296 THE PYOGENIC COCCI 
in man or animals/ but it is often present in the saliva;- occasionally 
it has been recovered from dento-alveolar abscesses.^ 
Morphology.— The organism occurs typically in tetrads, enclosed 
in transparent gelatinous capsules which require special staining 
methods for their demonstration. The individual cells are about 1 
micron in diameter. In artificial media the tetrad arrangement may 
disappear and the cocci occur chiefly in pairs and groups of three or 
four pairs. The tetrad arrangement and the capsule may be restored 
by passage through animals. The organism is non-motile, and 
possesses no flagella. It forms no spores and stains readily with ordinary 
anilin dyes. It is Gram-positive. 
Isolation and Culture.— Micrococcus tetragenus grows rather slowly 
in all ordinary media, particularly the first transfers from the tissues 
to artificial media. It can be isolated readily in pure culture in gelatin 
or agar plates; the colonies are small, round and graVish, 1 to 2 mm. 
in diameter. 
Growth in Media.— The organism does not liquefy gelatin, casein or 
blood serum. Acid is produced in glucose, lactose, saccharose and 
mannitol broths. A uniform turbidity is produced in plain and sugar 
broths; the growth is more luxuriant in the latter. Milk is slightly 
acidulated, but usually no coagulation or peptonization takes place. 
Micrococcus tetragenus is aerobic, facultatively anaerobic. The opti- 
mum temperature of growth is 37° C, the maximum about 44° C, 
the minimum about 12° C. The resistance to physical and chemical 
agents is undetermined. 
Products of Growth.- Unknown; no toxin has been described. 
Pathogenesis. Pathogenicity appears to be quite definitely asso- 
ciated with encapsulation, at least for mice."* The frequent occurrence 
of the organism in the sputum of the tuberculous and its occasional 
isolation from tuberculous cavities has led to the theory that Micro- 
coccus tetragenus may play a secondary part in the destruction of lung 
tissue. This is not definitely determined, however. It is also found 
in the saliva of healthy individuals. Less commonly it has been found 
in the pus of empyemas which follow pneumonia; but the organism can 
hardly be regarded as a human pathogen. 
Injected subcutaneously into white mice. Micrococcus tetragenus 
usually causes a fatal septicemia; the organism may be recovered 
from the heart blood, spleen and liver. House and field mice appear 
to be relatively immune. Intraperitoneal injection into guinea-pigs 
may cause a fatal peritonitis with much pus in which typical tetrads 
are found. Rabbits and dogs are not susceptible. Uncomplicated 
infections with the organism in man are so uncommon that nothing is 
> Miiller: Wien. klin. Wchnschr., 1904, 17, 815. 
2 Biondi: Ztschr. f. Hyg., 1887, 2, 194. Stoss: Baumgarten's Jahresb.. 1895, p. 48; 
1896, p. 757. 
3 Goadby: Mycology of the Mouth, 190.3, p. 101. 
^ Wreschncr; Zeitschr. f. Hyg., 1921, 93, 74. 
