The “S” and “R” Forms of Hemophilus Influenzae

Abstract

During the course of an experiment with a culture of Hemophilus influenzae, transplants were made on a plate of Levinthal's solid transparent medium. It was observed that the culture contained 2 types of colonies. Well defined small transparent colonies resembling the textbook description of the typical Pfeiffer bacillus colony were found but in addition there were larger, slightly opaque iridescent mucoid smooth colonies. Transplants were made from the transparent colonies on Levinthal agar and all the colonies were found to be identical with the mother colony. The same was true of transplants from the opaque iridescent colonies. It was thus possible to separate 2 cultures each of which produced typical colonies. Since the largest transparent colonies were found to have a rough surface the strain producing these colonies has been called an “R” strain; and since the iridescent colonies from the other culture had a smooth surface this strain has been called an “S” strain.

This culture and 2 others from which “S” and “R” forms were subsequently isolated had been seeded many times on blood or chocolate agar and the variation in the colonies had not been noticed. Yet the difference in the colony appearance was marked when transplants were made on the transparent agar plates. Very striking was the difference when the plate cultures were held before a 100 watt electric light bulb or in the sunshine.

The morphology of the bacteria from the 2 strains has also been found to be dissimilar and characteristic for each type. This is best noticed if the stained preparation is made from a solid medium culture which is not older than 24 hours. The bacteria from the “S” culture appear as small coccobacilli that vary little in size.