Growth medium

They remain solid, as very few bacteria are able to decompose agar (the exception being some species in the genera: Cytophaga, Flavobacterium, Bacillus, Pseudomonas, and Alcaligenes).

For microorganisms, they consist of providing trace elements and vitamins required by the microbe and especially defined carbon and nitrogen sources.

An undefined medium has some complex ingredients, such as yeast extract or casein hydrolysate, which consist of a mixture of many chemical species in unknown proportions.

The number of ingredients that must be added to a minimal medium varies enormously depending on which microorganism is being grown.

[7] Minimal media are those that contain the minimum nutrients possible for colony growth, generally without the presence of amino acids, and are often used by microbiologists and geneticists to grow "wild-type" microorganisms.

Media lacking an amino acid such as proline in conjunction with E. coli unable to synthesize it were commonly used by geneticists before the emergence of genomics to map bacterial chromosomes.

Gancyclovir is an exception to the rule, as it is used to specifically kill cells that carry its respective marker, the Herpes simplex virus thymidine kinase.

[12] This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators (such as neutral red, phenol red, eosin y, or methylene blue) added to the medium to visibly indicate the defining characteristics of a microorganism.

[16] In addition, the dependence of a cell line on a metabolic gene was shown to be affected by the media type.

[20] In contrast, serum-free media (SFM) offer standardized formulations that enhance reliability and reduce contamination risks.

They are designed to include essential nutrients like amino acids, vitamins, and glucose, but can sometimes provide weaker growth performance compared to serum-containing alternatives.

[22] The development of protein-free and chemically defined media is aimed at achieving greater consistency and control in cell culture processes.

An agar plate – an example of a bacterial growth medium*: Specifically, it is a streak plate ; the orange lines and dots are formed by bacterial colonies.
US Food and Drug Administration scientist tests for Salmonella
A culture of Salmonella bacteria
Physcomitrella patens plants growing axenically on agar plates ( Petri dish , 9 cm diameter)
Blood-free, charcoal-based selective medium agar (CSM) for isolation of Campylobacter
Blood agar plates are often used to diagnose infection. On the right is a positive Staphylococcus culture; on the left is a positive Streptococcus culture.
Four types of agar plate demonstrating differential growth depending on bacterial metabolism
UTI agar is a chromogenic medium for differentiation of main microorganisms that cause urinary tract infections (UTIs).