Lab 3: Streak Plate
Mixed cultures of bacteria can be found in patient and environmental specimens. Diagnosis of infection in a patient cannot be achieved unless the medical lab is able to isolate the specific bacteria responsible for the infection. The streak plate is an inoculation technique used to isolate individual types of bacteria from a mixed culture. When a mixed culture is "streaked" onto a Petri plate, individual bacterial cells are deposited on the surface of the agar. During incubation each individual cell will multiply to form millions of identical cells, or clones. This mass of identical cells, a pure culture, is called a colony.
The 3-quadrant streak plate method involves dragging the inoculating loop across the agar surface through 3 zones in a systematic manner.
Individual bacterial colonies can be seen with the naked eye. A bacterial colony is a visible mass of bacterial cells (on a media surface) that arose from a single bacterial cell. In lab you used the streak plate method to deposit individual bacterial cells onto the surface of a TSA plate. While your plates were being incubated at 37°C, the bacterial cell began to reproduce, or divide. All the bacterial cells within a colony are identical clones of the original bacterial cell. The colony is a pure culture: a culture that contains one species of bacteria.
Each species of bacteria produces its own unique bacterial colony with its own characteristics (shape, color, texture, etc). Colony morphology refers to the characteristic appearance of a colony of a specific species of bacteria. Several terms are used to describe colonies:
Whole Form – overall shape
Margin – shape of edge
Elevation – height observed at eye level
Surface – texture of the surface observed visually
Optical Characteristics – effects of light
opaque = not allowing light through
translucent = allowing some light through
glistening = shiny when observed from an angle
dull = not shiny when observed from an angle
Consistency – overall texture observe by dragging loop through the colony
butyrous = butter like
brittle = dry and fragile
viscid = growth follows loop
Pigmentation – color
red, yellow, off white, white, beige
Examples of each of these bacterial colony characteristics are illustrated below. Remember, colony morphology is unique to each species.
Cell morphology refers to the shape of individual bacterial cells. Bacterial shapes include cocci (circle), bacilli (rod), and spirals (corkscrew). In order to determine the cell morphology of a specific bacterial species, one must view a stained smear of the culture under the microscope. Individual bacterial cells are much too small to be seen with the naked eye.
Complete the following quiz to test your knowledge of streak plates!
1 Trypticase Soy Agar (TSA) plate
SA = Staphylococcus aureus broth culture
1. Use a sharpie to draw the letter "T" on the bottom of the TSA plate. Label the lid of the plate with your initials, date, and the name of the organism.
2. Heat the inoculating loop in the bacti-cinerator for 5 seconds. Allow loop to cool. Obtain a sample of culture using aseptic technique.
3. Steak Zone 1 on the TSA plate. Be careful not to dig into the agar. STERILIZE LOOP. Do not dip back into the culture.
4. Streak Zone 2 using two to three "pick-up" lines into Zone 1 and three to four "fall-off" lines in Zone 2. STERILIZE LOOP.
5. Streak Zone 3 using two to three "pick-up" lines into Zone 2 and three to four "fall-off" lines into Zone 3. Do not go into Zone 1. STERILIZE LOOP. Place loop in test tube rack.
6. Incubate the plate in an INVERTED position at 37°C or room temperature for 24 hours. By incubating plates upside down, the problem of moisture collecting on the cover is minimized. Moisture on the cover could drop down onto the surface of the agar and cause colonies to run together.
Figure 3.1 Three Quadrant Streak Plate Method
After observing your TSA streak plate for growth, answer the following questions:
Observe the following photos, then take the corresponding quiz to describe the colony morphology from each photo.
Preparing a Streak Plate
Streaking for Isolation