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Penicillins

Classify penicillins. write the mechanism  of action, adverse reaction and uses of benzyl penicillin.

Definition
Penicillins are antibiotics derived from a fungus called Penicillium
They are a group of Beta-lactam antibioitics
Beta-lactam is a particular ring of atoms; all the penicillins will have this ring of atoms in their structure
Some bacteria produce an enzyme called beta lactamase/penicillinase which inactivate the beta-lactam ring

Classification of Penicillins
Natural penicillins : (Penicillin V, Penicillin G, Benzathine Penicillin, Procaine Penicilline
Launched first
G is the basis of natural penicillins
Mainly useful against Gram + strains of Staphylococci and Streptococci and a few Gram -ve bacteria like meningococcus
Penicillin V is useful for anaerobic coverage in patients suffering from infections of oral cavity and Streptococcal pharyngitis

Penicillinase-Resistant Penicillins (Oxacillin, Dicloxacillin, Cloxacillin, Methicillin, Nafcillin, Cloxacillin, Dicloxacillin)
These do not have as broad an antibacterial spectrum as the Natural Penicillins
They are more specific for penicillinase-creating strains  of gram +ve cicci, and more specifically against staphylococcal species
Hence these are called anti-staphylococcal penicillins

Aminopenicillins (Amoxicillin, Bacampicillin, Ampicillin)
These are active against several gram -ve bacteria like H.influenzae and E.Coli
They have acid resistant properties
Hence oral forms
Used in mild infectons like sinusitis, tonsillitis, otitis media, bronchitis, bacterial diarrhea and UTIs.
For otitis media amoxicillin is the drug of choice

Extended Spectrum Penicillins
Also called as anti-pseudomonal  penicillins
Mezlocillin, piperacillin, azlocillin - acylaminopenicillins. 
Ticarcillin and carbenicillin - alphacarboxypenicillins
Many time combined with aminoglycosides
Vulnerable to  inactivation by beta-lactamases

Pharmacology
Pharmacology
Food does not interfere with absorption of amoxicillin
But penicillin G should be given 1 h before or 2 h after a meal.
Amoxicillin has generally replaced ampicillin for oral use because amoxicillin is absorbed better, has fewer GI effects, and can be given less frequently.

Penicillins are distributed rapidly in the ECF of most tissues, particularly when inflammation is present.

All penicillins except nafcillin are excreted in urine and reach high levels in urine. Parenteral penicillin G is rapidly excreted (serum half-life 0.5 h), except for repository forms (the benzathine or procaine salt of penicillin G); these forms are intended for deep IM injection only and provide a tissue depot from which absorption takes place over several hours to several days. Benzathine penicillin reaches its peak level more slowly and is generally longer-acting than procaine penicillin.

Benzylpenicillin, also known as penicillin G, is an antibiotic used to treat a number of bacterial infections. This includes pneumonia, strep throat, syphilis, necrotizing enterocolitis, diphtheria, gas gangrene, leptospirosis, cellulitis, and tetanus. It is not a first-line agent for pneumococcal meningitis.
As an antibiotic, penicillin G is noted to possess effectiveness mainly against Gram-positive organisms. Some Gram-negative organisms such as Neisseria gonorrhoeae and Neisseria meningitidis are also reported to be susceptible to Penicillin G.