Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Bacteroides Bile Esculin (BBE) Agar
Purpose
Bacteroides fragilis is the most abundant bacterium found in the human colon, reaching densities of 10^11 cells per gram of feces! It also is the most common anaerobic human pathogen. BBE Agar is a selective and differential medium used for the isolation and presumptive identification of B. fragilis and its close relatives (B. fragilis group).
Principle
Nutrition is supplied by a base medium of tryptic soy agar, which includes digests of casein (milk protein) and soybean meal. Other anaerobes in the sample are inhibited by oxgall (bile). Facultative anaerobes, also abundant in feces, compete with obligate anaerobes when grown anaerobically. These are inhibited by the antibiotic gentamicin. The medium also includes esculin, which B. fragilis is capable of hydrolyzing to produce esculetin. Esculetin in turn reacts with ferric ion in the medium to produce a brown coloration around B. fragilis growth (Figures 7-7 and 7-8). Presumptive identification of B. fragilis is based on its ability to grow on BBE and darken the medium (Figure 2-4).
2-4 BACTEROIDES BILE ESCULIN AGAR This is
a streak plate of a fecal specimen on BBE. The
larger colonies within the darkened medium are
presumptively identified as B. fragilis group. The
smaller, lighter colonies not producing darkening
of the medium are something other than B. fragilis
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
COMMENTS