Antibiotics: an ongoing war (part 3)
Antimicrobial drugs can be divided into different categories. Proper antibiotics are substances produced entirely by microorganisms, whereas antimicrobial chemotherapy stems totally from the laboratory. Finally, semi-synthetic antibiotics are created by modifying naturally occurring antibiotics). In general, they are all grouped under the term ‘antibiotics’. This subdivision derives from the fact that the bacteria themselves (and other organisms such as fungi) produce toxins (antibiotics) to kill other bacteria and thus have a so-called ‘evolutionary advantage’. The bacterium x that produces the toxin y is logically not susceptible to that toxin (it is “immune”).
How do antibiotics kill bacteria?
These substances can act by blocking some processes for cell division, thus preventing bacteria growth. In this case, we define them as bacteriostatic. Another mechanism of action is stopping some vital functions, inducing their death. This time they are labeled as bactericidal.
The goal is to obtain this effect while causing the least possible damage to the host, thanks to the selective toxicity, i.e. the ability of antibiotics to selectively hit a bacterial target that is not present in the host (we are going to discuss this in future articles). However, any molecule introduced into our body is harmful in some quantities.
«Omnia venenum sunt: nec sine veneno quicquam existit. Dosis sola facit, ut venenum non fit»
Paracelsus
«Everything is poison: nothing exists that is not poisonous. Only the dose prevents the poison from taking effect»
Selective toxicity can be predicted exploiting the therapeutic index, which depends on the therapeutic dose and the toxic dose of the substance. Given a group of patients who received a specific drug, the therapeutic dose corresponds to the necessary amount of the drug to carry out its therapeutic action in 50% of people. On the contrary, the toxic dose is the quantity that causes serious or irreversible toxic effects in 50% of patients. The therapeutic index is therefore the ratio between the toxic dose and the therapeutic dose. In other words, the higher the index, the better the antibiotic.
Another important aspect is the antibiotic spectrum. Broad-spectrum antibiotics act on numerous bacterial species, even very different from each other (for instance, on both Gram+ and Gram–), while narrow-spectrum ones are effective against a limited number and often very similar bacterial species (usually either Gram+ or Gram–).
Two other fundamental values in antibiotic chemistry are the MIC and the MLC. The first is the minimum inhibitory concentration, i.e. the minimum concentration of the antibiotic in an aqueous solution which prevents the growth and duplication of a specific pathogen. The second is the minimum lethal concentration, thus the minimum concentration of the antibiotic that causes cell death.
Thanks to these characteristics, we now have a general picture of each type of antibiotic.