Bacillus
subtilis. These gram positive, sporeforming rods produce colonies
which are dry, flat, and irregular, with lobate margins.
Solid, agar-based media can be used to identify colonial characteristics (shape, size, elevation, margin type), but can also serve to select for particular bacterial groups and differentiate between two or more different species. In Microbiology 201, we use trypticase soy agar (TSA) as an enriched medium for general bacterial isolation since most common species and even some fastidious forms will grow on this medium. Blood agar plates (BAP) are used both as enriched media and to differentiate between individuals on the basis of their ability to produce hemolysins, enzymes which lyse red blood cells. When used in combination with other forms of presumptive and confirmatory tests, the examination of colonial characteristics provides valuable information leading to precise identification. Click on images to see a larger view.
Bacillus
subtilis. These gram positive, sporeforming rods produce colonies
which are dry, flat, and irregular, with lobate margins.
Mycobacterium
smegmatis. These acid-fast rods produce irregular colonies with
lobate margins which are dry, flat, orange-yellow, and appear waxy due
to the high concentration of lipids in the cell wall.
Staphylococcus
epidermidis. Circular, pinhead colonies which are convex with
entire margins. The colonies of this gram-positive coccus appear
either the color of the agar, or whitish.
Staphylococcus
aureus. Circular, pinhead colonies which are convex with entire
margins. This gram positive coccus often produces colonies which have a
golden-brown color.
Micrococcus
luteus. Circular, pinhead colonies which are convex with entire
margins. This gram positive coccus produces a bright yellow, non-diffusable
pigment.
Rhodospirillum
rubrum. Pinpoint circular colonies which are convex with entire
margins. This gram negative spirillum produces a non-diffusable red
pigment.
Serratia
marcescens. These gram negative rods produce mucoid colonies
which have entire margins and umbonate elevation. Note that there
are both red and white colonies present on this plate. Some strains
of S. marcescens produce the red pigment prodigiosin in response
to incubation at 30o C, but do not do so at 37o C.
This is an example of temperature-regulated phenotypic expression.
Pseudomonas
aeruginosa. This gram negative rod forms mucoid colonies with
umbonate elevation. Some strains produce a diffusable green pigment
and a distinctive fruity odor. P. aeruginosa is an opportunistic
contaminant of burn injurys, wounds such as cuts and gunshot, and can cause
bacterial pneumonia. It is often nosocomial pathogen, easily transmitted
by hands and invasive medical procedures.
Salmonella
choleraesuis serovar typhimurium. This gram negative rod
is a component of the gastrointestinal tract of birds and reptiles and
is an agent of gastroenteritis in humans. It forms shiny, convex
colonies with entire margins.
Escherichia
coli. This gram negative rod (coccobacillus) forms shiny, mucoid
colonies which have entire margins and are slightly raised. Older
colonies often have a darker center.
Enterobacter
aerogenes. This gram negative rod is a common contaminant of
vegetable matter which forms shiny colonies with entire margins and convex
elevation.
Enterococcus
faecalis. This gram positive coccus forms pinpoint colonies with
convex elevation. This strain exhibits gamma (g)
hemolysis; growth on the BAP, but no lysis of the red blood cells in the
agar.
Streptococcus
pneumoniae. This gram positive streptococcus produces colonies
which are circular with entire margins, often raised with depressed centers.
This microbe exhibits alpha (a)
hemolysis; a partial breakdown of the red blood cells in the medium.
Streptococcus
pyogenes. The gram positive streptococcus produces pinpoint colonies
which are circular with entire margins. It exhibits beta (b)
hemolysis; the complete breakdown of the red blood cells surrounding the
colonies.