Overview:
ABGP Medium is a selective enrichment broth developed by Schubert (1966) specifically for the isolation of Streptococcus agalactiae (Group B Streptococcus, GBS) from clinical specimens (vaginal/rectal swabs, newborn samples) and raw milk. The combination of arginine, brilliant-green dye, and nalidixic acid strongly inhibits Gram-negative bacteria, enterococci, staphylococci, and most other streptococci, while GBS grows luxuriantly and produces a characteristic alkaline shift (purple ? yellow) due to arginine deamination followed by glucose fermentation.
Applications:
?? Selective enrichment of Streptococcus agalactiae (GBS) from mixed clinical and food samples
?? Screening of pregnant women for GBS carriage (35�37 weeks gestation)
?? Detection of GBS in raw milk and dairy products
?? Recommended in older European protocols (Germany, Switzerland, Austria) as an alternative or complement to LIM/Todd-Hewitt enrichment
Key Features:
?? Highly selective � brilliant-green + nalidixic acid suppress almost all non-GBS flora
?? Colour change indicator (purple ? yellow) signals GBS growth
?? Arginine stimulates heavy GBS growth and typical alkaline reaction
?? Still widely used in some European laboratories despite newer media
Composition (per liter):
Peptone  � 10.0 g
Glucose � 5.0 g
L-Arginine HCl  � 5.0 g
NaCl    � 5.0 g
Na?HPO?  � 2.0 g
Brilliant Green  � 0.01 g (10 mg)
Nalidixic Acid � 0.015 g (15 mg)
Bromocresol Purple� 0.02 g
Final pH: 7.2 � 0.2 at 25�C
Preparation:
Dissolve all components except nalidixic acid and bromocresol purple in 900 mL distilled water.
Adjust pH to 7.2.
Add bromocresol purple (dissolved in a few drops of 0.1 N NaOH).
Make up to 1 liter.
Dispense 5�10 mL into tubes.
Autoclave at 121�C for 15 minutes.
After cooling to <60�C, aseptically add filter-sterilized nalidixic acid (15 mg/L).
(Some commercial versions include heat-stable nalidixic acid derivatives.)
Incubation:
35�37�C for 18�24 hours (up to 48 h if necessary).
Positive reaction: medium turns yellow (GBS present).
Negative: remains purple.
Subculture:
Subculture yellow tubes onto Columbia CNA blood agar, Granada medium, or CHROMagar� StrepB for confirmation.
Storage:
Prepared broth (with nalidixic acid): 2�8�C, protected from light, use within 4 weeks.
For laboratory use only. Not for diagnostic use without proper validation.