The concept of a balanced intestinal microbiota enhancing
resistance to infection and reduction in resistance
when the intestinal microbiota is disturbed is important
in understanding the microbe-host relationship. What
constitutes the balanced and disturbed populations is not
clear; however, lactobacilli and bifidobacterial species
seem to be sensitive to stress, and these populations tend
to decrease when a bird is under stress. Proposed mechanisms
of pathogen inhibition by the intestinal microbiota
include competition for nutrients, production of toxic conditions
and compounds (volatile fatty acids, low pH, and
bacteroicins), competition for binding sites on the intestinal
epithelium, and stimulation of the immune system
(Fuller, 1989; Gibson and Fuller, 2000; Rolfe, 2000). These
are not mutually exclusive mechanisms, and some microorganisms
may effect change with a single mechanism,
whereas others may use several mechanisms.