Official reprint from UpToDate®
www.uptodate.com ©2017 UpToDate®

Overview of antibacterial susceptibility testing

Jatin M Vyas, MD, PhD
Mary Jane Ferraro, PhD, MPH
Section Editor
Stephen B Calderwood, MD
Deputy Editor
Allyson Bloom, MD


The microbiology laboratory serves as a valuable ally to clinicians in the diagnosis and treatment of infection. In particular, the isolation of bacteria from clinical samples yields useful information that is translated directly into therapeutic strategies for the patient. While fungi, viruses, and parasites are also identified in the laboratory, the technology to perform susceptibility testing is not routinely available. Thus, discussion of antimicrobial susceptibility testing usually addresses bacterial isolates.

The responsibility of the clinical microbiology laboratory is to identify the bacterial isolate and to determine antibiotic susceptibility patterns. However, not all bacteria isolated from clinical specimens have antibiotic susceptibility performed; some of the factors that influence this decision include:

  • Whether the antibiotic susceptibility of an isolate is predictable based upon the genus and species
  • Whether the organism is likely to represent normal flora in which case susceptibility testing is not routinely done
  • Whether sufficient numbers of colonies are present; if colony numbers are low (eg, urine, intravenous catheter tip), susceptibility tests are frequently not done.

Host factors also play a role in whether or not susceptibilities should be performed. If clinicians notify the laboratory that a patient is a transplant recipient or another immunocompromised host, all organisms will be identified and analyzed for susceptibility to antibiotics. The same organisms recovered from an immunocompetent patient may not be tested.

More detailed susceptibility testing is required for certain serious infections. As an example, determination of the minimum inhibitory concentration (MIC) may be necessary to choose optimal therapy for infections, such as streptococcal endocarditis [1] and pneumococcal meningitis [2].


Subscribers log in here

To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information or to purchase a personal subscription, click below on the option that best describes you:
Literature review current through: Aug 2017. | This topic last updated: Jun 19, 2013.
The content on the UpToDate website is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding any medical questions or conditions. The use of this website is governed by the UpToDate Terms of Use ©2017 UpToDate, Inc.
  1. Scheld WM. Therapy of streptococcal endocarditis: correlation of animal model and clinical studies. J Antimicrob Chemother 1987; 20 Suppl A:71.
  2. Doit C, Barre J, Cohen R, et al. Bactericidal activity against intermediately cephalosporin-resistant Streptococcus pneumoniae in cerebrospinal fluid of children with bacterial meningitis treated with high doses of cefotaxime and vancomycin. Antimicrob Agents Chemother 1997; 41:2050.
  3. Clinical and Laboratory Standards Institute. Performance standards for antimicrobial disk susceptibility tests; approved standard-tenth edition. Wayne, Pennsylvania, 2009 January. Report No.: M02-A9.
  4. Clinical and Laboratory Standards Institute. 2011. Performance standards for antimicrobial susceptibility testing; Twenty-first informational supplement; M100-S21. Clinical and Laboratory Standards Institute, Wayne, PA.
  5. Clinical and Laboratory Standards Institute. 2010. Performance standards for antimicrobial susceptibility testing; Twentieth informational supplement; M100-S20. Clinical and Laboratory Standards Institute, Wayne, PA.
  6. Clinical and Laboratory Standards Institute. 2010. Performance standards for antimicrobial susceptibility testing; Twentieth informational supplement; M100-S20. June 2010 Update. Clinical and Laboratory Standards Institute, Wayne, PA
  7. CLSI M100-S20 (2010) Cephalosporin and Aztreonam Breakpoint Revisions Fact Sheet http://www.clsi.org/Content/NavigationMenu/Committees/Microbiology/AST/CephalosporinandAztreonamBreakpointRevisionFactSheet/CephalosporinAztreonamBreakpointFactSheet.pdf (Accessed on January 26, 2011).
  8. Ericsson HM, Sherris JC. Antibiotic sensitivity testing. Report of an international collaborative study. Acta Pathol Microbiol Scand B Microbiol Immunol 1971; 217:Suppl 217:1+.
  9. Manavathu EK, Cutright JL, Loebenberg D, Chandrasekar PH. A comparative study of the in vitro susceptibilities of clinical and laboratory-selected resistant isolates of Aspergillus spp. to amphotericin B, itraconazole, voriconazole and posaconazole (SCH 56592). J Antimicrob Chemother 2000; 46:229.
  10. Royo P, Martín-Casabona N, Martínez E, Andonegui M. In vitro susceptibility of Mycobacterium kansasii to the difluorinated quinolone sparfloxacin using a broth microdilution and macrodilution MIC system. Int J Tuberc Lung Dis 1999; 3:349.
  11. Spangler SK, Jacobs MR, Appelbaum PC. Bactericidal activity of DU-6859a compared to activities of three quinolones, three beta-lactams, clindamycin, and metronidazole against anaerobes as determined by time-kill methodology. Antimicrob Agents Chemother 1997; 41:847.
  12. Jorgensen JH, Ferraro MJ. Antimicrobial susceptibility testing: general principles and contemporary practices. Clin Infect Dis 1998; 26:973.
  13. Clinical and Laboratory Standards Institute. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically; approved standard-eighth edition, 2009 January. Report No.: M07-A7.
  14. Ringertz S, Kronvall G. On the theory of the disk diffusion test. Evidence for a non-linear relationship between critical concentration and MIC, and its practical implications for susceptibility testing of Haemophilus influenzae. APMIS 1988; 96:484.
  15. Bauer AW, Kirby WM, Sherris JC, Turck M. Antibiotic susceptibility testing by a standardized single disk method. Am J Clin Pathol 1966; 45:493.
  16. Brown DF, Brown L. Evaluation of the E test, a novel method of quantifying antimicrobial activity. J Antimicrob Chemother 1991; 27:185.
  17. Couroux PR, Massey VE, Schieven BC, et al. Comparison of the E-test and reference agar dilution method for susceptibility of gram-negative anaerobic organisms. Am J Clin Pathol 1993; 100:301.
  18. Kelly LM, Jacobs MR, Appelbaum PC. Comparison of agar dilution, microdilution, E-test, and disk diffusion methods for testing activity of cefditoren against Streptococcus pneumoniae. J Clin Microbiol 1999; 37:3296.
  19. Prakash V, Lewis JS 2nd, Jorgensen JH. Vancomycin MICs for methicillin-resistant Staphylococcus aureus isolates differ based upon the susceptibility test method used. Antimicrob Agents Chemother 2008; 52:4528.
  20. Lee A, Gooden H. Improving communication of microbiology test results. Am J Clin Pathol 1982; 77:443.
  21. Clinical and Laboratory Standards Institute. Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria; Approved Guideline. Waye, Pennsylvania, 2006. Report No: M45-A.
  22. Patel M, Waites KB, Moser SA, et al. Prevalence of inducible clindamycin resistance among community- and hospital-associated Staphylococcus aureus isolates. J Clin Microbiol 2006; 44:2481.
  23. Fiebelkorn KR, Crawford SA, McElmeel ML, Jorgensen JH. Practical disk diffusion method for detection of inducible clindamycin resistance in Staphylococcus aureus and coagulase-negative staphylococci. J Clin Microbiol 2003; 41:4740.
  24. Clinical and Laboratory Standards Institute. Performance standards for antimicrobial susceptibility testing; M100-S16. 2006 Catalog Supplement. 2006;26(3).
  25. Howe RA, Wootton M, Walsh TR, et al. Heterogeneous resistance to vancomycin in Staphylococcus aureus. J Antimicrob Chemother 2000; 45:130.