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Related post: Morphological Characters, — The individual cells of different bacteria range from spherical or slightly ellipsoidal {mtcrococ- cus) to oblong or cylindrical {bacterium) or corkscrew-shaped {spirillum). Their size, mode of multiplication, and such internal structure as can be made out directly, or by the aid of reagents or staining fluids, furnish additional aids in the distinction of species. But each of the characters is usually variable within certain limits, in a given species, and the limits of variability of closely related forms not infrequently overlap, in one respect or other, so that the most experi- enced investigator might be pardoned for declining alto- gether to pronounce judgment on a species that he was perfectly familiar with, if it were presented in a fluid cul- ture, or even in a mounted slide, well stained, but from a source of which he knew nothing. But while the morphological characters of bacteria, as seen under the best microscopes, may fail when taken alone for discrimination between related species, what must be regarded as invisible structural differences, render it possi- ble to stain one species with a pigment that is not taken up, or is readily parted with, by another, very similar in other respects. In illustration, I need only recall the bacillus of tuberculosis, and the similar germ of leprosy— now distin- guished by their different behavior with reference to staining agents. Fluid Cultures. — Where the species are isolated in pure L FROM A ROTANTCAI, jtn- cultures, microscopic characters are often the most valuable attainable. For Septra Cost many purposes, cultures in suitable nutri' ent solutions in carefully sterilized and pluj^ged Hasks tubes, are useful, especially iii Buy Septra the study of species wtui form characteristic mycoderma peliicles, or diffusible . "^ ments. Cultures of this kind l.ave been much used by both advocates and opponents of the theory of spontaneous generation as applied to lower organisms, and we owe to them many of the brilliant results atlained by Pasteur and Tyndall, Indeed, the Pasteur school still makes constant use of fluid cultures. But a serious objection to tlieir em- ployment lies in the ease with which a foreign species, in- troduced into a flask by accident, may spread through and contaminate the whole. Potato Cultures. — For most purposes, pure cultures on solid media are preferable to those in fluids. The simplest of these is potato that has been thoroughly washed, rinsed in dilute corrosive sublimate, and steamed or slowly boiled so as to be cooked without falling to pieces. The potato is taken from the boihng water in a thoroughly disinfected spoon, halved with a sterilized knife, and placed in a dish under a glass cover — both of which, after being made chemically and optically clean, hi-.ve been well sterilized rinsing with a one-tenth per cent, solution of corrosive limate, or by exposure to a high temperature for some Ui By means of a platinum needle, cleaned and heated shortlv before it is used, the merest trace of fluid or zooglea to \->- Septra Iv cultivated, is transferred to the cut surface of the potato a-i distributed by making a slight scratch with the point of lin needle, after which the cover is replaced, and the culturv. duly numbered and labeled, is set aside to be examinedi inlervab, for the purpose of taking notes on the api of the inoculated scratches. Some cultures are fouiK thrive best when shielded from light, these are kept in ets; others stand on the tables of a well-lighted lal still others are maintained at elevated temperatures may be controlled within narrow limits by the use of " With all precautions, many cultures of this sort are 4 laminated by germs from the air, but in from one to I days the inoculation lines of most of them show, here | there, or for their entire length, minute, mostly hemia ical zooglea ma.sscs, some of which have developed ( isolated cells left by the inoculating needle. If the Septra Ds 800 cult BACTERIA FROM A BOTANICAL STANDPOINT. 121 contains more than one species, the fact is easily recognized, from the different appearance of the zooglea dots, each of which may be made the starting point for a new inoculation ; or, in case the common ''weeds" of the laboratory have in- truded, the desired species only is removed Septra Tablets to a new cul- ture, and the rest are destroyed. For greater security, potato that has been mashed and made into paste of suit- able consistency, after cooking, is sometimes put in plugged test-tubes, and sterilized, after which it is inoculated simi- larly to gelatinized media. As a result of the lobing and confluence of the colonies, their varying texture and surface as they mature or become dry, and the pigments that some of them secrete, many bacteria are easily known at sight, when grown on potato, though no study of them is complete even from a botanical standpoint until they have been examined living, under the best powers of the microscope, and a thorough comparison is made of permanent preparations, colored with suitable staining fluids. Transpofent Solid Cultures, — By far the best methods, however, are those introduced by Koch, and applied by Septra Ds Tablets him and his school to the investigation of many species of chromogene and pathogenic bacteria. In these, Septra Ds the potato is replaced by a transparent substance which can be im- pregnated with any desired nutrient solution, and sterilized thoroughly. Where the temperature need not be elevated, from five to ten per cent, of gelatine, neutralized or ren dered slightly alkaline by the addition of sodium carbon- ate, is added to a suitable decoction of meat or other organic matter, and, after cooking and filtering, a few cubic centimeters of the mixture is placed in each of a number of sterilized test-tubes, that are subsequently plugged with sterilized cotton, over which a layer of filter paper has been placed, in some cases to prevent the introduction of molds, the mycelium of which is sometimes able to pass through cotton alone. The contents of the tubes arc then sterilized by protracted boihng, Septra Ds Tablet or preferably by boiling for about ten minutes at a time on four or five successive days, which is more certain, inasmuch as a single boiling kills the vege-
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