Some
media support the growth of selective organisms (e.g. only
gram-negative or only gram-positive). Growth on certain media gives
preliminary information about specific biochemical characteristics (e.g.
ferments or does not ferment a particular sugar) that lead to
subsequent steps to take in the definitive identification of the
organism.
In
the laboratory sessions of this course, a variety of culture media will
be utilized to aid in the growth and identification of the clinically
important bacteria studied. In all instances, the media in the tube or
plate is identified by a label on the tube or on the bottom (agar
containing portion) of the petri dish. You are not expected to be able to identify any of the media introduced in these sessions without reading the label. It is expected
that you are responsible for verifying the identity of all media before
each use and inoculating it according to the protocols provided. A
description of each of these media is provided in the text of the Lab
session in which it is first introduced. A description of all media
encountered in these lab sessions is available in Appendix 2, Culture
Media and Biochemical Tests, of this laboratory manual.
Early
preliminary information regarding a patient’s culture results can often
be provided to the physician after observing the growth and colony
morphology on a specific agar media.
Clinical
microbiologists routinely make note of the colony characteristics of
bacterial growth on agar plates. The success or failure of bacterial
identification procedures often depends on the accuracy of the initial
observation of colony characteristics. Some of the criteria frequently
used to characterize bacterial growth on agar media include:
- Colony size (using relative terms such as pinpoint, small, medium, large)
- Colony color
- Colony shape (describing the form, elevation, and edges of a colony)
- Colony surface appearance (e.g. dull, opaque, moist/glistening)
- Changes in agar media resulting from growth (e.g. hemolytic pattern on blood agar, changes in media or colony color due to a change in pH indicators, pitting of agar surface)
- Odor (certain bacteria produce distinct odors)
Many
of these criteria are subjective. In our laboratory sessions as in the
clinical laboratory, careful determination of colony appearance is
important but should not be the only criteria for making a preliminary
identification. Notations in each Laboratory Section will be provided for colony characteristics that are useful in the identification of commonly encountered pathogenic bacteria including your
unknown specimens. To take full advantage of these notations, it is
important to spend some time viewing the demonstration plates provided
for each lab and making note of the characteristics that are described
in the text.