Complement Resistance Is Essential for Colonization of the Digestive Tract of Hirudo medicinalis by Aeromonas Strains

Abstract

From the crop of the medicinal leech, Hirudo medicinalis, only Aeromonas veronii bv. sobria can be cultured consistently. Serum-sensitive A. veronii mutants were unable to colonize H. medicinalis, indicating the importance of the mammalian complement system for this unusual simplicity. Complementation of one selected mutant restored its ability to colonize. Serum-sensitive mutants are the first mutant class with a colonization defect for this symbiosis.Aeromonas veronii bv. sobria and Aeromonas hydrophila are pathogens of fish (20,26) and humans, causing wound infections, septicemia, and probably diarrhea (2-4, 10, 12). In addition to being a pathogen, A. veronii bv. sobria is the digestive tract symbiont of the medicinal leech, Hirudo medicinalis (6). The symbionts are housed in the crop of the digestive tract, apparently to the exclusion of other culturable bacteria (5, 6). The symbionts are thought to aid in the digestion of the ingested blood, provide essential nutrients to the host, or prevent other bacteria from colonizing (5). Interestingly, they can also infect the wounds of patients during medical application if the leeches are attached to tissue with poor blood circulation and the patients do not receive preemptive antibiotics (1,3,9,18).A recent study evaluated the ability of clinical Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus isolates to colonize the medicinal leech and discovered that the strains tested either could not proliferate inside the medicinal leech or were killed (9). The concentration of the E. coli strain decreased inside the medicinal leech for the first 48 h after feeding, suggesting that the bacteria were killed (9). Heating the blood or the addition of EDTA or EGTA and Mg 2ϩ to the blood prevented the killing of the E. coli strain in vitro (9). These treatments indicate that a heat-sensitive and divalentcation (possibly Ca 2ϩ )-requiring property was responsible for the demise of the E. coli strain. This is suggestive of the membrane attack complex of the complement system. Heat treatment of the blood prior to feeding allowed the E. coli strain to proliferate inside the medicinal leech, suggesting that the complement system was also responsible for inhibiting its growth inside the leech. These results indicate that powerful antimicrobial properties of mammals contribute to the unusual specificity of this bacterium-invertebrate symbiosis.If the complement system contributes to the specificity of this symbiotic interaction, then A. veronii bv. sobria mutants that have an increased sensitivity to the complement system would be expected to have a reduced ability to colonize the medicinal leech. Several surface structures are responsible for the resistance of bacteria to complement-mediated killing, including lipopolysaccharide (LPS), outer membrane proteins, polysaccharide capsule, and S layer (15,22,24). A previous study generated serum-sensitive mutants derived from the A. veronii bv. sobria strain AH-1 (also called TF7), which is of the O11 serotype and produc...

Identification ofAeromonas veroniiGenes Required for Colonization of the Medicinal Leech,Hirudo verbana

“…We have previously shown the importance of intact LPS for serum resistance (6) and the importance of a type III secretion system to prevent phagocytosis (55). The characterization of the lpp mutant suggests that there are additional antimicrobial properties that prevent the colonization of sensitive bacteria.…”

“…As a consequence, serum resistance is essential in order for bacteria to successfully colonize the leech gut (6,25). The goal of this investigation was to discover new classes of mutants; hence, the 83 presumptive colonization mutants were first tested for serum sensitivity.…”

“…Bacterial surface structures can provide resistance to complement-mediated killing. For example, mutants containing a defect in lipopolysaccharide (LPS) have been shown to be serum sensitive; this was the first class of colonization mutants discovered in this symbiosis (6). The growth of Pseudomonas aeruginosa and Staphylococcus aureus strains inside the leech was inhibited in a complement-independent manner, suggesting the presence of other barriers that contribute to the specificity of this symbiosis (25).…”

Spatial and Temporal Population Dynamics of a Naturally Occurring Two-Species Microbial Community inside the Digestive Tract of the Medicinal Leech

“…The medicinal leech is a hematophagous parasite native to freshwater environments throughout Europe (38,41) and is increasingly being used postoperatively in modern medicine. Recent molecular studies indicate that H. verbana is a distinct but closely related species of Hirudo medicinalis (41) and that the animals used in our previous studies, as well as animals sold for medical applications, are now considered to be H. verbana (4,15,20). The digestive tract of the leech consists of two major compartments, the crop and the intestinum.…”

Aeromonas infection from river and playa lake waters in West Texas and southeastern New Mexico

“…11 Aeromonads have been detected in bottled natural mineral waters 12 and in various foodstuffs, including raw fish and shellfish, poultry, meat products, milk, and fresh vegetables. 13,14 Aeromonas is also found in association with a growing list of invertebrate and vertebrate animals, including molluscs (snails and mussels) 15,16 , annelids (leeches) [17][18][19] ,crustaceans (shrimp) 20 , insects (mosquitoes) 21 , fishes 22 , amphibians (frogs) 23 , reptiles (snakes and crocodiles) 24,25 , birds 26 , and mammals 27 . In some cases, the aeromonads form part of the host animal's normal microbial flora; in others, they act as invading parasites that can cause potentially lethal infections.…”