In the Dark Skies episode "The Last Wave," two characters hallucinate insects under their skin as a result of actually having alien bacteria in their bodies.

(Except Hugh was a colony of alien bacteria.)

A few alien bacteria in a mud puddle someplace would change science.

The Doctor has taken a sample of Johan's blood using a tiny cybernetic mosquito, and has found strands of alien bacteria.

"But that fireman was right, we were contami- nated with alien bacteria," Gloria said.

Unfortunately, there was no alien bacterium- or at least none she could find.

"But-" "If they are uncomfortable with alien bacteria and lichen," Muzorawa interrupted, "how do you think they feel about meeting intelligent aliens?"

The trails, we were informed, are washed every night to trap skin flakes bearing alien bacteria.

Carlyle was heavily conditioned to protect himself against alien bacteria.

If alien bacteria hadn't already gotten to them, then it was probably not going to-or so she prayed.