Complement resistance is essential for colonization of the digestive tract of Hirudo medicinalis by Aeromonas strains

This study (Braschler et al., Applied and Environmental Microbiology 2003) found that only Aeromonas veronii bv. sobria can be consistently cultured from the crop of the medicinal leech Hirudo medicinalis, and that serum-sensitive A. veronii mutants failed to colonize the leech while complementation of a selected mutant restored colonization, identifying resistance to the (mammalian, blood-meal-derived) complement system as essential for this symbiosis and describing serum-sensitive mutants as the first colonization-defective class for this partnership. This is directly relevant to hirudotherapy because Aeromonas is the leech-associated bacterium most implicated in post-leeching wound infections, so understanding why it uniquely colonizes the leech gut informs infection risk and antibiotic-prophylaxis rationale. Honest caveat: this is preclinical microbiology using bacterial mutants and the leech host to dissect a symbiotic mechanism; it does not study human infection rates or any clinical leech-therapy outcome.