Macromorphology
The figure shows blood agar plates with four different bacteria, which have been incubated during 24 h at 37°C. These bacteria (A, Erysipelothrix rhusipathiae; B, Streptococcus agalactiae; C, Staphylococcus aureus; D, Bacillus cereus) represent four different colony sizes. The length of the scale bar is equivalent to 5 mm. - Click on the image to enlarge it.
The figure shows closeups of the bacteria on the above plates. All four pictures were taken from the same distance The length of the scale bar is equivalent to 5 mm. - Click on the image to enlarge it.
The appearance of a bacterial colony on agar plates depends on, among other things, growth medium, nutritional availability, incubation time and temperature. There are many different culture media used for typing and/or selective cultivation of bacteria. One single bacterial species could, thus, have different appearance depending on the medium in the agar plate. The appearance of a bacterial colony is usually judged on a blood agar plate.
A bacterial colony usually originate from a single bacterium, but once it becomes visible it consists of several miljons of bacterial cells. Such a collection of bacteria is usually referred to as a colony forming unit (CFU), because you cannot know for certain that it originates from one single bacterial cell. Bacterial colonies differ in appearance regarding size, colour and shape etc. The bacteria included in VetBact are classified according the size of the colonies they form:
- Pin-point sized colonies (<1 mm in diameter), A in the figures
- Small colonies (1-2 mm in diameter), B in the figures
- Medium sized colonies (2-3 mm in diameter), C in the figures
- Large colonies (>3 mm in diameter), D in the figures
Other macromorphological characteristics of bacterial colonies that can be used for identification/classification, are the following:
The entire bacterial colony
- Circular (round)
- Irregular
- Swarming
- Mucoid
The shape of the margin of the colony
- Entire
- Irregular (undulate or lobate)
- Filamentous
The structure of the colony
- Smooth
- Wrinkled
- Wavy
The surface of the colony
- Shiny or dull
- Moist or dry
The profile of the colony
- Flat and low
- Flat and high
- Convex
- Cupol shaped
- Peak shaped
Furthermore, the following characteristics can also be used for identification/classification: for instance colour, apacity, odour and possible hemolysis.