For Gram-negative rods and staphylococci, the standardized disc-diffusion method (Kirby-Bauer) should be used.
No susceptibility testing is needed for S. agalactiae or N. meningitidis.
All strains of pneumococci should be tested on blood agar for susceptibility to chloramphenicol and penicillin. For the latter, the oxacillin (1 μg) disc is recommended (see “Lower respiratory tract infections”).
Strains of H. influenzae should be tested for susceptibility to chloramphenicol using chocolate agar or a supplemented blood agar. Most ampicillin-resistant strains produce a β-lactamase that can be demonstrated using one of the rapid tests recommended for the screening of potential β-lactamase-producing strains of gonococci.