THE CONTROL OF MICROBES

Good day my readers, it's my belief that microorganisms are present in our immediate environment and they are able to cause harm to susceptible individuals although in some cases these organisms can be beneficial.

These organisms are too small to be seen by the unaided eye, but with the assistance of the microscope, they can be viewed extensively. Their ability to cause harm to a susceptible host depends on the invasive of these agents. these disease-causing agents include the bacteria, fungi, viruses etc

Infectious diseases resulting in the destruction of cells are brought about by the uncontrolled microbial growth on or in such areas. It is worthy to note that the application of control measures helps in the prevention and spread of diseases.

Today, we will be describing various mechanisms used in the control of these agents (microorganisms) and the factors that affect these control mechanisms(methods) but before we continue with educating ourselves, I will describe the reproduction in a unicellular organism.

A single cell of Escherichia coli can reproduce under 48 hours to produce a mass of organisms greater than the mass of the earth. This could lead to the considerate destruction of valuable “nutrient materials” and thus microbial growth control is often necessary to stop such destruction. In the food industry for example, for spoilage can cause great economic loss and hence considerable resources are used to control spoilage.

Furthermore, to prolong the shelf life of perishable materials, it is necessary to inhibit microbial growth and thus control techniques like disinfection, sterilization of food materials and equipment and pasteurization of consumables liquids are often practiced.

Infectious diseases resulting in the destruction of cells are brought about by the uncontrolled microbial growth on or in such areas. Application of microbial control measures helps prevent the spread of such diseases or to bring cures to such ailments. Microbial control methods and agents help to better understanding of antibiotic resistance,epidermiology, antibiotic mechanisms, spore formation and microbial metabolism.

There are two major methods of controlling microorganism namely:

• The physical control method
• Chemical control methods

We will describe these methods by answering the following questions.What is Physical Control of Microorganisms?, What is sterilization?, What are the factors that affect the heat sterilization?

What is Physical Control of Microorganisms?

PHYSICAL CONTROL: In this method of knowledge of the minimum, maximum, and optimum conditions for growth of a particular microorganism to be dealt with is important. For instance, when the environmental temperature or microorganisms like bacteria is raised above the maximum it can withstand; it first of all ceases to multiply and after a few further degrees its dies.

The temperature at this time is Lethal Temperature. But if we subject the microorganisms to a temperature lower than the minimum, only multiplication is prevented and the effect is said to be static. So if in bacteria the effect is described as bacteriostatic. It must be noted that the lethal temperature varies from species to species hence the maximum growth temperature for a thermophilic bacterium may be the lethal temperature for a mesophilic one.

The physical control method comprises five broad divisions:

1. Sterilization by heat which includes moist heat and dry heat.
2. Desiccation
3. Osmotic pressure
4. Light effect(irradiation)
5. Mechanical removal by filtration

What is sterilization?

STERILIZATION

Sterilization means removing or killing all microorganisms and viruses or their dormant forms on or in a material. This so-called killed microorganisms or virus will thus not have the ability to reproduce even when favourable conditions are supplied (i.e. it is an irreversible loss of the ability to grow). It is possible that some killed microorganisms can “rescucitate” when their condition of growth is adjusted and that is the more reason why sterilization should hav![e the above property.

What is heat sterilization?

HEAT STERILIZATION

The principle of heating as one of the physical methods of sterilization is based on the fact that as the temperature rises past the maximum temperature for growth, there is denaturation of enzymes(refer effect of environmental factors on growth). To sterilize a microbial population, it will take longer at lower temperature than at higher temperature. Thus it is important to adjust the time and temperature to achieve sterilization.

Normally, moist heat has better penetrating power than the dry heat. Heat sterilization can be measured by the decimal reduction time or D. This is the time required for a 10-fold reduction in the population density at a given temperature while the thermal death time is the time at which all cells are killed at a given temperature.

Heat sterilization must involve the procedures for killing endospores because they are much more heat resistant than the vegetative cells. Also the nature of the medium in which heating takes place influences the killing of both negative cells and spores. Microbial death is more rapid at acidic pH and in acid foods like tomatoes, fruits e.t.c.

What are the factors that affect the heat sterilization?

The following factors affect heat sterilization.

i. Presence of endospores. It takes la onger time to kill vegetative cells of bacteria than to destroy the endospores because they are more heat-resistant.
ii. Nature of medium in which sterilization takes place .microbial death is more rapid at acidic pH and in acid foods like tomatoes and fruits.
iii. High concentration of sugars, proteins, and fats decrease penetration and usually increase the resistance of organisms to heat.
iv. High salt concentration may decrease or increase heat resistance depending on the organism.
v. Dry cells and spores are heat-resistant than moist ones.

What is moist heat sterilization?

Moist heat sterilization

The four methods here are

i. Use of autoclave.
ii. Tyndallization (fractional distillation).
iii. Use of boiling water and
iv. Pasteurization.

AUTOCLAVE

PRINCIPLE OF OPERATION

As far back as 1860, pressure cookers were first used for sterilization. They are made up of a vessel which contains heated water to attain pressures above those in the atmosphere with a safety valve provided to prevent an explosion,water and steam at temperatures above 100C could be produced to kill the most heat resistant microorganisms and heat-resistant endospores within a short time. These have to be done heating at a temperature above boiling point and use of steam under pressure.

The autoclave in essence is a sophisticated pressure cooker with mechanisms in place for adjusting the steam pressure and to make sure that air is completely removed from inside the chamber. The steam produced is normally at the temperature at which water boils for any given pressure. Air within chamber allows for an increase in pressure without necessarily an increase in temperature.

Both the pressure and temperature of the autoclave are choked intermittently, though the temperature parameter is most important in killing microorganisms. Hot air (similar to superheated steam or dry steam)is steam heated to temperature above the boiling point of water at a given pressure and may not be too satisfactory for the operation of the autoclave.

For sterilization, the conditions necessary are 15 pounds/square inch(Ib/in) steam pressure ,above atmospheric pressure and 121^OC(250F) for 15 minutes. Sterilization can also be achieved at 135^OC in 3 minutes. This is called flash autoclaving.

TYNDALLIZATION
This method was discovered by John Tyndall and it is done by using an apparatus which will not heat beyond 100^OC, e.g Arnold (Koch) sterilizer. In this live steam comes into direct contact with the materials to be sterilized for a predetermined or preset time. The medium is usually heated three times in three successive days hence fractional sterilization.

First day: All vegetative forms die while the spores remain. When incubated some of the spores germinated before the next day.

Second day: On exposure again to a steam at 100^OC, all vegetative forms die remaining those spores which did not germinate overnight.

Third day: The medium is exposed to steam again at 100^OC and at this time we expect that all the microorganisms in the material must have died.

Boiling: This does not offer real sterilization since the spores are not destroyed. Boiling here can be compared with first day treatment of Tyndallization.

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