INTRODUCTION

The term “antibiotic” was put forward by Vuillemin in 1889, to designate the active component involved in the process of ‘antibiosis’ or to the opposition of one living micro-organism to another . According to another school of thought – ‘antibiotics are nothing but the microbial metabolites which in relatively high dilution may inhibit the growth of micro-organisms’ .

Waksman proposed the widely cited definition that – ‘an antibiotics or an antibiotics substance is a substance produced by the micro-organisms , which has the capacity of inhibiting the growth and even destroying other micro-organism’ .

However , the restriction that an antibiotic must be a product of micro-organism is not keeping with common use.

Classification

The antibiotics will be discussed explicitly under the following four main heads , namely :

1. β – Lactum antibiotics
2. Aminoglycoside antibiotics
3. Chloramphenicol
4. Tetracyclines

β – LACTUM ANTIOBIOTICS

Beta – lactam anitibiotics are substrates for beta-lactamases and the interaction results in the destruction of thr antibiotics . There arenow known as agents which bind to the beta – lactamase enzymes and inactivate them, thus sparing β – lactam antibiotics from destruction .

The β – Lactum antibiotics may be further divided into two categories , namely :

(a) Penicillins

(b) Cephalosporins

1) Penicillins

Penicillin is the name assigned to the mixture of natural compounds having the molecular formula C16H18N2O4S

The different only in the nature of ‘R’.

Various silent features which ultimately determine the general structure of all penicillins :

1. The penicillins are all strong monobasic acids i.e., they form salts .

2. The penicillins are hydrolysed by hot dilute inorganic acids ; one carbon atom is eliminated as carbon dioxide and two products are obtained in equimolecular proportions , one being an amine , Penicillamine and the other an aldehyde , penniloaldehyde .

C9H11O4N2SR + 2H2O ―> CO2 + C5H11O2NS + C3H4O2NR

3. Combination of penicillamine and Penilloaldehyde in Penicillin

It has been observed that the hydrolysis of penicillin with dilute alkali or with the enzyme(penicillinase) yields penicillonic acid (a dicarboxlic acid), which readily eliminates a molecule of cabon dioxide to form penilloic acid, thereby suggesting that a carboxyl group is present in the β – position with regard to a negative group .

4. Probablen structures for penicillin

Based on the foregoing chemical evidences probable structures for penicillin are there:

Since penicillin is prone to undergo abrupt molecular rearrangement, e.g., on treatment with dilute acid, penicillin rearranges to penillic acid .

Therefore, it was absolutely necessary to examine the molecule by physical methods (thereby leaving the molecule intact). In fact, an intensive study of the penicillins was carried out with respect to their infra-red and X-ray diffraction analysis.

Different types of penicillin

Chemical reaction using penicillin

Semisynthetic Penicillins

• Penicilinase-resistant penicillins
• Carbapenems: very broad spectrum
• Monobactam: Gram negative
• Extended-spectrum penicillins
• Penicillins + b-lactamase inhibitors

2) Cephalosporins

The best known family of β – lactams are termed as the cephalosporins , wherein the β – lactam ring is strategically fused to a 6-membered dihydrothiazine ring system . The thiazine ring fusion makes the β – lactam system more stable but not fully resistant to opening by acids and penicillinase .

Classification

The cephalosporins may be classified under the following four categories :

• First generation
• Second generation
• Third generation
• Fourth generation

First generation

Cephalexin

Cefazolin

Cephalothin

Cephradine

Active against G+ cocci ( except.enterococci & MRSA ): s.pneumoniae, s.pyogenes,s. aureus, s.epidermidis