100 MANUAL OF MICROBIOLOGICAL METHODS 



recommended in the manuals published by the manufacturers of dehy- 

 drated and other ready-prepared culture media (Difco Laboratories, 

 Baltimore Biological Laboratory, and others). 



SELECTION OF MATERIALS 



As the maintenance of cultures is largely a mechanical task (for which, 

 however, no suitable machine has been devised), it is desirable to reduce 

 all mechanical factors to a minimum consistent with efficient manipula- 

 tion of the cultures. The size and number of culture vessels, their con- 

 venience in handling, the volume of culture media required, the storage 

 space for cultures, especially refrigerated storage, all must be taken into 

 consideration. For the conditions under which the American Type 

 Culture Collection operates, the following specifications for materials 

 have proved very satisfactory and are recommended to others, though re- 

 cognizing that other laboratories may have varied preferences of their own. 



Culture tubes. These are 125 by 16 mm with straight sides (without 

 flare) for cotton plugs or screw-cap tubes with rubber liners (plastic liners 

 are prone to drop out, at times they leak, and they may give off toxic 

 emanations that suppress growth of some cultures). These tubes hold 

 5-6 ml of culture media. For slow growers like Mycobacterium fMbercu- 

 losis, and those requiring special media, as marble chips for Nitrobacter or 

 a large surface of floating sulfur for Thiobacillus, larger culture tubes or 

 flasks are needed. 



Shell vials for lyophilizing. Outer vial is 85 by 14 mm; inner vial 

 35 by 9.5-10.00 mm (outside dimensions). 



Culture tube racks. For the size of tube specified, a standard zinc- 

 coated wire rack about 7^i by 3}i by 33^ in. will hold 40 tubes; two of 

 these j oined end to end will fit the shelf depth of average-sized household 

 or laboratory refrigerators. A storage space 40 by 16 by 16 in. will 

 accommodate 800 cultures. Special racks or trays for holding smaller 

 culture tubes or lyophiUzed specimens that are even more economical of 

 space are commercially available. 



Milk solution for lyophilizing. Although various fluids, including 

 serum (usually from beef blood), corn-steep liquor, and solutions of 

 lactose or glucose, have been proposed for obtaining cell suspensions for 

 lyophilizing, skim-milk powder, reconstituted at twice the original solid 

 content of milk, possesses general advantages. It is readily obtainable 

 and of uniform quality, can be stored without deterioration for many 

 months (refrigerated), and has no adverse chemical or physical effects on 

 most organisms. It is not suitable for all bacteria, however; for example. 

 Vibrio comma must be lyophilized from serum. Milk must be carefully 

 prepared for use in lyophilizing, as an excess of heat will cause clotting or 

 partial caramelization , making it difl&cult to obtain a uniform suspension 



