β-Lactam antibiotics


      Mechanism of action



      Clinical uses 


At the end of this session, students will be able to:

       Classify Penicillins

       Describe the mechanism of action of Penicillins

       Outline the pharmacokinetic profile of Penicillin

       Discuss  the clinical uses of Penicillins

β-Lactam antibiotics

       All of the drugs in this group contain a β-lactam ring in their structure

       Inhibit bacterial cell wall synthesis


       Obtained from Penicillium notatum

       At present obtained from Penicillium chrysogenum

       Consists of sulfur containing thiazolidine ring fused with β-lactam ring having a side chain at C-6

       6- amino pencillanic acid (6-APA)- active moiety

       Has intact β-lactam ring and amino group at C-6 joined at thiazolidine ring

Mechanism of action


       Inhibits bacterial cell wall synthesis

       Bacterial cell wall is composed of peptidoglycan

       Peptide chains are cross linked by pentaglycine bridge

       Process of trans bridging -  transpeptidation

       Catalysed by penicillin binding protein (PBP)

       Penicillin competes and inhibits PBPs

       Forms imperfect cell wall

       Osmatic drive of fluid

       Bacetrial death by swelling and lysis

       Bactericidal action

Resistance to penicillin

By any of the following mechanism

       Inactivation of β-lactam ring by β-lactamase

       Modification of PBPs

       Reduction in penicillin permeability to reach PBPs

       Activation of antibiotic efflux mechanism

Classification of Penicillins

       Based on antimicrobial spectrum

       Some are stable to gastric acid

       Some are acid labile (give parenterally)

       Some are resistant to β-lactamase

       Others are β-lactamase sensitive

Narrow spectrum penicillin

Extendend spectrum penicillin

Narrow spectrum penicillin

       Penicillin V and G- natural penicillin

       β-lactamase sensitive

       Penicillin G – acid labile

       Easily absorbed

       Wide distribution – except eye, prostrate and CNS

       Excreted through kidneys

Extended spectrum penicillin

       β-lactamase sensitive

       Aminopenicillin – acid stable

       Carboxypenicillin  and Ureidopenicillin – acid labile

       Good bioavailability

       Excreted through kidneys

Antimicrobial spectrum

Narrow spectrum penicillin

       Penicillin G and V - Effective against gram positive and gram negative bacteria

       Semisynthetic – Cloxacillin, dicloxacillin, methicillin and nafcillin

       Spectrum of action similar to natural penicillin

Extended spectrum penicillin

       Retains antimicrobial spectrum of penicillin G

       Extended spectrum against gram negative organism

       Amoxycillin – for eradicating H.pylori in duodenal and gastric ulcers

       Piperacillin- antipseudomonal penicillin

Clinical uses

       UTIs, typhoid fever, bacillary dysentry

       Amoxycillin – for eradicating H.pylori in duodenal and gastric ulcers

       Ampicillin – for meningitis by Listeria monocytogenes





       β-lactam antibiotics are the group of drugs that contain a β-lactam ring in their structure and they act by inhibiting bacterial cell wall synthesis

       They include – penicillins, cephalosporins, monobactams and cabapenems

       Penicillins are the drug initially obtained from fungus, now obtained by semisynthetic process

       They used in the treatment of UTIs, endocarditis, septicemia

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