Studies on Microorganisms in Simulated Room Environments

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Abstract
A theory of the action of the Wells centrifuge is developed, leading to certain equations which express the upper limit of its efficiency, % recovery, in terms of normal settling velocities of particles and of speed and dimensional constants of the machine. An exptl. technic is developed with which to test the validity of the theory. Living bacteria were sprayed into a closed exptl. chamber from which samples could be taken at frequent intervals, and upon the floor of which, prepared petri dishes could be exposed and removed as desired. Settling upon the floor appears to be at a rate which decreases geometrically. All our observations indicate that the particles have a downward velocity with respect to the air which in turn is sufficiently stirred by convection currents to maintain a uniform distribution within the chamber. The rate of settling, therefore, becomes the height of the chamber[long dash]91 inches[long dash]times the rate of removal expressed as % of remainder. Comparisons of recoveries by centrifuge and by settling for various periods of time gave somewhat confused results which were difficult to interpret. The technic was simplified by employing inert substances, uranine and phosphate, for which sufficiently delicate colorimetric tests could be devised. With these substances it is found possible to make a much more complete and satisfactory analysis of the data. These results are then employed in the further analysis of the data on bacteria. The sprayed material has a wide range of variation in size as measured by settling rate. The rate of recovery by settling can best be accounted for on the basis of a geometric distribution of the settling rates in the normal curve of probability. The normal distribution curve formulated includes 90% of the material within a range of settling rates of about 30-fold. Applying the theory of the centrifuge to this hypothetical distribution of particle sizes, class by class, and summing the total recovery gives indicated actual performances considerably in excess of actual results, both at the start and after settling in the chamber for several hrs. Owing to limitations of size of particle capable of carrying a bacterial cell, the actual distribution of sizes of such particles is truncated at about the middle of the size distribution curve. With this simplification and with approximate allowance for the known death rate and for the different quantitative measures employed in the 2 cases, settling rates observed for bacteria are in reasonable accord with the size distribution found to fit the inert material. Employing this same distribution and technic to test the recovery by centrifuge, we find a discrepancy similar to that noted with uranine. We have purposely dealt with finely atomized material, produced by a process simulating coughing and sneezing, and capable of evaporating to nuclei, small enough to remain suspended in the quiescent air of a room for an hour or more and large enough to contain bacteria. Within this range of sizes the maximum theoretical recovery by centrifuge cannot exceed 69% of the quantity capable of settling ultimately. Actual recoveries were from 24 to 41% of this same quantity. These represent the centrifugal recovery of freshly sprayed material. After a period of settling the average size of the material becomes smaller and the recovery proportionally less efficient. Material once settled and again raised into the air as dust may of course be of large size and will be recovered in larger proportion. Our data do not include this type of suspended matter.

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