Antibiotics

What are Antibiotics?

Antibiotics are substances processed by living organisms or produced in the laboratory, capable of causing the death of bacteria or preventing their growth.

Antibiogram: each disk is soaked in a different antibiotic; where the dark halo is missing, it means that the antibiotic is effective in killing the bacterium disseminated in the culture medium.

DIFFERENCE BETWEEN ANTIBIOTICS AND CHEMOTHERAPICS

Both are antibacterial drugs. The difference, originally, was based on the fact that chemotherapy drugs are synthetic drugs, while antibiotics have a "natural origin; the latter come, for example, from the metabolism of fungi (molds) or that of certain bacteria (streptomycetes).
Antibiotics represent a pharmaceutical category in constant evolution, for which many natural molecules have been chemically modified to obtain new drugs, called semi-synthetic drugs.

Functional classification

Depending on the effects on the microorganism, antibacterials are divided into:

BACTERIOSTATIC ANTIBIOTICS: they block the growth of the bacterium, facilitating its elimination by the organism.
ANTIBIOTICS BACTERICIDES: which determine the death of the bacterium.

BACTERICIDES

BACTERIOSTATS

Aminoglycosides
Betalactamines
Quinolones
Cycloserine
Cotrimoxazole
Daptomycin
Phosphomycin
Glycopeptides
Isoniazid
Nitroimidazoles
Pyrazinamide
Polypeptides
Rifamycins
Streptogramine

Fusidic acid
Amphenicoli
Dapsone
Ethambutol
Lincosamides
Macrolides
Nitrofurans
Novobiocin
Sulfones
Sulfonamides
Tetracyclines

Many times the bacteriostatic or bactericidal activity depends on the dosage of intake.

On the basis of the spectrum of action we speak of:

WIDE SPECTRUM ANTIBIOTICS: active against both gram positive and gram negative bacteria.
STRICT SPECTRUM ANTIBIOTICS: act only on certain bacteria.

Synergy and Antagonism

CONCEPT OF SYNERGISM: two antibiotics increase their activity when used together; in fact, they act on two different targets. The first, for example, inhibits protein synthesis, while the second that of nucleic acids.
CONCEPT OF ANTAGONISM: the activity of two antibiotics influences each other, as when they both act on the same biological target.

Combinations of several antibiotics can be used to treat polymicrobial infections, to prevent the appearance of resistant microorganisms, or to achieve a synergistic effect. For example, multi-therapy is used in the treatment of AIDS and for microorganisms that have frequent mutations.

Chemotherapy

They are drugs that act as antimetabolites and compete for the substrate with the enzyme that catalyzes a certain reaction.

SULPHAMIDES: act by inhibiting the synthesis of folates, substrates essential for the formation of nucleotides and amino acids. Man gets folate through his diet, while bacteria synthesize them starting from precursors (as the bacterial wall is impermeable to these substances). Thanks to this assumption, sulfonamides are toxic for the bacterium, but not for man. . The only microbe that escapes the action of these antibiotics is the intestinal enterococcus, which is able to absorb folic acid from the enteric kilo.
Sulfonamides have a structure similar to para-amino benzoic acid (a substrate necessary for the bacterial synthesis of folate) and compete with it for its enzyme (to which they bind by sequestrating it).

TRIMETHOPRIM: extremely widespread chemotherapy. It inhibits the bacterial production of folate, but, in the series of biochemical steps that lead to their synthesis, it acts at a different level than sulfonamides.

QUINOLONES: chemotherapeutic drugs derived from nalidixic acid. They act by inhibiting topoisomerase II; this protein, also known as gyrase, is made up of 2 subunits, A and B, which allow the unwinding and rewinding of bacterial DNA. Subunit A cuts the DNA in specific sites, while B acts in the so-called negative spiralization (DNA unwinding). Quinolones act by inhibiting the A subunit of gyrase and with it the replication of bacterial DNA (novobiacin is instead active on the B subunit and can therefore have a "synergistic action with quinolones).

Categories of antibiotics

Antibiotics can be distinguished on the basis of their biological target, therefore on the ability to:

inhibit cell wall synthesis (penicillins and cephalosporins)
break down the lipid structure of the cell wall (polymyxins)
inhibit protein synthesis by acting on the minor (30s) subunit of the ribosome (such as tetracycline and aminoglycosides, including gentamicin) or on the major (50s) subunit, such as chloramphenicol and macrolides.
inhibit the synthesis of nucleic acids by acting on the duplication of DNA (novobiocin) or on its transcription into RNA (rifamycins).