Diagnostic Antimicrobial Agents: Introduction, Working Mechanism and Uses

diagnostic antimicrobial agents

Diagnostic antimicrobial agents(Antibiotics)

There are some diagnostic antimicrobial agents that are used for the presumptive identification of various organisms.  They are the most common optochin, bacitracin( 10 µg), bacitracin (0.04µg), and novobiocin.

The most common antimicrobial agents and their uses:

Optochin (ethylhydrocupreine hydrochloride)-5µg:

Mode of action:

Optochin is a quinine derivative. It selectively inhibits the growth of S. pneumoniae at very low concentrations i.e. 5 mg/mL or even less. It may also inhibit viridans streptococci, but requires higher concentrations. It is water-soluble and diffuses readily into the agar medium.


It uses for the presumptive identification of Streptococcus pneumoniae, which is optochin-sensitive, from other alpha-hemolytic streptococci such as viridans streptococci, which are resistant.

Bacitracin (10U): 

Mode of action:

Bacitracin is a polypeptide antibiotic derived from Bacillus subtilis that functions to block cell wall formation by interfering with the dephosphorylation of the lipid compound that carries peptidoglycans to the growing microbial cell wall.


Put the polypeptide antibiotic bacitracin 10 Unit disc into chocolate agar to inhibit normal flora, including gram-positive bacteria, such as streptococci, and most species of Neisseria whereas to support the growth of Haemophilus which is resistant.

Bacitracin (0.04U):

Mode of action:

same as in  bacitracin 10U disc


It uses to differentiate Streptococcus pyogenes from other ß hemolytic streptococci, Streptococcus agalactiae . S. pyogenes is sensitive whereas S. agalcatiae resistant.

Novobiocin (5µg):

Mode of action and use:

The mechanisms of novobiocin resistance include inhibition of cell wall synthesis as well as inhibition of protein and nucleic acid synthesis. Staphylococcus saprophyticus is an intrinsic resistance. This disc uses to differentiate S. saprophyticus from other coagulase-negative staphylococci by the overnight incubation disc test method.

Further Readings

  1. Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University press.
  2. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  3. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  4. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr and Sommers H.M.
  5. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  6. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.

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