Lecture 12 - Control of Microoranisms

There are two main methods of controlling microorganisms: physical methods and chemical methods. To determine which method to use, we look at the organism. The method will depend on the structure of the microorganism (gram, spores, capsules), the age of the culture (growth curve), and the resistances that the target acquires due to plasmids or mutations.

Cidal
If a method of control is cidal, it kills the target, hopefully all the organisms. This is especially useful because more and more people are immunocompromised (caused by AIDS, cancer, or anti-inflammatory drugs), and no work is required from the immune system if all the microorganisms are dead.

Static
A static method will not kill the bacteria, but will stop the growth of bacteria. This allows the immune system, if competent, to eliminate the bacteria.

Inhibitor
An inhibitor will slow the growth of an organism in order to help the immune system get ride of the organism.

Ineffective
Organisms that are resistant will survive and reproduce at their normal pace.

Amount
In chemicals, the amount is determined by concentration. Generally, a higher concentration will increase the effectiveness, but this is not always true. For example, 100% alcohols are not effective because at least some water is needed to be effective. Alcohol will take about 15 minutes to kill vegetative cells, and is not effective against spores. The gold standard for wipes is now chlorhexidine, and is used by the CDC. (i.e. phenol)

For phisical methods, the amount is determined by intensity. The higher the intensity, the more effective it is.

Time
No method of control is instantaneous. Log death phase will always drop down at a steady rate. Alcohol isn't used to disinfect skin anymore because it evaporates bery quickly, and has no residual effect, unlike chlorohexidine. The autoclave runs at 121.5°C for 15 minutes to sterilize a sample, but can be run for 20 minutes just to be sure that everything is dead. Generally, the longer the time used for treatment, the more effective the agent will be.

Temperature
An increase in temperature will result in an increase of the reaction rate. Every 10°C, the reaction speed is affected by a factor of 2. An increase will double, and a decrease will half the reaction speed (i.e. autoclave). If something is heat labile, it is easily changed by heat. If you want to disinfect a wound, you would not use an autoclave because you'll also destroy the host's body.

Load
The load is the amount of cells. A Higher load will be take more time to control, and methods will generally be less effective. A disinfectant will kill a set percentage of organisms at a set time interval because the capsules of dead bacteria will protect the cells of the living. One can reduce the load of bacteria by scrubbing and washing.

Kind of Organism
Gram negative bacteria and spores are traits of resistant bacteria. Viruses such as Hepatitis B can last up to a week on a benchtop, and AIDS can last 1.5 days if inside a cell, or 6 hours if isolated.

Maturity of Organism
Mature organisms are most resistant to to control agents because they have a low metabilism already has formed, as opposed to the younter organisms which have a high metabolism and are constantly synthesizing new cell components. Old organisms past the stationary phase are more susceptible because they are damaged by the toxins that they release (such as alcohol).

Penetration
The larger the volume, the longer the treatment will take because the medium stops the control agents from reaching all parts of the medium. The penetration is also hindered by a high density, i.e. agar is denser than broth.

Colloids
A colloid is a heterogeneous mixture that have tiny particles in suspension. Some parts of the colloid mixture will absorb the treatment agent and deactivate it. To compensate, treatment agents are added until there is an excess.

pH
Iodine and other halogens only work in an acidic environment, but alkaline environments will compromise their ability to kill bacteria.

Enzymes
Enzymes will break down antibiotics over time. In a human body, the blood is filtered by the kidneys and then excreted. Renal dipeptidases will break down a number of carbapenems. To compensate for this, some drugs will include another chemical to block the enzymes from breaking down the drug.