PRESERVATION OF CULTURES 61 



understood, they can pass unnoticed for considerable periods by persons 

 who are otherwise good culture workers. Mites will crawl from petri dish 

 to petri dish, leaving behind them a trail of bacterial and mold contamina- 

 tions, as well as streams of eggs which develop rapidly into more mites that 

 actually destroy the colonies. Since mites have preferences as to food, 

 some species are invaded and others avoided. To reach an attractive food 

 supply a mite will frequently go through a cotton plug as ordinarily made 

 and occasionally seems to get through even a paraffined plug. As a factor 

 in the mixing of strains of molds in a laboratory collection, mites must not 

 be ignored. 



Similar mixing and contamination occurs frequently whenever a labora- 

 tory becomes infested with ants, roaches, or other insects. 



Poisoning Cotton Plugs 



Mites: Entire elimination of mites by sanitary measures is possible but 

 often not attained. As a precaution in the preservation of stock cultures, 

 some scheme of poisoning should be used. One of these formulas consists 

 of dipping the tips of the cotton plugs in a solution of the following com- 

 position: 



95% alcohol 95 cc. 



Bichloride of mercury 0.5 gm. 



Glycerine 5 cc . 



Color with any aniline dye. 



Care must be taken that the solution does not come in contact with the 

 colony. The cultures must be allowed to develop into typical colonies 

 before poisoning. An antiseptic formula for the purpose needs alcohol to 

 insure penetration of the plug, a poison to destroy the mites, glycerine to 

 prevent the crystallization of the poison as the alcohol evaporates, and the 

 dye to insure the destruction of the cotton plugs when removed from the 

 tubes. In our own experience we have consistently made it a practice to 

 wipe off the outside of all culture tubes with the above, or some other 

 sterilizing solution, and to poison all plugs before cultures are replaced in or 

 added to the collection of stock cultures. 



Molds: Under humid laboratory conditions, cotton plugs, especially if 

 made from the absorbent type of cotton, absorb moisture. Careless han- 

 dling in preparation and care of such plugs often adds enough nutrients to 

 support growth. Steam sterilization tends to distribute nutrients. Dirt, 

 bacteria, and molds fall from the air upon the exposed portion of the plug. 

 Handling detaches spores from the colony within so that both ends of the 

 plug are commonly well seeded. Spore germination, therefore, may begin 

 at either or both ends. Outside molds may grow through and drop into the 



