Posted: September 12th, 2022

Colony and cell morphology

Fall 2022
Answers to Lab 1: Colony and cell morphology
Table 1.1. Unique colony morphologies of three common Gram positive bacterial species on TSA.
Organism Colony Morphology
Enterococcus faecalis
Form Circular,
Margin entire
Surface Texture smooth
Colour cream
Transparency opaque
Staphylococcus aureus
Form Circular, pinpoint
Margin entire
Surface Texture smooth
Colour yellow
Transparency opaque
Staphylococcus epidermidis
Form Circular, punctiform (pinpoint, small)
Margin entire
Surface Texture smooth
Colour white
Transparency opaque
Table 1.2. Unique colony morphologies of three common Gram negative bacterial species on TSA.
Organism Colony Morphology
Escherichia coli
Form circular
Margin Entire or irregular (undulate)
Surface Texture smooth
Colour cream
Transparency opaque
Klebsiella aerogenes
(a.k.a. Enterobacter aerogenes)
Form circular
Margin Entire or irregular edge (undulate)
Surface Texture smooth
Colour cream
Transparency opaque
Proteus vulgaris
Form circular
Margin entire
Surface Texture smooth
Colour Cream, plate dark in areas
Transparency opaque
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Fall 2022
Table 1.3. Gram reaction and cell morphologies of six common bacterial species viewed under 1000X (total
magnification).
Organism
Gram Reaction
(Colour and +/-)
Cell Shape/Arrangement
Enterococcus faecalis
Gram positive
Purple
Elongated cocci
Pairs (diplo) or short chains (strepto)
Staphylococcus aureus
Gram positive
Purple
Cocci (spheres)
Clusters (staphylo)
Staphylococcus epidermidis
Gram positive
Purple
Cocci
Clusters (staphylo)
Escherichia coli
Gram negative
Pink
Bacilli (rod)
Single/pairs (diplo)
Klebsiella aerogenes
Gram negative
Pink
Bacilli (rod)
Single/pairs (diplo)/chains (strepto)
Proteus vulgaris Gram negative
Pink
Bacillus (rod)
Single
Part I, Exercise A
1. Upon first glance, what is one discernable difference between the colonies of the Gram-positive species
compared to that of the Gram-negative species?
Gram positive colonies are smaller than Gram negative.
2. What does this difference tell you about the growth rate of the bacteria on the plate? (Hint: think about how a
colony is formed).
The Gram positive bacteria grow slower on the agar compared to Gram negative bacteria because their colonies have
less cells in them indicating that less replication has occurred.
Part II, Exercise A
1. What is the total magnification when looking at a specimen under the 100X objective lens?
1000X (10X ocular, 100X objective).
2. What is the resolution of the 10X objective lens? (wavelength of light = 500 nm)
Resolution = 500/(2×0.25) = 1000 nm
3. Draw what you would see under the 10X objective lens if the bacterial cells were situated:
a. 3 μm apart: separate cells
b. 500 nm apart: overlapping cells
c. 1000 nm apart: separate cells
Part II, Exercise B
1. What is an observation you made based on the cells Gram reaction and their shape?
All of the Gram negatives are rod while the Gram positives are cocci.
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Fall 2022
2. What would be an advantage to knowing an organisms Gram reaction in case you want to identify it using
subsequent tests?
Some tests are only used for Gram negative organisms and others only for Gram positive. Some media selects against
one Gram or the other so for example, it would be useless to test a Gram positive species on agar that selects for Gram
negatives (i.e. MAC).
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