The ability to degrade glucose differs between bacteria. No species can ferment glucose, some are oxidative while others cannot do either (nonsaccharolytic).
The medium of Hugh and Leifson is formulated for detection of weak acid production associated with oxidation. The agar is semisolid with low peptone and relatively high glucose concentrations.
Testing:
(1) one tube is inoculated and left open to the air (with a cap)
(2) one tube is inoculated then covered with 1 cm of sterile mineral oil or paraffin
Reading:
(1) acid production indicates glucose degradation and converts bromthymol blue to yellow
(2) failure to degrade glucose means that minimal acid is produced and leaves bromthymol blue green or blue
Open Tube
Covered Tube
Interpretation
yellow
green
oxidative
yellow
yellow
fermentative
green or blue
green or blue
nonsaccharolytic
A slow-growing organism may require incubation for 3 or more days.
Causes of a false-positive reaction:
(1) mixed isolate
Causes of a false-negativ result:
(1) reading the test too soon
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