The dynamics of mesophilic and thermophilic bacterial population was studied in the bulk soil and rhizosphere of the grass Aristida adscensionis L. in a caatinga of the semi-arid Brazilian Northeast. Mesophilic heterotrophic bacteria, sporeforming bacteria and actinomycetes were significantly more abundant than thermophiles, and their counts were 10(6) - 10(7) CFU g-1 dry soil, showing weak fluctuations over the one-year study period. Thermophiles were below 10(6) CFU g-1 dry soil and, in general, demonstrated stronger fluctuations over the studied period. Among them, thermophilic actinomycetes showed the greatest decline during the dry season. No significant differences in microbial number were observed between bulk and rhizosphere soil. Mesophilic bacteria were represented mainly by Gram positive, filamentous organisms (actinomycetes) and by Gram positive, endospore-forming, rod-shaped organisms. All isolates of thermophilic bacteria were Gram positive, endospore-forming rods. Most of mesophilic isolates produced amylases and proteases, and 38% of isolates produced all studied extracellular enzymes: amylases, proteases, cellulases and chitinases. Among thermophilic bacteria, all isolates showed proteolytic activity and 77% produced amylases, but no one produced cellulases or chitinases.
mesophilic bacteria; thermophilic bacteria; actinomycetes; extracellular enzymes; semi-arid; caatinga
