Growth patterns on agar and colony types:
As a working microbiologist, it is of utmost importance to be able to recognize the different bacterial growth morphologies on agar plates and slants and even in broths. This is important in the identification process. Slants are usually used for cultivating pure cultures, whereas plates are often subcultured to get pure cultures. Bacteria grow as distinct morphologies, shapes, colors and consistencies on agars.
We start out with a single, lone bacterial cell, which is deposited on a solid nutrient medium agar. It divides at a logarithmic pace until a colony appears on the plate. One colony typically equals about 1,000 bacterial cells. This is expressed in colony-forming units per milliliter (CFU/mL). Gram stains are performed to determine purity of the culture. If the culture is not pure, suspect colonies must be subcultured to another plate to gain a pure, unmixed culture for further testing. Characteristics of the colonies are viewed with the naked eye or with assistance of lights, magnifying glasses, eyepieces, or colony counters. Bacterial growth is dependent upon a variety of factors including:
We start out with a single, lone bacterial cell, which is deposited on a solid nutrient medium agar. It divides at a logarithmic pace until a colony appears on the plate. One colony typically equals about 1,000 bacterial cells. This is expressed in colony-forming units per milliliter (CFU/mL). Gram stains are performed to determine purity of the culture. If the culture is not pure, suspect colonies must be subcultured to another plate to gain a pure, unmixed culture for further testing. Characteristics of the colonies are viewed with the naked eye or with assistance of lights, magnifying glasses, eyepieces, or colony counters. Bacterial growth is dependent upon a variety of factors including:
- temperature
- available nutrients
- oxygen or carbon dioxide
- incubation time
Colony characteristics on Agar and Agar Slants:
Categories of colony growth and appearance include:
- shape
- margin or edge
- elevation or depression
- color
- texture
- round, circular
- irregular
- punctiform (tiny, pinpoint)
- Entire (smooth, no irregularities)
- Undulate (wavy)
- Lobate (lobed)
- Filamentous
- Rhizoid (branched)
- Swarming
- Spreading
- "Feet" (ex: Candida albicans yeast)
- Double zone of hemolysis
- Complete zone of hemolysis (beta)
- Partial zone of hemolysis (alpha)
- No hemolysis (gamma)
- Convex (rounded)
- Concave (depressed/sunken in)
- Flat
- Raised
- Pulvinate (very convex)
- Umbonate (raised in the center)
- Target-shaped
- Dull
- Shiny
- Mucoid (slimy)
- Dry
- Crusty
- Friable
- Moist
- Rough
- Smooth
- Crumbly
- Sticky
- Stringy
- Molar Tooth
- Powdery
- White
- Cream/Buff
- Purple
- Gray
- Yellow
- Red
- Green/Blue-Green
- Pink/Rose
- Orange
- Black centers
- Opaque
- Transparent
- Translucent
- Shiny
- Dull/Matte
- Flowery
- Fruit-like
- Bleach
- Ammonia
- Malodorous
- Putrid
growth patterns in broth:
Bacteria display characteristic growth patterns in broth that aid in their identification. Microbes cultivated in broth show different growth characteristics.
- Some float on top of the medium and produce a membrane at the surface called a pellicle.
- Some sink to the bottom (sediment).
- Some produce uniform growth and cloud the broth (turbidity). These organisms are usually motile.
- Some produce clumps in the broth (flocculence).
Agar deep stabs:
Agar deep stabs are agars in test tubes that are prepared with Trypticase Soy Agar (TSA) enriched with yeast extract. This basically promotes the growth of a wide range of microbes. Oxygen is removed from the medium during its preparation and autoclaving. When the agar cools and solidifies, it diffuses back in, creating a gradient of oxygen concentrations in the medium. This makes the top aerobic and the bottom anaerobic to support a large range of microorganisms.
Agar deep stabs are stab-inoculated with a sterile inoculating needle. This is to introduce as little air as possible and the location of growth that you will see develop in the tube will indicate the microbe's aerotolerance.
Agar deep stabs are stab-inoculated with a sterile inoculating needle. This is to introduce as little air as possible and the location of growth that you will see develop in the tube will indicate the microbe's aerotolerance.