Antisepsis and disinfection are techniques for preventing infection.

A disinfectant is a toxic substance which kills living organisms. It does not distinguish between human cells or bacteria but will kill both. These substances are not always effective against bacteria or viruses it depends upon the species and the stage it is in. Bacteria often enter a spore stage in which they are dormant and in this condition they are highly resistant to disinfectants.

An antiseptic is used on the skin and may be used to clean wounds. The antiseptic is less toxic to human tissue than it is to the bacterial cells.

Antibiotics are substances originally derived from mould fungus. They are secreted by the mould as part of its interspecific competition to prevent the growth of competitive species especially bacteria on its food supply. Antibiotics have no action against viruses because they do not have any biochemistry to disrupt. Fleming discovered the first antibiotic, penicillin which is secreted by the mould Pencillium. The use of moulds by native medicine men in Africa and South America has been well documented and they only choose moulds at a precise stage of development. Currently the ant is being investigated as a source of antibiotic material since they a rarely affected by microorganisms and have hade many millions of years to develop systems to protect themselves. Initial studies are very promising. It is now possible to design an antibiotic material by using knowledge of the structure of the proteins which are to be attacked. The antibiotic can be genetically engineered to have a precise affect upon a particular microorgansim whilst having the minimum effect upon the human system. The antibiotics commonly used are able to attack bacteria in a number of ways.

Destruction, or prevention of manufacture, of the cell wall structure. Penicillin is an example of this type. It prevents peptide bonding in gram posistive bacteria cell walls and this results in the cells bursting.

Inhibition of RNA transcription. Rifampicin can bind to the RNA polymerase found in bacteria and prevent the production of RNA which inhibits protein manufacture by the bacterium. The Rifampicin does not bind to human RNA polymerase.

Inhibition of RNA translation. Streptomycin will bind to bacterial ribosomes which are slighlty smaller than eukaryote ones which it fails to bind to. This will prevent the attachment of messenger RNA anf therefore inhibit protein production.

Inhibition of DNA replication. Anthracyclines are used to prevent DNA replication in all cells but can be used as an anticancer treatment by applying the drug at times when cnacer cells are dividing whilst normal cells are not.

Damaging cell membranes. Amphotericin binds to substances in the membrane of fungi and this alters the normal ionic exchange mechanism of the cells. Polymixin will bind with gram negative bacteria and damage them, but will also attack the kidney and nervous system membranes.

One of the main problems with antibiotics is that once they are used they will naturally select for resistance in the population of organisms they are attacking. The development of resistance plasmids in bacteria can be quite rapid and very recent evidence suggests that mutation rates on the plasmids can increase when antibiotic attack happens and that this increases the chances of a successful mutation occurring. Up until now it was generally held that the mutation must already be prsent in the population at a very low level before the selection process occurred. Bacteria may become resistant to an antibiotic in a year or two and it usually takes about ten years to develop and trest an antibiotic. We are therefore running out of antibiotics to attack certain organisms. The failure to use a cocktail approach to drug adminestration is the main cause of this problem. If three drugs are used simultaneously and in a suitable dose rate they are unlikely to cause the development of a resistanst strain because it is highly improbable that a bacterium will have resisistance genes to all three drugs just by chance. When using a single drug any organisms with the single resistance gene will survive grow and spread, then a second drug is used and we select for a type that has a mutation for that particular drug having givien the bacterial population time to randomly mutate its resistance alleles. We end up with a population of organisms resistant to all of our antibiotics. This has happened with Gonnorrhea, Streptococcus aureus and Tuberculosis.

The method by whiich resistance occurs can be the development of an enzyme to destroy the antibiotic, eg penicillinase. The targetted enzyme may mutateand thus prevent inhibition by the antibiotic. changes in the wall structure may prevent the entry of the antibiotic.

Antibiotics can be classified as broad spectrum or narow spectrum. A broad spectrum antibiotic will attack a wide range of organisms, chloramphenicol and tetracycline are examples which are active against both gram positive and gram negative bacteria. Narrow spectrum antibiotics attack only particular types of bacteria, penicillin will only be active against gram positive types, it cannot attack the murein in the cell wall of a gram negative type because of the protective lipids covering it.

Interferons.

These are substances produced in minute amounts by the human lymphocyte system. They attach to cells which put up markers indicating that they are invaded by viruses. They inhibit protein synthesis in the cells they attach to and therefore prevent the viral replication process by preventing the viral protein coats being made.

Monoclonal antibodies are produced from b-lymphocytes which have been fused to mouse cancer cells. The cancer cell is immortal and will grow reapidly in culture solutions, while the lymphocyte is short lived and impossible to grow in any numbers. Thus the heterocyte formed from the fusion of the two different cells solve the problem. Each lymphocyte is fused with a mouse cancer cell by placing the mtogether in a solution with a very high Calcium ion concentration. This causes cell membranes to fuse together. The individual cell is then placed in a cultuire solution and its numbers increase. It will release antibodies of one particular type. By culturing thousands of different lymphocyte types in this way it is possible to test their individual antibodies against selected antigens and produce effective passive immune systems to protect us from disease. The use at the moment for these antibodies is mainly directed to specific targetting of drugs or radiation to particular tissues. A toxin can be attached to the antibodies which react with antigens found only on particular types of cancer cell. These will become contrated in the cancer tissue and the build up of toxins could kill the tumour. These are the so-called magic bullets but they have met with very limited practical success thus far.

ELISA technique

The ELISA technique is an assay method which can determine the precence of specific antibodies in body fluid samples. An antigen is fixed to a surface. Ablood sample is placed over the antigens. The antigens will react with a specific antibody if it is present in the blood sample. The blood sample is then gently washed away leaving the antibodies fixed. Next the antigen area is covered with a solution of antihuman antibody antibodies (yes that’s right!). The AAA’s react with the tail ends of human antibodies and carry a marker on them, if they stick to the antigen area then there are antibodies present therefore we know the blood sample contained them.