If a reliable, standardized method for testing a particular bacterial genus or species does not exist, the ability to produce accurate and meaningful data is substantially compromised. Although standard methods exist for most of the commonly encountered bacteria (see Tables 1 to 3), clinically relevant isolates of bacterial species for which standard testing methods do not exist are encountered. In these instances, the dilemma stems from the conflict between the laboratory’s urge to contribute in some way by providing data and the lack of confidence in producing interpretable and accurate information.
Table1. Summary of Broth Dilution Susceptibility Testing Conditions
Table2. Summary of Agar Dilution Susceptibility Testing Conditions
Table3. Summary of Disk Diffusion Susceptibility Testing Conditions
Many organisms not listed in Table 4 grow on the media and under the conditions recommended for testing commonly encountered bacteria. However, the ability to grow and the ability to detect important antimicrobial resistance patterns are not the same thing. For example, the gram-negative bacillus Stenotrophomonas maltophilia grows extremely well under most susceptibility testing conditions, but the results obtained with beta lactam antibiotics can be widely variable and seriously misleading. This organism produces potent beta lactamases that seriously compromise the effectiveness of most beta-lactams, yet certain isolates may appear susceptible by standard in vitro testing criteria. Therefore, even though testing may provide an potential answer, the answer may be incorrect.
Table4. Categorization of Bacteria According to Need for Routine Performance of Antimicrobial Susceptibility Testing *
Given the uncertainty surrounding the testing of bacteria for which standardized methods are lacking, two approaches may be used. One is to not perform testing, but rather to provide physicians with information based on clinical studies published in the medical literature about the antimicrobial agents generally accepted as the drugs of choice for the bacterial species in question. This approach is best handled when the laboratory medical director and infectious disease specialists are involved. The other option is to provide the literature information and perform the test to the best of the laboratory’s ability. In this case, results must be accompanied by a message indicating that testing was performed by a nonstandardized method and results should be interpreted with caution. When such tests are undertaken, customized antimicrobial batteries, including the agents most commonly used to eradicate the bacterial species of interest, need to be assembled and used. Recently CLSI has published the document M45 to provide guidelines for the testing of certain less frequently encountered bacteria.
