For each of the following types of contamination suggest the

For each of the following types of contamination, suggest the most likely point in preparation (or later) at which the contaminant was introduced.

A. Growth in all broth tubes
B. Growth in one broth tube
C. Growth only on the surface of a plate
D. Growth throughout the agars thickness on a plate
E. Growth only in the upper 1 cm of agar in an agar deep tube
F. All plates in a batch have the same type and density of contaminants
G. Only a few plates in a batch are contaminated, and each looks different.

Solution

A. Failure to correctly sterilize medium after dispensing.
B. Tube closure become insufficient.
C. Contaminate fell into plate while dispensing.
D. Contamination of agar while pouring the medium which includes a drop of water from water bath used to temper the medium before meting out
E. Contaminate entered at some stage in dishing out.
F. Contamination of inventory bottle of medium because of failure to dry field and not flaming the mouth of the field.
G. Dust entered plates while pouring the plates. Or some plates have been contaminated prior to pouring the medium.