Final answer:
Microbiologists count plates with 30 to 300 colonies for statistical accuracy. Using a plate with less than 30 colonies may not provide reliable estimates of the true number of live bacteria, while plates with more than 300 colonies can be difficult to count accurately. A standard known concentration is used for comparison to ensure a uniform cell count.
Step-by-step explanation:
When conducting a viable plate count in microbiology, using a plate with less than 30 colonies is statistically inaccurate. Microbiologists aim to count plates with 30 to 300 colonies to ensure statistical reliability. Plates with fewer than 30 colonies do not provide a solid basis for estimating the true number of live bacteria in a culture due to the potential for statistical anomalies at such low counts. On the other hand, plates with more than 300 colonies often lead to difficulties in counting and can result in underestimation if multiple cells form a single colony.
To ensure a uniform number of cells in each test, microbiologists prepare suspensions of cells that can be visually compared to a 0.5 McFarland standard, equivalent to 1 x 108 to 2 x 108 CFU/ml of E. coli. The colony-forming units per milliliter (CFU/mL) are then calculated after conducting dilutions and plating a known volume on a suitable growth medium. This process is crucial for achieving accurate measurements of bacterial populations in research and clinical settings.