Final answer:
Researchers collect low amounts of bacteria for culture using techniques like membrane filtration for samples such as drinking water, and the streak plate method for isolating single colonies. Enrichment culture is used to amplify bacterial populations from food samples.
Step-by-step explanation:
Researchers use several different methods to collect a low amount of bacteria for culture, depending on the type of sample and the concentration of bacteria present. These methods facilitate studying various bacteria and their behavior in controlled environments.
For collecting bacteria from environments with low bacterial numbers such as drinking water, the membrane filtration technique is employed. This involves filtering known volumes of the sample through a membrane that traps microorganisms due to its small pore size. The membrane is then placed on a Petri plate containing a suitable growth medium. After incubation, colonies are counted, allowing for the calculation of bacterial cell density in the original sample.
Another method is to use a sterile swab, which is soaked in sterile water and rubbed across the sample area. The swab is then streaked onto an agar plate using a technique known as the streak plate method. This dilutes the bacteria and allows for the growth of single colonies, each of which originates from a single bacterial cell. Such colonies can be subcultured to generate a pure culture.
Finally, researchers sometimes amplify small quantities of bacteria via enrichment culture, particularly for pathogen detection in food samples. In this process, the food sample is incubated in a non-selective medium to promote the growth of bacteria, which can then be transferred to selective and differential media for further analysis.