l. Smooth mucoid colonies developed in anthrax cultures held
for long periods at 37°C. in a number of different media. Similar
colonies developed in strains attenuated at 42°C.
2. All these smooth mucoid colonies yielded rough variants which
were usually less virulent than the smooth parent and which in some
cases showed a complete and abrupt loss of virulence.
3. The loss of virulence and smoothness was associated with a
loss of the ability to produce capsules both in vitro and in vivo.
4. A number of these rough avirulent daughter strains produced
immunity to anthrax in guinea pigs.
5. Fully virulent and freshly isolated anthrax strains always
grew smooth mucoid on serum agar in carbon dioxide.
6. These virulent strains rapidly developed rough daughter
colonies in the carbon dioxide, in the same way that attenuated
smooth mucoid strains developed rough daughters under ordinary
conditions (summary 1 and 2).
7. These rough variants obtained on serum agar in carbon dioxide
were completely avirulent and uncapsuled in vitro and in vivo.
8. These rough a virulent dissociants were able to produce a
high degree of immunity in guinea pigs and preliminary immunity
tests on sheep were very promising.
9. Evidence is brought forward concerning the significance of
the capsule in virulence and immunity.
10. The above findings are discussed.