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SUMMARY: When the avirulent strain of Mycobacterium tuberculosis H37 Ra was repeatedly subcultured in a liquid medium containing low concentrations of ‘Triton A20’ or other chemically similar non-ionic surface-active polyoxyethylene ethers, the morphology changed from the amorphous growth pattern characteristic of avirulent tubercle bacilli to the formation of microscopic ‘cords’ similar to those shown by virulent strains such as H37Rv. Once developed, the cord-forming property of H37 Ra persisted when cultures were transferred to liquid medium devoid of surface-active agent, even when such transfer was made after passage through mice; after further subcultures, however, a tendency to reversion was apparent. No difference in virulence was detected between cord-forming H37 Ra and the unmodified strain when tested in mice.Previous work had indicated that a lower member of the polyoxyethylene ether series, the dimer ‘D4’, inhibited amorphous-growing avirulent tubercle bacilli, including H37 Ra, but not the growth of naturally cord-forming strains, exemplified by the virulent H37 Rv and attenuated strains derived from it. The growth of cord-forming avirulent organisms, developed from H37 Ra in the present experiments, was also found to be relatively unaffected by D4.

Changes in Morphology of an Avirulent Strain of Mycobacterium tuberculosis under the influence of Non-ionic Surface-active Agents