Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


pen.gif benzathine-penicillin-g-2-4mega_1986226-20090610015408[1].jpg fermenter02_02[1].jpg On the left a mould is killing bacteria, using a chemical substance.

Because one living organism is working against another living organism, the chemical is called an antibiotic.

The mould is called Penicillium, and the antibiotic is called penicillin.

When the mould is grown in a fermenter, penicillin can be extracted and purified.

Antibiotics can control bacterial infections

Do not confuse antibiotics with antibodies
domestos.jpg Antibiotics are specific chemical substances that are produced by one type of micro-organism (usually a bacterium or fungus, rarely other types of living organism), which kill or prevent the growth of another species of microorganism, usually bacteria. Luckily, antibiotics are many times more poisonous to bacteria than they are to humans! However, some people are allergic to certain antibiotics.

The antibiotic is extracted from a culture medium in a fermenter and purified, then possibly chemically modified in some way and converted into tablet or liquid form, like many other drugs.

Examples of antibiotics include penicillin, ampicillin, amoxicillin, streptomycin, chloramphenicol, cephalosporin, tetracyclin(e), vancomycin, avoparcin, (and many more besides).

Other, less specific, ways of controlling microbial growth include chemotherapeutic agents of purely chemical origin, such as sulphonamides. Sometimes these are loosely included alongside true antibiotics.

Production of penicillinopens in new frame
The penicillin molecule in 3Dopens in new frame
Bacteriaopens in new frame
In practice then, antibiotics are used as medicines to prevent the growth of infectious bacteria inside the body.

Bacteria have cells which are very different from the cells of the human body, and they also carry out some of their basic biochemical processes - metabolism - in a different way. This means that it should be possible to use these chemicals to control bacteria without harming the human body.

Some antibiotics destroy bacteria by interfering with processes that bacteria perform in a special way, such as making their cell wall. Others merely prevent them from reproducing.

Antibiotics cannot be used to control viruses

Examples of human viral diseases: Common cold, Flu (influenza), Mumps, Measles, Rubella, Chickenpox, Shingles, HIV/AIDS, HPV, Herpes, Viral gastroenteritis (stomach flu), Viral hepatitis, Viral meningitis, Viral pneumonia and many more. Follow this link for more details

Virusesopens in new frame
This is because viruses do not have cells or carry out biochemical processes which can be targeted. However viruses can enter cells of the body and take them over as they reproduce. This means that they cannot be destroyed without harming the body's cells.

In fact most Biologists do not classify viruses as living (although they may use this word when viruses enter their host cells), so it is debatable whether they can be killed!

Specific antibiotics need to be used to control specific bacteria

Bacteria are said to be 'sensitive' to a particular antibiotic if they can be controlled by it.
Some antibiotics are called broad-spectrum antibiotics. These can be used against a wide range of bacterial types to treat the infections they cause. Examples include the penicillins e.g. amoxicillin

Other narrow-spectrum antibiotics e.g. vancomycin are used for more specific purposes against certain species of bacteria.

Bacteria are divided into two broad groups: Gram positive and Gram negative. These have different cell wall components, and because of this they are affected differently by different antibiotics.

Bacteria can be tested for sensitivity to different antibiotics by a simple laboratory test. This is especially important if there is a suspicion that the strain of bacterium is resistant to some (or all) antibiotics.
Antibiotic sensitivity testingopens in new frame

Some antibiotics are described as first-line antibiotics. These are the one that ought to be tried first. Second-line antibiotics can be used if these do not work, but they generally cost more!

Antibiotics have greatly reduced deaths from bacterial diseases

Penicillin - the first real antibiotic - was discovered by Alexander Fleming in 1928, but it took several years development before it was available for general use (1942-1948 in various parts of the world). The chemotherapeutic Prontosil (a sulphonamide drug) was released in 1935-1937.

Before this time there were very few reliable chemical methods of controlling bacterial infections, so relatively small injuries could quickly develop into life-threatening situations, especially if the injury involved contamination with dirt or gut contents. However for the last 60 years there has been a great reduction in the numbers of human deaths resulting from bacterial infections. Much of this can be explained by the availability of antibiotics to treat these conditions, although improved nutrition, living conditions and hygiene have played a part.

This can be seen in the following charts for septicaemia (general bacterial infection causing "blood poisoning") and puerpal fever (childbed fever - as seen in the article on Semmelweis). Note, however the different time scales (and mortality scales). The rise in mortality in the mid 1920s might be due to the economic depression.


Tuberculosisopens in new frame
Worldwide Distribution of TBopens in new frame
Prevention and control of TBopens in new frame
Certain bacterial infections were uncontrollable until appropriate antibiotics (and means of delivering them) were developed. Some continue to this day. For example, it is said that one third of the world's population is infected with the bacterium that causes tuberculosis.

Antibiotic-resistant strains of bacteria

Never use the word 'immune' to describe bacteria that cannot be controlled with antibiotics.
If antibiotics are not used carefully, there is a chance that antibiotic-resistant strains of bacteria can develop.

Non-serious conditions (e.g. sore throat?) should not be treated with antibiotics.

Antibiotics are not instant cures; they are prescribed as a course lasting several days, weeks, or even months. If patients stop taking antibiotics before the intended end of the treatment, it is likely that not all the target bacteria will be killed, so some will survive and continue to reproduce in their body and cause further problems.

How do antibiotic-resistant strains of bacteria develop?

Bacteria reproduce asexually, so a colony of bacteria derived from one cell ought to be effectively a clone, with no genetic difference between them and therefore all with the same physical characteristics as one another. This is because genetic material - DNA - is normally copied perfectly when cells divide.

However mutations are very rare (about one in a million) small random changes in DNA. When a bacterial infection occurs, there is such a large number of bacterial cells that there is a chance that some may have a mutation which enables them to survive in the presence of the antibiotic. For example, these mutants may have an enzyme that breaks down the antibiotic concerned, or have some other difference, e.g a modified cell wall.

In the presence of an antibiotic, all of the susceptible bacteria die. However the mutant resistant forms are unaffected. These then reproduce and pass on the resistance to all the offspring. The result is an antibiotic-resistant strain of bacteria. These will continue to cause the disease in the affected patient as they are not controlled, and they may also be passed to other patients.

New antibiotics must be developed

As infectious organisms become resistant to commonly-used antibiotics, it is considered necessary that new antibiotics are developed. Otherwise we could be in the situation that existed before antibiotics were discovered. It has been said that simple infections could become life-threatening and that many hospital procedures considered normal today (transplant surgery, chemotherapy, joint replacement) could be compromised.

The pace of development of new antibiotics by pharmaceutical companies is slowing down, whilst the emergence of antibiotic-resistant strains of bacteria shows no real reduction.

By the 1960s 14 classes of antibiotic had been introduced, but only 5 in the last 43 years. It is thought that only two antibiotics are currently in development, and that pharmaceutical companies find the development of antibiotics difficult and potentially unprofitable.

Other information on this site

More information about Types of antibiotics, together with links to several interactive examples showing molecular structure in 3D which can be moved, highlighted etc.

Diseases caused by bacteria : some information about different bacteria, what they look like, what diseases they cause, and something about problems with antibiotic resistance.

Web references

Antibiotics - BUPA factsheet

European Antibiotic Awareness Day European Antibiotic Awareness Day from the European Centre for Disease Prevention and Control
- good videos: examples from several countries
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