January 28, 2006

The Earth’s fossil records show that bacteria appeared more than 3.5 billion years ago and enabled the development of other life forms by providing oxygen to the atmosphere. If all the bacteria were to die today, life will cease to exist on our planet.

If one judges the evolutionary success by the yardstick of survival, bacteria are indeed highly evolved, remembering that in evolution there is no inexorable trend towards higher forms of life. They have survived extremes of temperature and climate for billions of years and continue to exist as the most prolific forms on Earth. Their success is at least partly attributable to “lateral gene transfer” — obtaining genes from each other.

They can acquire gene sequences through conjugation (DNA transfer as a result of bacterial mating), transformation (when bacteria pick up DNA which is no longer part of another organism) and transduction (in which bacterophage transfer DNA fragments).

Bacteria are to us, both a friend and a foe. Let us first look at the help and assistance they provide. Inside the intestinal tract they produce enzymes and vitamins. The enzymes are particularly important for the herbivores, as without them, the plant cells could not be broken down. If one kills all the resident microbes in the termite intestines, the termites die because the wood splinters remain undigested.

The bacteria are highly efficient scavengers. They break down all the organic matter that falls on the ground, so that dead animals and plants release their valuable chemicals for use by the other organisms. In fact, bacteria have been identified which can degrade even heavy metals such as iron.

The nitrogen cycle, essential for life on Earth; is maintained by bacteria. Plants are unable to use gaseous nitrogen from the atmosphere. The principal way in which nitrogen becomes available to them is through nitrogen-fixing bacteria, such as Rhizobium. These bacteria are able to convert gaseous nitrogen into nitrates and release them into the soil which the plants then utilize.

Some plants, such as legumes, have evolved structures which can house these useful bacteria in their tissues. In return, plants supply carbohydrates to the bacteria. This is an excellent example of two different life forms coexisting in a symbiotic relationship.

As bacteria have a short life span, they provide us with an excellent tool to study the nature of life and the laws that govern it. With the help of recombinant DNA technology (genetic engineering), bacteria are now used for producing important medicinal substances such as hormones. For a long time, insulin, used by diabetics was extracted from bovine or porcine pancreas. Now it is produced by bacteria. The harnessing of bacteria in this respect is no less important than the domestication of wild animals, which took place 9,000 years ago.

Bacteria are found on our skin, in the nose, mouth and vagina. In the intestinal tract, the bacterial count progressively increases from the stomach downwards, reaching its maximum in the colon (large intestine). The high acidity in the stomach (pH 3), prevents most bacteria from surviving, but a spiral bacterium, Helicobacter pylori lives in it, sometimes causing ulcers. In view of this, H. pylori is of great medical importance.

There are so many bacteria in, and on our body, that about 10 per cent of our body weight consists of them. It has been estimated that the number of Escherichia coli (a species living in the gut) in a single individual, would be greater than the total number of humans that have ever lived on Earth.

The normal flora of bacteria on the skin and in the intestinal tract protect us from other dangerous organisms. The use of antibiotics can disturb this balance and make us more prone to infections.

Bacteria range in size from 0.1µm (micrometre) to about 600 µm. The largest bacteria so far discovered is the Epulopiscium fishelsoni, found in the gut of the sturgeon fish, which lives in the Red Sea and off the coast of Australia. E. fishelsoni is so large that it can be seen by the naked eye, appearing as a small dot.

Bacteria come in three main shapes. Rod-shaped called bacilli, round-shaped called cocci and spiral-shaped called spirochetes. All bacteria divide by simple binary fission in which the mother cell separates into two equal daughter cells.

Based on their gaseous requirements, bacteria can be classified into three groups.

— Aerobic, dependent on oxygen for their survival;

— Anaerobic, that cannot tolerate oxygen;

— Facultative anaerobic — prefer growing in the presence of oxygen but can also grow without it. Most bacteria fall into this class.

Bacteria lack the membrane-bound nucleus of the eukaryotes and their DNA appears as a tangled web in the centre, known as the nucleoid. In addition, bacteria contain plasmids, or small loops of extra chromosomal DNA. These can be transmitted from cell to cell by conjugation.

Antibiotic resistance is carried by the plasmids, and can spread rapidly between bacterial species by this method. Once resistance is acquired it is rarely lost, which means that the pool of antibiotic resistant bacteria is continuously growing, posing a danger to the human population.

In all bacteria, the plasma membrane is surrounded by a cell wall, excepting Mycoplasma, which are tiny organisms (0.2 to 0.3 um) and are variable in shape. The composition of the cell wall differs among species and is used for classifying bacteria into two major groups, using Gram’s stain (named after a Danish bacteriologist).

The bacteria with cell wall made up of peptidoglycan, stain purple with crystal violet (a dye), and are designated as G+. Other bacteria with an outer wall of carbohydrates, proteins and lipids do not stain with crystal violet, and are designated as G-. Gram’s staining is used very widely for diagnostic purposes.

Bacteria can form a resistant stage or spore. In this, a part of the genetic material of the cell gets concentrated in one place and then gets surrounded by a protective coat. This makes the spore impervious to chemical agents, dessication and temperature, which would normally kill the vegetative stage.

Some bacteria produce poisonous substances known as toxins. Toxins can be divided into two categories. Those that can easily be separated from the bacterial cell are known as exotoxins and those that remain coupled with the bacterial cell are known as endotoxins.

The endotoxins are usually associated with G-bacteria. Exotoxins, on liberation from the bacteria, circulate in the blood and can damage organs distant from the site where the bacteria are primarily located.

For instance, in diphtheria the causative organisms (Corynebacterium diphtheriae) are located in the throat, while the toxins damage the heart muscle. By chemical treatment, exotoxins can be made non-toxic, and yet retain their immunogenecity, when they are known as toxoid. Toxoid is used for immunization against two dangerous diseases, diphtheria and tetanus.

Most bacteria can be cultivated or grown outside the host in artificial culture media. If the medium is rendered solid by the addition of a gelling agent known as agar, the bacteria form visible masses of growth known as colonies. Colonies of different species vary in size, shape, texture and colour. Colonial characters are therefore used for the identification of bacterial species.

When bacteria act as foes, their first step is to come in contact with the host tissues. The indigenous flora are already in contact with the host and infection can result from them, but only in special circumstances.

Infections, resulting from this local pool are said to be “endogenous”. An example of this is the urinary tract infection caused by Escherichia coli, from the intestinal tract. This occurs more often in females, because of the close proximity of the anal opening to the female urethra, and its small size.

E. coli, which were non-pathogenic (not disease-producing), in their normal habitat (intestine), become pathogenic in their new or abnormal habitat, the urinary tract. A common example of endogenous infection is dental caries. This is characterized by demineralization of the enamel and destruction of the organic matrix of teeth. The primary pathogen is Streptococcus mutans.

Organisms acquired from external sources are labelled as “exogenous”, and are responsible for many serious diseases. This kind of transmission can occur from human to human, or from animals to human. In the latter case; the infection is called zoonosis.

The major routes of transmission of exogenous infections are:

— Direct contact, including sexual intercourse;

— Inhalation or droplet infection;

— Ingestion;

— Inoculation, including arthropod bites, and;

— Vertical transmission, from mother to foetus.

The body is constantly exposed to microorganisms from its environment, and these exposures, in most cases, do not result in disease. To establish themselves in a new host, the microorganisms need to survive, multiply and overcome the host defences. If the host defences are impaired, as in AIDS, then it becomes easier for the microbe to establish itself in the new host.

Direct contact in the form of sexual intercourse is a highly effective form of transmission. Slowly but surely the infection spreads in a community. Social and cultural behaviour plays a very important role in the spread.

There are three important bacterial infections which are spread sexually. These are gonorrhea (caused by Neisseria gonorrhoeae), Syphilis (caused by Treponema pallidum), and non-gonoceal urethritis (caused by Chlamydia trachomatis). Vaccines are not available for any of these, and there is at present no possibility of eradicating them from the human population.

Inhalation is an extremely common mode of infection and may result in the upper or lower respiratory tract involvement. Lower respiratory tract involvement is less common, but more likely to cause serious illness and even death. Transmission is more rapid in groups sharing living quarters.

A large number of bacteria can cause respiratory tract infections and one which has been in the limelight recently is Legionella pneumophilia. This bacterium is transmitted via air-conditioning ducts or by hotel shower exposure and can lead to pneumonia. Mycobacterium tuberculosis is also acquired, in most cases, by inhalation.

In the developing world, tuberculosis is probably the most important bacterial infection. In developed countries, its incidence had decreased since the last century, with improved standard of living. The situation has now changed because of AIDS and the disease has re-emerged as major public health problem in the developed world as well. In addition, the bacteria have become resistant to many anti-tuberculosis drugs, compounding the problem of effectively treating the disease in AIDS cases.

Ingestion of contaminated water and food is particularly common in third world countries, because of poor food hygiene. If the main water source gets contaminated, major epidemics can ensue. In the past, cholera has spread in this manner and has led to the death of millions of people all over the world.

The contamination of food is closely connected with house-fly population, which acts as a mechanical vector, picking up bacteria from faeces and transferring it to food. Typhoid fever, caused by Salmonella typhi, is a very important food-borne infection in developing countries.

Inoculation of bacteria by arthropods is less common than in viral infections. Tiny Gram negative bacteria, known as Rickettsiae, are transmitted by the bites of tick, mite, louse and flea. A variety of illnesses are produced as a result of infection by Rickettsiae, and at least 11 species of bacteria are involved.

Historically, the most famous bacterium transmitted by the flea is Yersinia pesos, which is the causative organism of plague. Plague, under the name of Black death, caused death and destruction across Europe in the middle ages.

In 1975, a new bacterial disease was recognized in rural children in Lyme, Connecticut, USA. The causative agent of Lyme disease is Borrelia burgdoferi. The infection is transmitted by ticks. In 5 to 15 per cent of untreated cases, cardiac, nerve and joint involvement occurs.

Inoculation of bacteria into tissues can also occur as a result of trauma and accident. The soil bacteria can enter the tissues and can cause life threatening diseases. Clostridium tetani, an anaerobic spore-forming organism, enters the tissues in this way and causes tetanus.

The article has been published with permission from World Scientific, the publishers of Prof Zaman’s book Life sciences for the non-scientist

Please Visit our Sponsor (Ads open in separate window)