Investigations on the growth of Rhodococcus rubra in relation to the formation of stable biological foams

The growth characteristics of a foam-forming species, Rhodococcus rubra were studied on different substrates. The basic medium contained Czapek (3.34%), yeast extract (0.2%), potassium dihydrogen phosphate (0.12%), dipotassium hydrogen phosphate (0.25%) and ammonium chloride (0.1%). This was supplemented with varying concentrations of glucose (0-2%). The same basic medium was also used to examine the growth of R. rubra in combination with varying concentration of n-hexadecane (0.0-0.5%) as a source of energy while varying the concentration of ammonium chloride in the range 1-3 g l(-1). Studies based on determining the biomass concentration, the surface activity related to the cell suspensions and measuring the variations in broth pH revealed that glucose encouraged the growth of R. rubra, compared to the control. However, increasing the glucose concentration from 0.1 to 2.0% had no further effect on growth. The surface activity of the cell suspensions increased with increasing glucose concentration. Results similar to glucose were exhibited by the addition of n-hexadecane, suggesting same degree of growth among different concentrations with higher surface activity increasing with increase in substrate concentration. Results have also shown that the pH of all the culture broths decreased as the ammonium chloride concentration increased, suggesting that there was a production of hydrogen ions during the course of its metabolism.