Re: How are bacteria dependent on streptomycin?

A Streptomycin-dependent mutant can mean one of two things.

1. Most commonly this is what is meant. That the mutant is maintained by 
growing the organism on streptomycin, and thus streptomycin-dependent. 
Streptomycin is an antibiotic that is the enemy of many bacteria, but if 
the bacteria harbors a resistance gene to streptomycin on a plasmid then 
the bacteria is not harmed by streptomycin. A plasmid is a piece of DNA, 
typically circular, that is not part of the chromosomal DNA and is 
typically only found in a subset of bacteria. A bacteria can have one to 
several different kinds of plasmids, each with a different resistance gene. 
Also more than one resistance gene can occur on the same plasmid. More 
importantly for a streptomyicn-dependent mutant, other genes can also be 
located on that plasmid. That other gene is dependent upon the maintainance 
of the plasmid in the bacteria. If the bacteria is not grown on streptomycin
 that plasmid will eventually be lost, and so will all the other genes on 
that plasmid.

2. Bacteria only like to produce genes when they are needed. Just like we 
don't go shovelling the walks in the summer time to remove the snow, 
bacteria don't want to produce genes that are not needed. It takes a lot 
of work to make a gene. So bacteria have regulatory proteins - proteins 
that sense certain things are 'around' and thus a gene would be needed. 
Just like we have eyes to see whether there is snow on the walks to shovel. 
But sometimes a regulatory protein controls more than one thing. In our 
case this it is possible that a gene that senses the presence of an 
antibiotic such as streptomycin and produces the streptomycin resistance 
gene would also control the expression of another gene. This gene would 
then be dependent upon the presence of streptomycin for expression even 
though it has nothing to do with streptomycin resistance.

David Beck
MadSci Admin