THE STAPHYLOCOCCUS GROUP 297 
definitely known of human susceptibility and immunity. Vaccines 
have been tried in a very few cases with somewhat i)romising results. 
Bacteriological Diagnosis.- The finding of Gram-positive cocci about 
1 micron in diameter in pus, which occur habitually in tetrads, usu- 
ally suffices to establish a satisfactory bacteriological diagnosis. The 
saliva occasionally contains tetracocci which resemble Micrococcus 
tetragenus very closely, but it is claimed by many that these organisms 
are not necessarily Micrococcus tetragenus. Isolation and identifica- 
tion by cultural methods must be resorted to in suspected cases. 
Micrococcus Ovalis. — Synonym. — Enterococcus.^ 
Historical.— Micrococcus ovalis was described by Escherich,^ who 
found it very commonly in the intestinal tracts of nurslings and 
bottle-fed infants. 
Morphology.— The organism is oval in outline, measuring 0.6 to 
0.9 microns in the lesser diameter, and it occurs habitually in pairs, 
with a tendency for the proximal ends to be slightly flattened, and the 
distal ends to be somewhat pointed. In this respect Micrococcus 
ovalis resembles the pneumococcus very closely. In fluid media, 
particularly sugar broths, the pairs of organisms tend to remain 
adherent in chains of greater or lesser length giving rise to a diplo- 
streptococcus arrangement which is precisely like that exhibited by 
the pneumococcus under the same conditions. 
Micrococcus ovalis is non-motile and possesses no flagella. It forms 
no spores. According to Lewkowicz'' and others, capsules are produced 
when the organism is isolated directly from lesions. The organism 
stains readily with ordinary anilin dyes, and it is Gram-positive. 
Isolation and Culture.— Micrococcus ovalis grows with moderate vigor 
on agar plates, better in glucose or lactose agar. The colonies after 
forty-eight hours' incubation at 37° C. are round, translucent, color- 
less, and measure about 1 to to 2.5 microns in diameter. They are not 
distinctive. Colonies on gelatin plates are very small and develop 
slowly. The medium is not liquefied. Blood agar appears to be a 
better medium for isolation of Micrococcus ovalis than any other; 
the colonies are 1 to 3 mm. in diameter even after eighteen hours' 
incubation, grayish and succulent. No hemolysis takes place. A 
slight turbidity, which soon settles, forms in plain broth; the addi- 
tion of glucose or lactose greatly enriches the growth. Milk is usually 
coagulated in one to three days (acid coagulation), but the coagulum 
does not become digested. 
Micrococcus ovalis is an aerobic, facultatively anaerobic organism. 
The lower limit of growth is about 8° C., the optimum from 37° to 
39° C., and the maximum about 45° C. Its resistance to chemical 
and phj'sical agents is about the same as that of the staphylococcus. 
1 Thicrcelin: Th^se de Paris, 1894; Compt. rend Soc. do bioL, 1899, 5, 55, 269. Also 
Dible: Jour. Pathol, and Bacteriol., 1921, 24, 3. 
- Darmbakterien des Sauglings, Stuttgart, 1886, p. 89. 
3 Centralbl. f. Bakteriol., 1901, 29, 635. 
