60 90

Time (min)

Figure 5.2 The Relationship Between the Numbers of Initial Microorganisms and the Time It Takes to Kill Them The D value is the time it takes to reduce the population by 90%.

survivor. It would take 20 minutes (10 D values) to reduce a population of 1010 cells to only one survivor. Removing organisms by washing or scrubbing can minimize the time necessary to sterilize or disinfect a product.

Environmental Conditions

Factors such as pH, temperature, and presence of fats and other organic materials strongly influence microbial death rates. For example, a solution of sodium hypochlorite (household bleach) can kill a suspension of M. tuberculosis in 150 seconds at a temperature of 50°C; whereas it takes only 60 seconds to kill the same suspension with chlorine if the temperature is increased to 55°C. The hypochlorite solution is even more effective at a low pH.

The presence of dirt, grease, and organic compounds such as blood and other body fluids can interfere with heat penetration and the action of chemical disinfectants. For this reason, it is important to thoroughly clean items before disinfection or sterilization. Meticulously cleaning a product also substantially decreases the number of microorganisms, which, as mentioned previously, shortens the time required to sterilize or disinfect the product.

Potential Risk of Infection

To guide medical biosafety personnel in their selection of ger-micidal procedures, items such as surgical instruments, endoscopes, and blood pressure cuffs are categorized according to their potential risk of transmitting infectious agents. Those that pose the greatest threat of transmitting disease must be subject to more rigorous sterilization procedures.

■ Critical items come into direct contact with body tissues. These items, including needles, scalpels, and

5.3 Using Heat to Destroy Microorganisms and Viruses 113

biopsy forceps, must be sterilized to avoid transmission of all infectious agents.

■ Semicritical items come into contact with mucous membranes, but do not penetrate body tissue. These items, including gastrointestinal endoscopes and endotracheal tubes, must be free of all viruses and vegetative bacteria including mycobacteria. Low numbers of endospores that may remain on semicritical items pose little risk of infection because mucous membranes are effective barriers against their entry into deeper tissue.

■ Non-critical instruments and surfaces pose little risk of infection because they only come into contact with unbroken skin. Countertops, stethoscopes, and blood pressure cuffs are examples of non-critical items.

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