4.3.3 Duration of incubation

In the diagnostic setting, the incubation period is the time elapsing between the inoculation of the media and the planned visual inspection of the bacterial growth.

How does bacterial growth appear in liquid and on solid media?

Answer

In liquid media, the growth may appear as a turbid (cloudy) medium, as a sediment on the bottom of the tube or a meniscus on the surface, with a colour change if a chemical growth indicator is present.

On solid media, the bacterial growth appears as a confluent growth or as discrete colonies along the inoculated areas (Figure 8).

Described image
Figure 8 Bacteria grown in liquid media making it turbid (top), and in solid media forming discrete colonies or a confluent growth on the surface of the medium (bottom).
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The first image shows bacteria grown in liquid media making it turbid. The second image shows bacteria grown in solid media forming discrete colonies or a confluent growth on the surface of the medium.

Figure 8 Bacteria grown in liquid media making it turbid (top), and in solid media forming discrete colonies or a confluent growth on the...

The time of inspection varies depending on the expected bacterial species and the bacterial load in the inoculum. Fast-growing bacteria such as Escherichia coli double in number every 10–20 minutes and require short incubations to reach a visible growth, while slow-growing organisms such as pathogenic mycobacteria have a generation time of 12–20 hours and require long incubation.

Long incubation times increase the likelihood of growth of contaminants such as environmental bacteria, and other organisms that require prolonged incubation times.

4.3.2 Incubation temperature

4.4 Procedures for bacterial isolation