Evidence for more than one division of bacteria within airborne particles.

When the protocol that we had used to demonstrate a single division of bacterial cells in airborne particles was changed to one that increased the glycerol content of the atomizer fluid from 1 to 5% (vol/vol), thus producing larger particles, more than two (and nearly three) divisions of bacteria occurred within 6 h of aerosol time.

Evidence that bacteria can form new cells in airborne particles.

Serratia marcescens incubated for 8 h at 31 degrees C in a chemically defined medium contained in shake flasks was aerosolized into rotating-drum aerosol chambers at 30 degrees C and saturated humidity. Cells furnished tryptone (Difco) and glycerol just before aerosolization increased (in viable numbers and countable cells) almost twofold within 1 to 2 h after becoming airborne, whereas cells not furnished additional tryptone decreased in viable numbers at a faster rate than the number of particles removed by gravitational settling. Limited tests with a Coulter Counter showed that cell volume changes occurred in growing cells that did not occur in the nongrowing population.

Evidence for incorporation of thymidine into deoxyribonucleic acid in airborne bacterial cells.

As part of an effort to discover whether bacteria might propagate within airborne particles, we studied the incorporation of thymidine into the trichloroacetic acid-insoluble fraction of airborne cells of Serratia marcescens to seek evidence of the possible formation of new DNA. Two aerosols, one of S. marcescens and another of [3H]thymidine ([3H]dT) suspended in growth medium were caused to aggregate in air just prior to directing the aerosols into rotating-drum aerosol storage chambers.

Evidence for propagation of aerobic bacteria in particles suspended in gaseous atmospheres.

One factor involved in the possibility that airborne microbes might contaminate the Jovian atmosphere, is whether microbes have the capacity to propagate in air. Prior to these studies, the evidence was that the airborne state was lethal to microbes. By mixing an aerosol of aerobic bacteria with another containing 14C glucose, we were able to detect the presence of 14CO2, showing that the airborne cells were metabolically active.