What are the principles of proper antibiotic usage? Name three important classes of antibiotics, give a specific example for each and indicate its main use in clinical practice.

Outline:
•	Principles of antibiotic usage:
- avoid usage in viral infections.
- use of single-agent & narrow-spectrum antibiotics.
- judicious use of chemoprophylactics.
- avoid topical applications.
•	Classes of antibiotics:
- penicillins: Benzylpenicillin- pneumococci, meningococci & S. pyogenes infections
- aminoglycosides: Streptomycin – tularemia, plaque, TB
- tetracyclines: Tetracycline – intracellular bacterial infections

Suggested Answer:

	The basis of antibiotic usage is to arrest of the growth of the causal pathogen and if possible, eradicate it and remove it from the body completely. There is now a vast choice of antibiotic agents available for the treatment of infectious disease. In making the choice of an agent, the physician has to take into account the likely pathogen, possible adverse effects and the selection of resistant microorganisms. The choice and subsequent management of antimicrobial chemotherapy is governed by clinical, microbial and pharmacological considerations.

	Proper use of antibiotics is important in slowing the development of antimicrobial resistance. The clinical evidence for infection should be reviewed meticulously before prescription of an antibiotic. An adequate history should be sought and the patient properly examined. Many patients are unnecessarily treated with antimicrobial agents for conditions that are not caused by a microbial agent. In some infections such as viral respiratory tract infections, the patient is unlikely to benefit from antimicrobial therapy. Some infectious diseases do not response to specific therapy at all, e.g. diarrhea caused by cryptosporidia. In these cases, it will be more prudent to provide only symptomatic relief instead of prescribing antibiotics. When antibiotic usage is indicated, a single agent and narrow-spectrum antibiotic should be used instead of a broad spectrum one unless it has been suspected that another pathogen may be present which warrant the its use. However, in treating diseases like tuberculosis when a long-course of antibiotic therapy is required, it is necessary to use more than one antibiotic to eradicate the pathogen completely and not to allow for the survival of resistant strains. Prophylactic use of antibiotics should be restricted to the period of maximum risk in patients undergoing procedures in which prophylaxis is of proven benefit. The use of topical antibiotics is only rarely justified. Infections involving superficial structures are better treated systematically, with the exception of conjunctival infection. The skin is an important site where antibiotic resistance can develop. Non-absorbable antibiotics delivered into the gastrointestinal tract are another form of topical antibiotic use that may promote antimicrobial resistance. Use of fixed combination of antimicrobial agents are not advisable as it promotes the development of antimicrobial resistance. The same antibiotic should not be used repeated for treating the same infection. Alternative antibiotic should be used if the original one is not effective.

	Penicillins are a group of beta-lactam antibiotics derived from benzyl penicillin. Their action is by inhibition of cross-linkage of bacterial cell wall peptidoglycan. Benzylpenicillin remains the drug of choice for severe pneumococcal, meningococcal and Streptococcus pyogenes infections. It is also used in treating infections due to anaerobes (except B. fragilis), spirochaetes, bacillus and Corynebacterium diphtheriae. Derivatives of benzylpenicillin such as the aminopenicillins and carboxypenicillins are more effective against Gram-negative bacilli at the expense of reduced activity against Gram-positive cocci. The penicillins are one of the most widely used group of antibacterial antibiotics. 

	Aminoglycosides are a group of potent antibacterial aminosugar-aminocylitol compounds which inhibit bacterial protein synthesis by binding to receptors on the 30S subunit of bacterial ribosomes. Streptomycin is used in clinical practice for tularemia and plaque. It is used with penicillin G or ampicillin in Streptococcus. faecalis endocarditis. Streptococcus is also employed in anti-TB therapy though it is seldom used now. Due to their toxicity, monitoring of serum concentration of the drug is necessary.

	Tetracyclines are group of antibiotics with useful activity against bacteria and some protozoal parasites. The group includes tetracycline, oxytetracycline and doxycycline. Tetracycline is used in the treatment of rickettsial, mycoplasma and chlamydial infections.