, Volume 72, Issue 1, pp 7-15

Degradation of bacteria by Mytilus edulis

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Abstract

The uptake of several species of bacteria by the common mussel Mytilus edulis (L.) and the subsequent fate of some polymers of the bacteria have been investigated in a study carried out during 1981. Bacteria (Escherichia coli, Micrococcus luteus, M. roseus, Bacillus cereus, Staphylococcus aureus and a marine pseudomonad, 1-1-1) were radiolabelled by growth in medium containing 3H-thymidine and the uptake of bacteria by Mytilus edulis was monitored. Labelled and unlabelled bacteria, at initial concentrations of 0.5 to 1x107 bacteria ml-1, were cleared at similar, exponential rates with no significant difference in the rates for different bacteria: 90% of bacteria were cleared in a mean time of 1.93±0.12 h (SEM, n=63). Those bacteria with cell walls which were sensitive to M. edulis lysozyme were rapidly degraded by the mussel and 3H-labelled DNA was released in a form not precipitable by 10% trichloroacetic acid. Lysozyme-resistant bacteria (Micrococcus roseus and S. aureus) were cleared from suspension by Mytilus edulis but most were rejected intact. By measuring the rate of release of 3H-thymidine-labelled material from the mussel the rate of degradation of lysozyme-sensitive bacteria by M. edulis was found. For different bacteria the degradation rate varied from approx 2x108 to 27x108 bacteria h-1 with an overall mean of 10x108 bacteria h-1. A thymidine- and diaminopimelicacid-requiring auxotroph of E. coli was radiolabelled with 3H-thymidine, 3H-diaminopimelic acid or 14C-glucose and fed to M. edulis. Bacteria were cleared and degraded by the mussel; 3H-diaminopimelic acid-labelled or 14C-glucose-labelled polymers were retained, whereas 3H-thymidine-labelled polymers were released into the surrounding water. Extracts of the digestive gland of M. edulis degraded lysozyme-sensitive bacteria to release 3H-thymidine-labelled material, but did not release 3H-thymidine-labelled material from lysozyme-resistant bacteria. It is concluded that M. edulis can select lysozymesensitive bacteria for subsequent processing and discriminate between bacterial polymers to reject DNA. Also, bacteria could provide a substantial fraction of the carbon requirement of the mussel.

Communicated by J. Mauchline, Oban