Mode of action of Pseudomonas fluorescens strain CL145A, a lethal control agent of dreissenid mussels (Bivalvia: Dreissenidae).

Pseudomonas fluorescens strain CL145A (Pf-CL145A) has demonstrated promise as an efficacious and selective agent for the control of macrofouling Dreissena spp. mussels. Herein, we report trials to investigate the mode of action of this biocontrol agent against Dreissena polymorpha, the zebra mussel. Exposure to dead Pf-CL145A cells achieved the same temporal pattern and percentage mussel mortality as did live cells, thereby excluding infection as the possible lethal mode of action. Histological analysis revealed pathologies consistent with the cause of death being intoxicating natural products associated with Pf-CL145A cells. Irrespective of whether the mussels were exposed to live or dead Pf-CL145A cells, examination of tissues from histological sections revealed that: (1) at the end of the 24-h treatment period there was massive hemocyte infiltration into the lumina of both the digestive gland and stomach; and (2) mussel deaths occurred following lysis and necrosis of the digestive gland and sloughing of stomach epithelium. These trials provide strong evidence that the lethal mode of action of Pf-CL145A is intoxication.

Pseudomonas fluorescens strain CL145A - a biopesticide for the control of zebra and quagga mussels (Bivalvia: Dreissenidae).

Zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) are the "poster children" of high-impact aquatic invasive species. In an effort to develop an effective and environmentally acceptable method to control their fouling of raw-water conduits, we have investigated the potential use of bacteria and their natural metabolic products as selective biological control agents. An outcome of this effort was the discovery of Pseudomonas fluorescens strain CL145A - an environmental isolate that kills these dreissenid mussels by intoxication (i.e., not infection). In the present paper, we use molecular methods to reconfirm that CL145A is a strain of the species P. fluorescens, and provide a phylogenetic analysis of the strain in relation to other Pseudomonas spp. We also provide evidence that the natural product lethal to dreissenids is associated with the cell wall of P. fluorescens CL145A, is a heat-labile secondary metabolite, and has degradable toxicity within 24 h when applied to water. CL145A appears to be an unusual strain of P. fluorescens since it was the only one among the ten strains tested to cause high mussel mortality. Pipe trials conducted under once-through conditions indicated: (1) P. fluorescens CL145A cells were efficacious against both zebra and quagga mussels, with high mortalities achieved against both species, and (2) as long as the total quantity of bacterial cells applied during the entire treatment period was the same, similar mussel mortality could be achieved in treatments lasting 1.5-12.0 h, with longer treatment durations achieving lower mortalities. The efficacy data presented herein, in combination with prior demonstration of its low risk of non-target impact, indicate that P. fluorescens CL145A cells have significant promise as an effective and environmentally safe control agent against these invasive mussels.