SUMMARY: Capsule formation by virulent strains of on nutrient agar is known to depend on incubation in air with added CO as well as the addition of serum or bicarbonate to the medium. The minimum effective concentration of CO varies with the pH of the medium in a way which shows that capsulation depends on a threshold concentration of bicarbonate in the medium. Serum is more effective than bicarbonate and appears to act by binding an agent which inhibits capsule formation since it is replaceable by activated charcoal. The inhibitor might be a fatty acid since certain acids prevented capsule formation. Capsules are formed on nutrient agar containing added bicarbonate only after the culture has become very dense which suggests that the organisms either inactivate the inhibitor or become resistant to its action as their growth rate falls on approaching the stationary phase.


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