Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 833

16.6.1. Major Classes of Interactions in Mixed Cultures
The major interactions between two organisms in a mixed culture are competition,
neutralism, mutualism, commensalism, amensalism, and prey e predator interactions.
Competition is an indirect interaction between two populations that has negative effects on
both. In competition, each population competes for the same substrate. Two populations or
microorganisms with similar nutrient requirements usually compete for a number of
common, required nutrients when grown together. The outcome of competition between
two species for the same growth-limiting substrate in an open system (e.g. a chemostat) is deter-
mined by the specific growth-rate limiting substrate concentration relationship. Two
different cases can be distinguished in a mixed culture of two competing species A and B:

(1) m B . 'The organisms with the fastest growth rate will displace the

others from the culture. This is known as the exclusion principle.
(2) Crossover in

m A is always greater than

m net S relationship. In this case, the faster growing organism is determined
by the dilution rate. Depending on the dilution rate, three different cases may be
identified:
(a) At the crossover point D ¼m X k d ; S ¼ S X , two species could be maintained in a
chemostat at D ¼m X k d . However, this is an unstable operating point.
(b) If D

> m X k d , then m A k dA > m B k dB , and B will be washed out; Awill dominate.

m B k dB > m A k dA , and Awill be washed out; B will dominate.
In a batch system, both species would exist in culture media. The ratio of number density
of species at a given time will be determined by the relative magnitudes of the specific
growth rates and the initial concentrations of cells.
Neutralism is an interaction where neither population is affected by the presence of the
other. That is, there is no change in the growth rate of either organism due to the presence
of the other. Neutralism is relatively rare. One example of neutralism is the growth of yogurt
starter strains of Streptococcus and Lactobacillus in a chemostat. The total counts of these two
species at a dilution rate of 0.4/h are quite similar whether the populations are cultured sepa-
rately or together. Neutralism may occur in special environments where each species
consumes different limiting substrates and neither species is affected by the end products
of the other.
Mutualism and protocooperation are more common than neutralism and may involve
different mechanisms. In both cases, the presence of each population has a positive effect
on the other. For mutualism, the interaction is essential to the survival of both species. In
protocol operation, the interaction is nonessential. One mechanism is the mutual exchange
of required substances or the removal of toxic end products by each organism.
The metabolisms of partner populations must be complementary to yield a mutualistic
interaction. An example is the growth of a phenylalanine-requiring strain of Lactobacillus
and a folic-acid-requiring strain of Streptococcus in a mixed culture. Exchange of the growth
factors phenylalanine and folic acid produced by partner organisms helps each organism to
grow in a mixed culture, while separate pure cultures exhibit no growth.
Another example of mutualistic interaction exists between aerobic bacteria and photosyn-
thetic algae. Bacteria use oxygen and carbohydrate for growth and produce CO 2 and H 2 O.
The algae convert CO 2 to carbohydrates and liberate oxygen in the presence of sunlight.

(c) If D < m X k d , then