The technique is simple to describe and surprisingly difficult to execute well when you are first starting out. The most common failure modes are predictable: too much inoculum carried forward from one quadrant to the next, not flaming the loop between streaks, rotating the plate by the wrong amount. The result in each case is dense growth across the entire plate rather than the well-separated isolated colonies you need in the final quadrant.
This page explains exactly how the technique works and why, walks through the step-by-step procedure with attention to the details that actually matter, covers colony morphology and how to read a plate after incubation, describes the most common errors and how to avoid them, and discusses how the choice of agar changes what you see and why.
Why Streaking Works: Serial Dilution on a Surface
The streak plate is essentially a serial dilution performed on a solid surface instead of in liquid. Each time you streak across a new area of the plate, you are carrying a smaller number of bacteria forward than you started with. The key is that the dilution happens by physical deposition: as the loop moves across the agar, it deposits bacteria at each point of contact. With each subsequent movement, fewer bacteria remain on the loop. By the third or fourth quadrant, the density should be low enough that individual bacteria land far enough apart from each other that their growing colonies do not merge.
The inoculating loop is a critical part of this process. A standard loop holds a fixed, reproducible volume of liquid (typically 1 to 10 microlitres depending on diameter). For solid specimens, a small amount of material is emulsified in saline or water before looping, or the loop is used directly on a colony or a colony-rich area of a plate. The quality of the dilution across the plate depends on the consistency of technique: how much material is picked up, how firmly the loop contacts the agar surface, and how many times the loop crosses back into the previous streak to pick up organisms for the next series.
Step-by-Step Streak Plate Procedure
Preparing the loop: Use a 10-microlitre disposable plastic inoculating loop or a Nichrome wire loop that has been flamed to red-hot in a Bunsen burner flame and allowed to cool. Cooling is essential: a hot loop will kill bacteria on contact. Allow 10 to 15 seconds for a nichrome loop to cool after flaming, or test by touching it to the agar surface away from the streaking area (it should not sizzle or cause any visible damage to the agar).
Loading the loop: Touch the loop to the colony or specimen to pick up a small amount of material. For a clinical swab, roll the swab briefly across a small area of the plate first (primary inoculation area), then use the loop to pick up from that area.
Primary streak (Quadrant 1): Hold the plate lid slightly ajar and tilt the plate away from you. Streak the loop across approximately one quarter of the plate in a series of tight, parallel back-and-forth lines. Do not press so hard that you gouge the agar. Aim for 6 to 8 passes across the quadrant. The loop should be nearly parallel to the agar surface, not held upright. After the primary streak, close the plate.
Flaming and cooling: Flame the nichrome loop until red-hot. Allow it to cool for 10 to 15 seconds. This step is not optional. It removes the bacteria from the loop so that you carry forward only the small number that were deposited at the end of Quadrant 1.
Secondary streak (Quadrant 2): Rotate the plate 90 degrees. Open the plate and streak through the last two or three lines of Quadrant 1 into the fresh area of the plate, again in tight parallel passes. This picks up a small number of bacteria from the edge of the primary streak and carries them into the new zone.
Repeat for Quadrants 3 and 4: Flame the loop between each quadrant. Rotate the plate 90 degrees each time. Streak through the edge of the previous quadrant into the fresh area. By Quadrant 4, the density should be low enough to produce isolated colonies. In Quadrant 4, some microbiologists use a slightly different approach: instead of parallel lines, they use a single sweeping continuous line across the quadrant to spread bacteria as far apart as possible.
Incubation: Invert the plate (agar side up) and incubate at the appropriate temperature for 18 to 24 hours (or longer for slow growers). Inverting prevents condensation from dripping onto the agar and disturbing colonies.
Reading a Streak Plate After Incubation
After incubation, the results in each quadrant tell you whether the technique worked.
Heavy, continuous growth across Quadrant 1 is expected and normal. Lighter, more separated growth appears in Quadrant 2. Scattered individual colonies or near-isolated colonies in Quadrant 3. Fully isolated, separated colonies in Quadrant 4 indicate a successful isolation.
Each isolated colony represents growth originating from a single bacterium (or a very small cluster of bacteria that were deposited together). These are the colonies to use for further work: subculture to a pure culture plate, set up biochemical identification tests, or pick into broth for further testing.
Colony morphology on the streak plate is a useful preliminary identification guide. Record the following features for each distinct colony type: size (pinpoint, small, medium, large, measured in millimetres), shape (circular, irregular, filamentous, rhizoid), elevation (flat, raised, convex, umbonate), margin (entire, undulate, lobate, serrate, filamentous), surface texture (smooth, rough, granular, wrinkled), colour (cream, white, yellow, orange, grey, black, translucent), opacity (transparent, translucent, opaque), consistency (butyrous, viscous, dry, brittle), and haemolysis on blood agar (alpha-haemolysis: green discolouration, partial lysis; beta-haemolysis: clear zone of complete red blood cell lysis; gamma-haemolysis: no haemolysis).
Choosing the Right Agar for Your Specimen
The choice of agar affects which organisms grow, how they appear, and how easy it is to separate them from others in a mixed specimen.
Blood agar supports the growth of almost all clinically relevant bacteria and shows haemolysis patterns. It is the base medium for most clinical specimens.
Chocolate agar is blood agar heated to lyse the red blood cells, releasing growth factors (haemin and NAD) needed by fastidious organisms like Haemophilus influenzae and Neisseria. It is used routinely for respiratory and CSF specimens.
MacConkey agar selects for gram-negative organisms and differentiates lactose fermenters (pink colonies: E. coli, Klebsiella) from non-fermenters (colourless colonies: Salmonella, Shigella, Pseudomonas).
MRSA chromogenic agar allows direct screening for methicillin-resistant Staphylococcus aureus: MRSA colonies appear as a specific colour (mauve, pink, or green depending on the agar brand) distinguishable from methicillin-sensitive S. aureus and other bacteria.
CCDA (Campylobacter Blood-Free Selective Agar) under microaerophilic conditions at 42 degrees Celsius isolates Campylobacter from stool specimens while inhibiting most other faecal flora.
XLD and DCA agars are used for Salmonella and Shigella from stool: Salmonella colonies produce hydrogen sulphide and appear with black centres on XLD.
Common Errors and How to Fix Them
Confluent growth across all quadrants: Either the initial inoculum was too heavy, the loop was not flamed between quadrants, or not enough passes were made in each quadrant to reduce the bacterial density before moving on. Ensure the loop is flamed and cooled between every quadrant and increase the number of passes per quadrant.
No growth anywhere on the plate: The inoculating loop may have been applied to the agar while still hot from flaming, killing all bacteria on contact. Alternatively, the organism is slow-growing or fastidious and needs a different medium, longer incubation, or different atmospheric conditions.
Growth only in Quadrants 1 and 2, nothing in 3 and 4: The dilution was not carried forward effectively. Ensure the loop passes through the last 2 to 3 lines of the previous quadrant when starting each new quadrant.
Agar surface gouged or pitted: Too much pressure on the loop. Hold the loop at a flatter angle and use gentler contact.
Only one colony type even though the specimen should contain multiple organisms: This can indicate that one organism dominated and outcompeted others during plating, or that the selective medium suppressed all but one type. Consider using a non-selective medium in parallel.
Frequently Asked Questions
What is the purpose of the streak plate method?
The streak plate method isolates individual bacteria from a mixed population by spreading them across an agar surface to separate them. Each isolated colony grows from a single bacterium and provides a pure culture that can be identified and tested.
Why do you flame the loop between quadrants?
Flaming the loop removes all bacteria from it so you carry forward only the small number deposited at the edge of the previous streak. Without flaming, you would carry the same large number of bacteria into each new quadrant and never achieve the dilution needed for isolated colonies.
What is colony morphology?
Colony morphology describes the visible characteristics of a bacterial colony grown on agar: size, shape, elevation, margin, colour, opacity, consistency, and haemolysis pattern. Different species produce characteristic colony morphologies on specific media. Colony morphology is the first step in identifying an isolate and helps distinguish between different colony types on a mixed-growth plate.
What does beta-haemolysis look like on blood agar?
Beta-haemolysis appears as a clear zone around the colony where the red blood cells in the agar have been completely lysed by bacterial haemolysins. The zone is clear rather than the opaque red of intact blood agar. Group A Streptococcus (S. pyogenes) and many strains of S. aureus produce beta-haemolysis.
How long do you incubate a streak plate?
Most clinically relevant bacteria are incubated for 18 to 24 hours at 35 to 37 degrees Celsius. Slower-growing organisms such as Brucella may require 48 to 72 hours. Fungi require up to 2 to 4 weeks. Mycobacterium tuberculosis requires 3 to 6 weeks.
What is the difference between a pure culture and a mixed culture?
A pure culture contains only one type of microorganism, grown from a single isolated colony. A mixed culture contains two or more types of organisms growing together. The streak plate method is used to obtain pure cultures from mixed specimens.
Can you use the streak plate for quantitative counts?
No. The streak plate is a qualitative isolation technique, not a quantitative one. For counting bacteria, you need serial dilution and plate counting, where a known volume is plated and colonies are counted to calculate CFU/mL.
What is the T-streak method?
The T-streak is a simplified version of the four-quadrant streak. The plate is divided into three sections with an imaginary T shape. The first section is inoculated heavily, and the loop is flamed and used to streak into the second and then third sections. It is faster but produces fewer isolated colonies than a four-quadrant streak.
What happens if you do not rotate the plate between quadrants?
If you do not rotate the plate between quadrants and streak in the same direction each time, you will simply continue the original streak without changing direction. The bacteria will not be diluted effectively into new areas, and the plate will show continuous heavy growth rather than progressive dilution toward isolated colonies.
How do you know which colonies to pick for further testing?
Pick colonies from the area of the plate showing isolated, well-separated growth (Quadrant 4 of a four-quadrant streak). Choose colonies that appear distinct from neighbouring colonies, with consistent morphology. If you are looking for a specific organism (for example, MRSA from a screening swab), use selective or chromogenic agar and pick colonies with the morphology or colour characteristic of that target organism.