Thymidine incorporation of bacteria sequentially extracted from soil using repeated homogenization-centrifugation

Bacteria were sequentially extracted from soil into a water suspension after shaking soil with water or mixing it in a blender followed by a low-speed centrifugation. Bacteria, which were released only after several cycles of homogenization-centrifugation, had higher growth rates as judged from thymidine and leucine incorporation, whereas bacteria that were more readily released by a gentle shaking procedure had the lowest growth rate. This indicated that bacteria more tightly bound to soil particles were growing faster than those that were more easily released into the water suspension. The same pattern was found both in an agricultural and a forest soil, with contrasting pH and organic matter content, and irrespective of whether the bacteria were labeled before or after the centrifugation steps. The different growth rates of the bacteria could not be explained by different partitioning of label between different macromolecules, different cell size, different viability of the bacteria, or different dilution of the added radioactive substrate in the different homogenization-centrifugation fractions. The total amount of phospholipid fatty acids per bacterial cell was also similar in the different fractions. Different composition of the bacterial communities in the different homogenization-centrifugation fractions was indicated by a gradually altered phospholipid fatty acid pattern of the extracted bacteria, and an increased hydrophobicity of the bacteria released only after several homogenization-centrifugation treatments.