Laboratory Workup: Gram NEG Cocci
*Neisseria (meningitidis & gonorrhoeae)
*: Most medically relevant of this category. They occur as diplococci and possess the enzyme cytochrome c oxidase (thus are oxidase +).
To distinguish the gram negative cocci from each other, we can look at the oxygen requirements for growth of the organism. Most gram negative organisms are aerobes. Scroll over the boxes below to discover which gram positive cocci is anaerobic. We won't be mentioning this organism for the rest of this lecture as it is not as medically relevant as the other gram negative (diplo)cocci.
In addition to oxygen requirements, some species require special media for growth. Blood agar is a media that contains trace metals and fatty acids which inhibit the growth of certain species (scoll over box to reveal which species). To grow these organisms, it requires "chocolate agar" which is normal blood agar plate that has been heated to 80C to inactivate the growth inhibitors.
Non-selective chocolate agar is used for sterile sites (CSF, blood, synovial fluid). A special chocolate agar (Thayer-Martin or Martin-Lewis agar) is used for other sites prone to contamination (such as urethral cultures). It contains a selective agent that inhibits the growth of other organisms and is used for urethral cultures.
To distinguish N. meningitidis from N. gonorrhoeae, other carbohydrate utilization tests are used. Explore the fermentation patterns below.
Presence of a Capsule
MICRO & IMMUNO
Gram POS Rods
Gram NEG Rods