Thermotolerant clostridia as an airborne pathogen indicator during land application of biosolids

Since biosolids could harbor a variety of potentially infectious microorganisms, there are concerns of human exposure to these microbial pathogens in population centers surrounding sites where biosolids are land applied. A field study was conducted at a large commercial biosolid application site to determine if thermotolerant clostridia could be employed as a microbial indicator in determining the presence of such fecal contamination in aerosols. Even though the applied biosolids harbored as much as 10(7) MPN fecal and total coliforms per gram wet weight, these traditional indicators were undetectable at locations having the greatest potential for aerosolization. Thermotolerant clostridia and bacteriophages, however, were detectable in 73% and 53% of the samples, respectively. Hydrogen sulfide producing bacteria as a group, were detected in 93% of the samples. Even at sites directly in the vicinity of the biosolid application, thermotolerant clostridia were detected in 26% of the samples, as compared with the fecal and total coliforms which were detected in only 1 out of 15 (6.6%) samples. Since municipal sewage sludges usually undergo anaerobic digestion before land disposal, it would favor the selection of thermotolerant clostridia within these biosolids. Moreover, clostridia also could be ribotyped using the 16S-23S interspacer region length polymorphism to identify the origins or sources of aerosol contamination. Thermotolerant clostridia could thus serve as a reliable indicator to determine the presence of microbial pathogens and biosolid derived microbial populations in aerosols being generated from biosolid application programs.