The discovery suggests that airborne microbes play a key role not just in hailstorms, but in other weather events.
All precipitation - rain, hail, snow or sleet - begins with ice crystals forming around cloud particles. Dust grains and pollution droplets may both serve as "nucleating particles". But the new find has helped confirm suspicions that in many cases living micro-organisms cause the rain to fall.
Lead researcher Dr Alexander Michaud, from Montana State University in the US, said: "Bacteria have been found within the embryo, the first part of a hailstone to develop. The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone."
Dr Michaud's team analysed hailstones over 5cm in diameter that were collected after a storm in June last year.