth of Pseudomonas C, they remained in the fermenter independent of the dilution rate, but as a very low percentage of the total population (about 1%). When pathogenic bacteria (Staphylococcus aureus and Salmonella typhimurium), which are unable to utilize methanol as a sole carbon source, were added separately to a pure culture of Pseudomonas C in a chemostat, they too remained in the fermenter independent of the dilution rate. However, they constituted less than 1% of the population in the culture broth but a high percentage of the population on the fermenter wall. When added to a mixture of Pseudomonas C and bacteria unable to utilize methanol, the pathogenic bacteria could not be found in the fermenter after a few medium changes. The results suggest that operation of a continuous culture of Pseudomonas C at high dilution rates serves to prevent contamination with other methylotrophs that may have lower yields. A mixture of Pseudomonas C and heterotrophs from soil is relatively resistant to invasion by pathogens.