How do bacteria reach the stratosphere?

Culturable bacteria have recently been found in the stratosphere (at heights up to 41km). How do such bacteria cross the tropopause to reach such heights? Although possible mechanisms are suggested here, such transfer remains difficult to explain. It is however, likely to more easily achieved by sub-micron bacteria. Such small forms have been found in terrestrial environments and have the potential to seed the stratosphere. Large, ten micron clumps of bacteria have also been found in the stratosphere and it is suggested that these are incoming to Earth from space. Finally, the possibility will be discussed that the presence of bacteria in the stratosphere has led to an increase in the rate of evolution.