Special Media

Specialized media is usually required for the propagation of anaerobes. Anaerobes usually grow slowly; sometimes, overgrowth of aerobic bacteria makes it difficult for medical microbiologists to cultivate and isolate anaerobic colonies. In addition to the setup of aerobic culture media, the following are recommended for recovering anaerobes.⁷

What is distinctive about PRAS media?

During normal manufacturing methods involving sterilization of microbiology plates or tube media, oxidized products (organic peroxides) are formed. These tend to inhibit the growth of anaerobes. PRAS media are processed in a reduced condition; PRAS remain reduced up to (and after) inoculation.

Many medical microbiology laboratories no longer make their media; PRAS commercial media have emerged as superior in the ability to recover anaerobes from inocula containing a small number of bacterial cells.

Anaerobic blood agar is supplemented with peptones, which supply nitrogenous substances and amino acids to help anaerobes grow.

Choose at least one of the following to propagate anaerobic growth:

Note about blood culture bottles: Blood cultures are obtained as a set (aerobic and anaerobic bottles). The anaerobic bottle assists in the recovery of anaerobic and facultative anaerobic bacteria from venous blood (and also normally sterile body fluid, depending on manufacturer claims). In general, anaerobic bottles contain carbon dioxide in the presence of nitrogen under vacuum, while aerobic bottles contain more oxygen.

Selective and differential media are used to select for growth; some provide distinctive biochemical characteristics of a particular bacteria.

• Phenylethyl alcohol (PEA)-sheep blood agar inhibits the overgrowth of gram-negative bacilli and reduces swarming
• Kanamycin-vancomycin-laked (KV) blood agar aids in the selection of pigmented anaerobes
• Bacteroides bile esculin agar (BBE) for the selection of B. fragilis and other specific anaerobes