CULTURE MEDIA 45 



Some new glassware when first received into the laboratory may still 

 contain enough alkali to change considerably the hydrogen ion concentration 

 of the medium to be employed. It may be necessary to heat this glassware 

 with strong acids before using it for careful work, although it may still be 

 used for many of the more tolerant organisms without further treatment. In 

 many laboratories all glassware is treated with a strong oxidizing agent, such 

 as an acid solution of sodium or potassium bichromate, commonly called 

 cleaning solution. This tends to neutralize any free alkali in the glass as well 

 as to destroy any life or organic material. This is especially necessary if dry 

 sterilization of the glassware is expected, as failure to remove all organic 

 material will result in a deposit of carbon which reduces visibility and is 

 almost impossible to remove later, although it probably does not interfere with 

 the growth of the organism. 



After treatment with cleaning solution (if necessary), the glassware 

 should be thoroughly rinsed with distilled water. If dry sterilization takes 

 place after hard tap water has been used for rinsing, a deposit of salt may be 

 baked in, causing decreased visibility and, rarely, toxicity. The alternative 

 method of rinsing with the solution to be used is not recommended in ordi- 

 nary practice, a^ some liquid may adhere to the top of the tube of the neck 

 of a flask and cause trouble in later procedures. 



Sterilization. — Many methods have been developed in the last half cen- 

 tury, but none is equally good for everything, consequently the principles 

 underlying each should be thoroughly understood and the proper method 

 selected for the material in hand, with a clear understanding of its limitations. 



Chemical Methods (Disinfectants, Antiseptics). — These methods as com- 

 monly applied in the laboratory consist in treating the material with a toxic 

 chemical substance for a sufficient time to destroy all life, then removing the 

 chemical substance and preserving the material from further contact with 

 microorganisms until it is used. The methods are difficult and little used when 

 other methods will serve. The chemical nature of the disinfectant must al- 

 ways be considered, as well as its fungicidal power. Disinfectants may be 

 grouped as halogens, salts of heavy metals, and organic compounds. 



Halogfens. — Fluorine and bromine are rarely used under present condi- 

 tions. All of the halogens corrode metals and are difficult to handle. They may 

 prove toxic to living tissue, although they are very valuable in some pro- 

 cedures. Chlorine usually is applied as a solution of a hypochlorite which 

 slowly gives off the chlorine in intimate contact with the material to be dis- 

 infected. Improvements of this technic have been found useful as antiseptic 

 dressings, disinfection of seeds in laboratory practice, etc. Calcium hypo- 

 chlorite (bleaching powder, chloride of lime, sodium or potassium hypochlorite) 

 is also used as a deodorant and disinfectant in outhouses and for sterilizing 

 white clothing where other methods are not easily available. 



Iodine is usually used in alcoholic or potassium iodide solution or in the 

 organic compound iodoform. This is one of the common disinfectants for 



