Fate of GFP-expressing Escherichia coli in the midgut and response to ingestion in a tick, Ornithodoros moubata (Acari : Argasidae)

Ticks are well-known vectors of various pathogens but migration of the pathogens in the tick midgut is not fully understood. In the present study, the fate of microbes in the midgut of Ornithodoros moubata was observed using green fluorescent protein (GFP)-expressing Escherichia coli. Fluctuations in the percentage of hemocytes in the hemolymph (Hc) and expression of an antimicrobial peptide, defensin, in the midgut was also investigated. Most E. coli gradually disappeared in the midgut after ingestion fluctuations in Hc coincided with the changes. Expression of defensin was also confirmed and slightly up-regulated after E. coli ingestion. Moreover, it was demonstrated that E. coli can not pass through the tick midgut epithelium after ingestion by the hemolymph cultures. It is known that various pathogens and host immunoglobulins ingested with a blood meal can enter into the hemocoel, which suggests the presence of unique and complex passage mechanisms for each molecule and organism. The results obtained here help to clarify that digestion enzymes is an important function of the tick midgut to protect against invading molecules and organisms.