CHROMOGENIC TRYPTONE BILE X-GLUC (TBX) AGAR
A chromogenic medium for the detection of E. coli in foodstuff.
Code: CB1/1115a
Typical formula g/l
| Tryptone | 20.0 |
| Bile salts No. 3 | 1.5 |
| Agar | 14.0 |
| X-GLUC | 75.0 mg |
pH 7.2 ± 0.2
Directions
Weigh 35.6g in 1000ml of cold distilled water. Heat to boiling with agitation until complete dissolution and autoclave at 121°C for 15 minutes. Cool to 50°C and distribute into sterile Petri dishes.
Description
Chromogenic Tryptone Bile X-GLUC (TBX) Agar is a selective differential medium for the enumeration and immediate identification of Escherichia coli in foodstuff. It is prepared according to the formulation given by ISO 16649-2. The medium contains bile salts for the complete inhibition of Gram-positive bacteria and X-GLUC (5-bromo-4-chloro-3 indolyl â-D-glucuronide) for the detection of â-glucuronidase. Among the Enterobacteriaceae only E. coli, together with some strains of Salmonella and Shigella, is one of the few â-glucuronidase positive species, so cultivates on the plates with blue or green-blue colonies. â- glucuronidase negative bacteria grow with colourless colonies.
Method
Transfer 1ml of the test sample into a sterile Petri dish if liquid, or 1ml of the initial suspension (10-1), in the case of other products. Inoculate 2 plates per dilution. Repeat the procedure with further decimal dilutions if necessary. Pour into each Petri dish about 15ml of TBX Agar pre-cooled to 47-50°C. Mix inoculum well with the medium. Invert the inoculated dishes and incubate at 44°C for 18-24 hours. In cases where stressed colonies are suspected incubate for 4 hours at 37°C before incubation at 44°. Do not incubate over 45°C. Count as presumptive all E. coli the blue or green-blue colonies. Report the results as UFC/g considering the "dilution factor". The medium can be inoculated even on the surface with the usual methods.
Quality Assurance (44°C - 24 hrs)
Productivity control
E.coli ATCC 25922: growth, blue-green colonies
Specificity control
S.typhimurium ATCC 14028: growth, colourless colonies
Selectivity control
E.faecalis ATCC 19433: inhibited
Storage
Dehydrated medium: 2-8°C
User prepared flasks: 1 month at 2-8°C
Reference
ISO 16649-2 (2001). Microbiology of food and animal feeding stuffs - Horizontal method for the enumeration of presumptive E.coli- part 2: colony count technique at 44°C using 5-bromo-4-chloro-3-indolyl-beta-D-glucuronic acid.