When steam is admitted to the sterilizing chamber, it promptly condenses upon contacting cold objects. As steam condenses, it liberates a great amount of latent heat, simultaneously heating and wetting the materials and thereby providing the two requisites for the thermal destruction of microbial life, namely moisture and heat. These two factors of moisture and heat must always be present for effective sterilization.
The process by which bacteria are destroyed when subjected to heat is not clearly understood. The traditional theory is that death at elevated temperatures is closely linked with the alteration of proteins, involving some irreversible protoplasm change within the bacterial cell.
To be sure, it is known that moist heat is a more efficient sterilizing agent than dry heat because in the presence of moisture, bacteria are destroyed at considerably lower temperatures than when moisture is absent. This phenomenon has been explained on the basis that all chemical reactions, including the coagulation of proteins, are catalysed by the presence of water. In brief, the generally accepted theory is that death by moist heat is caused by the denaturation and coagulation of some essential protein or enzyme-protein system within the bacterial cell, whereas death by dry heat is primarly an oxidation or slow burning up process.
Today, in every modern hospital, there may be found a variety of sterilizers, each performing a vital service in protecting the patient against infection, but all are dependant upon the application of certain fundamental principles allied with the use of steam as a sterilizing agent. We use steam under pressure, rather than atmospheric steam, for the sole purpose of attaining higher temperatures. It should be understood that pressure of itself has nothing whatsoever to do with the microbicidal properties of steam.
Since atmospheric, or what may be called “non-pressure” steam has a maximum temperature of 100 C, it is no value for the sterilization of surgical supplies.
Boiling water should not be employed as a sterilizing agent. Several investigations have shown that heat-resistant bacterial spores will withstand boiling water for many hours of continuous exposure.
At best, the boiling water processes could be described as a sanitizing medium. Those factors which have established pressure steam as the most realible medium for the sterilization of surgical supplies are : its power of penetration; its microbiological efficiency; and the ease of regulation or control for economical operation. Saturated steam possesses the singular property of being able to heat materials and permeate porous substances by the rapid process of condensation as opposed to the very slow process of heat absorption, as in the case of hot air or when any other gas is used as the heating medium. Also, saturated steam at a temperature of 134 C equivalent to 210 kPa pressure will destroy the most heat-resistant form of microbail life within a brief interval of exposure. Bacterial spores are recognised as the most resistant of all living organisms in their capacity to withstand external destructive agents. Anthrax spores, for example, dried on silk threads, have been found viable after sixty years. Other viable spore-formers have been recovered from canned and hermetically sealed food after a lapse of 115 years. The magnitude of resistance to saturated steam is illustrated by the fact that certain spore cultures all withstand a temperature of 115 C for more than three hours, whereas the vegetative forms of most bacteria are killed in a few minutes at temperatures ranging from 54 C to 65 C. Today, most authorities concur that no living thing can survive 10 to 15 minutes direct exposure to saturated steam at 121 C. To my knowledge, none of the pathogenic organisms have been known to be resistant to an exposure of even three minutes at 134 C.