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 se veral 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 r eleased into the water suspension. The same pattern was found both in an agricultural and a forest soil, with contrasting pH and organic mat ter content, and irrespective of whether the bacteria were labeled bef ore or after the centrifugation steps. The different growth rates of t he bacteria could not be explained by different partitioning of label between different macromolecules, different cell size, different viabi lity of the bacteria, or different dilution of the added radioactive s ubstrate 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 bacte rial communities in the different homogenization-centrifugation fracti ons was indicated by a gradually altered phospholipid fatty acid patte rn of the extracted bacteria, and an increased hydrophobicity of the b acteria released only after several homogenization-centrifugation trea tments.