The use of membrane filters for sampling bacterial population in air


Thesis (M.A.)--Boston UniversityThe membrane filter air sampler is a simple and economical apparatus for sampling bacterial population in air. The literature indicated efficiency approaching 100 per cent in recovery or organisms. The organisms on the membrane filter grew rapidly after cultivation on a suitable medium. Colony counts were made usually after 24 hours incubation at 37 C in a moist environment. After incubation the membrane can be preserved, stained, and filed for record purposes. Because of the difficulty with spreaders, the PM2X Medium (Millipore Filter Corporation) was modified by the addition of one per cent agar. Experience indicated that this reduced trouble from spreading organisms which, if present, tend to decrease the accuracy of the bacterial counts. The most efficient sampling time with an air flow rate of ten liters per minute was six minutes. A series of ten runs was taken routinely in order to collect large samples of air without the risk of lose of bacteria due to dehydration. The total volume filtered in this way was 600 liters or 21.19 cubic feet. For collecting simultaneous air samples, which is very easy to do with the membrane filter sampler, two filters were connected to a single vacuum pump through a Y tube and test runs made. The same experiment was repeated using two separate vacuum pumps. One pump with a Y tube proved to be satisfactory for collecting simultaneous air samples. It was found that the greatest number of bacteria was recovered at lower levels and air samples were taken approximately two and one-half feet above the floor. Four different media- selective for staphylococci were tested to see if one might be found which would be selective for all staphylococci present in the air and which could be used to identify the potentially pathogenic Staphylococcus aureus directly, the criteria chosen being the production of pigment and coagulase. Staphylococcus Medium No. 110 recovered staphylococci only. Mannitol Salt Agar and Egg Yolk Agar seemed to grow more staphylococci, but were slightly leas selective - only about 98 per cent of the colonies on these media proved to be staphylococci. Pigment production by Staph. aureua on Mannitol Salt Agar and coagulase-positive activity did not correlate upon checking with the tube coagulase test. Preliminary experimente indicated that Tellurite Glycine Agar is too inhibitory. Also there was no correlation between color of the colony on the membrane filter and coagulase activity. It seems best to omit the use of selective media for Staph. aureus For greatest accuracy it would be better to culture the membrane filter for total bacteria, picking the suspected Staph. aureus colonies, and checking them for coagulase activity by tube test with plasma

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