This article opens the laboratory methods pillar of the TAAG Learning Center. Continue with what microbiological enrichment is.
What is traditional microbiological culture?
It is the oldest and still most widespread method for detecting and quantifying microorganisms: it consists of plating a sample (or a dilution of it) onto a culture medium that provides the necessary nutrients, incubating it at a defined temperature and time, and then observing whether visible colonies grew. Each colony is interpreted as the result of the growth of an original colony-forming unit (CFU).
Why does it remain the "reference method"?
Most regulatory frameworks (FDA, USDA-FSIS, ISO standards) still consider traditional culture the method against which alternative methods — including PCR — are validated. The reason is conceptual: culture demonstrates that the microorganism is alive and capable of multiplying, the most direct definition of "viable" in microbiology. A new method, however fast, generally needs to demonstrate equivalence against this standard before being accepted as an alternative.
Culture media: general vs. selective
General (or non-selective) media allow a wide variety of microorganisms to grow and are used for total counts, such as aerobic mesophiles. Selective media, on the other hand, include substances that inhibit the growth of most microorganisms and favor only the one being sought — for example, a medium with high salt concentrations favors Staphylococcus aureus and inhibits many others.
The main drawback: time
A culture requires the microorganism to multiply enough to form a visible colony, a process that — depending on the microorganism and the medium — can take from 24 hours to several days. For pathogens that require a prior enrichment step, the total time to a confirmed result can exceed 4-5 days, the main reason the industry adopted molecular methods for rapid screening.
Biochemical and serological confirmation
A colony with the expected morphology is not, by itself, a definitive identification: it is usually confirmed with biochemical tests (which evaluate which substances the microorganism metabolizes) and, for certain pathogens such as Salmonella, with serological tests that identify specific antigens on the bacterial surface.
Conclusion
Traditional microbiological culture remains the standard against which any new method is measured, precisely because it directly demonstrates viability. Its main cost is time, which is why it increasingly coexists with — rather than being replaced by — rapid molecular screening methods.
About TAAG
Discover how TAAG's laboratory services combine reference culture with rapid molecular detection according to each customer's regulatory requirement.
Frequently asked questions
Will traditional culture disappear in favor of molecular methods?
It is unlikely in the short term: it remains the regulatory reference method in most frameworks, although it is increasingly used in combination with rapid methods that perform the initial screening.
Does a selective medium guarantee that only the target microorganism will grow?
Not absolutely; it drastically reduces the growth of other microorganisms, but some non-target species can partially tolerate the selective conditions.
Why do some cultures require more than one medium in sequence?
Because different stages serve different purposes: a non-selective enrichment medium first, then a selective one to specifically isolate the target microorganism.