Medicine

Introduction

Previous chapters looked at different types of microorganisms. They can be harmful, pathogenic or beneficial. Beneficial microorganisms outnumber the harmful ones. People have been 'employing' microoganimsms for centuries. This chapter looks at the way microorganisms are used for medical purposes.

Medical biotechnology

The industrial application of living organisms is called biotechnology. Humans have been 'employing' microorganisms for centuries and today biotechnology is a fast- developing industry.

Biotechnology is the use of plants, animals and microorganisms, such as bacteria and fungi, to produce medicines, vaccines and to develop disease testing techniques. The biotechnological technique called DNA profiling plays a very important role in forensic science and identification. DNA profiling is the process of testing to identify DNA patterns or types. Sometimes it is also referred to as genetic fingerprinting.

Antibiotics

Antibiotics are natural substances that can be used to fight bacterial infections. They are produced and secreted naturally by bacteria and fungi. Different biotechnological techniques are used to produce antibiotics in pure forms and large quantities so they can be used for treating bacterial infections. Antibiotics do not affect viruses.

The search for antibiotics began in the late 19th Century when it was accepted that some diseases are caused by tiny organisms.

The first antibiotic-producing organism was discovered by Alexander Fleming in 1929 - apparently by accident. The antibiotic-producing mould 'contaminated' and killed some bacterial cultures that were part of another scientific experiment. Instead of throwing the contaminated dish away, Fleming examined the mould. He identified the mould as Penicillium notatum and named the substance that it produced, penicillin.Fleming found that it was effective against many types of bacteria. Penicillin was extracted and purified only in the mid-20th century. Today it is still the main infection-fighting antibiotic. The development of penicillin-resistant bacteria now limits its effectiveness.

Penicillin kills bacteria by preventing the formation of their cellular walls. Pre-existing cells are unaffected, but all newly-produced cells grow abnormally and are very fragile and easily destroyed in a process called osmotic lysis. See image 1.

The success of penicillin led to the search for other antibiotic-producing microorganisms, especially from soil environments. Today, several hundreds of antibiotics have been isolated from different microorganisms but only a few of them are clinically useful. The reason for this is that only compounds with selective toxicity can be used clinically. Selective toxicity means that an antibiotic will destroy only some organisms. Clinically useful antibiotics must be effective against pathogens but have minimal toxicity to humans and human beneficial microflora. In practice, this is expressed in terms of the therapeutic index - the ratio of the toxic dose to the therapeutic dose. The larger the index, the better its therapeutic value is.

Many of the antibiotic-resistant genes of some pathogenic bacteria are carried on their plasmids - circular DNA strands containing non-essential genetic information. This genetic information can be exchanged in a process called conjugation (see Topic 1, Chapter 4 of this unit).

Vaccine

The name 'vaccine' comes from the Latin 'vacca' which means 'cow' because the very first vaccine was created for smallpox using cows infected with this disease.

Pathogenic microorganisms like bacteria and viruses, can make people sick by destroying body cells or by releasing harmful toxins that destroy body cells. When these same pathogens are 'served' to humans in the form of a vaccine, they can help the body to develop immunity against them. A vaccine is a mixture of dead or weakened pathogens used to induce the formation of antibodies against this pathogen.

Vaccines work by triggering the body's immune response without making it sick. In cases where disease is caused by toxins produced by pathogens, the vaccine contains its weakened toxin. A vaccine is introduced to the body either by injection or within a small drink.

According to the type of pathogens used in the mixture, all vaccines are divided into four main groups:

• Live vaccines are made from live but 'disabled' pathogens. This type of vaccine produces long-term immunity.
• Inactivated vaccines are made of dead pathogenic organisms. Inactivated vaccines produce short-term immunity.
• Toxoids are made of toxic substances produced by pathogenic organisms.
• A subunits mixture is not made of whole pathogenic organisms, but their fragments.

The process of vaccine distribution is called vaccination. See image 2.