Summary: The bacterium C was grown in a chemostat on methanol as sole source of carbon and energy. At a dilution rate of 0.1 h, other methanol-utilizing bacteria ( 1 and 135), when added separately at a steady state, became dominant in the fermenter and C was excluded. At a dilution rate of 0.3 h, however, 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 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 ( and ), which are unable to utilize methanol as a sole carbon source, were added separately to a pure culture of 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 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 C at high dilution rates serves to prevent contamination with other methylotrophs that may have lower yields. A mixture of C and heterotrophs from soil is relatively resistant to invasion by pathogens.


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