Lecture 14 - Physical Methods - Other

Mechanical
Mechanical methods is the mode of action that reduces the load by physically removing some organisms.

Scrubbing
A bland soap is a soap that does not have a germicide in it. The soap acts as a surfactant to emulsify the bacteria and dirt and helps wash them away with water.

A germicidal soap is a soap that is enhanced with usually tryclosan (a phenol compound), and helps kill even more bacteria by sticking to your hands. This is referred to as residual action. It's also difficult to get rid of these compounds; they stay in the environment. According to the CDC, germicidal soap is just as effective as bland soap for everyday cleaning.

Alcohol based soaps and hand wipes do not have residual action because it evaporates quickly, and is only recommended to be used when there is no visible soil on the target area. Additionally, alcohols are poor surfactants and overall poor at disinfecting, so is rarely the preferred method.

Sedimentation
Sedimentation is the first step in water purification. This method removes suspended particles, including bacteria, by letting gravity separate the suspended particles over a long period of time. Ferric sulfate can be added to help smaller particles settle out, and enzymes can be added to digest some particles.

Filtration
Filtration, the second step in water filtration, puts a gas or liquid through some screen or porous materials. You aim to remove the particles suspended in the solution. Filtration is the second step in water treatment. The advantage of filtration is it can be used to treat heat labile things. The main downside to filtration is that filters often get clogged, and cease working afterwards. Some filters use rocks and soil.

Membrane Filtration
Membrane filtration uses a cellulose paper that looks like a screen, and filters out the small particles. In order to sterilize a sample, the holes in the membrane must be small enough to filter out bacteria. These first two steps remove 90.95% of all microorganisms, and the final step is adding chlorine. Adding too much chlorine
 * 0.45 μm removes all eukaryotes (yeast) and some large prokaryotes (E. coli and Staph aureus).
 * 0.2 μm removes all prokaryotes including mycoplasma.
 * 0.02 μm removes viruses, but get clogged easily.
 * 0.01 μm still fails to remove prions, but get clogged even more.

Environmental Conditions
By changing the environmental conditions, it is possible to make an environment unsuitable for growth for some bacteria. However, not all bacteria respond the same to the changes in environmental conditions, which makes these methods highly inconsistent and unable to sterilize any sample.

Dessication
Dessication works by taking the water necessary for hydrolysis. It is a natural way to clean the environment, and used as a method of food and drug preservation. It can be used to treat yeast infections, and can be even more effective than anti-fungal creams like Nystatin. Dry sponges or paper towels will spread the infections organisms less than a wet sponge. Powdered eggs may have salmonella, and cereals may contain traces of Staphylococcus aureus.
 * Syphilis dies in a few minutes
 * Gonorrhea dies in 1-2 hours
 * E. coli, salmonella, and Staph aureus last 2-3 months
 * TB lasts up to 8 months
 * Spores are resistant to dryness
 * HIV/AIDS last 6 hours without a cell, and 1.5 days inside a dry cell
 * Hepatitis B can last for up to 1 week on a dry surface.

Cold
Bacteria are prokaryotes are usually not killed by freezing, unlike eukaryotes. Refrigerators at 4°C slow down enzymatic hydrolysis and other metabolic functions. Freezers at 0°C will remove the water necessary for enzymatic hydrolysis because ice does not react as readily as water.

Osmotic Pressure
By raising the osmotic pressure of the surroundings, one can control most bacteria by causing plasmolysis. A brine of up to 30% salt or a high sugar jelly (50-60%) can be used to preserve food. However, halophiles can grow up to 36% salt, and saccharophilic yeasts can grow in up to 70% sugar.

Sunlight
Not much UV light is actually used by sunlight to destroy bacteria. The main mode of action is photo-oxidation, which happens mostly during the summer when it's really hot. Kills Mycobacterium in a matter of hours.

Ultraviolet Light
Ultraviolet light that have wavelengths between 100-400nm are especially useful for disinfecting. Light with a wavelength of 253.7nm will create pyrimidine dimers. With enough mutations accumulated, any bacteria or spore will be destroyed. However, this is not considered sterilizing because of the poor penetration that the UV light gives.

Common uses for UV light are water and air purification. However, some structures like protozoan cysts (Giardia) are more resistant to UV radiation.

Infra-Red and Mirowaves
These radiations have a longer wavelength and lower energy than UV and visible light. However, these radiations readily convert to heat energy when intensity is increased. Similar to plain heating, when moist, the mode of action is protein coagulation, and when dry, the mode of action is protein oxidation. Microwaves are used to disinfect plastic contact lenses, urinary catheders, and milk.

Ionizing Radiation
Gamma, cathode, and X-rays are in the common group of ionizing radiation. They are extremely high energy particles that can produce hyperactive ions that can break down DNA, causing highly damaging mutations.
 * Gamma rays are produced by atomic bombs and Cobalt 60, which are highly penetrating. They vary in dosage; medium dosages are FDA cleared for disinfecting food products and increasing their shelf life. High dosage is only cleared for astronaut food. FDA cleared means that it's safe for use, and FDA approved means that it's effective.
 * Cathode rays are high energy electron beams used to disinfect sutures.
 * X-rays are very penetrating, dangerous, and no longer approved for routine use.