Antimicrobial Sensitivity, Kirby Bauer Technique

Background & Introduction: 

Many soil bacteria and some fungi secrete substances, called antibiotics that inhibit the growth of other bacteria.  Paper discs impregnated with different antibiotics can be placed on a special agar medium (Mueller-Hinton) that has been spread with a particular strain of bacteria.  After incubation a bacterial lawn will arise on the agar medium.  If the strain is sensitive to the antibiotic in the disc there will be clear zone of inhibition (no bacterial growth) around the disc.  This type of procedure allows physicians to prescribe appropriate and effective antibiotics for patients.  

Different antibiotics "work" by attacking certain targets on or in bacteria.  Because different antibiotics have different targets, they are selectively toxic.  That is, certain organisms can be killed by an antibiotic while others or only inhibited or not affected at all. 

Target or mechanism 

Example of an antibiotic with this target or mechanism 

Often used to treat 

Bacterial cell wall 

penicillin G 

Gram-positive bacteria* 

Inhibit protein synthesis 

tetracycline, streptomycin, erythromycin and gentamicin 

A broad-spectrum of bacteria 

Bacterial cell membrane 

nystatin and polymyxin 

polymyxin-Gram-negative bacteria                       nystatin-fungal infections 

Affect DNA or RNA synthesis 

naladixic acid 

A broad-spectrum of bacteria 


sulfa drugs (sulfonamides) (considered antibiotics even though they are synthetic agents and not microbially synthesized) 

A broad-spectrum of bacteria* 

*Because of improper use and overuse, an increasing number of bacteria are becoming resistant. 

Terms and Definitions 

  • Antibiotic: a substance produced by one living organism that kills or inhibits the growth of another organism. 

  • Bactericide: a substance that kills bacteria. 

  • Bacteriostatic: a substance that inhibits the growth and reproduction of bacteria. 

  • Antimicrobic: any agent that kills or inhibits the growth of microorganisms. 



E. coli

P. aeruginosa

S. aureus

Kirby Bauer results for E. coliKirby Bauer results for P. aeruginosaKirby Bauer results for S. aureus