Growth media help

[TTfutureMD]

Im not getting this auxotrphic stuff. My TPR books say ( if you have it, its page 104) says that a bacterium noted arg- implies that the bacterium cannot grow in media that does not have arginine. But then in the last paragraph, it says that a bacterium incapable of growing in lactose only medium is denoted lac-. My question is what does the (-) mean? From the first example, I thought arg- meant that the bacterium requires a supplemental substance to survive. But then in the lac example, the bacterium cannot grow with lac as the only source of carbon. Can someone please help me understand this. Please write the way you would explain to your grandma who has no background info. LOl! Thanks

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[IamSuperDoctor]

i think that the lactose case is different. some bacteria can use lactose as a substitute for glucose

 

[ready2rumble10]

Im not getting this auxotrphic stuff. My TPR books say ( if you have it, its page 104) says that a bacterium noted arg- implies that the bacterium cannot grow in media that does not have arginine. But then in the last paragraph, it says that a bacterium incapable of growing in lactose only medium is denoted lac-. My question is what does the (-) mean? From the first example, I thought arg- meant that the bacterium requires a supplemental substance to survive. But then in the lac example, the bacterium cannot grow with lac as the only source of carbon. Can someone please help me understand this. Please write the way you would explain to your grandma who has no background info. LOl! Thanks

The way I understand it, these are two different bacterial phenotypes. Arg- means that the bacterium has a mutation somewhere in the Arg synthesis pathway (i.e. it can't convert some intermediate into arginine), so it can only grow on medium that contains arginine. Lac- means that the bacterium isn't capable of metabolizing lactose (i.e. can't break it down into galactose and glucose--and we all know how important glucose is), so it won't grow on a plate where lactose is the only carbon source.

Hope this helps (or is remotely right).

 

[Mithril]

(-) means that the gene(s) involved for that particular pathway are dysfunctional.

Arg- means that the genes involved in arginine synthesis are dysfunctional, so the cell can't make its own arginine.

Lac- means that the genes involved in lactose metabolism are dysfunctional.