Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Endo Agar
Purpose
Endo agar is used to detect fecal contamination in water and dairy products. Whereas its current use
is to isolate and identify the presence of enteric lactose fermenters (coliforms), its original use was to isolate and identify Salmonella typhi, a lactose nonfermenter (noncoliform).
Principle
Endo Agar contains color indicators sodium sulfite and basic fuchsin (which also double as Gram-positive inhibitors). Lactose is included as a fermentable carbohydrate. Lactose fermenters (Figure 2-10) growing on the medium will appear red or pink and darken the medium slightly due to the reaction of sodium sulfite with the fermentation intermediate acetaldehyde. Refer to the Appendix for more details on fermentation. Lactose nonfermenters produce colorless to slightly pink growth (Figure 2-13). Some lactose fermenters, such as Escherichia coli and Klebsiella pneumoniae produce large amounts of acid, which gives the colonies a metallic sheen (Figure 2-14).
2-13 ENDO AGAR Endo Agar inoculated with Escherichia coli (top),
Enterobacter aerogenes (lower right) and Shigella sonnei (lower left).
Notice the difference in the intensity of the pink between
E. aerogenes (a coliform) and S. sonnei (a noncoliform).
2-14 METALLIC SHEEN Endo Agar streaked with
Escherichia coli to illustrate the metallic sheen resulting
from large amounts of acid produced during lactose fermentation.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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