Lactose and Non-lactose fermenting colonies of Escherichia coli on CLED Medium

CLED (Cystine-Lactose-Electrolyte-Deficient) medium is commonly used in microbiology for the culture and differentiation of urinary pathogens based on their ability to ferment lactose. Escherichia coli (E. coli), being a typical inhabitant of the intestinal tract and a common urinary pathogen, is often tested on this medium.
Lactose-Fermenting E. coli
On CLED agar, lactose-fermenting strains of E. coli typically produce colonies that are bright yellow due to the fermentation of lactose, which lowers the pH and leads to a change in the indicator color present in the medium. This bright coloration makes it easy to distinguish these colonies from non-lactose fermenters.
Non-Lactose Fermenting E. coli
Non-lactose fermenting strains of E. coli, on the other hand, produce colorless or bluish colonies on CLED agar. These strains do not ferment lactose and therefore do not cause the pH of the medium to change, which would otherwise result in a color change of the pH indicator.
Purpose and Benefits of CLED Medium
The CLED medium is particularly useful because it:
Prevents the swarming of Proteus species, which can obscure other colonies.
Clearly distinguishes between lactose fermenters and non-fermenters.
Helps in the identification and enumeration of urinary pathogens in a clinical setting.
This selective and differential medium provides valuable diagnostic information for microbiologists, aiding in the quick identification of E. coli strains and potentially guiding antibiotic treatment decisions.