Methods to Identify Anaerobes Clinical Microbiology Review 201
METHOD EXPLANATION
Gram stain
Growth on media
Special-potency antimicrobial disks
Rapid tests
Conventional tubed biochemicals
Biochemical multitest systems
Preformed enzyme-based systems
Gram reaction; morphology; presence, location, & shape of spores provide clues to ID. Some
GN anaerobes stain faintly with safranin. Recommended to extend time of counterstaining to
3–5 min or use 0.1% basic fuchsin. Some GP anaerobes, e.g., Clostridium, may stain pink.Which media organism grows on, pigmentation, hemolysis, & colonial morphology provide
clues to ID.Kanamycin, vancomycin, & colistin disks can be used to differentiate anaerobes & ensure that
over-decolorized Clostridiumis not misidentified as GNR. Disks placed on 1st quadrant of plate.
After incubation, observe if organism is susceptible or resistant.For presumptive ID, e.g., fluorescence; catalase; spot indole; urease; motility; SPS, nitrate, &
bile disks; lecithinase, lipase, & proteolytic rxn on egg-yolk agar.Test tubes containing variety of media inoculated & incubated in anaerobic environment. Rxn
leads to change in pH. Expensive & time consuming. Largely replaced by multitest systems.Trays or strips are inoculated & read after 24–48 hr incubation in anaerobic environment. Code
number is obtained & ID determined from codebook. Only contains codes for most commonly
isolated anaerobes.Detect preexisting enzymes. Panels or cards are inoculated & incubated in room air. Color
changes are read in 4 hr. Code number obtained & ID determined from codebook. Only contains
codes for most commonly isolated anaerobes.continued...