"[The] problem is surprisingly difficult to solve. There are over 3.5 million possible routes to choose from, and a regular computer must try them out one at a time to find the one that visits each city only once. Alternatively, a computer made from millions of bacteria can look at every route simultaneously."
The scientists did mention that programing the bacteria is pretty difficult.
An interesting side note to this story was the mention of a database of the basic biological designs for these types of machines called the Registery of Standard Biological Parts. I looked around this site for a while and it was facinating. It quickly went over my head, but all the parts and concepts that have already been developed was mind boggling. It's kind of like looking through an electronics catalog. You don't know what all the parts are for or how they are made, but you still look and wonder.
Link to the original article in the Journal of Biological Engineering.
[Image captured from http://www.guardian.co.uk/science/blog/2009/jul/24/bacteria-computer]