Tube 1: No color
change or red coloration to the slant. If
the medium did not change color at all, the probably no growth occurred
on the medium (look carefully for evidence of bacterial growth. If growth is apparent, then there probably is also a deeper red
coloration in the slant due to aerobic metabolism of amino acids.
Tube 2: Yellow
coloration of slant and butt. This
indicates fermentation of lactose (and most likely glucose as well).
The relatively high concentration of lactose (10g/L) in the
medium leads to strong acidification of the medium turning the pH
indicator yellow.
Tube 3: Gas formation. Indicates
that gas was also a product of fermentation.
Tube 4: Red slant
and yellow butt. Bacteria
can only metabolize glucose, which, when metabolized aerobically in the
slant is quickly exhausted (being present at a concentration of only
1g/L). Bacteria in the slant
then ‘resort’ to using amino acids which raises the pH of the
medium, causing the pH indicator to turn red.
Tube 5: Black
coloration indicates reduction of sulfur and production of H2S.
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