In direct selection methods used to identify possible carcinogens (example: Ames testing), what does it mean when a chemical can cause a bacterium to develop resistance to some environmental condition?

the chemical is a mutagen.

Explanation:

A mutagenic chemical is any type of agent that when exposed to cells has the ability to generate mutation. In other words, damage to genetic material (DNA) that is not repaired in the process of cell replication is passed on to offspring. This is what happens when a bacteria develops resistance to some environmental condition, ie the bacteria has been exposed to a mutagenic chemical.