On Skim milk agar (aerobic incubation) an excellent
colony dierentiation between S. thermophilus and L.
delbrueckii subsp. bulgaricus was achieved. The former
gave circular opalescent white colonies with well-de®ned
borders, while, L. delbrueckii subsp. bulgaricus gave irregular
¯at translucent colonies, bigger in size than the
colonies of streptococci, with non-de®ned borders. After
an anaerobic incubation, it was not possible to dierentiate
between both types of colonies, because of the
opalescent aspect of the Petri dish due to the precipitation
of casein.