A total of 34 thermophilic isolates identified as members of the genus
Thermoactinomyces by a range of chemotaxonomic, microscopic and deter
minative biochemical tests, were isolated from two acid soils. Growth
studies in shake flask and fermenter identified the isolates to be mod
erately acidophilic with growth occurring between pH 4.5 and 6.0 with
optima at pH 5.0. The isolates differed considerably from known Thermo
actinomyces cultures in their pH profile, colony morphology and in sev
eral biochemical tests. Extracellular enzyme activities are identified
and partially characterized in terms of their thermostability, pH and
temperature profiles from crude supernatant fluid samples. Optimal pr
otease, amylase and pullulanase activity was observed at pH 5.0-5.5 an
d 75-80 degrees C with each showing T-(50) values of 10, 30 and 30 min
, respectively. A highly thermotolerant extracellular esterase was als
o identified which retained 50% activity after 8 h at 90 degrees C. Th
is is the first report of an acidophilic member of the genus Thermoact
inomyces.