Antibiotics and antivirals against bacteria and viruses

Bacteria and viruses are very different infectious agents that do not cause the same diseases, and cannot be treated with the same drugs. Antibiotics are only active against bacteria, and antivirals only against viruses.

What is an infection?

An infection is a disease caused by the invasion of certain organs by an infectious agent. This agent generally has a route of entry into the body (mouth, nose, wound on the skin, etc.) from which it can reach different organs where it multiplies.

Each infectious agent generally has one or more preferred organs in which it develops. For example, the tuberculosis bacillus can infect numerous organs, but the most common form in our latitudes is infection of the lungs.

As it multiplies, it disrupts the functioning of the organ, activating the immune system.

and can produce toxins that are released into the bloodstream. It is these elements that cause the symptoms of infection.

The relationship between certain infectious agents and diseases already known in humans was established in the 2nd half of the 19th century:

• by Louis Pasteur for rabies;
• and Robert Koch for tuberculosis.

Infections can also affect animals and plants.

What is an infectious agent?

An infectious agent is a germ capable of causing an infection. Bacteria and viruses, which are responsible for a large number of human diseases, are the most common.

However, fungi (e.g. those responsible for skin mycoses) and parasites (e.g. Plasmodium, responsible for malaria) are also infectious agents.

Bacteria

Bacteria are very common because they can be found everywhere. A distinction is made between

• Aerobic bacteria, which need oxygen to develop;
• anaerobic bacteria, which prefer to live without oxygen.

They are generally made up of a single cell and have the ability to develop autonomously; in other words, they reproduce from themselves.

Viruses

These infectious agents, which are also very widespread, are also made up of a single cell.

But unlike bacteria, viruses cannot reproduce on their own. They have to infect an organism, then enter and live inside those cells, then use what's in the cell to make new viruses. This makes them dependent on the organism they infect; if the organism's cells die, the viruses will no longer be able to multiply and will also die. On the other hand, this gives them an advantage over bacteria, as they are more difficult to detect once inside the cells.

Some bacteria are useful to humans

Some germs can infect us without causing any illness; they are said to be harmless.

Some bacteria are even indispensable to humans. For example, several billion bacteria in the human intestine are necessary for digestion; they are known as the intestinal microbiota.

Similarly, the human skin contains an average of 1 million bacteria per square centimetre, which are essential to its healthy balance. If these bacteria are destroyed, they can be replaced by other germs with which the body cannot coexist. The result is an infection.

The bacteria naturally present in the body therefore act as a bulwark against other infectious agents.

What are the symptoms of an infection?

Each infection has its own specific symptoms.

However, the activation of the body by the infectious agent means that the classic signs of inflammation are almost always present, to a greater or lesser extent and for a greater or lesser length of time:

• redness at the site of infection. For example, the appearance of a painful red pimple (furuncle) in the case of a skin infection, or a red and painful throat in the case of angina;
• pain of varying intensity and more or less localised;
• localised heat or fever. This can usually be accompanied by aches and stiff joints.

The diagnosis cannot be made on the basis of these signs alone. During a consultation with the GP, the latter will ask the patient about the conditions in which the disease started and how it is progressing. The doctor may also need to arrange for further tests to be carried out; for example, to determine whether a sore throat is bacterial or not, a rapid diagnostic test is required.

How does an infection develop?

Factors in the development of an infection

The course of an infection depends mainly on three factors:

The dangerousness and virulence of the infectious agent

Some agents are more dangerous than others, for example:

• the tuberculosis bacillus progressively spreads to different organs and requires prolonged treatment, whereas the treatment of cystitis caused by Escherichia Coli is short;
• the rabies virus is responsible for an illness that can be fatal, while the gastroenteritis virus is responsible for an illness that is usually mild.

A dangerous bacterium or virus is unlikely to cause an infection that heals spontaneously. What's more, some infectious agents multiply more rapidly in the body than others; they are said to be more virulent. The more rapidly they multiply, the faster the infection occurs.

The quantity of bacteria or virus infecting the person

The more bacteria or viruses enter the body, the greater the risk of a rapid onset of infection.

The person's state of health

This is a very important factor in determining the body's resistance, both at the time of infection and subsequently, in the fight against infection.

Progression of an infection

Depending on the duration and severity of the infection, there are four possible courses of progression:

• Infections that are not serious and that heal spontaneously and fairly quickly. This is the case, for example, with simple viral angina or viral gastroenteritis.
• Infections that are not serious and never heal. The virus remains in the body for life, and the disease progresses from time to time, in flare-ups. This is the case, for example, with herpes labialis or cold sores. But this type of infection can also sometimes develop into a serious form if the person's immune defences are weakened. The virus then spreads through the body, and in particular to the brain, where it can cause herpetic meningitis.
• Serious infections, which do not heal spontaneously and can progress very rapidly. This is the case, for example, with certain forms of bacterial meningitis or pneumonia.
• Serious infections, which do not heal spontaneously and progress very slowly. The human immunodeficiency virus, or HIV, for example, can take up to 15 years to completely destroy a person's immune system.

Bacterial or viral infection: the difference in treatment

Depending on the type of infectious agent, different drugs are used:

• antibiotics are active against bacteria;
• antivirals against certain viruses.

Anti-infective drugs are used to slow or halt the progress of the infection. Their aim is to destroy or prevent the development of infectious agents.

Preventing certain infections through vaccination

Some infections can be prevented by vaccination. Its aim is to prevent infection by teaching the body to defend itself before encountering the virus or bacteria.

Vaccines are best known for their use against viruses (measles, mumps, hepatitis, influenza, Covid, etc.), but there are also vaccines against the bacteria that cause cholera, whooping cough, meningitis, etc.

How is a bacterial infection treated?

The anti-infectives used against bacteria are antibiotics. The first to be used in medicine was penicillin in 1941.

Antibiotics can:

• kill bacteria;
• or prevent them from proliferating.

An antibiotic is only active against one or more very specific bacteria. The bacteria that are dangerous to humans and on which the antibiotic is active constitute its spectrum of activity.

Bacteria that are naturally insensitive to the antibiotic are outside its spectrum of activity. What's more, bacteria are capable of developing resistance mechanisms. This explains why some bacteria become resistant to certain antibiotics after being exposed to them. These drugs lose their effectiveness over time. Their spectrum of activity is said to narrow.

Using antibiotics: rules to follow

To ensure that antibiotics remain effective, there are a number of rules to follow when using them.

No antibiotics for a viral infection

Antibiotics are never used to treat a viral infection.

Antibiotics should not, for example, be used to treat influenza or viral sore throat (which account for almost 80% of sore throats). Such treatment would have no effect whatsoever. In the case of flu, if your doctor prescribes an antibiotic, it is to treat a superinfection caused by bacteria, in addition to the flu virus.

Antibiotic chosen according to the bacteria identified

Antibiotics should only be used against bacteria that are sensitive to them. In other words, those that fall within their spectrum of activity.

Compliance with the medical prescription for antibiotics

• Antibiotics should be taken for the shortest possible time, strictly as prescribed. This avoids excessively prolonged contact between the bacteria and the antibiotic, and therefore limits the risk of the bacteria developing resistance.
• Antibiotics must be used only in the doses prescribed. A higher dose would not be more effective. A lower dose would increase the risk of the bacteria surviving. As a result, the infection would not be effectively cured, and there would be a very high risk of the bacteria becoming resistant to the antibiotic.
• Finally, the antibiotic treatment must be taken for the full prescribed period, even if the symptoms of the infection have disappeared. The disappearance of symptoms does not necessarily mean that all the bacteria have been eliminated. Stopping treatment before the end could therefore lead to a resumption of the infection.

To ensure that these rules are observed and that antibiotics remain as effective as possible, most of these medicines are only available with a doctor's prescription.

Side effects of certain antibiotics

When treating an infection, certain antibiotics can also eliminate bacteria naturally present in the body, leading to side effects. This explains the diarrhoea sometimes caused by certain antibiotic treatments, as they also eliminate the beneficial bacteria naturally present in the intestine.

This situation is not serious, but may require monitoring.

How is a viral infection treated?

The anti-infectives used against viruses are known as antivirals. These are drugs that were discovered more recently than antibiotics.

Antivirals are prescribed only when necessary, depending on the patient's state of health and the virus involved. Viral infections usually heal without treatment (colds or gastro-enteritis, for example).

Antivirals do not kill viruses. They prevent them from multiplying, making it easier for the body to eliminate them. One of the main ways of monitoring the effectiveness of antiviral treatment is to measure the viral load, i.e. the number of viruses present in the blood. The effectiveness of the treatment is judged by its ability to reduce the number of HIV viruses in the blood, for example.

Like bacteria, viruses are widespread in our environment and there are many different types. Antivirals are therefore, like antibiotics, specialised according to the type of virus on which they are active. The viruses on which an antiviral is active constitute its spectrum of activity.

Different antivirals depending on how the virus develops

Viruses need to enter the body's cells in order to multiply. Antivirals can block the obligatory stages in the development of viruses.

A distinction is made between:

• Antivirals that block the virus from entering the cell. This is the case with enfuvirtide, for example, which prevents HIV from entering the body's immune cells.
• Antivirals, which block the virus from using the cell. For example, aciclovir prevents the herpes virus from using the cell, once it has entered it, to manufacture new viruses.
• Antivirals that prevent viruses from leaving the cell. Osetamivir, for example, blocks the flu virus in the cell and prevents it from leaving to infect other cells.
• Antivirals, which prevent the production of the virus's genetic material. Sofosbuvir, for example, is used in the treatment of chronic hepatitis C to prevent the genetic material of new hepatitis C viruses from being manufactured in the body.

Using antivirals: rules to follow

As with bacteria in relation to antibiotics, viruses can develop resistance to antivirals. Some strains of HIV, for example, are now resistant to several drugs, including nevirapine and rilpivirine. This means that multitherapies (triple therapies, for example) have to be introduced, combining several antivirals to ensure 100% efficacy against the virus.

As in the case of antibiotics, this phenomenon of resistance means that rules for use have to be observed.

Choosing the right antiviral medicine

• You should never use an antiviral drug unless you are absolutely certain that the infection is viral, and without knowing exactly which virus is involved. Nor should antiviral medication be used if the infection is not serious and the person's state of health does not make them vulnerable to the virus. For example, it is the doctor who decides whether to prescribe an antiviral, Paxlovid® (nirmatrelvir / ritonavir) in the case of Covid 19.
• Antivirals should only be used against viruses that are sensitive to them. An antiviral used against the herpes virus will not be effective against the virus responsible for hepatitis B or HIV. Each antiviral should only be used within the strict limits of its spectrum of activity.

Respect the antiviral prescription

• Antivirals must be taken strictly for the prescribed duration. This limits the risk of developing resistance, and ensures complete treatment of the infection.
• They must be used only in the doses prescribed. A higher dose would not be more effective. A lower dose would reduce the effectiveness of the drug, while increasing the risk of the virus developing resistance.

To maximise the effectiveness of antivirals, and because of the toxicity of some of these drugs when taken over long periods, antivirals are only available with a doctor's prescription.

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