A spread plate is a microbiological technique in which a measured volume of a diluted sample is evenly distributed across the surface of an agar plate to isolate and count viable microorganisms.
Explanation
When culturing bacteria, yeasts or fungi, it is often necessary to determine how many viable cells are present. In the spread plate method, a small aliquot—typically 0.1 mL or less—of a sample or serial dilution is pipetted onto the centre of a solid growth medium. A sterile glass or plastic spreader is then used to gently spread the drop over the entire surface of the agar in a circular motion. The plate is allowed to absorb the liquid and is incubated at an appropriate temperature. Because the inoculum remains on the surface rather than being embedded in the agar, colonies grow as discrete spots that can be counted or picked. By plating different dilutions, one obtains plates with countable numbers of colonies, enabling calculation of colony‑forming units per mL of the original sample. The spread plate method is also used to evenly distribute cells when selecting transformants on antibiotic‑containing media or screening clones on indicator plates.
Applications and examples
Environmental microbiologists use spread plates to enumerate Escherichia coli in water samples by plating serial dilutions on selective media. Food safety testing laboratories spread aliquots of homogenised food onto selective agars to detect contamination with Salmonella or Listeria. Virologists perform plaque assays by spreading bacteriophage dilutions over a lawn of susceptible bacteria and counting the resulting clear plaques to calculate plaque‑forming units. Molecular biologists plate transformed E. coli on agar containing antibiotics and colour indicators to identify recombinant clones. Spread plates are also employed in clinical laboratories to estimate the bacterial load in urine cultures or quantify yeast cells in fermented beverages. Unlike the streak plate, which aims to isolate single colonies through successive dilution across quadrants, the spread plate produces an even lawn from a measured inoculum.
By producing countable, well separated colonies from a known volume, the spread plate remains a fundamental tool for quantifying and isolating microorganisms.
Related Terms: Colony-forming unit, Streak plate, Serial dilution, Agar plate, Microbial enumeration.