Antibiotics are an important group of medicines that treat and prevent bacterial infections of various types. Currently, over 100 different antibiotics are available to treat a wide range of infections, from minor to life-threatening.

An antibiotic is a chemical compound created to destroy bacteria. It can be produced naturally or synthetically. The role of an antibiotic is to stop bacterial reproduction through various methods.

Some antibiotics only block bacterial metabolism, allowing the immune system to destroy them, while others aim to kill bacteria. Antibiotics can also be used preventively, for example, to prevent infections before surgeries.

Bacteria are single-celled organisms in search of food sources; when they manage to bypass the immune system's filters, once inside the body, they begin to multiply, most often causing various ailments.

How did antibiotics appear?

It was the Scottish bacteriologist Alexander Fleming who accidentally discovered, in 1928, the strange bacteria-destroying properties of the "juice" secreted by a fungus of the Penicillium class. Upon returning from a two-week vacation, he found his staphylococcus culture affected by Penicillium, which, fortunately, came from a laboratory one floor below. Fleming observed that the bacteria he was studying were present throughout the culture medium but not in the moldy area. Thus, the "intruder" was responsible for inhibiting bacterial growth.

Do antibiotics also work against viruses?

Antibiotics do not act on viruses. Viruses are essentially a chain of DNA or RNA, without their own metabolism, considered not to be living organisms. There is nothing for an antibiotic to destroy.

However, there are cases where doctors may prescribe antibiotics even if the nature of the illness is viral. This happens because the patient's body may be so weakened that the doctor considers a bacterial infection possible. Therefore, in certain situations, as a preventive measure, antibiotics can also be useful in viral diseases.

Bacterial resistance to antibiotics

One of the extremely serious problems facing humanity is the increasing resistance of bacteria to existing antibiotics. Antibiotic resistance means either that the antibiotic no longer has any effect on the bacteria or that the effect is not significant enough to stop the disease. Bacteria become immune to antibiotics not only because of humans, but sometimes it happens as a result of antibiotic attacks between different types of bacteria.

There are three ways to acquire antibiotic resistance. The first way is the spontaneous mutation of the bacteria. Most of the time, this mutation leads to the death of the bacteria, but there are cases when this mutation is beneficial, in the sense that it confers immunity to it when the antibiotic is administered. Another way of immunization is the transfer of genetic information between bacteria; a non-immune bacterium, through the process called "conjugation," receives immunizing genetic material from another bacterium. Finally, the bacteria can acquire antibiotic resistance after an attack by a bacteriophage virus, which, if it does not kill it, can provide it with the genetic material necessary for survival in the fight against an antibiotic. Once immunity is acquired, bacteria transfer it to future generations.

How quickly do antibiotics work and for how long should they be taken?

Of course, the doctor's advice is paramount. However, as a general idea, the antibiotic begins to take effect after about 24 hours. If after 72 hours from taking the first pill there are no visible signs of health improvement, it is very likely that the antibiotic is not effective in combating the disease, requiring a new medical consultation and probably the prescription of another type of medication (or another type of antibiotic).

As a rule, the minimum period for which the antibiotic must be administered is 5 days. If you stop treatment immediately when you feel you have recovered (which can happen, in the case of a sore throat, for example, after only two days from taking the first pill), it is very possible that the remaining bacteria in the body are vigorous enough to multiply again and resume the attack against the body, with the initial symptoms returning.

• The Scottish bacteriologist Alexander Fleming discovered the first natural antibiotic, penicillin, in 1928.

• Antibiotics do not act on viruses.

• Appropriate use of antibiotics is essential to reduce antibiotic resistance.

• Fleming predicted the rise of antibiotic resistance. Bacteria become resistant to antibiotics over time, which reduces their effectiveness in controlling infection.

• Antibiotics work either by killing bacteria or by stopping their growth.

Mode of action of antibiotics

Antibiotic treatment aims to completely eliminate pathogenic bacteria from the body. The objective of an antibiotic molecule is to reach the surface or inside the bacterial cell and selectively target certain molecular or biological structures within it, leaving the animal organism unaffected.

There are different types of antibiotics, which act in two ways:

• Kill bacteria (bactericidal effect) - such as penicillin. These antibiotics interfere either with the formation of the bacterial cell wall or with its cellular content.

• Stop bacterial growth without destroying them (bacteriostatic effect) - such as tetracyclines.

• Due to the very high diversity of antibiotics, there is a multitude of mechanisms of action by which they exert their neutralizing or toxic effect on bacterial organisms. The objective of an antibiotic molecule is to reach the surface or inside the bacterial cell and selectively target certain molecular or biological structures within them, leaving the animal host cells unaffected. In this case, as can be seen in the diagram on the right, antibiotics can act at the level of the cell wall, cell membrane, nucleic acids, proteins, or bacterial ribosomes.

• Also, through their mechanism, antibiotics can belong to two broad classes, so they can have a bacteriostatic effect (inhibiting the growth of microorganisms but not destroying them) or a bactericidal effect (causing the death of bacterial cells), but there are examples of antibiotics that produce both effects.

Cell wall

• A relatively large number of antibiotics exert their effect by blocking the synthesis, organization, or final formation of the cell wall, especially at the binding sites of peptidoglycan (which is its main component), but do not interfere at all with other intracellular components. As a result of inhibiting cell wall synthesis, alterations occur in the intracellular composition of the microorganism, due to changes in osmotic pressure. As long as the intracellular processes can take place, they try to compensate for the internal pressure created by the absence of the cell wall, but until the cell is destroyed. At the same time, the lack of the wall will allow the entry of other antibacterial agents.

Some examples of antibiotics that act on the cell wall are: all beta-lactam antibiotics (penicillins, cephalosporins, monobactams, carbapenems, by binding to enzymes responsible for peptidoglycan synthesis), glycopeptide antibiotics (by preventing the binding of peptidoglycan layers), and bacitracin (by preventing the transport of peptidoglycan precursors)

Cell membrane

Another category of antibiotics is one that can produce direct or indirect damage (by inhibiting the synthesis of some constituent molecules) to the cell membrane, an effect that can manifest in both bacteria and some fungal species. Many molecules target membrane phospholipids, which are lipid components with anionic character and will attract cationic antimicrobial peptides, leading to physical deformations. Examples of such peptides include polymyxins (polymyxin B and colistin, which increase membrane permeability through detergent-like action) and daptomycin (which binds to the membrane and causes its depolarization)

Nucleic acids

• Some antibiotics have the effect of blocking the synthesis of nucleic acid molecules (DNA, RNA), the function of ribosomes, or the enzymes involved in the synthesis of bacterial proteins, which leads to the formation of defective proteins. If DNA and RNA synthesis is considered, there are several types of enzymes involved in their synthesis that can be inhibited. First, quinolones inhibit topoisomerase type II (DNA gyrase) and topoisomerase type IV, absolutely necessary for DNA replication, recombination, and DNA repair processes. The same mechanism of DNA gyrase inhibition is also exhibited by aminocoumarins (novobiocin).

• Other mechanisms of enzymatic inhibition may also exist. Rifamycins, a class that includes rifampicin, bind to RNA polymerase, which results in the inhibition of RNA synthesis initiation. Another mechanism is presented by nitroimidazoles and nitrofurans, which insert into the bacterial cell and are reduced under the action of bacterial nitroreductases to cytotoxic compounds that affect the structure of nucleic acids and proteins.

• Ribosomes and protein synthesis

• Finally, most classes of antibiotics are capable of inhibiting processes occurring at the level of the bacterial ribosome, which is structurally different from the eukaryotic one. Ribosomes are organelles with a role in cellular protein synthesis, so by disrupting their function, either protein synthesis will stop, or non-functional, aberrant proteins will be formed.

There are 10 classes of antibiotics:

• Penicillins. Also called beta-lactam antibiotics because they contain a beta-lactam nucleus in their molecule. This category includes penicillins, cephalosporins, monobactams, and carbapenems. They act by inhibiting bacterial cell wall synthesis, having a bactericidal effect (killing bacteria).

• Cephalosporins. They have a bactericidal effect (destroy bacteria) and act similarly to penicillins.

• Carbapenems. They are structurally related to penicillins. They have the broadest spectrum of action among beta-lactams. They are considered reserve antibiotics for moderate/severe infection cases. They are reserved as a last line of treatment to prevent the development of resistance.

• Macrolides - Erythromycin was frequently used as an alternative to penicillin treatment.

• Tetracyclines - They have a bacteriostatic effect (inhibit bacterial growth without destroying them).

• Quinolones - They have a bactericidal effect (kill bacteria) and a broad spectrum of action.

• Lincomycins - They have a bacteriostatic effect.

• Sulfonamides - They have a bacteriostatic effect (association with trimethoprim makes them bactericidal) and a broad spectrum of action.

• Glycopeptides

• Aminoglycosides