Recommendations for susceptibility tests on fastidious organisms and those requiring special handling

Fastidious organisms present problems in antimicrobial susceptibility testing related to particular cultural requirements or slow growth. Methods for commonly isolated fastidious organisms, including haemolytic streptococci, Neisseria spp. and Haemophilus spp., are included in the description of the standardized disc diffusion method. However, some further information is given here for Haemophilus influenzae and Neisseria spp. Other less commonly isolated fastidious organisms are detailed and modifications of the disc diffusion method are given for Campylobacter spp. and rapidly growing anaerobes. For those organisms where disc diffusion methodology is not recommended, Brucella spp., Helicobacter pylori and Leigionella spp., MIC determinations are often needed, although disc diffusion test results may be useful as screens for resistance. Where disc diffusion can be used it is important to use the correct medium and to include appropriate controls. Methodology is also given for testing isolates of Stenotrophomonas maltophilia, which are affected by both medium and temperature.