. Pathogenic microörganisms; a practical manual for students, physicians, and health officers . -tinguish them from submerged, colonies of the colon group; they arefinely granular, with a sharp margin and a yellowish-brown color. Thesuperficial colonies, however, particularly when young, are often quitecharacteristic; they are transparent, bluish white in color, with an irreg-ular outline, not unlike a grape leaf in shape. Slightly magnified they 336 TYPHOID BACILLUS appear homogeneous in structure, but marked by a delicate networkof furrows. Surface colonies from some varieties of colon baci
. Pathogenic microörganisms; a practical manual for students, physicians, and health officers . -tinguish them from submerged, colonies of the colon group; they arefinely granular, with a sharp margin and a yellowish-brown color. Thesuperficial colonies, however, particularly when young, are often quitecharacteristic; they are transparent, bluish white in color, with an irreg-ular outline, not unlike a grape leaf in shape. Slightly magnified they 336 TYPHOID BACILLUS appear homogeneous in structure, but marked by a delicate networkof furrows. Surface colonies from some varieties of colon bacilli give asimilar picture. In stick cultures in gelatin the growth is mostly on the surface, appear-ing as a thin, scalloped extension, which gradually reaches out to thesides of the tube. In the track of the needle there is but a limitedgrowth, which may be granular or uniform in structure, and of ayellowish-brown color. Growth in Bouillon.—^This medium is uniformly clouded by the typhoidbacillus, but the clouding is not so intense as with the colon bacillus. When the bouillon is somewhat. alkaline a delicate pellicle issometimes formed on the surfaceafter eighteen to twenty-fourhours growth. Growth on Agar.—^The streakcultures on. agar are not distinc-tive; a transparent, filiform, gray-ish streak is formed. Growth on Potato.—The growthon this medium was formerly ofgreat importance in identification,but now other media, giving morespecific characteristics, have beendiscovered. When characteristic,the growth is almost invisiblebut luxuriant, usually coveringthe surface of the , the growth may be quiteheavy and colored yellowishbrown, and with a greenish halo,when it is very similar to the growth of the colon bacillus. Thesedifferences of growthon potato appear to be chiefly due to variationsin the substance of the potato, especially in its reaction. For thecharacteristic growth the potato should be slightly acid. A new lotof potato should always be tested wi
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