114 DISINFECTION 



Miindel (1937) has found that the addition of 2 per cent, washing soda (NagCOg) greatly 

 increases the disinfecting power of boiUng water, as well as reducing the tendency of metal 

 instruments to rust. Thus, spores in a 0-35 per cent, suspension of earth in water resisted 

 boihng for about 10 hours, but were killed in a 2 per cent, solution of soda at 98° C. in 

 10 to 30 minutes. 



It is on account of the resistance of spores to boiling water that the autoclave 

 has largely displaced the steamer in laboratory practice. Steam is still employed 

 at atmospheric pressure for the sterilization of certain media, the physical or 

 chemical composition of which would be altered by steam under pressure, but 

 where this is necessary we take advantage of Tyndall's observation, and submit 

 the medium to steaming for 30 minutes on 3 successive days ; any sporing 

 organisms that have not been killed on the first day germinate, and thus become 

 susceptible to exposure on the second day. It must be realized that tyndallization 

 can be successful only if the nature of the medium and the conditions to which 

 it is subjected between successive heatings are such as to enable all the spores 

 to germinate. It is quite inapplicable, for instance, to the sterilization of bacterial 

 suspensions in a non-nutrient fluid. Similarly, it is unsuitable for the destruction 

 of anaerobic spore-bearing bacteria the spores of which are unable to germinate 

 aerobically, or of thermophilic spore-bearing organisms which fail to grow at a 

 temperature below 50-55° C. 



Steam under pressure, on the other hand, is so effective that a single steriliza- 

 tion usually suffices. In the autoclave the steam, while being submitted to pres- 

 sure, still remains saturated with moisture. This is most important. Steam 

 which is superheated behaves like a gas, and condenses very slowly on objects 

 cooler than itself. Steam that remains saturated with moisture is much more 

 effective, as it rapidly condenses on objects cooler than itself, and, by giving up 

 its latent heat, quickly raises them to its own temperature. All air must be expelled 

 from the autoclave before the pressure is allowed to rise ; otherwise the temperature 

 developed by a given pressure of steam will be less than that reached by saturated 

 steam (see Kishworth 1938, Spooner and Turnbull 1942). Though there are a few 

 exceptions, it is safe to say that saturated steam under a pressure of 15 lbs. per square 

 inch, i.e. with a temperature of about 120° C, is sufficient to sterilize any medium 

 in 30 minutes. This is therefore the exposure to which the usual media are 

 submitted. 



The higher the temperature, provided the steam remains saturated, the more 

 rapid is the sterilization. This is clear from the results of Bigelow and Esty. 

 But there are certain factors other than temperature that affect the time necessary 

 for sterilization by steam. One of the most important is the H-ion concentration 

 of the medium. 



It will be remembered that Pasteur in his experiments on spontaneous generation (see 

 Chapter 1) found that boiling was more lethal in an acid than in an alkaline medium. 

 This has since been confirmed repeatedly. Bigelow and Esty (1920), for examj)le, working 

 with the spores of thermophilic organisms, found that when suspended in an acid medium 

 of pH 4-6 they were destroyed by a temperature of 120° C. in 2 minutes, whereas in a less 

 acid medium of pH 6-1 it required 9 minutes to destroy them. Chick (1910) likewise 

 found that minute quantities of acid or alkali, too small of themselves to produce any 

 germicidal action, had a very marked influence on the power of disinfection by hot water. 

 With these substances the rate of disinfection was increased, but much more with the acid 

 than with the alkali. Thus, working with Salm. typhi suspended in distilled water, she 



