Do Slow Growers Exist?

Abstract

Many microorganisms in nature appear to grow slowly. Slow growth may be a potential strategy for surviving stressful environmental conditions. æHowever, our lab found that slow growth may not be a typical strategy of environmental bacteria. Long-term cultivation of environmental isolates demonstrated that the majority of isolates initially grew slowly, but most grew quickly upon subcultivation. The remaining isolates required over 180 days to grow. Here we examine this minority to determine if they represent true slow growers or, alternatively, that the apparently long generation times were due to the lack of trivial nutritional factors. æIsolates were marine and soil strains that repeatedly required over 6 or 1-2 months to form visible growth, respectively. Strains were grown on the original isolation medium and on media optimized for their cultivable kin. Soil strains grew faster than the original isolates, forming visible growth in less than one month. Marine strains grew even faster, forming visible biomass in 1-5 days. For one strain we determined optimal growth required the presence of magnesium and sodium chloride. Our data support that slow growth is not an inherent characteristic of the majority of environmental isolates. Initial slow growth may be due to several reasons, including suboptimal media design, but not the innate inability to proliferate quickly. æOur conclusion is general since the microorganisms tested are spore- and non-spore forming soil and marine strains. These findings may relate to the phenomenon of uncultivability since initial slow growth may be difficult to detect, resulting in misclassification as ñunculturableî.