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Effect of the insect pathogenic bacterium Photorhabdus on insect phagocytes
1Department of Biology and Biochemistry, University of Bath, Bath, BA2 7AY, UK.
Abstract:
Photorhabdus are insect pathogenic bacteria that replicate within the insect haemocoel following release from their entomopathogenic nematode symbionts. To investigate how they escape the cellular immune response we examined the effects of two strains of Photorhabdus, W14 and K122, on Manduca sexta phagocytes (haemocytes), in vitro and in vivo. Following injection of Esherichia coli into Manduca larvae, these non-pathogenic bacteria are rapidly cleared from the haemolymph and the number of free haemocytes transiently increases. In contrast, following injection of either strain of pathogenic Photorhabdus, the bacteria grow rapidly while the number of haemocytes decreases dramatically. In vitro incubation of haemocytes with either Photorhabdus supernatant reduced haemocyte viability, and the W14 supernatant caused distinct changes in the actin cytoskeleton morphology of different haemocyte cell types. In phagocytosis assays both Photorhabdus strains can inhibit their own phagocytosis whether the bacterial cells are alive or dead. Further, the supernatant of W14 also contains a factor capable of inhibiting the phagocytosis of labelled E. coli. Together these results suggest that Photorhabdus evades the cellular immune response by killing haemocytes and suppressing phagocytosis by mechanisms that differ between strains.
Insights
Photorhabdus bacteria evade insect immune defenses by killing host phagocytes and inhibiting their own engulfment. These mechanisms, involving bacterial toxins and secreted factors, differ between Photorhabdus strains.
Area of Science:
- Insect pathology
- Bacterial pathogenesis
- Innate immunity
Background:
- Photorhabdus bacteria are symbionts of entomopathogenic nematodes and infect insect hosts.
- Understanding how Photorhabdus evades the insect immune system is crucial for developing novel biocontrol agents.
Purpose of the Study:
- To investigate the mechanisms by which Photorhabdus bacteria evade the cellular immune response in Manduca sexta.
- To compare the immune evasion strategies of two Photorhabdus strains (W14 and K122).
Main Methods:
- In vitro and in vivo experiments using Manduca sexta larvae and their haemocytes.
- Assessing haemocyte viability and morphology after exposure to Photorhabdus.
- Phagocytosis assays to evaluate bacterial engulfment by haemocytes.
- Analysis of Photorhabdus supernatant for immune-modulating factors.
Main Results:
- Photorhabdus infection led to a dramatic decrease in haemocyte numbers, unlike non-pathogenic E. coli.
- Photorhabdus supernatant reduced haemocyte viability and altered haemocyte actin cytoskeleton.
- Both Photorhabdus strains inhibited their own phagocytosis, regardless of viability.
- Photorhabdus W14 supernatant suppressed the phagocytosis of E. coli by haemocytes.
Conclusions:
- Photorhabdus employs multiple strategies to evade insect cellular immunity, including haemocyte cytotoxicity and phagocytosis suppression.
- Strain-specific mechanisms contribute to Photorhabdus immune evasion.
- These findings provide insights into insect-bacterial interactions and potential targets for pest control.