There is always a risk. However, these bacteria evolved in an extremely hostile environment. The probability that they developed mechanisms to infiltrate a multicellular organism and fight off its immune system is practically infinitesimal, although non-zero.
There is likely zero evolutionary drive in such an environment to evolve above basic anaerobic life sustaining adaptations and it could even be the case that any evolution that isn't geared towards maximizing survivability in such an environment are actually detrimental. Chances are that any functional mutations will either optimize current processes (and thus help survivability) or create new processes or chemical pathways. The latter would usually expend more energy and could very well cause the mutated species to die off prematurely because their "improvement" is too much to handle in this low energy environment.
Of course, this doesn't preclude the bacteria being so strange that our immune system wouldn't know what to do about it but after 25 million years of sequestration with no "predators", it is unlikely that they developed such a uniqueness.
There is likely zero evolutionary drive in such an environment to evolve above basic anaerobic life sustaining adaptations and it could even be the case that any evolution that isn't geared towards maximizing survivability in such an environment are actually detrimental. Chances are that any functional mutations will either optimize current processes (and thus help survivability) or create new processes or chemical pathways. The latter would usually expend more energy and could very well cause the mutated species to die off prematurely because their "improvement" is too much to handle in this low energy environment.
Of course, this doesn't preclude the bacteria being so strange that our immune system wouldn't know what to do about it but after 25 million years of sequestration with no "predators", it is unlikely that they developed such a uniqueness.