SUMMARY: Auxotrophic mutants of Bacillus anthracis with a specific requirement for adenine in their phenotype were isolated and the block in the purine pathway determined by enzymic activity of preparations obtained from the individual strains. The adenine-dependent mutants could be classed into two groups: one group was of strains devoid of any adenylosuccinate synthase (EC 220.127.116.11) activity; the presence of adenylosuccinate lyase (EC 18.104.22.168) activity was not detected in the strains of the other group. In these latter strains a single gene locus controls the two enzyme activities involved in the purine pathway, as established for other organisms.\r
None of the adenine-dependent strains was capable of killing mice although they were given a toxic dose of adenine at challenge. Reversion to prototrophy restored full virulence. The lack of proliferation of the adenine-dependent bacteria in the host could not be explained simply by a shortage of available adenine in the body of the mouse, since the adenine intake provided a certain concentration of this base at the inoculation site, in the peritoneal cavity, and in the blood stream. In spite of the fact that adenine-dependent mutants were producing both of the known aggressins, namely capsule and toxin, they were not capable of invading the blood stream or of multiplying there. It is assumed that something essential for invasion of the host was not produced by adenine-dependent mutants. The lack of this hypothetical factor rendered these organisms highly vulnerable to the defence mechanism of the host. In contrast to the adenine-dependent mutants, purine auxotrophs blocked either in the early part of the purine pathway or in the conversion of xanthylic acid to guanylic acid were found to grow readily in the body of the mouse, even in the absence of an exogeneous supply of purine bases. It is suggested that purine bases needed by these mutants for their growth might originate from an influx of purines from tissues damaged by the bacteria.
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