MicroStation System, MicroLog Version 4.2 - DTU Systems Biology ...

Preparing Samples
Preparing an
inoculum
directly from
a mixedgrowth
plate
will cause
identification
problems.
Use the
colony
magnifier
lamp to
closely
examine
colonies.
• Do not allow cultures to grow for too long. Maximum growth is 24
hours for most bacteria, 72 hours for yeasts, and 10 days for most
fungi. Some exceptionally slow-growing or fastidious bacteria may
require 48 hours of growth or the use of multiple growth plates.
Checking to see if your culture is pure
Colonies on the plate that seem to be isolated, may in fact be the result of
mixed growth. This is especially true with Staphylococcus species.
Careful visual examination is essential to ensuring that a culture is pure. It
may be useful to use a colony magnifier lamp to assist in the examination.
If a colony shows any hint of pleomorphism, it is probably not a pure
culture and requires additional re-streaking and isolation.
Carefully examine areas of confluent growth. If the lawn is not uniform in
texture and color, this may indicate that the culture is not pure. Once
again, restreak for isolation.
Note: Unless your organism is a very slow grower, we don’t recommend
using lawn growth.
The opposite problem can also occur; sometimes a culture may be pure,
but give the appearance of heterogeneity. This is due to a rather common
phenomenon whereby microorganisms produce more than one colony
type. To be certain of its identity, purify and test each colony type
individually. Be aware that different colony types of the same strain may
ID but give different phenotype profiles.
Gram Staining
A Gram stain is essential in order to select the proper media and
MicroPlates for testing bacteria. Once your initial culture has been
incubated sufficiently, perform a standard Gram stain. Determine the
following:
• Is the microbe a bacterium or a yeast?
• Is the microbe gram positive or negative?
• Are the cells cocci or rods?
• Do the cells form spores?
Based on the source of the sample, the initial growth conditions, and the
Gram stain, you should know what basic type of microbe you have. See
pages 10.1-10.2 for specific Gram stain information. Refer to a basic
microbiology textbook if you are unsure of these procedures.
Section 4 � Page 2 MicroStation System/MicroLog User Guide Apr 07