Phospholipid fatty acid composition of size fractionated indigenous soil bacteria

The soil bacterial community is dominated by small cells (< 0.4 μm dia), and most of these “dwarf” cells are unculturable. We have tried to elucidate the character of these cells by determining the amounts and types of phospholipid fatty acids (PLFAs), in comparison with the whole microbial community. Indigenous soil bacteria were extracted from soil and fractionated according to size by sequential filtration, and the PLFA pattern was determined. The PLFA pattern of the different size fractions were different when compared using principal component analysis. This separation was partly due to an increase in branched fatty acids like a15:0 and br18:0 and a decrease in relative amounts of the PLFA 18:1ω7 in the < 0.4 μm size fraction, compared to the whole community, indicating a higher proportion of Gram-positive bacteria among the smaller cells. The amount of PLFA per cell indicated the possibility of intact cell membranes in these small cells.