Final answer:
The method used to thin a sample for bacterial growth and counting is serial dilution, often followed by either pour plate or spread plate methods to form countable colonies. Membrane filtration is used for samples with low bacterial concentration. Ensuring 30-300 colonies per plate leads to accurate and statistically reliable counts.
Step-by-step explanation:
The method described for thinning a sample to allow enough space for bacterial colonies to grow and be counted is known as serial dilution. This technique involves adding a fixed volume of the original culture into a series of dilution tubes with a known volume of sterile diluent to achieve a range of diluted samples. Microbiologists can then use the pour plate or spread plate methods to plate these diluted samples onto agar plates. After incubation, the colonies that form are counted to estimate the number of cells in the original sample. This is a crucial method in microbiology for obtaining colony-forming units (CFUs) from a given sample, which allows for the estimation of viable bacterial counts.
When working with samples with low bacterial concentration, such as drinking water, a membrane filtration technique is employed to first concentrate the bacteria before counting. After filtering and incubating on a growth medium, the colonies are counted and used to calculate the cell density.
For both methods, it is important that the number of colonies on a plate lies between 30-300 for reliable counts, as too few or too many colonies can skew the results. Several dilution levels are often plated to ensure that at least one plate will fall within this ideal range. Counts outside of this range may be unreliable due to the statistical significance or the physical constraints of accurately counting the colonies.