Lesson 7 - Microorganism Detection and Control SAQ
This lesson allows you to test yourself on the material covered in the Microorganism Detection and Control lesson, and determine the level to which you have attained the outcomes.
Suggested Response - Q2
** Discuss how we traditionally isolate contaminating microorganisms within the brewery environment.
Response:
To isolate a pure culture of a microorganism for further identification, we must collect samples. We often need to concentrate the organisms and grow them on or in media to levels that we can use for subsequent testing.
Sampling is done aseptically and can be from a liquid, solid (surface), or gas.
Liquid samples are taken from beer or rinse water after vessel or pipe cleaning. We can use alcohol swabbing, steaming, or we can flame the sample point to ensure that we do not sample environmental bacteria. The sample is collected into a sterile bottle through a sample tap or we can pierce a membrane septum with a sterile hypodermic needle.
Solid surfaces are swabbed with a sterile cotton bud and returned to the sterile bud holding tube.
Gases are sampled by bubbling the gas into sterile water then sampling the water, or directly through a sterile membrane filter.
Liquid samples, where no or low levels of contaminants are expected (e.g. beer or water), can be vacuum filtered through a membrane filter to isolate the organisms on the filter.
Samples are then grown on liquid (broth) or solid (agar) media aerobically or anaerobically to allow for enumeration (counting) or identification. Membrane filters are placed directly onto the agar plate and incubated to enable growth.
General purpose or basal media are used for primary isolation and support the growth of many microorganisms. These are usually liquid media.
Selective and enrichment media are designed to inhibit unwanted contaminants from growing and to select the strain required for isolation. This is important when, for example, we want to select a specific type of bacteria such as Lactobacillus from a general population of bacteria. A general purpose medium can be made selective by adding antibiotics, chemicals, dyes, changing pH, or a combination of these. In this context, solid media are referred to as selective and liquid media are referred to as enrichment media. We can prevent the growth of brewing yeast by adding cycloheximide.