Microbiology Laboratory : MacConkey Agar Media
If the microbiology laboratory has the resources to support the use of a
third plate for subculture, MacConkey agar should be used, particularly if
the specimen was obtained from a patient with fever of unknown origin, when typhoid fever (S. typhi) is
suspected symptomatically, or if a Gram stain of blood culture broth reveals gram-negative bacilli.
MacConkey agar is designed to grow gram-negative bacteria and stain them
for lactose fermentation, in order to distinguish those that can ferment
the sugar lactose (Lac+) from those that cannot (Lac-). It contains bile
salts (to inhibit most gram-positive bacteria, exceptEnterococcus and some species of Staphylococcus, i.e., Staphylococcus aureus), crystal violet dye (which also inhibits
certain gram-positive bacteria), neutral red dye, (which stains microbes
fermenting lactose), lactose, and peptone.
1. Transfer approximately 0.5 ml of the blood culture broth onto MacConkey
agar and streak for isolated colonies.
2. Incubate the MacConkey agar for 18-24 hours at 35-37°C with ~5% CO2 (or
in a candlejar).
3. Inspect plate for growth and identify the bacteria (Figure 9).
Macroscopic examination of colonies
Presumptive identification of N. meningitidis, S. pneumoniae, and H. influenzae can be made on
the basis of growth and colony morphology on a BAP and CAP and a Gram stain of the organisms.
Table 1. Presumptive identification of N. meningitidis, S. pneumoniae, and H. influenzae based on growth on primary culture media and Gram stain results
Young colonies of N. meningitidis are round, smooth, moist, glistening, and convex, with a clearly defined edge on a BAP (Figure 13). Some colonies appear to coalesce with other nearby colonies. Actively growing colonies of N. meningitidis on a BAP are grey and unpigmented.
Older cultures (> 24 hours) become more opaquely grey and sometimes cause the underlying agar to turn dark. On a CAP, N. meningitidis appear similar to H. influenzae (see description above). While H. influenzae produce a pungent indol smell that can differentiate it from N. meningitidis, plates should not be opened in order to smell the cultures
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