170 
Journal o f Agricultural Research 
Vol. XXIX, No. 4 
few days as a finely granular streak, 
then disappears. 
Agar slants. —On peptone-beef agar 
growth is smooth to slightly pitted. 
Margins definite, entire. Transparent 
to translucent. White, very viscid. 
On 2 per cent agar the growth is some¬ 
what restricted. On 1 per cent agar 
the growth quickly spreads over the 
surface and forms a pellicle over the 
water in the V and a white, fine grained 
sediment is deposited. 
On beef agar there is often a lack of 
uniformity in the interior structure of 
the growth, ‘some areas being quite 
translucent, others opaque. This 
gives a mottled appearance to the 
growth. 
Beef bouillon. —In peptone-beef 
bouillon+ 14 at 24° C. there is thin 
clouding and a thin pellicle in 18 hours. 
Clouding is best at the surface and 
never becomes heavy. Pellicles are 
white, membranous, and viscid, falling 
when even slightly disturbed and 
another pellicle forming promptly. As 
many as five definite pellicles have 
formed in one culture tube. Sediment 
consists of a scanty amount of white 
substance, small crystals, and fallen 
pellicles. Pellicles eventually disin¬ 
tegrate and form a viscid, translucent 
mass. The medium also becomes 
viscid. 
Gelatin plates. —In +10 peptone- 
beef gelatin at 18° to 20° C. the colonies 
grow slowly. They have the same 
character in regard to center and border 
as colonies in agar plates. The borders 
remain thin and narrow and are 
slightly lobed at the margin. Lique¬ 
faction was noted first after 5 to 7 
days as a narrow rim about the colony. 
The margin of liquefaction is smooth 
and definite. Eventually the colony 
floats in a clear liquid. 
Gelatin stabs. —At 18° to 20° C. in 
+ 10 gelatin liquefaction becomes visi¬ 
ble in 48 hours and proceeds at a mod¬ 
erate rate. In 5 to 6 days the lique¬ 
fied layer is 5 to 7 mm. deep. There is 
a thin, white pellicle, thinly clouded 
liquid and a small amount of white 
sediment. Four to six weeks are re¬ 
quired for complete liquefaction of 
tubes containing 10 cc. of gelatin. 
Blood serum. —On Loeffler’s blood 
serum at room temperature (25° to 
27° C.) growth is abundant, smooth, 
white, opaque. The medium becomes 
translucent, light amber in color, and 
the consistency of soft jell} 7 . 
Potato cylinders. —Growth on 
steamed potato is scanty. A very 
thin, white growth over the potato 
and slight white deposits along mar¬ 
gins where the potato touches the tube 
are the only indications of growth. 
The water remains clear, sometimes 
slightly browned. The potato dis¬ 
colors slightly. The diastasic action 
is feeble. 
Potato agar. —Potato broth with 
agar. Growth on this medium is lim¬ 
ited. Thin, white, becoming dull 
white. No change in the color of the 
medium. 
Potato agar with dextrose 6 .— 
Colonies in plates are at first smooth, 
shiny, hemispherical, like tiny drops of 
boiled starch. In 2 to 3 days the 
surface is finely wrinkled or coarsely 
pitted, and a trace of brown color 
has appealed. In 5 to 6 days the col¬ 
onies are rather umbonate and brown 
at the margins. In 10 to 12 days the 
whole colony is brown and widely sep¬ 
arated colonies have developed a new, 
thin, cream-colored marginal growth. 
Colonies in thickly sown plates are less 
likely to turn brown than well isolated 
colonies. 
On slants and stabs the growth is 
first white, opaque, and slightly con¬ 
toured. Later the growth is very 
abundant and the surface becomes 
smooth and shining. The color of 
the growth in most isolations becomes 
tawny to russet brown, the color 
usually showing first at points where 
the growth touches the wall of the tube. 
On soft, moist agars the growth is 
often quite smooth from the beginning. 
The growth is butyrous in consistency. 
Some isolations produce a dark-brown 
growth, others become only slightly 
brown. Microscopic examination of 
the brown growth shows the color to 
be due partly to brown bacteria which 
vary in size from small coccilike forms 
to abnormally large rods; and partly 
to brown, spherical bodies (PI. 5, J) 
the character of which is yet to be 
determined. They may be degenerate 
bacteria, or a substance formed by 
bacterial action. 
The brown bacteria and the brown 
spheres occur together in pseudozoo- 
gloeal masses, the surrounding growth 
consisting of normal, hyaline bacteria. 
There is also produced a definite 
change in the medium as evidenced by 
an area of increased translucency 
bordering the brown growth. This 
forms a halo about plate colonies, 
rapidly spreading out until all the 
agar in the plate is changed. In stab 
and slant tube cultures a deep layer 
6 A modified Thaxter’s potato agar. Potato 500 grams, sliced and steamed, in 1,000 cc. of distilled 
water, strained, 13 ^ per cent agar added, steamed again, filtered, 2 per cent dextrose added, tubed, 
and sterilized. 
