Interaction between innate immune cells and a bacterial type III secretion system in mutualistic and pathogenic associations
Basic science published in PNAS (2007)
Abstract
Animals house a community of bacterial symbionts in their digestive tracts that contribute to their well being. The medicinal leech, Hirudo verbana, has a remarkably simple gut population carrying two extracellular microbes in the crop where the ingested blood is stored. This simplicity renders it attractive for studying colonization factors. Aeromonas veronii, one of the leech symbionts, can be genetically manipulated and is a pathogen of mammals. Screening transposon mutants of A. veronii for colonization defects in the leech, we found one mutant, JG752, with a transposon insertion in an ascU homolog, encoding an essential component of type III secretion systems (T3SS). Competing JG752 against the wild type revealed that JG752 was increasingly attenuated over time (10-fold at 18 h and >10,000-fold at 96 h). This colonization defect was linked to ascU by complementing JG752 with the operon containing ascU. Fluorescence in situ hybridization analysis revealed that at 42 h 38% of JG752 cells were phagocytosed by leech macrophage-like cells compared with <0.1% of the parental strain. Using mammalian macrophages, a lactate dehydrogenase release assay revealed that cytotoxicity was significantly reduced in macrophages exposed to JG752. In a mouse septicemia model, JG752 killed only 30% of mice, whereas the parent strain killed 100%, showing the importance of T3SS for both pathogenesis and mutualism. Phagocytic immune cells are important not only in defending against pathogens but also in maintaining the mutualistic symbiont community inside the leech, demonstrating that animals use similar, conserved mechanisms to control bacterial populations, even when the outcomes differ dramatically.
Abstract sourced from PubMed (NCBI) for the cited record. See the original publication for the authoritative version.
Summary
Transposon insertion in ascU (T3SS) attenuates Aeromonas veronii colonization 10,000-fold in leech crop and ~70% killing in mouse septicemia.
Why This Matters for Hirudotherapy
This study investigated the interaction between the medicinal leech (Hirudo verbana) innate immune cells and the type III secretion system (T3SS) of Aeromonas veronii, a symbiotic gut bacterium. It is relevant to the biological understanding of medicinal leeches by demonstrating how leech macrophage-like cells use conserved mechanisms to control bacterial populations, dictating whether the symbiont acts as a mutualist or pathogen. The research showed that a mutation in the T3SS led to massive phagocytosis by leech immune cells and decreased virulence in a mouse model. However, the scope is limited to leech gut microbiology and immunology, offering no direct insight into hirudotherapy, the leech secretome, or therapeutic applications for human patients.
Citation
Interaction between innate immune cells and a bacterial type III secretion system in mutualistic and pathogenic associations.
Silver AC et al. · Proceedings of the National Academy of Sciences of the United States of America, 2007
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