Tong, Yongyi, and Bruce Lighthart. 1999. Life state of airborne bacteria. In S.G. Pandalai, (editor) Recent Research Developments in Microbiology. Vol. 2, pp. 499-517.
On entering the atmosphere, it is hypothesized that a bacterium may
sequentially lose some of its functions depending upon their sensitivity
to the airborne condition (or stress). Thus the bacterium may be in a
continuum of life states starting with an entire repertoire of capacities
to the loss of some capacity with concurrent inability to form colonies on
culture medium (i.e., viable but not culturable, VBNC) and might be
resuscitated, and with further loss of capacity to a final state where
cellular reproduction under any circumstance is not possible, i.e., it is
non-viable or dead. When airborne the bacterial cell is subjected to a
number of environmental factors: more-or-less rapid dehydration depending
upon the prevailing relative humidity, damaging or lethal solar radiation
during the day and possible resuscitating photoreactivation, high or low
temperature, and toxic (or possibly life promoting) chemical reactions
from air pollutants. How all these factors affect the life state of
natural alfresco atmospheric bacteria is open to speculation. Some or all
may act independently or in concert. They may be synergistic (additive or
multiplicative) or antagonistic. It seems logical that if factors had
negative effects on survival that they would increase the proportion of
airborne bacteria in the VBNC and non-viable states but if they were
positive in effect than either resuscitation and multiplication or no
effect would occur, i.e., the proportion of viable bacteria would increase
or remain constant.
On entering a moist environment (conceivably, a cloud) , many of the dehydrated viable cellís membranes could be damaged unless they were dehydrated from liquid containing trehalose and other similar chemicals, and then they would become VBNC or non-viable. With this scenario it would seem most likely that live bacteria entering the atmosphere would quickly be rendered non-viable and if not on entering, then upon departing during rehydration. The dynamics of the proportion of dead and live bacteria probably is closely integrated with the local meteorological conditions.
Finally, the question remains: what proportion of bacteria with their full capacity entering the natural alfresco atmosphere have these potentials on departing the atmosphere?