Efficacy of Disinfection and Sterilization

Following last week’s article, I will like to talk about what are some factors that affect the efficacy of disinfection and sterilization in health-care facilities. As we all know that the concentration of germicide has far exceeded the bactericidal concentration; however, we also know that germicide can reduce susceptibility due to a number of factors. Some factors are intrinsic qualities of the organism, other factors are the chemical and external physical environment. If we are aware of these factors, we will be able to utilise the maximum effectiveness of the disinfection and sterilization process.

There are seven main factors in Guideline for Disinfection and Sterilization in Healthcare Facilities, 2008. First, the number and location of microorganisms. The larger the number of microbes, the more time a germicide needs to destroy all of them. Professor Spaulding demonstrated that it took 30 minutes to kill 10 Bacillus atrophaeus spores but took 3 hours to kill 100,000 Bacillus atrophaeus spores. The guideline recommends cleaning before disinfection or sterilization reduces the number of microorganisms and increases the safety margin of the germicide’s exposure time. The location of microorganisms must also be considered as medical instruments can be disassembled into many pieces. Medical instruments with multiple pieces must be disassembled; equipments such as endoscopes that have crevices, joints, and channels are prone to aggregation of microorganisms. As penetration of the disinfectant of these instruments can be limited due to limited surfaces, disinfection and sterilization process can be compromised.
Second, the innate resistance of microorganisms. As intrinsic resistance mechanisms in microorganisms to disinfectants vary, all disinfection strategies implicitly target the most resistant types of microorganisms to achieve complete destruction. The germicidal resistance demonstrated by gram-positive and gram-negative bacteria is similar with some exceptions. For example Pseudomonas aeruginosaRickettsiaChlamydia, mycoplasma, and Coxiella burnetti. These microorganisms behaved more resistant to disinfectants than other microorganisms.
Third, the concentration and potency of disinfectants. The more concentrated the disinfectant, the greater its efficacy and the shorter the time necessary to achieve microbial kill; however, must be noted that not all disinfectants are similarly affected by concentration adjustments. The disinfection time depends on the potency of the germicide as Professor Spaulding exemplified using Mucin-loop test that 70% isopropyl alcohol destroyed 10000 Mycobacterium tuberculosis in 5 minutes, whereas a simultaneous test with 3% phenolic required 2-3 hours to achieve the same level of microbial kill.
Fourth, physical and chemical factors. There are several factors in consideration that will influence disinfectant procedures: temperature, pH, relative humidity, and water hardness. Activity of most disinfectants increases as temperature increases, but some exceptions as excess increase in temperature causes disinfectants to degrade and weakens its germicidal activity. An increase in pH improves the antimicrobial activity of some disinfectants (i.e. glutaraldehyde, quaternary ammonium compounds) but decreases the antimicrobial activity of others (i.e. phenols, hypochlorites, iodine). Relative humidity is the single most important factor influencing the activity of gaseous disinfectants/sterilants (i.e. EtO, chlorine dioxide, formaldehyde). Water hardness reduces the kill rate of certain disinfectants because divalent cations in hard water interact with the disinfectant to form insoluble precipitates.
Fifth, organic and inorganic matters. Organic matters in the form of serum, blood, pus or fecal material can interfere with antimicrobial activity of disinfectants in two ways. Commonly, chemical reaction occurs between germicide and organic matter resulted into complexes that are less germicidal or non-germicidal thus reduced the active germicides available. Another interaction is organic material protects microorganisms from attack by acting as a physical barrier. Inorganic matters act as protective agents of microorganisms to all sterilization processes which result from occlusion in salt crystals. This further emphasised the importance of thorough cleaning of medical devices before any sterilization or disinfection procedures.
Sixth, duration of exposure. Medical devices and equipments must be exposed to germicide for the appropriate minimum contact time. Since all lumens and channels instruments must contact the disinfectant, air pockets must be eliminated during submersion. The disinfectant must be introduced reliably into the internal channels of the devices; overall, longer contact times are more effective than shorter contact times.
Seventh, biofilms. Biofilms are microbial communities that are tightly attached to surfaces and cannot be easily removed. Microorganisms may be protected from disinfectants by biofilms. Resistance include multiple mechanisms: physical characteristics of older biofilms, genotypic variation of the bacteria, microbial production of neutralising enzymes, and physiologic gradients within the biofilms. The presence of biofilms can significantly compromise the sterilization and disinfection procedures.
Disinfectants available to healthcare personnels and household uses are more similar than different. Disinfectants are not interchangeable and incorrect concentrations and inappropriate disinfectants can result in excessive costs. In this part I will give an overview of the performance characteristics of the two commonly used chemical disinfectants.
Chlorine and chlorine compounds, also known as “household bleach”, is one of the most widely used disinfectant. Hypochlorite is the most widely used of the chlorine disinfectants and is available in liquid (i.e. sodium hypochlorite). They have a broad spectrum of antimicrobial activity, do not leave toxic residues, unaffected by water hardness, are inexpensive and fast acting, remove dries or fixed organisms and biofilms from surfaces, and have low incidence of serious toxicity. Disadvantages of household bleach are ocular irritation, oropharyngeal burn, oesophageal burn, and gastric burn. Other disadvantages include metal corrosion in high concentration, inactivated by inorganic matter, discolouring of fabrics, release of toxic chlorine gas when mixed with ammonia or acid, and relative stability. The mode of action is free chlorine inactivates microorganisms. Microorganisms become inactive due to oxidation of sulfhydryl enzymes and amino acids, ring chlorination of amino acids, loss of intracellular contents, decreased uptake of nutrients, inhibition of protein synthesis, decreased oxygen uptake, oxidation of respiratory components, decreased adenosine triphosphate production, breaks in DNA, and depressed DNA synthesis.
Information of Sodium Hypochlorite
At low concentration, available chlorine have a biocidal effect on mycoplasma (25 ppm) and vegetative bacteria (<5 ppm). At high concentration (1000 ppm), chlorine can kill M. tuberculosis. 100 ppm will kill 99.9% of B. atrophaeus spores within 5 minutes and destroys mycotic agents in <1 hour. 5000 ppm of bleach can inactivate 1000000 Clostridium difficile spores in <10 minutes. Study has also shown that 200 ppm available chlorine can inactivate 25 different viruses in 10 minutes. 500 ppm of chlorine showed inhibition of Candida after 30 seconds of exposure. Data are available for chlorine dioxide that support manufacturers’ bactericidal, fungicidal, sporadical, tuberculocidal, and virucidal claims.
Hypochlorites are widely used in healthcare facilities in a variety of settings. Inorganic chlorine solution is used for disinfecting tonometer heads, spot disinfection of countertops, and floors. Household bleach and tuberculocidal disinfectant have been recommended for decontaminating blood spills. Small spills of blood can be treated with 1:100 dilution on 5.25%-6.15% sodium hypochlorite; however, large spills of blood must be treated with 1:10 diluted final concentration of sodium hypochlorite. CPR training manikins should be decontaminated by at least 500 ppm available chlorine for 10 minutes. Chlorine has long been used as disinfectant of water treatment; Legionella-contaminated hospital water system resulted in dramatic decrease (30% to 1.5%) in the isolation of L. pneumophilia from water outlets and a cessation of healthcare-associated Legionnaires’ disease in an affected unit. Hypochlorite solutions should be stored at room temperature (23 degrees Celsius) in closed opaque plastic containers but can lose up to 40-50% of free available chlorine over 1 month.
Some common bacterial resistance data
Glutaraldehyde is a saturated dialdehyde that has gained wide acceptance as a high-level disinfectant and chemical sterilant. Aqueous solutions of glutaraldehyde are acidic and generally not sporadical in this state. Only when the solution is activated (made alkaline) it becomes sporicidal. However, once activated the shelf life will be limited to 14 days. Later designed glutaraldehyde formulations have a life-span of 28-30 days while generally maintaining excellent microbial activities. The biocidal activity of glutaraldehyde results from its alkylation of sulfhydryl, hydroxyl, carboxyl, and amino groups of microorganisms, which alters RNA, DNA, and protein synthesis. The microbicidal activity can differs by different exposure time. With at least 2 minutes of exposure, glutaraldehyde can effectively kill vegetative bacteria. Within 10 minutes, M. tuberculosis, fungi, and viruses; within 3 hours, spores of Bacillus and Clostridium species. Glutaraldehyde is commonly diluted to 2% as high-level disinfectant and used for medical equipments, spirometry tubing, dialyses, transducers, anaesthesia and respiratory therapy equipment, hemodialysis proportioning and dialyse delivery systems, and reuse of laparoscopic disposable plastic tracers.
Glutaraldehyde Chemical Label
Glutaraldehyde safety handle should be very cautious. Healthcare personnels should be aware that usage of glutaraldehyde can exposed harmful vapour in a poorly ventilated room, so keeping a good ventilation is important. Acute or chronic exposure can result in skin irritation or dermatitis, mucous membrane irritation, or pulmonary symptoms.

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