Sampling Devices for Air-borne Bacteria

Abstract

Researches have resulted in 2 types of efficient bacterial air-samplers, each of which collects the airborne bacteria on nutrient media in standard petri dishes. One of these is designated as the Radial Jet Sampler. A stationary flat metal drum-type enclosure is located above a slowly rotating petri dish. Air passes through a narrow radial slit in the bottom of the metal drum and impinges, at a velocity of approx. 100 ft. per sec, on the culture medium in the petri dish. This sampler collects > 95% of the bacteria in the air passing through and distributes the air-borne organisms uniformly over the nutrient in the petri dish. The only important disadvantage of this sampler is the need for a vacuum pump to produce the air-flow, thus reducing its portability. The 2d type of sampler is designated as the Duplex Electrostatic Air-Sampler. By means of a small blower operated by an electric motor, air is drawn through the center of a fairly flat metal cone located over the petri dish in a plastic enclosure. The petri dish rests on a metal plate, and by means of a high-voltage vacuum-tube rectifier a DC electrostatic field of approx. 7000 volts is impressed between this metal plate and the upper metal cone. Two separate sampling units operate in parallel! The electrode under the petri dish is negatively charged in one unit and positively charged in the other. Positively and negatively charged organisms, dust, etc., respectively, are collected in the 2 units. The air-rate through each unit is 0.5 cu. ft. per minute, hence 1 cu. ft. is sampled by each unit in 2 minutes and the total bacterial content is detd. by incubating the dishes and adding the colonies collected on both. Tests indicate a collection efficiency of > 95%. This sampler is fairly quiet in operation and weighs only about 12 pounds. Data are included to show results obtained.