Antimicrobial susceptibility testing: Part 2 - Bacterial organisms

Ðurđa Slavić                                                                                                                           

Animal Health Laboratory, University of Guelph, Guelph, ON                       

AHL Newsletter 2022;26(1):8.

When performing antimicrobial susceptibility testing (AST), all diagnostic laboratories in North America are expected to follow the standards and guidelines established by Clinical Laboratory Standard Institute (CLSI).  The goal of this institute is to promote accurate and reproducible AST results, as well as appropriate reporting and interpretation by standardizing all aspects of AST.  A sub-committee on veterinary antimicrobial susceptibility testing (VAST) establishes guidelines for veterinary laboratories that include: testing methods, bacterial density, media and drug types, drug dilutions, incubation times, quality control (QC) requirements, and - most importantly - interpretative criteria.  

Some of these recommendations are being reviewed in a series on antimicrobial susceptibility testing in AHL newsletters, beginning with the article on test methods published in December 2021.  The second article in our series, presented here, pertains to selection of bacterial organisms for antimicrobial susceptibility testing.  The full document is available in LabNote 64: 

1. It is acceptable to proceed with empirical treatment for bacterial species that have a predictable susceptibility with low level or no known resistance to certain drugs.

  1. Penicillin is the recommended treatment choice for beta haemolytic streptococci including Streptococcus equi subsp. equi, S. zooepidemicus, and S. canis.  It is also a preferred treatment option for Trueperella pyogenes.
  2. It is recommended to test a bacterial organism that contributes to an infectious process requiring treatment if its susceptibility cannot be reliably predicted based on bacterial identification.
  3. AST is most often indicated for bacterial species known to have the ability to develop resistance to commonly used antimicrobial agents (i.e., E. coli, Staphylococcus spp., Pseudomonas aeruginosa).
  4. Both AST methods have been developed for routine testing of fast-growing aerobic bacteria and are modified for testing of some fastidious bacteria such as Actinobacillus pleuropneumoniae, Mannheimia haemolytica and Histophilus somni.

2. It is discouraged to perform AST on normal bacterial flora such as coagulase negative Staphylococcus spp. from skin samples.

3. CLSI guidelines are not available for all bacterial species.  Breakpoints are not established for certain veterinary pathogens because they may be difficult to grow under standard testing conditions (i.e., Glaesserella (Haemophilus) parasuis), they require a higher biosafety level than may be available in veterinary diagnostic labs (i.e., Brucella spp.), or if they are newly discovered.   AHL