Michel Brehélin

Bacillus thuringiensis has been widely used for 40 years as a safe biopesticide for controlling agricultural pests and mosquitoes because it produces insecticidal crystal proteins. However, spores have also been shown to contribute to overall entomopathogenicity. Here, the opportunistic properties of acrystalliferous B. thuringiensis Cry(-) and Bacillus cereus strains were investigated in an insect species, Galleria mellonella, and in a mammal, BALB/c mice. In both animal models, the pathogenicity of the two bacterial species was similar. Mutant strains were constructed in which the plcR gene, encoding a pleiotropic regulator of extracellular factors, was disrupted. In larvae, co-ingestion of 10(6) spores of the parental strain with a sublethal concentration of Cry1C toxin caused 70% mortality whereas only 7% mortality was recorded if spores of the DeltaplcR mutant strain were used. In mice, nasal instillation of 10(8) spores of the parental strain caused 100% mortality whereas instillation with the same number of DeltaplcR strain spores caused much lower or no mortality. Similar effects were obtained if vegetative cells were used instead of spores. The cause of death is unknown and is unlikely to be due to actual growth of the bacteria in mice. The lesions caused by B. thuringiensis supernatant in infected mice suggested that haemolytic toxins were involved. The cytolytic properties of strains of B. thuringiensis and B. cereus, using sheep, horse and human erythrocytes and G. mellonella haemocytes, were therefore investigated. The level of cytolytic activity is highly reduced in DeltaplcR strains. Together, the results indicate that the pathogenicity of B. thuringiensis strain 407 and B. cereus strain ATCC 14579 is controlled by PlcR.

Eggs of an immune suppressive strain (= virulent) of the parasitoid Leptopilina boulardi are encapsulated neither in resistant nor in susceptible strains of Drosophila melanogaster but are encapsulated in Drosophila yakuba. Eggs of a nonimmune suppressive strain (= avirulent) of the same parasitoid are encapsulated in a resistant strain of D. melanogaster and in D. yakuba but are not encapsulated in a susceptible strain of D. melanogaster. Egg chorion in the 2 parasitoid strains showed the same morphology and the same modifications after egg laying whatever the host strain. When a capsule is built, a small dotted dense layer was first spread on the chorion, followed by accumulation layers of cells (plasmatocytes and lamellocytes) and lastly necrosis of the inner haemocytes. The encapsulated eggs darken only at the time of necrosis of haemocytes. In susceptible hosts, neither the tiny dense layer nor haemocyte accumulation occurred. We concluded that (1) this tiny dense layer was present before the deposition of the first haemocytes, (2) inhibition of deposition of this dense layer was the initial event of the induced immunosuppression, (3) haemocytes other than lamellocytes were engaged in capsule formation, (4) the immunosuppressive factors did not target only the lamellocytes but also the plasmatocytes, (5) darkening of the encapsulated eggs was due to cell necrosis rather than to extracellular melanin deposition.

Infectious hypodermal and haematopoietic necrosis (IHHN) is one of the most important viral diseases of cultured penaeid shrimps and is potentially a limiting factor in the development of farming projects for some species of these shrimps. Although the IHHN agent was recognized early as being viral in origin, attempts to characterize it were inconclusive because of difficulties in obtaining sufficient amounts of purified virions to permit its characterization. Recent improvements of purification procedures have allowed the physicochemical characterization of this virus. Purified IHHNV is a non-enveloped icosahedral particle averaging 22 nm in diameter, exhibiting a mean buoyant density of 1.40 g/ml in CsCl. The genome is a single molecule of ssDNA with an estimated size of 4.1 kb by molecule length measurement in transmission electron microscopy. As determined by SDS-PAGE, the particle contains four polypeptides with Mrs of 74K, 47K, 39K and 37.5K, respectively. From its characteristics, this virus could be a member of the Parvoviridae family.