What is an antibiotic?

Molecules of biological origin, which are derived from soil-fungi or bacteria that are inhibitory or lethal against other bacteria are termed antibiotics. Many antimicrobial drugs are either modification of natural molecules or synthetically made.

Who coined the term "antibiotic"?

Selman Abraham Waksman, who also discovered Streptomycin, coined the term antibiotic.

Do all antibiotics kill bacteria?

Some antibiotics are bactericidal whereas some other are bacteriostatic.

What are the different mechanisms of action of antibiotics?

• Inhibition of cell wall synthesis.
• Alteration of cell membrane function.
• Inhibition of protein synthesis.
• Inhibition of nucleic acid synthesis.

Name the antibiotics that inhibit cell wall synthesis.

beta-lactam antibiotics (such as Penicillins and cephalosporins), vancomycin, bacitracin and cycloserine.

Name the antibiotics that alter cell membrane function.

Colistin, Polymyxin, Amphotericin B and Nystatin.

Name the antibiotics that inhibit protein synthesis.

Chloramphenicol, Erythromycin, Clindamycin, Tetracycline and Aminoglycosides.

Name the antibiotics that inhibit nucleic acid synthesis.

Sulphonamide, Trimethoprim, Quinolones and Rifampin.

What are the different methods of drug resistance in bacteria?

• Production of enzymes that inactivate the drug. Eg: β-lactamase, Chloramphenicol transacetylase.
• Production of new or modified targets with little or no affinity for the drug. Eg: Resistance to penicillin by MRSA, resistance to Streptomycin and Erythromycin.
• Decreased membrane permeability and active efflux. Eg: Resistance to Tetracycline.

What is the genetic basis of drug resistance?

• Chromosomal mutations.
• Acquisition of plasmid or transposon, containing gene coding for drug resistance.

What is intrinsic and acquired resistance?

• A bacterium found resistant to an antibiotic since the very beginning is intrinsically resistant to that antibiotic. Eg: Novobiocin resistance in S.saprophyticus.
• A bacterium acquiring resistance to an antibiotic through plasmid, transposon or due to a chromosomal mutation is acquired resistance. Eg: Penicillin resistance.

What are the methods of antibiotic susceptibility testing?

• Micro-broth and Macro-broth dilution method.
• Agar dilution method.
• Disk diffusion method.
• E test
• Automated systems

What are MIC & MBC?

• MIC is minimum inhibitory concentration. It is the highest dilution of an antibiotic that inhibits the growth of bacteria.
• MBC is minimum bactericidal concentration. It is the highest dilution of an antibiotic that kills the bacteria.

Name two drugs, when given in combination, are additive or synergistic.

Penicillin and Gentamicin; Amoxycillin and Clavulanic acid; Trimethoprim and sulphamethoxazole. Several drugs are given together in the treatment of tuberculosis to achieve synergistic killing and to prevent the emergence of drug resistance against any one of them.

Name two drugs, when given in combination, are antagonistic.

Penicillin and Tetracycline (since one is bactericidal and the other is bacteriostatic).

Name some bactericidal and bacteriostatic drugs.

Penicillins, Cephalosporins, Aminoglycosides etc., are bactericidal while Chloramphenicol and Tetracycline are bacteriostatic.

Which is the enzyme that degrades penicillin ring?

β-lactamase ( penicillinase and cephalosporinase). There are many types of beta-lactamases such as Extended Spectrum Beta-Lactamases, AmpC beta-lactamases, Oxacillinases and Carbapenemases.

When is a bacterium called “Multi- drug Resistant”?

When a bacterium develops acquired resistance to three or more standard antibiotics, it is said to have developed multi drug resistance. This feature is common in Staphylococcus aureus, Salmonella typhi and Mycobacterium tuberculosis. There are organisms which exhibit extended-drug resistance and pan-drug resistance.

Name some Beta-lactamase inhibitors.

Natural or derived molecules such as Clavulanic acid, Sulbactam and Tazobactam. Avibactam is a synthetic beta-lactamase inhibitor.

Name antibiotics used for topical application?

Bacitracin, Neomycin and Mupirocin.

How is the antibiotic susceptibility test reported?

Bacteria may be susceptible (sensitive), moderately (intermediate) susceptible or resistant to an antibiomicrobial drug.