If the only problem the guppies have is the spinal deformity, the affliction is not fish TB. I would never encourage the breeding and spread of guppies seriously infected with Mycobacterium.

Fish TB is probably found in minute quantities in far more aquariums than we realize. The bacterium seems to have a benign stage (which can last for years) where they live in and off of the detritus in the bottom of the tank. If conditions decline in that aquarium, they may multiply at the same time as dirty water weakens immune systems.

While that Drs. Foster and Smith site suggests that it is nor very contagious and can be treated effectively with antibiotics, there are a number of aquarium writers and experienced aquarists who would strongly disagree.

Rainbowfish seem especially vulnerable to Fish TB and the cure rate, with one happy exception, is zero. It has been the practice to euthanize everything in the aquarium (better a clean death than a horrible, long one), tear down the tank, throw out, boil or bleach everything and then to wipe down everything in rubbing alcohol.

"Mycobacteria are especially resistant to medications because they are cased in a triple cell wall uniquely rich in waxy lipids, which repel water-soluble medications." Evidentially even bleach doesn't always get them, hence the wipe down with rubbing alcohol, which seems to open these cells.

The one successful limit to the spread of Mycobacteriosis involved using a UV (ultra-violet) unit to zap any free-floating bacteria in the water. This seemed to keep the numbers on any one fish at a level where their immune systems appear able to cope. That aquarium, in a sense, will continue its existence in a stare of siege though. :(

The lesson to me? Don't overcrowd tanks. Don't overfeed. Be as faithful as possible with the partial water changes. Those of course are the same things we try to do to prevent every other aquarium malady too. ;)

See also
http://www.skepticalaquarist.com/docs/health/bactin.shtml
http://www.members.optushome.com.au/chelmon/Myco.htm
The rainbowfish archive keeps previous messages. Unfortunately, it is several months behind.
http://www.peter.unmack.net/archive/rml/
Notice the way they use Google to search.

Another time we can talk about how open cuts on one's hand, can lead to "Fishkeeper's Finger." Aquarists can indeed catch one of these forms of TB, but initially only on their extremities.