Final answer:
The 30-300 rule is a microbiological guideline for accurately estimating the number of live bacteria in a sample by ensuring colony counts on a petri dish fall within this range. It aids in minimizing errors that arise from too few or too many colonies, which can affect the statistical reliability and the precision of the counts respectively. The rule utilizes dilution factors and averages from multiple plates to provide a more accurate representation of bacterial concentration.
Step-by-step explanation:
The 30-300 rule in microbiology refers to the ideal range of colony-forming units (CFU) on a petri dish when counting microbial colonies to estimate the size of a bacterial population. Counts below 30 may lead to statistical unreliability, while counts above 300 can cause difficulty in distinguishing individual colonies, leading to inaccuracies. This rule is important for ensuring a representation of the actual number of live bacteria in a sample, as it minimizes the chances of counting multiple bacteria clustered together as one colony.
To calculate the number of CFU per mL in the original culture, microbiologists use the dilution factor. For example, if 50 colonies are counted on a plate from a 1:10,000 dilution and only 0.1 mL was plated, the number of bacteria in the original culture is estimated by multiplying the colony count by both the inverse of the plated volume and the dilution factor. Thus, 50 × 10 × 10,000 gives an estimated concentration of 5 million cells/mL.
Uniformity in the number of cells plated is crucial for reproducibility, and for this, a spectrophotometer or visual comparison to a known concentration standard like the 0.5 McFarland standard is used. Dilution series assist in achieving the desired cell concentration for accurate counting, and a sample from each dilution is plated on solid medium. Counts from multiple plates are averaged to obtain a reliable measurement of cell concentration.