A study of the effect of dilution rate on bacterial populations

Population levels of a mixed culture that was ambiently exposed to bacteria in the atmosphere was studied at various constant dilution rates. Random fluctuations in the free swimmer population were noted. An aseptic continuous flow system proved of no great advantage over that which had ambient inoculation, however, slime production did not follow identical growth patterns in three identical growth vessels.

Coulter counts were made on a pure culture of B. cereus grown in a continuous flow system at various dilution rates. These counts did not follow continuous flow theory for a pure culture in that there was no sharp drop in count at the critical dilution rate but gradual reductions over a range of several dilution rates.

Total population counts by the Coulter Counter were made on a defined mixed culture of B. cereus and A. aerogenes. After a several days Petroff-Hausser counts showed that 70% of the population was A. aerogenes. Total population was reduced from that of a pure culture of B. cereus. Steady-state populations were noted at various dilution rates. These steady-states, however, did not follow continuous flow theory for a pure culture in that larger counts were noted at a higher dilution rate than some counts at a lower dilution rate. It seemed that slime on the growth vessels’ walls continually changed the environment had a greater effect and made it impossible to control all the environmental factors even in continuous flow processes.