Ammonia production from arginine was tested in two
media; arginine broth , and B.C.P.A.. In arginine broth , the
detection of alkaline reaction was done by a color change reaction
of Nessler's reagent . B.C.P.A. detect s pH change. It is purple or
dark blue at neutral pH, however, when I t comes i n contact with
acid (pH < 5.2), i t turn s yellow . Thus, all colonies on B.C.P.A.
appear whit e at first , they all form yellow zones with in firs t
two days of growth, and the narginine utilizing bacteria ( S .
lactis ) eliminat e the yello w zones , indicatin g an alkalin e
reaction .