Population Dynamics of Soil Bacteria Cultured Under Different Nutrient Conditions

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Abstract
Bacteria in paddy soil were cultured at 10-, 100-, 10,000-fold dilutions of the conventional nutrient broth (NB/10, NB/100, NB/10,000). After about 3 weeks of incubation, the organisms which grew on the full strength nutrient broth (NB) (NB organisms) became predominant in the NB/10 culture. On the other hand, those which grew on the NB/100 medium but not on the NB medium (DNB organisms) became predominant in the NB/10,000 culture. In the NB/100 culture, the number of NB and DNB organisms was similar. In order to compare predominance of microorganisms growing at different dilutions of nutrient broth, the cell morphology and fatty acid composition was studied in 49 to 68 isolates obtained from each culture. It was observed that the organisms which predominated in the NB/10 culture were markedly different from those in the NB/10,000 culture. Based on growth responses of NB and DNB organisms to the media at various concentrations, the former were considered to be zymogenous organisms and the latter autochthonous organisms. Such a marked difference in the populations was neither recognized between the NB/10 and NB/100 cultures, nor between the NB/100 and NB/10,000 cultures.

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