Madical brain Notes 10

  Sterilisation One of the oldest forms of antimicrobial treatment is that of heating, and in most cases this remains the pre-ferred means of sterilisation, provided that it does not cause damage to the material in question. The benefits of boiling drinking water have been known at least since the 4th century  BC , when Aristotle is said to have ad-vised Alexander the Great to order his troops to take this precaution. This of course was many centuries before the existence of microorganisms had been demonstrated or perhaps even suspected. Sterilisation by heat Boiling at 100  ◦  C for 10 minutes is usually enough to achieve sterility, provided that organisms are not present in high concentrations; in fact most bacteria are killed at about 70  ◦  C. If, however, endospores of certain bacteria (notably  Bacillus  and  Clostridium ) are present, they can resist boiling, sometimes for several hours. As we saw, the causative agents of some particul...

  Sterilisation by heat Boiling at 100  ◦  C for 10 minutes is usually enough to achieve sterility, provided that organisms are not present in high concentrations; in fact most bacteria are killed at about 70  ◦  C. If, however, endospores of certain bacteria (notably  Bacillus  and  Clostridium ) are present, they can resist boiling, sometimes for several hours. As we saw, the causative agents of some particularly nasty conditions, such as botulism and tetanus, are members of this group. In order to destroy the heat resistant endospores, heating beyond 100  ◦  C is required, and this can be achieved by heating under pressure in a closed vessel (Table 13.1). A typical laboratory treatment is 15 minutes at a pressure of 103 kpa (15 psi), raising the temperature of steam to 121  ◦  C. This is carried out in an  autoclave , which is, to all intents and purposes, a large-scale pressure cooker (Figure 13.1). Air is driven out o...

  Sterilisation by irradiation Certain types of irradiation are used to control the growth of microorganisms. These include both ionising and non-ionising radiation. The most widely used form of non-ionising radiation is ultraviolet (UV) light. Wave-lengths around 260 nm are used because these are absorbed by the purine and pyrimidine components of nucleic acids, as well as certain aromatic amino acids in proteins. The absorbed energy causes a rupture of the chemical bonds, so that normal cellular func-tion is impaired. You will recall that UV light causes the formation of  thymine dimers  (Figure 11.21), where adjacent thymine nucleotides on the same strandare linked together, inhibiting DNA replication. Although many bacteria are capable of repairing this damage by enzyme-mediated photoreac-tivation, viruses are much more susceptible. UV lamps are commonly found in food preparation areas, operat-ing theatres and specialist areas such as tissue culture facilities, where ...

  Sterilisation using ethylene oxide Generally, chemical methods achieve only disinfection ; the use of the gas ethylene oxide, however, is effective against bacteria, their spores and viruses. It is used for sterilising large items of medical equipment, and materials such as plastics that would be damaged by heat treatment. Ethylene oxide is particularly effective in sterilising items such as dressings and mattresses, due to its great powers of penetration. In the food industry, it is used as an antifungal fumigant, for the treatment of dried fruit, nuts and spices. The materials to be treated are placed in a special chamber which is sealed and filled with the gas in a humid atmosphere at 40–50  ◦  C for several hours. Ethylene oxide is highly explosive, so it must be used with great caution; its use is rendered safer by administering it in admixture (10 per cent) with a non-flammable gas such as carbon dioxide. It is also highly toxic, so all items must be thoroughly fl...

  Disinfection Disinfection, by comparison, allows the possibility that some organisms may survive, with the potential to resume growth when  conditions become more favourable.  A  disinfectant  is a chemical agent used to disinfect inanimate objects such as work surfaces and floors. In the food and catering industry, especially in the USA, the term sanitisation  is used to describe a combination of cleaning and dis-infection. Disinfectants are incapable of killing spores within a reasonable time period, and are generally effec-tive against a narrower range of organisms than physical means.  Decontamination  is a term sometimes used interchangeably with disinfection, but its scope is wider, encompassing the removal or inactivation of microbial products such as toxins as well as the organisms themselves. The lethal action of disinfectants is mainly due to their ability to react with microbial proteins, and therefore enzymes. Consequently, any chemi...

  The kinetics of cell death When microorganisms are exposed to any of the treatments outlined in the preceding pages, they are not all killed instantaneously. During a given time period, only a certain proportion of them will die. Suppose we had 1000 cells (an unrealistically small number,  but it keeps the arithmetic simple) and that 10 per cent were killed each minute.  After one minute, 900 cells would remain, and after the second minute 10 per cent of these would die, leaving us with 900  −  90  =  810 survivors. After a further minute, another 10 per cent of the survivors would be killed, so 810  −  81  =  729 would be left. A plot of the surviving cells against time of exposure gives a graph such as Figure 13.8. The curve is exponential; theoretically, there will never be zero survivors, but after a while we are going to have less than one cell, let us say one tenth of a cell, which clearly can not happen. What this really me...