discontinuous method. Broth may be autoclaved, as may potato, but all these media are best sterilized by streaming steam. (4) Sterilization at Low Temperatures.-The vegetative forms of most bacteria are easily destroyed at low temperatures (55°-60° C.). Advantage is taken of this fact in sterilizing blood serum and other fluids which coagulate at 75°-100° C. The serum collected under aseptic precautions is kept at a temperature of 57° 58° C. for an hour on five or six successive days. The temperature adopted does not coagulate the serum, which may be used in the fluid condition if desired, and more important still may be kept in sterile flasks of convenient size for indefinite periods, care being taken to avoid certain infection of the plugs with air-borne spores. Roux's paper caps may be advantageously used, or better still "sterilized milk " bottles or rubber caps may be placed on the tubes. Sterilization of Instruments.-The various instruments used for post-mortem examinations, injection syringes, &c., are sterilized by boiling in water in a suitable copper vessel fitted with a perforated tray. A small quantity of carbonate of soda is added to the water to prevent rusting. A quarter of an hour is generally conconsidered sufficient exposure to boiling water for all practical purposes. Dressings, bandages, and the like, may be sterilized in the autoclave, or by the hot air method; the former is preferable, dry air being afterwards passed through the apparatus. The platinum wires used in the inoculation of media during the process of making cultivations, forceps and various other small articles that are not injured by heat are sterilized by heating in the bunsen flame. The platinum or platino-iridium inoculating needles must be heated in the flame till red hot before and after use, to prevent the contamination of the culture or the dissemination of the organisms in the culture tube. The platinum needle is first heated to redness and then the glass or aluminium handle passed through the flame also. It is essential that no wire should be laid down under any consideration whatever without previous sterilization. It is not perhaps out of place to note here that the majority of dental instruments may be sterilized by boiling with water containing 1 per cent. of sodium carbonate, in the manner adopted for other surgical instruments. With some of the finer instruments it is better to substitute pure almond oil for the water in a small sterilizer; the edge and temper are not affected in the least. ITT FIG. 11.-UNGLAZED PORCELAIN FILTERS (Pasteur-Chamberland). Sterilization by Filtration. This method is largely adopted in the preparation of the soluble products of bacterial activity, such as toxines and enzymes. The material, broth cultures, for instance, is placed in a specially constructed hollow cylinder of unglazed porcelain (fig. 11). The cylinder is fitted into the mouth of a sterile filter flask by means of an india-rubber washer previously sterilized by boiling, and negative pressure developed by means of a filter pump. The canals of the porcelain are so minute and tortuous that the fluid alone can pass through, the bacteria being arrested. To sterilize the "filter candle" after use it may be heated to redness in a muffle, using great caution, or hot alkaline permanganate solution may be filtered through, by which means the bacteria remaining in the canals of the filter are dissolved. Hydrochloric acid may also be used, but considerable care is required to wash away the acid afterwards. Many fluids may be prepared for cultural purposes by filtration in this way if the process of sterilization at 100° C. damages them. Various forms of water filters are constructed of unglazed porcelain and form the only efficient bacteriological filters. These filters do not however work indefinitely, as in about a week the bacteria which are arrested by the windings of the canals have grown to such an extent that the filtrate becomes contaminated. ANTISEPTICS AND DISINFECTANTS. The terms antiseptic and disinfectant are somewhat misleading in that a substance which will certainly destroy bacteria or their spores in a fairly strong solution (disinfectant) will only inhibit their growth when used in higher dilutions (antiseptic). It follows then that antiseptics only hinder the growth, while disinfectants destroy the life, of bacteria. A large number of chemical substances have been used from time to time, many of these substances eventually proving to be of little. value, not perhaps so much on account of the inefficiency of the chemical to destroy bacteria as on account of the wasting of the disinfectant by reason of other substances present. It may often happen that owing to the presence of some body with which the disinfectant easily forms compounds-such for example as mercuric chloride and albumin-a large quantity of the supposed disinfectant is rendered inert by precipitation. Permanganate of potash readily oxidises all organic matter, whether bacteria or proteid compounds; it is therefore necessary in choosing an antiseptic to obtain the most efficient one for the special purpose for which it is to be used, having regard to the particular local conditions. It must also be remembered that many antiseptics and disinfectants are at least as injurious to the cells of the body as to the bacteria they are employed to destroy, and a solution used in such strength actually favours the entrance of the organisms by lowering the tissue vitality. This effect of antiseptic solutions is often overlooked, and it follows that a great deal more may be done by preventing the access of organisms than by attempting to destroy them when they have once gained a footing. The following list of antiseptics and disinfectants gives some of the more common ones in use with their relative strength as determined practically by laboratory experiments. These tests, however, are more favourable to the antiseptic used than the organism tested, which is growing artificially and not in its usual habitat. The use of spores and their death as determined by absence of germination is more reliable; the spores to be tested are dried upon sterile silk threads and immersed for various periods of time in the antiseptic to be tested, then washed with boiled distilled water to remove traces of antiseptic and transferred to a culture tube. Another method is to add various quantities of the antiseptic under investigation to broth cultivations of the organisms experimented with; in this case care must be taken to avoid fallacies due to the neutralisation of the antiseptic by the medium used. The cultivations may be either fully developed ones, or one or more loop-fuls (öse) of culture may be inoculated into the nutrient medium containing the antiseptic to be tested. In making the subsequent sub-cultivations to test the destruction or inhibition of the organisms care must be taken to use a sufficiently large quantity of nutrient medium, otherwise the amount of antiseptic in the öse may invalidate the result. Control tubes should invariably be made. Sternberg recommends mixing the standard culture and the diluted antiseptic in equal proportions; thus 10 cc. of sterile broth containing 1 in 200 carbolic is added to 10 cc. of a twenty-four hours' broth culture of the given organism (= 1 in 400); plate cultivations are then made at given intervals. Many substances, such as concentrated solutions of sugar or common salt, prevent the development of bacteria but do not kill them. Disinfection of Hands, &c.-The bacterial flora of the skin is of a varied nature, and owing to the cracks and fissures of the epidermis, particularly the hands, it is difficult to remove the bacteria; moreover the bacteria actually live upon the dead epithelial cells, rarely however penetrating the true skin. The best method to adopt is first thorough scrubbing with a nail brush (boiled and kept in 2 per cent. lysol), soap and hot water, to remove as much of the dry epidermal scales as possible. The hand should be then soaked for two minutes in some antiseptic solution such as the one recommended by Lockwood1, 1 of biniodide of mer 1 Brit. Med. Jour., Jan. 11, 1896. cury in 500 of methylated spirit, which is subsequently washed off in 1 in 3,000 biniodide solution. This treatment does not cause the same roughness that mercuric chloride or carbolic so often produces. It need hardly be added that unless a perfectly sterile towel is used the disinfection is of no avail, and in operative surgery they are discarded entirely, and should not be used by the dental surgeon to dry his hands before the operation of extraction. The following short list of antiseptics and disinfectants gives the most useful; for other and extensive lists the reader is referred to Sternberg's "Bacteriology," Macfarland's "Pathogenic Bacteria," and Hueppe's exhaustive article in "Principles of Bacteriology." Formalin (40 per cent. sol. of formic aldehyde gas in water) Bichloride of Mercury ... ... ... ... 1 in 40,000 1 35,000 1 14,000 1,000 These figures give the relative strengths of solutions which will restrain the growths of bacteria but will not always destroy them. Salicylic acid and quinine are also powerful antiseptics, whilst iodoform, so commonly used, must be first changed into iodine-a somewhat rare thing-before it is effective. Its chief action is the neutralisation of the products of the organisms. The various mineral acids are strong disinfectants, and 5 per cent. HCl added to mercuric chloride greatly increases its efficiency. Lysol, consisting of coal tar oil, phenol and soap, is advantageous in that it is strongly alkaline and dissolves grease. It is used largely in general laboratory routine, especially for soaking used slides, the coverslips becoming detached by solution of the balsam as soap. A jar containing a 2 per cent. solution should be kept on the laboratory bench. |