Overview:
Bromocresol Purple Azide Broth is a highly selective liquid medium used for the confirmation and MPN enumeration of enterococci (Enterococcus faecalis, E. faecium, etc.) in water, milk, and foods. Sodium azide strongly inhibits Gram-negative bacteria and most Gram-positive organisms, while only enterococci grow and produce a characteristic acid reaction (purple ? yellow) from glucose fermentation.
Applications:
Confirmation of presumptive enterococci from azide-containing primary media
MPN determination of faecal enterococci in bathing/recreational water
Traditional European method (especially Germany, Netherlands, Switzerland)
Alternative to Bile Esculin Azide Broth in some national standards
Key Features:
Sodium azide 0.02�0.04 % ? excellent selectivity
Acid production from glucose ? clear purple-to-yellow colour change
Positive within 18�48 h at 37�44 �C
No Durham tube required (colour change is the indicator)
Composition (per liter):
Tryptone/Peptone 20.0 g
Glucose 5.0 g
NaCl 5.0 g
K?HPO? 2.7 g
KH?PO? 2.7 g
Sodium Azide 0.2�0.4 g
Bromocresol Purple (0.1 % solution) 32 mL (0.032 g)
Final pH: 7.0 � 0.2 at 25�C
Preparation:
Dissolve all components except sodium azide and indicator in ~900 mL water.
Adjust pH to 7.0.
Add azide (dissolved separately) and bromocresol purple solution.
Make up to 1 L.
Dispense 5�10 mL into tubes (single- or double-strength for MPN).
Autoclave at 121�C for 10 minutes only (azide is heat-labile).
Incubation:
37 �C (or 44 �C for faecal enterococci) for 24�48 h.
Interpretation:
Positive (enterococci): purple ? yellow colour change + turbidity
Negative: remains purple or only slight turbidity
Storage:
Prepared tubes: 2�8 �C, protected from light, use within 4 weeks.
Safety note: Contains sodium azide � highly toxic and explosive with metals.
For water and food microbiology use only. Traditional European reference method (DIN, NEN, SFS).