Types of bacteria on blood and chocolate agar and the immediate cause of these types

The action of certain bacteria on blood agar is so striking and constant a phenomenon that it has come to be of considerable importance in diagnosis. This is especially true in the case of the cocci. The effect on blood agar of bacteria other than the cocci has been described from time to time but no systematic study of a large number of types seems to have been made. Paul-son and Brown (1931) have recently described colonies of colon bacilli of the alpha, beta, and gamma types. The present paper deals with the effect on blood agar of some 260 strains of Gram-negative bacilli and 14 selected strains of various types of streptococci and pneumococci. The probable cause of the green coloration produced by some of these bacteria has been determined, and the various factors which influence the production of the green coloration have been investigated. A study has also been made of the effect on chocolate agar of both bacilli and cocci. It must be emphasized at this time that the types of bacterial colonies in blood agar about to be described are all deep colonies. Many deep colonies of the gamma (non-hemo-lytic) type may be considerably more hemolytic as surface colonies. The appearance of different types of colonies of streptococci in blood agar has been described by a number of investigators and especially by Brown (1919) in whose paper wrln be found a I The writer wishes to express his appreciation to Dr. J. Howard Brown for many helpful suggestions, and kindly criticism