Methylobacterium

Interestingly, the bacteria most consistently identified as active, not just present, but metabolically thriving, were organisms you wouldn't expect to find in the sky.

Groups like Sphingomonas, Pseudomonas, and Methylobacterium, bacteria that are classified as leaf bacteria, organisms that normally live on the surfaces of plants.

They apparently have the right physiological toolkit to survive in clouds because plant leaf surfaces are themselves subjected to UV exposure, temperature swings, desiccation, and humidity shifts, conditions that aren't so different from what you'd face floating inside a cloud droplet.

The sky, in a sense, is just another extreme surface, and some bacteria were already pre-adapted for it.