Control of bacterial infections in the hard tick Dermacentor variabilis (Acari: Ixodidae): evidence for the existence of antimicrobial proteins in tick hemolymph.

The ability of hard ticks to prevent infection by bacteria was investigated. During a 72-h period, virtually all Dermacentor variabilis females survived inoculation with Bacillus subtilis, Escherichia coli, and Staphyloccocus aureus but few survived infection with Pseudomonas aeruginosa. The hemocyte population increased to peak abundance at 48 h to approximately 6 times that of the uninfected controls. In contrast, the soluble hemolymph protein content decreased inversely as the hemocytes increased. D. variabilis hemolymph was found to be constitutively antimicrobial (i.e., hemolymph from bacteria-naive individuals inhibited bacterial growth). Infection with various bacterial species enhanced this innate capability. When hemolymph fractions separated by high-pressure liquid chromatography were tested for their ability to inhibit microbial growth, activity against the gram-positive bacterium, B. subtilis, was found in 2 polar fractions. Antimicrobial activity was lost when the fractions were incubated with protease. The least polar fraction contained 1 major protein, M(r) 14.5 kDa, that comigrated with human lysozyme. This protein, tentatively identified as tick lysozyme, was abundant in bacteria-naive ticks but increased greatly (43%) following challenge with B. subtilis. The identity of the other, more polar protein is unknown. Studies to characterize these antimicrobial proteins are planned.