CUIiTURE MEDIA 47 



violet end of the spectrum, at present belongs rather to the realm of thera- 

 peutics than to laboratory practice and need not be considered here, although 

 it may play a great part in hygiene. 



Hot dry air is one of the oldest methods employed and is still used for 

 glassware, such as Petri dishes. The clean material, usually heat resistant 

 glass, is put into a gas oven (rarely electric) heated to 150°-180° C. for from 

 a half hour to an hour or more, depending upon the material to be sterilized 

 and the length of time necessary to kill spore-forming organisms which may 

 be found in a given laboratory. 



Passing material through a flame for a varying period is also a very old 

 practice, now mostly used for sterilizing inoculating tools, such as needles, 

 platinum spatulas, etc. The metal is ordinarily heated to redness in the flame, 

 then allowed to cool to room temperature without touching any object which 

 might contaminate it until used. The former surgical practice of cautery with 

 red hot iron probably owed its success in part to this method of sterilization. 

 It is obvious that this method is not available for tempered tools, such as 

 scalpels, etc. A variant of this practice is to dip in alcohol and ignite, prob- 

 ably less efficient but adequate in many cases. 



Steam. — None of the above-mentioned methods are useful for most cul- 

 ture media, and it was only with the use of steam that culture media in the 

 modern sense began to develop rapidly. Steam may be applied at atmospheric 

 pressure in which case the medium reaches 100° C. and is held at that tem- 

 perature for a period. This is usually carried out in an Arnold steam steri- 

 lizer, a simple apparatus for generating steam with a minimum loss of water 

 during the process. When spores were discovered in bacteria, Tyndall ap- 

 plied this knowledge by proposing discontinuous sterilization, by heating the 

 medium for a definite period, sufficiently long to kill all the organisms in the 

 active vegetative state, then incubating sufficiently long to allow the spores 

 to develop the vegetative stage, and heating again. This process must be 

 repeated until the medium remains sterile on incubation. Under ordinary 

 conditions the usual procedure is to heat in steam at 100° C. for an hour each 

 day for three successive days. If the period between sterilizations is too long, 

 the spores may have germinated and formed new spores, and if too short 

 they may not all have germinated. The long time necessary to prepare media 

 by this method, as well as several more theoretical objections, has tended to 

 eliminate this method from the laboratory. However, some biologic products 

 used as media are profoundly altered by higher temperatures, and it is neces- 

 sarj^ to employ this method for these substances. 



Superheated Steam. — In this method the steam is confined in an autoclave 

 up to any pressure desired, instead of being allowed to escape as in the ordi- 

 nary steam sterilizer. A good steam pressure gauge on the autoclave is 

 requisite, and a thermometer is not only desirable but also an additional safe- 

 guard. The temperature ordinarily employed is 115°-125° C. or about 10-20 

 pounds pressure per square inch. An exposure to about 120° C. (or 15 lb.) 

 for 15-20 minutes will sterilize most media, unless some very heat resistant 



