Final answer:
Precultures in Luria-Delbrück fluctuation assays are used to synchronize bacterial cells in the same physiological state, allowing an accurate assessment of mutation rates by reducing variability due to differences in growth phases.
Step-by-step explanation:
Precultures are done in Luria-Delbrück fluctuation assays to ensure that the bacterial cells are in the same physiological state before the actual assay is started. This is crucial because it allows for a controlled start of the experiment, where all the cells are at the exponential phase of growth, which is essential for assessing the mutation rates accurately. The assay essentially measures the number of resistant colonies that arise in a given bacterial population, revealing important insights into bacterial mutation rates and mechanisms of genetic variation. Precultures help to synchronize the cells, reducing variability due to differences in growth phases, which could otherwise lead to inaccurate results.
The Luria-Delbrück experiment initially involves growing a small number of bacteria in a non-selective medium, where all bacteria can grow regardless of whether they have the resistance mutation. This pre-culture step allows the bacterial cells to reach an optimal growth phase. After this pre-culture period, the cells are then distributed in several replicates, where they continue to grow before being exposed to the selective agent (antibiotic). The reasoning is that mutations occur spontaneously and independently of selection pressure during the growth phase. As a result, by the time the bacteria are exposed to the antibiotic, mutants may already be present.
The central premise behind the pre-culture step is that if mutations are indeed random and not induced by the selective agent (in this case, the antibiotic), one would expect a fluctuation, or variation, in the number of resistant colonies between different cultures. Some will have no resistant mutants, while others might have one or two, and yet others will have many. This variability is due to mutations arising at different times during the replication of the bacterial cells in the pre-cultures.