Several studies have revealed the existence of diverse microorganisms in the atmosphere especially around urban areas. A major problem encountered in the literature is the lack of well-defined methods for the detection, isolation and characterization of airborne bacteria, making the interpretation and inter comparison of results difficult. In this study, we employed both culture dependent and non culture dependent methods to characterize the aeromicrobiological population from two urbanized areas, one located in Bamako, Mali and the other in Washington, D.C. Using culture dependent methods, the number of bacteria in each sample was determined, and each isolate was classified into family groups by its phenotypic characteristics. In Mali, the number of bacteria found ranged from 19.8 to 84 colony forming unit per cubic meter (CFU/m 3) of air sampled with an average of 56.71 CFU/m3. Representatives of seven different families were found in ten samples with the most frequently isolated being Bacillaceae (63%), Micrococcaceae (11%), Enterobacteriaceae (8%) and Neisseriaceae (6%). Phylogenetic analysis of the isolates revealed distinct clusters within the families, Bacillaceae, Staphylococcaceae, Myroidaceae, Microbacteriaceae and Moraxellaceae. Isolates belonging to the genera Exiguobacterium, Myroides, Acinetobacter and Staphylococcus were also identified. Two species Acinetobacter baumannii and Staphylococcus sciuri that have not been previously reported to occur in the ambient air of Mali were also identified. In D.C., the number of bacteria found ranged from 0.44 to 19.81 CFU/m3 with an average of 8.07. The most frequently isolated bacteria and their percent of occurrence, as revealed by phenotypic characterization, were Bacillaceae (70%), Enterobacteriaceae (19%) and Micrococcaceae (10%). Phylogenetic analysis delineated them into eight families, Bacillaceae, Staphylococcaceae, Microbacteriaceae, Planococcaceae, Pseudomonadaceae, Vibrionaceae, Caulobacteriaceae and Enterobacteriaceae. The number of culturable bacteria was correlated with meteorological conditions. In both urban areas, we found that the temperature was not a major factor affecting the number of culturable bacteria but an inverse correlation was observed for percent relative humidity. We also observed that the time of collection was an influential factor, with bacterial counts rising higher than average during the evening periods. They were also influenced by the origin of the air masses, which suggests a transport pattern of microorganisms. In an attempt to determine the complete bacterial community (culturable and non-culturable), the membrane filters used to collect air samples in Washington, D.C. were subjected to several methods for direct DNA extraction. Modification to the Rusch et al., (PLoS Biol. 5:e77, 2007) extraction protocol was found to be more effective than the methods tested. The dominant bacteria were Bacillus spp., taxa already known to be predominant in the background air, while such bacterial species as Pasteurella pneumotropica, Ralstonia taiwanensis, Stenotrophomonas maltophilia and Cryobacterium spp., and several uncultured species that could not be detected by culture dependent method were also identified. The results of this research highlights the importance of applying both culture based and non-culture based methods in aeromicrobiology.
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