Selection of Antimicrobials for Antimicrobial Testing

By Acharya Tankeshwar •  Updated: 05/30/21 •  2 min read

Antimicrobial agents that are chosen for testing against a particular bacterial isolate are referred to as the antimicrobial battery or panel. Selecting the appropriate antimicrobials for susceptibility testing can be difficult given the vast numbers of antimicrobial agents  available.

  1.  Certain microorganisms can be intrinsically resistant to particular antimicrobial classes; therefore it is unnecessary and misleading to test certain agents for activity in vitro. Antimicrobials to which the organisms are intrinsically resistant are routinely excluded from the test battery (eg vancomycin vs. gram-negative bacilli).
  2. Antimicrobials in the same class may have similar in-vitro activities against selected bacterial pathogens. In these cases, a representative antimicrobial should be selected that predicts susceptibility to other members of the same class.
  3. If the resistance to particular agent is common, the utility of the agent may be sufficiently limited so that routine testing is not warranted and only more potent antimicrobials must be included in the test panel. 
  4. Antimicrobials developed for use against particular organisms must be included in the test panel of that organism only (eg. ceftazidime is developed for use against Pseudomonas aeruginosa but not against Staphylococcus aureus).
  5. Antibiotics must be able to achieve anatomic approximation.  Nitrofurantoin achieves effective levels only in the urinary tract so it should not be included in batteries tested against bacterial isolates from other body sites.
  6. Antimicrobial agent used in the test panel must be available in the local market/pharmacy.
  7. Use of antimicrobial susceptibility testing method also limits the use of antimicrobials as resistance may not be detected for that particular agent from this specific method in use.
  8. The number of antimicrobials to be tested should be limited in order to ensure the relevance and practicality of AST.

Periodic review of microorganisms that are currently predictably susceptible to certain antimicrobial agents is recommended to ensure that emergent, unexpected resistance is detected. Emerging resistance may also be suspected following poor response to a standard antimicrobial treatment regime.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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2 responses to “Guidelines for Routine Antibiotic Susceptibility Testing (AST)”

  1. Ishani says:

    it should be P. aeruginosa NCTC 10662

  2. daisy says:

    what is antibiotic susceptibility testing

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