TPR Biology Passage 34 - Microbiology and growth plates #2

[tshank]

Hi all, I'm wondering why in question #2 the say that Lac is both necessary for growth, as seen in III. and unnecessary in I.? What did I miss? The book explanation isn't very helpful. Question and answer below.

Q:
Which of the following genotypes could grow on Plate D in Experiment 1? Plate D shows growth on a plate with glucose, leucine, threonine present

1. Lac+ Arg+ Leu+ Thr+
2. Lac- Arg- Leu+ Thr+
3. Lac- Arg+ Leu- Thr-

1. I only
2. I and II only
3. I and III only
4. II and III only
   

"Item I: True. This bacterium could grow on only
minimal medium plus a carbon source, i.e.,
glucose or lactose. Arg+ Leu+ Thr+ just means it
could grow without arginine, leucine, or
threonine.
Item II: False. Arg- means arginine is required for
growth.
Item III: True. Plate D supplies glucose, leucine,
and threonine, just what this bacterium needs
for growth, since it can't make leucine or
threonine and can't metabolize lactose."
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[kraskadva]

They're not saying Lactose is necessary for growth in III. They said III can't metabolize Lactose (Lac-), so lactose as the carbon source is useless to III.
Bacteria I can use Lactose (Lac+), but that's irrelevant here, since the plate only has glucose.
But the real point is that Lactose is a red herring here. The plate supplies glucose (which pretty much everything can use as a carbon/energy source). The only thing you should be focusing on here is the Amino Acids.

The plate gives Leucine and Threonine, so Leu +/- & Thr +/- are both irrelevant.
The plate does not give Arginine, so the bacteria that can't synthesize Arginine (Arg-) won't be able to grow.
Exclude II, and voila...

 

[tshank]

Thats a great explanation. Thank you very much. The red herring certainly threw me off. Especially because it doesn't say in the passage that all of the species are unable to digest lactose or somehow produce it from glucose (if its possible). Either way, I had some over analytical, out of scope thoughts going on. Thanks!