Abstract
Honeybee (Apis mellifera) imagines are resistant to the Gram-positive bacterium Paenibacillus larvae (P. larvae), causative agent of American foulbrood (AFB), whereas honeybee larvae show susceptibility against this pathogen only during the first 48 h of their life. It is known that midgut homogenate of adult honeybees as well as a homogenate of aged larvae exhibit strong anti-P. larvae activity. A bioactivity-guided LC-HRMS analysis of midgut homogenate resulted in the identification of 1-oleoyl-sn-glycero-3-phosphocholine (LPC) pointing to a yet unknown immune defence in adult honeybees against P. larvae. Antimicrobial activity of LPC was also demonstrated against Melissococcus plutonius, causative agent of European Foulbrood. To demonstrate an AFB-preventive effect of LPC in larvae, artificially reared larvae were supplemented with LPC to evaluate its toxicity and to assess whether, after infection with P. larvae spores, LPC supplementation prevents AFB infection. 10 μg LPC per larva applied for 3 d significantly lowered mortality due to AFB in comparison to controls. A potential delivery route of LPC to the larvae in a colony via nurse bees was assessed through a tracking experiment using fluorescent-labelled LPC. This yet undescribed and non-proteinous defense of honeybees against P. larvae may offer new perspectives for a treatment of AFB without the utilization of classic antibiotics.
Highlights
Defence strategies in insects against pathogens rely on individual innate mechanisms, such as the action of antimicrobial peptides, e.g., apidaecins in honeybees[1], or induced immune responses that infer an immunological memory, including transgenerational immune priming effects[2,3,4]
A suppression of pathogenic bacteria by the intestinal probiotic lactic acid bacterial community can be observed in Apis mellifera[5,6]
American foulbrood (AFB) is a devastating bee disease that is caused by the Gram-positive spore-forming bacterium Paenibacillus larvae (P. larvae), for which several genotypes including ERIC I and ERIC II have been identified, which show different pathogenic behaviour[11,12]
Summary
Defence strategies in insects against pathogens rely on individual innate mechanisms, such as the action of antimicrobial peptides, e.g., apidaecins in honeybees[1], or induced immune responses that infer an immunological memory, including transgenerational immune priming effects[2,3,4]. Honeybee larvae are susceptible to P. larvae spores only during the first instar stage and become resistant against the infection after the first approximately 48 h of life This age-dependent decrease in susceptibility, which according to some authors goes along with tissue differentiation in the peritrophic matrix[13], has intrigued many researchers and has formed the starting point of our research. Tests of homogenized larvae (day 3 and older) revealed a positive correlation between larval age and antibacterial activity[25] This antimicrobial activity was retained even after heat treatment, ethanolic precipitation, and the application of proteases, prompting us to focus our attention on a non-proteinogenic source of P. larvae resistance
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