Origins and fate of fungi and bacteria in the gut of Lumbricus terrestris L. studied by image analysis

Abstract

The effect of the passage through the gut of the earthworm Lumbricus terrestris L. on fungi and bacteria ingested with decomposing leaves of Taraxacum officinale and with soil was quantified using image analysis tools. Both leaf and soil material were labeled with fluorescent latex microbeads to allow a quantification of the food sources in the fore-, mid-, and hindgut of the earthworms. The content of leaf material in the gut varied in a range between 4 and 59% of the total gut content in different earthworms and the different parts of the intestine of individual animals. Filamentous fungi in the gut compartments were found to originate mainly from leaf material (7700±1800 μg (g leaf (dry wt.))−1), however, the major part was disrupted before arriving in the intestine. Remaining hyphae in the foregut with a biomass of up to 900±150 μg (g gut content (dry wt.))−1 were completely digested during passage through the earthworm gut. Spores of fungi were not detected in our studies. Bacterial cell numbers in the gut compartments ranged from 63±5×108 to 327±16×108 (g gut content (dry wt.))−1 and were significantly higher than the numbers found in the soil (50±1×108 cells (g soil (dry wt.))−1). Cell numbers usually increased from fore- to hindgut. This increase was not correlated to contents of organic material and only partially due to a multiplication of bacterial cells. Numbers of dividing cells accounted in total for approximately 12% of all bacteria, increasing significantly from fore- to hindgut, counts were from 10±1×108 to 25±2×108 (g gut content (dry wt.))−1, respectively. Average cell volumes of bacteria calculated from cell size distributions in leaf and soil material differed significantly, being 0.197 and 0.063 μm3, respectively. In the gut compartments, average cell volumes ranged from 0.043 to 0.070 μm3, which may indicate the disruption of large cells originating from the leaves before arriving in the foregut.