Culture Media and Laboratory Diagnosis

Key points

• • Culture media are nutrient preparations, solid or liquid, used to grow microbes from a specimen so they can be identified and tested.
• • Media are classified by purpose: enriched media (such as blood agar) grow fussy organisms, selective media favour one organism while holding others back, and differential media make organisms look different.
• • MacConkey agar is both selective and differential: it favours Gram-negative bacteria, especially enteric rods, and turns pink those that ferment lactose.
• • Some organisms need special media, such as chocolate agar for fastidious organisms like Haemophilus and Neisseria, and Lowenstein-Jensen medium for the tuberculosis organism.
• • Bacteria are identified by how their colonies look, by the Gram stain, by whether they need oxygen, and by biochemical tests such as the catalase and coagulase tests.
• • Faster modern methods detect an organism's genes (for example by PCR) or its proteins, which can give an answer without waiting for growth.