22 DIFCOMANUAL 



advertised soapless cleansers commonly employed for cleaning laboratory glass- 

 ware were investigated and found quite bacteriostatic for many bacteria. Glass- 

 ware on which two of the three cleansers were used required two thorough rinses, 

 and that on which the third detergent was used required three complete rinses in 

 cold water to remove the toxic effect of the cleanser for bacteria. Clean glassware 

 is particularly required in media for microbiological assay procedures where ex- 

 tremely small amounts of material may give rise to erratic results. For example, 

 in the assay of Vitamin B12, as small a quantity as 0.01 millimicrogram will give 

 rise to a definite growth response. The importance of clean glassware in the assay 

 of this vitamin was stressed by the U.S. Pharmacopeia Vitamin B12 Study Panel, 

 since they stated that glassware for this purpose required special handling, and 

 as many as twelve rinses appeared to be necessary for satisfactory results. 



Following rehydration, Difco culture media require no filtration. In some 

 cases the medium contains a slight flocculent precipitate, such as, for example, 

 Levine E.M.B. Agar and Bacto-Niacin Assay Medium. Removal of these precipi- 

 tates will result in an inferior growth response. The filtration of any medium 

 through cotton is especially to be avoided. Wright^ showed that cotton-wool con- 

 tained a material inhibitory to the growth of the pneumococcus. Drea^ showed 

 that absorbent cotton stoppers contained inhibitory material for the tubercle 

 bacilli. Boyd and Gasman^ filtered 100 ml. quantities of Tryptose Agar through 

 10 gram portions of 15 different commercial brands of absorbent cotton and 

 failed to obtain growth from a dilute inoculum of Brucella abortus while the un- 

 filtered medium supported good growth of the test organism. They were able to 

 extract a fatty acid-like substance from the cotton which completely inhibited 

 growth when added to the medium in a concentration of 1 mg. per liter, as did 

 unsaturated fatty acids from cottonseed oil. The addition of starch to the medium 

 neutralized this toxic effect. The same observation was also made with centri- 

 fuged and cotton filtered Potato Infusion Agar for B. abortus. The centrifuged 

 medium gave excellent growth while medium filtered through cotton was in- 

 ferior. 



Adjustment of the reaction of the medium is not required. The final reaction 

 of the sterilized medium at 25 °C. is shown on the label of each bottle. Tenipera- 

 ture and method of standardization of the potentiometer in the determination of 

 pH is important. The potentiometer should be standardized with a standard 

 buffer solution, near the same reaction of the medium. The temperature of the 

 standard buffer used in standardizing the instrument should be the same as the 

 medium when making readings. The ionization constant increases with a rise in 

 temperature. For example, the pH of distilled water free from carbon dioxide at 

 25°G. is practically pH 7.0, but at 40°C. is 6.7. 



Uniform standardized media are readily prepared in large or small amounts 

 from Bacto Dehydrated Gulture Media. By the use of these products microbiolo- 

 gists are able to provide themselves daily with freshly prepared and wholly satis- 

 jfactory media of known composition and definite reaction. 



ij. Path. Bact., 38:499= i943- 



2 J. Bact., 44:149:1942. 



8 Public Health Reports, 66:44:1951. 



