A slit sampler for collecting and counting air-borne bacteria

Abstract

1. A slit sampler for air-borne bacteria is described, in which air is sucked through a narrow slit on to the surface of agar in an ordinary Petri dish.2. This method has proved very rapid and simple in use, and has appeared more efficient than other methods against which it has been tested.3. The collection efficiency for an aerosol consisting ofStaph. albus, sprayed from distilled water as single cocci, is about 96%. The method thus collects the finest bacteria-carrying, particles almost as efficiently as coarser ones.4. In contrast to this, the exposure of plain open Petri dishes appears to collect the larger particles in the air of crowded rooms at least 200 times as efficiently as it does single washed bacteria. This highly selective effect renders it advisable to use great caution in evaluating tests made in plain open dishes.5. The methods of determining sampling efficiency, and their errors are discussed in detail.