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Medicine and health science fair project:
Which disinfectants are the best at destroying germs?

Science Fair Project Information
Title: Which disinfectants are the best at destroying germs?
Subject: Medicine & Health
Grade level: Elementary School - Grades 4-6
Academic Level: Ordinary
Project Type: Experimental
Cost: Low
Awards: First Place, Canada Wide Virtual Science Fair ($80)
Affiliation: Canada Wide Virtual Science Fair
Year: 2010
Materials: Sterile petri dishes, distilled water, nutrient agar, surgical gloves, cotton swabs.
Description: Microbe samples were collected from diferent surfaces and introduced into the agar by cotton swabs. The same way different disinfectants and cleaning agents were introduced into the Petri dishes; Petri dish changes were monitored over time.
Link: http://www.virtualsciencefair.org/2010/ssaixs2
Short Background


Disinfectants are substances that are applied to non-living objects to destroy microorganisms that are living on the objects. Disinfection does not necessarily kill all microorganisms, especially non resistant bacterial spores; it is less effective than sterilisation, which is an extreme physical and / or chemical process that kills all types of life. Disinfectants are different from other antimicrobial agents such as antibiotics, which destroy microorganisms within the body, and antiseptics, which destroy microorganisms on living tissue. Disinfectants are also different from biocides the latter are intended to destroy all forms of life, not just microorganisms.

Sanitisers are substances that simultaneously clean and disinfect.

Bacterial endospores are most resistant to disinfectants, but some viruses and bacteria also possess some tolerance.

Disinfectants are frequently used in hospitals, dental surgeries, kitchens, and bathrooms to kill infectious organisms.

There are arguments for creating or maintaining conditions which are not conducive to bacterial survival and multiplication, rather than attempting to kill them with chemicals. Bacteria can increase in number very quickly, which enables them to evolve rapidly. Should some bacteria survive a chemical attack, they give rise to new generations composed completely of bacteria that have resistance to the particular chemical used. Under a sustained chemical attack, the surviving bacteria in successive generations are increasingly resistant to the chemical used, and ultimately the chemical is rendered ineffective. For this reason, some question the wisdom of impregnating cloths, cutting boards and worktops in the home with bactericidal chemicals.

In the 1940s and early 1950s, further studies showed inactivation of diverse bacteria, influenza virus, and Penicillium chrysogenum (previously P. notatum) mold fungus using various glycols, principally propylene glycol and triethylene glycol. In principle, these chemical substances are ideal air disinfectants because they have both high lethality to microorganisms and low mammalian toxicity

Alcohols, usually ethanol or isopropanol, are sometimes used as a disinfectant, but more often as an antiseptic (the distinction being that alcohol tends to be used on living tissue rather than nonliving surfaces). They are non-corrosive, but can be a fire hazard. They also have limited residual activity due to evaporation, which results in brief contact times unless the surface is submerged, and have a limited activity in the presence of organic material. Alcohols are most effective when combined with purified water to facilitate diffusion through the cell membrane; 100% alcohol typically denatures only external membrane proteins. A mixture of 70% ethanol or isopropanol diluted in water is effective against a wide spectrum of bacteria, though higher concentrations are often needed to disinfect wet surfaces. Additionally, high-concentration mixtures (such as 80% ethanol + 5% isopropanol) are required to effectively inactivate lipid-enveloped viruses (such as HIV, hepatitis B, and hepatitis C). Alcohol is, at best, only partly effective against most non-enveloped viruses (such as hepatitis A), and is not effective against fungal and bacterial spores. The efficacy of alcohol is enhanced when in solution with the wetting agent dodecanoic acid (coconut soap). The synergistic effect of 29.4% ethanol with dodecanoic acid is effective against a broad spectrum of bacteria, fungi, and viruses. Further testing is being performed against Clostridium difficile (C.Diff) spores with higher concentrations of ethanol and dodecanoic acid, which proved effective with a contact time of ten minutes.

See also: http://en.wikipedia.org/wiki/Disinfectant

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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