Physical Methods for the Control of Microorganisms

Physical Methods for the Control of Microorganisms

Make a discussion of the methods

Physical Methods for the Control of Microorganisms are often used to achieve decontamination, disinfection and microbial sterilization.

Control of Microorganisms

HEAT as a Physical Method for the Control of Microorganisms

Exposure to boiling water for 10 minutes is sufficient to destroy vegetative cells, but it is not sufficient to destroy endospores. It does not sterilize.

The effectiveness of heat as an antimicrobial agent can be expressed as the Thermal Death Time (TMT), which is defined as the shortest time necessary to destroy the microorganisms in a suspension, at a specific temperature and in defined conditions. However, since the destruction is logarithmic, it is not possible to completely eliminate microorganisms from a sample. There are several methods of controlling microorganisms by means of heat:

 

Steam sterilization (moist heat or autoclave):

The water is brought to a boiling point so that steam fills the chamber, displacing cold air. When all the air is expelled, the safety valves are closed and the steam saturates the whole chamber, which increases the pressure, until the desired values ​​are reached (121 ° C and 15 lb. pressure).

Under these conditions all the vegetative cells and endospores are destroyed in a time that is usually 15 minutes. It is thought that humid heat degrades nucleic acids, denatures proteins and also alters cell membranes. If the right conditions are not met, there is no sterilization.

To control the proper functioning of the equipment, a biological control or a chemical indicator can be included with the sterilization. The biological indicator consists of a sterile ampoule with a medium and a paper covered with spores of Bacillus stearothermophilus or Clostridium. After sterilization, the ampoule is broken and incubated for a few days. The chemical indicator consists of a special tape with letters or lines that change color after sufficient treatment with heat.

 

Pasteurization: Physical Methods for the Control of Microorganisms

It is used for substances or media that can not be heated above its boiling temperature. A brief heating at 55 or 60 ° C will destroy the pathogenic microorganisms and decrease the causes of the decomposition of the substance. DO NOT sterilize.

There are variations that are used in the milk industry: rapid pasteurization (HTST high temperature short-term) which consists of heating at 72 ° C for 15 seconds. And the ultra-high temperature (UTH ultrahigh temperature) pasteurization that heats at 140-150 ° C for 1 to 3 seconds. c. Tindalization or steam sterilization: it is used for chemicals or biological material that can not be carried over 100 ° C. It is heated at a temperature of 90 ° C to 100 ° C for 30 minutes for three consecutive days and incubated at 37 ° C with each heating.

The first heating destroys vegetative cells but not spores, so they germinate at 37ºC and then they are eliminated with the following heating.

Physical Methods for Microorganisms Control

d. Dry heat:

Ovens or stoves are used at a temperature of 160-170 ° C for 2 or 3 hours. It is less effective than moist heat, but does not corrode metallic utensils. It is slow and can not be used for thermo sensitive material. and. Incineration: Completely destroys microorganisms. (heat the handles on the lighters).

e. Low temperatures:

Refrigeration and freezing, are only bacteriostatic. In general, the metabolism of bacteria is inhibited at temperatures below 0 ° C. However, these temperatures do not kill microorganisms, but they can keep them for long periods of time.

This circumstance is also used by microbiologists to conserve microorganisms indefinitely. The cultures of microorganisms are kept frozen at -70 ° C or even better in tanks of liquid nitrogen at -196 ° C. g. Drying: It is bacteriostatic and the spores remain viable.

FILTRATION: Physical Methods for the Control of Microorganisms

it is used for thermosensitive materials. to. Depth filters: Fibrous or granulated materials are used to form a thick layer with very small diameter channels. The solution is vacuumed and the microorganisms are retained or are adsorbed by the material. Diatoms, unglazed porcelain, asbestos are used.

b. Membrane filters: They are circular with a thickness of 0.1 mm and very small pores, about 2 μm so that microorganisms can not pass through. They are made of cellulose acetate, polycarbonate, polyvinyl fluoride or other synthetic materials.

Physical Methods for the Control of Microorganisms

RADIATION:

a. Ultraviolet: It is lethal for all classes of microorganisms because of its short wavelength and high energy. It is lethal at 260 nm since it is the wavelength that is most effectively absorbed by DNA.

The primary mechanism of DNA damage is the formation of thymine dimers which inhibits their function and replication. They are poorly penetrating and are used for surfaces.

b. Ionizing:

Low levels can produce mutations and indirectly result in death, high levels are lethal. Specifically, they cause a series of changes in the cells: rupture of hydrogen bonds, oxidation of double bonds, destruction of rings, polymerization of some molecules, generation of free radicals.

The destruction of DNA is the biggest cause of death. It is excellent sterilizing and with deep penetration in different materials, so they are used to sterilize thermolabile (thermosensitive) materials such as disposable syringes, probes, etc. They are not used for culture media or protein solutions because they produce alterations of the components.

Physical Methods for the Control of Microorganisms

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