A petri dish is a shallow, circular container used in laboratories to culture microorganisms or cells on a growth medium. Accurate and permanent identification of these dishes is essential for experimental integrity. Poor labeling practices can quickly lead to confusion, invalidating data and causing research delays. Establishing a clear labeling protocol ensures every culture can be traced back to its origin and specific experimental conditions.
Essential Information for Identification
The goal of labeling is to provide immediate and complete traceability for the culture. Every label must include the date the dish was inoculated, which establishes the age of the culture for analysis. It is also necessary to clearly write the initials or name of the individual or group responsible for the experiment.
A unique sample or experiment identification code must be present to distinguish the specific test being conducted. This identifier often references the source of the culture, such as a patient swab number or an environmental sample location. Finally, the type of growth medium used, such as Tryptic Soy Agar (TSA) or Luria Broth (LB) agar, should be noted, as the nutrient base influences microbial growth.
Proper Label Placement and Technique
The physical location of the label is crucial and must always be written on the bottom section containing the agar. Writing on the lid is a common mistake because the lid can be easily separated, misplaced, or swapped, invalidating the identification. Labeling requires the use of a permanent, alcohol-resistant marker, such as an industrial-grade felt-tip pen.
Materials like pencil or grease pencil are unsuitable because they can smudge, fade, or wash away when exposed to moisture or cleaning agents. The writing should be placed clearly around the outer edge of the dish bottom. This positioning leaves the central area of the agar clear for unobstructed observation and counting of microbial colonies as they develop. Occasionally, a small alignment mark is drawn on the lid to ensure the correct top is replaced, but this is secondary to the primary label on the base.
Handling Labeled Dishes During Incubation
Once labeled and inoculated, the dish is prepared for incubation by turning it upside down, a process known as inversion. This technique is applied universally to prevent condensation from interfering with the culture. As the medium warms, water vapor condenses on the cooler lid, forming droplets that can fall onto the agar surface.
These droplets can cause colonies to merge or spread contamination, potentially ruining the growth pattern. Inverting the dish ensures that any condensation forms on the bottom, away from the agar surface. Since the label was written on the bottom, inversion places the label facing up, allowing personnel to easily read the dish’s identity without disturbing the growing culture or opening the lid.