Colony characteristics of bateria in CLED (Cysteine Lactose Electrolyte Deficienct Media)

In CLED (Cysteine Lactose Electrolyte Deficient) agar, which is commonly used for the isolation and differentiation of urinary pathogens, bacteria grow in distinctive ways. The characteristics of bacterial colonies on CLED agar depend on the organism's ability to ferment lactose and produce certain by-products.

CLED agar is deficient in cysteine, which inhibits the growth of certain fastidious organisms, especially some gram-negative rods. Therefore, bacteria that require cysteine for growth may not grow or may show slower growth.

Lactose fermenters produce colonies that appear as yellow, whereas colonies of non-lactose fermenters appear blue on CLED agar. [1]

Photograph 1. Plate A in the illustration does not contain any bacteria growing on it; instead, it displays CLED agar plates. Proteus vulgaris and Escherichia coli, respectively, have been grown on plates B and C. Take note of the colour shift on plate C, which has E. coli growing on it. Swarming may occur in some sections of plate B if the colonies are relatively near to one another and the plate has been incubated for an extended period of time. Proteus species typically do not swarm on CLED agar. Staphylococcus pseudointermedius has been grown in plate D.

[Photo courtesy of Lise-Lotte Fernström and Karl-Erik Johansson (BVF, SLU).][2]

Organism	Colony Characteristics
1. Escherichia coli	Large, elevated yellow, opaque, with a center slightly darker. The agar is yellow .
2. Enterobacter species	Similar to E. coli but mucoid and larger in size. Colour of agar is yellow.
3. Klebsiella spp.	Large, yellow or yellowish-white. Highly mucoid and elevated. It can present a light blue shade. Yellow agar.
4. Proteus spp.	Proteus species, particularly Proteus mirabilis, can show a characteristic swarming growth pattern on CLED agar, where the bacteria spread over the surface in concentric rings. The colonies may appear yellow, but the swarming can obscure the exact color.
5. Pseudomonas spp.	Pale bluegreen. Typical matte surface and irregular edges. "Sweet" odour.
6. Salmonella, Shigella and Serratia	Blue-green agar.
7.Providencia	Blue to intense blue.
8.Streptococcus faecalis	Very small, from 0.4 mm, yellow, opaque. Colour of agar is yellow.
9.Staphylococcus spp.	Small, yellow intense colour, opaque. Colour of agar turns yellow.
10. Corynebacterium spp.	Very small, gray.

References

1. https://www.vetbact.org/?displayextinfo=50#:~:text=Lactose%20fermenters%20produce%20colonies%20that,the%20agar%20will%20turn%20yellowish.
2. https://www.vetbact.org/