Commensalism is a type of symbiotic relationship in which one bacterium gains from the association and the other is neither harmed nor benefited.
In this type of relationship, the waste product of one microbe serves as a substrate for the other.
Example:
NITRIFICATION
The oxidation of ammonium ions to nitrite by Nitrosomonas and the subsequent oxidation of nitrite to nitrates by Nitrobacter
Nitrobacter benefits from this type of association as its uses nitrite to obtain energy for its growth.