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Dehydrated Culture Media

BILE AESCULIN AGAR

Code: CM0888

A differential medium for the isolation and presumptive identification of enterococci / Group D streptococci.

Typical Formula*

gm/litre

Peptone

14.0

Bile salts

15.0

Ferric citrate

0.5

Aesculin

1.0

Agar

15.0

pH 7.1 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 44.5g in 1 litre of distilled water and bring gently to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
The major use of Bile Aesculin Agar is to differentiate between enterococci / Group D streptococci and non Group D streptococci. It may also be used for the presumptive identification of other groups of organisms.

Enterococci / Group D streptococci hydrolyse aesculin to form aesculetin and dextrose. Aesculetin combines with ferric citrate in the medium to form a dark brown or black complex which is indicative of a positive result. Bile salts will inhibit Gram-positive bacteria other than enterococci / Group D streptococci.

The value of bile tolerance together with hydrolysis of aesculin as a means of presumptively identifying enterococci / Group D streptococci is widely recognised1,2,3,4,5.

The use of these parameters forms the basis of Bile Aesculin Agar and was described by Swan6 who concluded that the use of this medium is a valid alternative to Lancefield grouping for the recognition of enterococci / Group D streptococci.

Facklam7 further confirmed its usefulness in differentiating enterococci / Group D streptococci from non Group D streptococci while other workers have used the medium for presumptive identification of the Klebsiella-Enterobacter- Serratiagroup amongst the Enterobacteriaceae8,9,10.

Technique
Using a sterile loop inoculate the medium with 4-5 colonies and incubate at 37°C for 18-24 hours.
The result is positive for bile salt tolerance and aesculin hydrolysis if blackening of the medium occurs.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw-brown coloured gel

Quality control

Positive controls:

Expected results

Enterococcus faecalis ATCC® 29212*

Good growth; brown coloured colonies with aesculin hydrolysis

Enterobacter aerogenes ATCC® 13048* Good growth; brown coloured colonies with aesculin hydrolysis

Negative control:

 
Streptococcus pyogenes ATCC® 19615* No growth
* This organism is available as a Culti-Loop®

References
1. Facklam R. R. and Moody M. D. (1970). Appl. Microbiol. 20, 245-250.
2. Isenberg H. D., Goldberg D. and Sampson J. (1970). Appl. Microbiol. 20, 433-436.
3. Sabbaj J., Sutter V. L. and Finegold S. M. (1971). Appl. Microbiol. 22, 1008-1011.
4. Facklam R. (1972). Appl. Microbiol. 23, 1131-1139.
5. Facklam R. et al (1974). Appl. Microbiol. 27, 107-113.
6. Swan A. (1954). J. Clin. Path. 7, 160-163.
7. Facklam R. (1973). Appl. Microbiol. 26, 138-145.
8. Wasilauskas B. L. (1971). Appl. Microbiol. 21, 162-163.
9. Lindell S. S. and Quinn P. (1975). J. Clin. Microbiol. 1, 440-443.
10. Chan P. C. K. and Porschen R. K. (1977). J. Clin. Microbiol. 6, 528-529.

 
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