Contamination often occurs when pouring or inoculating agar plates, even when researchers follow proper sterile technique. Bacterial and fungal spores in the air and on surfaces can make freshly poured plates unusable, or can obscure results on inoculated plates. Being able to recognise these contaminants before using a compromised plate can save valuable time.
Look for signs of fungal contamination. Fungal contamination will appear as fuzzy, filamentous, or hairlike growths, and should be visible to the unaided eye. Fungal contamination often occurs right along the edge of an agar plate.
Inspect for signs of bacterial contamination. If the plate has not been inoculated, the presence of any bacterial colonies indicates contamination. On an inoculated plate, look for colonies that display morphology different than what you would expect from the type of bacteria used to inoculate the plate.
If you believe a group of plates may have been contaminated during pouring due to improper sterile technique, place them in a 37-degree Celsius incubator or other warm location overnight, then inspect for contamination as described above.
Use a magnifying glass to help identify minute differences in bacterial colony morphology.