Characteristics of coagulase positive and coagulase negative staphylococci in serum-soft agar.

Preliminary experiments revealed that coagulase positive staphylococci could be distinguished culturallv from coagulase negative staphylococci in a soft agar medium containing a small amount of normal human or rabbit plasma. In a medium of suitably low viscosity, staphylococci produce elongated diffuse feathery colonies. When coagulable plasma is incorporated in such a medium, the colonies of coagulase positive staphylococei are compact and spherical, whereas colonies of coagulase negative staphylococei are essentially unaltered. It was considered, at this point, that coagulase elaborated by microcolonies affected the plasma resulting in increased viscosity in the immediate colonial environment which, in turn, influences the morphology of the developing colony. Although this premise may be untenable, some observations made in an attempt to elucidate the mechanism of this phenomenon are of interest and may find application in studies of staphylococei and other bacteria. The use of soft or semisolid agar media in the differentiation of microorganisms is not unique. Pike (1946) used a modification of the soft agar medium of Ward and Rudd (1938) in differentiating streptococci. MIorphological changes of colonies in a semisolid medium were used as an indication of pneumococcus transformations (MIcCarty et al., 1946). This technique was later applied by Ravin (1954). Pittman and Davis (1950) used growth characteristics in semifluid media to distinguish species of Haemophilus. A double layer technique with antiserum-containing soft agar was utilized by Stocker (1949) in studies on flagellar antigenic phases in Salmonella. Knox et al. (1956) recently reported on characteristics of growth of Mycobacterium tuberculosis in semisolid agar media. Lankford et al. (1955) successfullly used soft agar, with and without antiserum, for demonstrating antigenic variation