Fundamentals of control

Fundamentals of control

The term death, as used in microbiology, is defined as the irreversible loss of the ability to reproduce.The viable microorganisms are capable of multiplying;the dead microorganisms do not multiply (grow). The determination of death requires laboratory techniques that indicate whether growth occurs when the sample is inoculated into a suitable medium. The failure of a microorganism to grow when inoculated into an appropriate medium indicates that the organism is no longer able to reproduce, and the failure to reproduce is the criterion of death. A complicating factor in this definition is that the response of the organism may not be the same in all media. For example, a suspension of Escherichia coli exposed to a heat treatment may yield a greater number of survivors if a plating medium of trypticase soy agar is used rather than a medium containing bile salts such as deoxycholate agar.

The rate of death of bacteria

When one drops a suspension of bacteria into a bottle of hot acid or an incinerator, the bacteria may all be killed so fast that it is not possible to measure the death rate. However, less drastic treatment may result in the cells being killed over a longer period of time, at a constant exponential rate that is essentially the inverse of their exponential growth pattern.

Exponential death can be understood easily in terms of a simple model. Imagine that each cell is a target and that a large number of bullets (i.e., units of a physical or chemical agent) are being sprayed at them in a random manner, as with a machine gun; that is, no one is aiming the gun directly at a target. Common sense dictates some rules about the way bacteria die under these conditions. To begin, we assume that a single hit kills a bacterium.

The probability of hitting the target is proportional to the number of targets, i.e., the number of bacteria, present. Intuition tells us that we shoot randomly at many targets not yet hit decreases steadily and it becomes harder and it is harder to hit the remaining ones. (Hitting a target again and again does not count; a bacterium can be killed only once.)Let us take the simple numerical example. and assume that we have an initial population of 1 million targets. We shower them with the bullets for 1 min and manage to hit the 90 percent so that there are now 100,000 survivors left. Then we shower them with bullets 1 min more, but since we only have one-tenth as many targets as in the first round, we hit only one-tenth as many. In other words, this time, we hit the 90,000 of the targets and have 10,000 survivors. We shower these with bullets another minute, and since we have only one-tenth as many, or 9,000. This pattern repeats itself until there are no targets left. But notice that it is just as hard to kill the last nine bacteria as it was to kill the first of 900,000. In fact, we can never be sure that we have killed the last one; all that we can do is give the targets enough to overkill for there to be a good chance that the last has been hit.

The probability of hitting a target is also proportional to the number of bullets shot, i.e., the concentration of the chemical or intensity of the physical agent. Intuition again tells us that the more bullets we shoot in a given time, the faster the targets will be hit. If the targets are the bacteria and the bullets are x-rays or ultraviolet light, it stands to reason that the cells will be killed faster as the intensity of the radiation increases. If bullets are molecules of some chemical agent, the cells will be killed more rapidly as the concentration of the agent increases (unto a certain limit, of course).

The longer we shoot, the more targets we hit, but the more targets we have, the longer it takes to hit them all. This is an obvious restatement of the exponential death pattern, it simply means that it takes the time to kill the population, and if we shall have many cells, we must treat them for a longer time to be reasonably sure that all of the bacteria are dead.

Conditions influencing antimicrobial action

Microorganisms are not simple physical targets. Many biological characteristics influence the rate at which microorganisms are killed or inactivated by various agents. Many factors must be considered in the application of any physical or chemical agents used to inhibit or destroy microbial populations. It is not possible to prescribe one agent that will be effective for the control of microorganisms for all materials and all circumstances. Hence, it is necessary to evaluate each situation separately in order to select a process that research and experience have shown accomplish the desired result. Some of the biological characteristics of the cells, as well as environmental conditions which influence the efficacy of antimicrobial agents (physical and chemical). More specific information is described where the supplication of a particular antimicrobial agent is described.

Environment

The physical or chemical properties of the medium or substances carrying the organisms, i.e., the environment has a profound influence on the rate as well as the efficacy of microbial destruction. For example, the effectiveness of heat is much greater in acid than in alkaline material. The consistency of the material (aqueous or viscous will) will markedly influence the penetration of the agent, and high concentration of carbohydrates generally increase the thermal resistance of organisms.

The presence of extraneous organism matter can significantly reduce the efficacy of an antimicrobial agent by inactivating it or protecting the microorganism from it. An increase in temperature, when used with another agent such as chemical, hastens the destruction of microorganisms.

Kinds of microorganisms

Species of microorganisms differ in their susceptibility to physical and chemical agents. In spore-forming species, the growing vegetative cells are much more susceptible than the spore forms; bacterial spores are extremely resistant. In fact, bacterial spores are the most resistant of all living organisms in their capacity to survive under adverse physical and chemical conditions. The relative resistance of bacterial spores in comparison with other microorganisms.

Physiological state of cells

The Physiological state of cells may influence susceptibility to an antimicrobial agent. Young, actively metabolizing cells are apt to be more easily destroyed than old, dormant cells in the case of an agent that cause damage through the interference with metabolism: growing cells would not be affected. A comparison of the susceptibility of young and old cells to a lethal agent.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.