COMPARISON OF METHODS FOR DETECTION AND ENUMERATION OF AIRBORNE MICROORGANISMS COLLECTED BY LIQUID IMPINGEMENT

Bacterial agents and cell components can be spread as bioaerosols, pro ducing infections and asthmatic problems. This study compares four met hods for the detection and enumeration of aerosolized bacteria collect ed in an AGI-30 impinger. Changes in the total and viable concentratio ns of Pseudomonas fluorescens in the collection fluid with respect to time of impingement were determined, Two direct microscopic methods (a cridine orange and BacLight) and aerodynamic aerosol-size spectrometry (Aerosizer) were employed to measure the total bacterial cell concent rations in the impinger collection fluid and the air, respectively. Th ese data were compared with plate counts on selective (MacConkey agar) and nonselective (Trypticase soy agar) media, and the percentages of culturable cells in the collection fluid and the bacterial injury resp onse to the impingement process were determined, The bacterial collect ion rate was found to be relatively unchanged during 60 min of impinge ment. The aerosol measurements indicated an increased amount of cell f ragments upstream of the impinger due to continuous bacterial nebuliza tion, Some of the bacterial clusters, present in the air upstream of t he impinger, deagglomerated during impingement, thus increasing the to tal bacterial count by both direct microscopic methods, The BacLight s taining technique was also used to determine the changes in viable bac terial concentration during the impingement process, The percentage of viable bacteria, determined as a ratio of BacLight live to total coun ts was only 20% after 60 min of sampling. High counts on Trypticase so y agar indicated that most of the injured cells could recover. On the other hand, the counts from the MacConkey agar were very low, indicati ng that most of the cells were structurally damaged in the impinger. T he comparison of data on the percentage of injured bacteria obtained b y the traditional plate count with the data on percentage of nonviable bacteria obtained by the BacLight method showed good agreement.