Structure consists of a cyclohexane rings and amino sugars
Some examples include Streptomycin, Kanamycin, Neomycin, Tobramycin synthesized by Streptomyces
Gentamycin is produced by Micromonospora purpurea
They bind to 30S ribosomal subunit and inhibit protein synthesis by causing misreading of the mRNA
Bactericidal
Effective against gram negative pathogens
Tetracyclines
4 ringed structure with different side chains
Oxytetracline and Chlortetracycline are natural antibiotics produced by Streptomyces
Some other semi-synthetic drugs are also available
Bind 30S ribosomal subunit and blocks binding of aminoacyl tRNA at the A site of ribosome. Hence, inhibits protein synthesis
Bacteriostatic
Broad spectrum being active against gram-negative and gram positive bacteria, rickettsias, chlamydiae and mycoplasmas
Used to treat acne
Macrolides
Contains 12-22 C lactone rings and sugars
Erythromycin binds to 23S rRNA of 50S ribosomal subunit and inhibits peptidyl transferase reaction. Peptidyl transferase adds the growing peptide chain at the P site of ribosome to the next amino acid held by the tRNA at A site. This prevents growth of the peptide chain
Used for whooping cough and diarrhoea
Other examples include clindamycin, azithromycin
Chloramphenicol
First obtained from Streptomyces venezuelae but now synthesized chemically
Binds to 23S rRNA of 50S ribosomal subunit and inhibits peptidyl transferase reaction. Peptidyl transferase adds the growing peptide chain at the P site of ribosome to the next amino acid held by the tRNA at A site. This prevents growth of the peptide chain
Being highly toxic it is used a drug of choice during aplastic anaemia, leukopenia and other life-threatening situations
Inhibitors of Nucleic acid Synthesis
Made up of quinolone rings
Do not have a selective toxicity as prokaryotes and eukaryotes do not differ greatly with respect to nucleic acid synthesis
Synthetic antibiotics
Examples are Nalidixic acid (quinolone), fluroquinolones – ciprofloxacine, ofloxacine, norofloxacine
Quinolones inhibit DNA gyrase which introduces negative twist in DNA and helps separate its strand. Inhibition of DNA gyrase disrupts DNA replication and repair and chromosome separation during cell division
Fluoroquinolone inhibits topoisomerase II that untangles DNA during replication and affects in the same way as quinolones
Quinolones are broad spectrum antibiotics and usually administered orally
Metabolic Antagonists
These drugs are bacteriostatic but broad spectrum
Sulfonamides
Synthetic antibiotic
Structurally related to sulphanilamide
They are analogous to para-aminobenzoic acid, thus competitively inhibit the enzyme that uses PABA as its substrate and consequently inhibits folate synthesis
Folate is an important vitamin to bacteria used for synthesis of purines and pyrimidines that form the building blocks of DNA and RNA. Absence of folic acid causes them to die
Have high therapeutic index since eukaryotes do not synthesize folate hence very safe to eukaryotic cells
Trimethoprim
Synthetic antibiotic
Broad spectrum antibiotic
Binds to Dihydrofolate reductase that converts dihydrofolate to tetrahydrofolate in folic acid synthesis. This inhibits this metabolic process and causes death of the pathogens
Both drugs are given together for synergistic effect
