• Antibiotics (Greek anti, “against”; bios, “life”) are chemical
compounds used to kill or inhibit the growth of infectious
organisms. Originally the term antibiotic referred only to organic
compounds, produced by bacteria or molds, that are toxic to other
• They are more toxic to an invading organism than they are to an
animal or human host.
• first antibiotic was penicillin, discovered accidentally from a mold
• Antibiotics only treat bacterial infections. Antibiotics are useless
against viral infections (for example, the common cold) and fungal
infections (such as ringworm).
• classification by their mechanism of action:
1. Inhibit bacterial cell wall synthesis: Penicillin
2. Cause leakage from cell membrane: Polypeptides
3. Cause misreading of m-RNA code
and affect permeability : Aminoglycosides
4. Inhibit protein synthesis:
5. Inhibit DNA gyrase:
6. Interfere with metabolism :
7. Interfere with DNA function : Rifampin
8. Interfere with DNA synthesis : Metronidazole
1. Β-Lactam antibiotics:
Carbapenems : Imipenem, Meropenem, Faropenem
A. PENICILLIN: Penicillin was the first antibiotic to be used clinically in 1941.
It was originally obtained from fungus Penicillium notatum.
Mechanism of action:
Binds to Penicillin-binding protein
Inhibit the Transpeptidase activity in
cell wall formation
Transpeptidation i.e cross linking
between peptides not occur.
Peptidoglycan cell wall not form.
bacterial cell wall synthesis inhibited.
• Eg: Penicillin G
B. CEPHALOSPORIN : These are group of semisynthetic antibiotics
derived from Cephalosporin-C obtained from a fungus
Cephalosporium. They are chemically related to penicillin. Its
nucleus consist of β-lactum ring.
• Their MOA is same as penicillin, i.e inhibition of bacterial cell wall
synthesis. All cephalosporins are bactericidal.
• Ist generation:
Broad spectrum than Penicillin.
Active against both gram+ve and gram-ve organisms.
• IInd generation:
Active against many gram-ve organisms which are resistant to
• IIIrd generation:
Gram+ve : less active than Ist generation
Gram -ve : more active.
• IVth generation:
Gram +ve : more active than IIIrd generation.
Gram –ve : excellent active against gram –ve.
2. AMINOGLYCOSIDES: Aminoglycosides are the drugs effective
against gram-ve bacteria. All aminoglycosides are produced by
soil actinomycetes. Aminoglycosides are bactericidal.
Mechanism of action : Cause misreading of m-RNA code and affect
permeability. Other antibiotics which inhibit protein synthesis are
Diffuse across outer coat of gram-ve bacteria (etransport, O2 dependent, only aerobes)
Bind to 30s (streptomycin), 50s (others), 30s-50s
interface. Protein synthesis initiation stopped.
Cause distortion of mRNA codon recognition.
Misreading of mRNA code. Wrong amino acid enters the
Abnormality occur. Bacteria become more permeable
and leak out ions, amino acids and protein.
• Uses : Tuberculosis
Subacute bacterial endocarditis
pseudomonas, proteus or klebsiella infection
Meningitis caused by gram-ve bacteria
Neomycin is used topically for infected wounds, ulcers, burn,
external ear infection, conjuctivitis, post operative infections.
3. MACROLIDES : Macrolides are the antibiotics having a
macrocyclic lactone ring with attached sugar. They are
bacteriostatic. Erythromycin is the first member discovered.
• Mechanism of action : Macrolides bind to 50s ribosome and
interfere with translocation.
When peptide bond forms between amino acid & peptide
chain at acceptor site (A).
The elongated chain is translocated back to peptidyl site (P).
Thus site (A) becomes available for next amino acyl-t-RNA
attachment. This is called translocation.
Erythromycin binds to 50s ribosome and inhibit this
Thus protein synthesis inhibited.
• Macrolides covers mostly gram+ve and a few gram-ve baterias.
Erythromycin is highly active against:
Uses : severe campylobacter enteritis
pneumonia, sinusitis, bronchitis
Brands : ALTHROCIN – ALEMBIC (erythromycin)
EROATE – LUPIN (erythromycin)
AZEE – CIPLA (azithromycin)
AZRO – PIRAMAL (azithromycin)
CELEX – GSK (clarithromycin)
4. TETRACYCLINES: These are the antibiotics having a nucleus of four
cyclic rings. They are obtained from soil actinomycetes. They are
Mechanism of action :
Binds to 30s ribosomes
Attachment of aminoacyl-t-RNA to the
mRNA ribosome complex is interfered
Peptide chain fails to grow
protein synthesis inhibited
Cocci : N. gonorrhoeae & N. meningitides
Bacilli : Gram+ve : Corynebateria, Propionibacterium acnes, B.anthracis
Gram-ve : V. cholera, Yersinia pestis , Helicobacter pylori
5. CHLORAMPHENICOL :Chloramphenicol was initially obtained
from Streptomyces venezuelae.
It is primarily bacteriostatic. Active against gram+ve & gram-ve
organisms same as tetracyclin.
Like tetracyclin it is inactive against Mycobacterium, Pseudomonas,
many proteus, viruses and fungi.
Mechanism of action:
binds to 50s ribosome
inhibit the transfer of elongated peptide chain to the
newly attached aminoacyl -tRNA at ribosome mRNA
Protein synthesis inhibited.
Gas gangrene : death & decay of wound tissues infected by soil
bacterium Clostridium. Toxins produced cause tissue decay and
Whipple’s disease : less absorption of digested food in intestine,
occur in males.
• Brands: CHLOREXIN CAP- CIPLA
PERAXIN – PIRAMAL
CHLOROMYCETIN CAP- PFIZER
6. GLYCOPEPTIDES :
It is active against MRSA. It is bactericidal to gram+ve cocci, Neisseria,
clostridia and diphtheroids.
• Mechanism of action:
It inhibits the bacterial cell wall synthesis.
binds to terminal dipeptide 'D-ala-Dala'
sequence of peptidoglycan unit
prevents the assembly and cross linking of
cell wall not form
Uses: Staphylococcal enterocolitis Septicemia, Osteomyelitis.
Brands : VANCOGEN – ALKEM
VANLID – CIPLA
VANCORIN – EMCURE
7. OXAZOLIDINONE :
Linezolid : active against only gram+ve bacterias. It is primarily
• Mechanism of action:
It inhibits bacterial protein synthesis ta an early step and different
site than other.
binds to 23s fraction of 50s ribosome
interfere with the formation of Nformylmethionine-tRNA-70s complex
thus tRNA binding site distorts
Protein synthesis not occur
• Uses : uncomplicated skin and skin structure infection.
Community acquired pneumonia, Nocosomial pneumonia.
• Brands: ALZOLID – ALEMBIC
LIZOLID – GLENMARK
LINOSPAN – CIPLA
Clindamycin : it is similar in MOA to erythromycin (inhibit protein
synthesis by binding to 50s ribosome).
• Uses :
Acne, Bacterial vaginosis.
• Brands :
ACNESOL – SYSTOPIC
CLINDAC A – GALDERMA
CTOP – INTAS
They are quinolone antimicrobials.
Mechanism of action:
• FQs act by inhibiting the enzyme bacterial DNA gyrase.
• DNA gyrase consist of A and B subunits.
• A subunit cause nicking of strand, while B subunit introduce –ve
supercoil into the strand, and then A subunit reseals the strand. This
is necessary to prevent the excessive positive supercoiling of DNA
strands, hence DNA replication occur.
• FQs bind to A subunit and interfere with the strand cutting and
• It is a novel antibiotic produced through fermentation of
Pseudomonas fluorescens. It inhibits isoleucyl transfer-RNA
synthetase and arrestbacterial protein synthesis.
Primary skin infection- impetigo, folliculitis, furunculosis, ecthyma.
Secondary skin infection- eczema, abrasion, lesions, insect bites,
• Brands: BACTROBAN – GSK
SUPIROCIN - GLENMARK
Polymyxin-B & COLISTIN
• Active against only gram-ve bacteria. They are bactericidal. They
cause distortion in bacterial cell membrane and cause pseudopore
formation, so that ions, amino acids leak out. And bacteria dies.
• Uses: ocular, autic and skin infections.
• Brands: NEOSPORIN – GSK
CADIPRIM – CADILA
ORIPRIM – ZYDUS CADILA
Mostly active against gram+ve bacterias. Acts by inhibiting cell wall
synthesis. Highly toxic so not given parenterally.
Resistance to antibiotics:
Resistance of bacteria to the effects of antibiotics has become a
major problem in the treatment of disease. Bacteria that are not
killed or stopped by antibacterial drugs may change in form so that
they resist attack against their cell walls or even produce enzymes
that kill the antibiotics.
Resistance: When any
Types of Resistance:
antibiotic does not show
affect against specific
microbes then it means drug
resistance is developed.
Natural: some antibiotics are
naturally resistant to specific
Eg- Penicillin is resistant to
Tetracyclin is resistant to M.
Acquired: microbes become
resistant due to use of
antibiotics for a long period of
Eg- Sulfonamides are resistant