Biology of Paenibacillus larvae, a deadly pathogen of honey bee larvae

Swarming motility and biofilm formation of Paenibacillus larvae, the etiological agent of American Foulbrood of honey bees (Apis mellifera)

American Foulbrood is a worldwide distributed, fatal disease of the brood of the Western honey bee (Apis mellifera). The causative agent of this fatal brood disease is the Gram-positive, spore-forming bacterium Paenibacillus larvae, which can be classified into four different genotypes (ERIC I-IV), with ERIC I and II being the ones isolated from contemporary AFB outbreaks. P. larvae is a peritrichously flagellated bacterium and, hence, we hypothesized that P. larvae is capable of coordinated and cooperative multicellular behaviors like swarming motility and biofilm formation. In order to analyze these behaviors of P. larvae, we firstly established appropriate functional assays. Using these assays we demonstrated that P. larvae ERIC II, but not P. larvae ERIC I, was capable of swarming. Swarming motility was hampered in a P. larvae ERIC II-mutant lacking production of paenilarvin, an iturin-like lipopeptide exclusively expressed by this genotype. Both genotypes were able to form free floating biofilm aggregates loosely attached to the walls of the culture wells. Visualizing the biofilms by Congo red and thioflavin S staining suggested structural differences between the biofilms formed. Biofilm formation was shown to be independent from paenilarvin production because the paenilarvin deficient mutant was comparably able to form a biofilm.

Proteinase pattern of honeybee prepupae from healthy and American Foulbrood infected bees investigated by zymography

American foulbrood disease (AFB) is the main devastating disease that affects honeybees' brood, caused by Paenibacillus larvae. The trend of the research on AFB has addressed the mechanisms by which P. larvae bacteria kill honeybee larvae. Since prepupae could react to the infection of AFB by increasing protease synthesis, the aim of this work was to compare protease activity in worker prepupae belonging to healthy colonies and to colonies affected by AFB. This investigation was performed by zymography. In gel, proteolytic activity was observed in prepupae extracts belonging only to the healthy colonies. In the prepupae extracts, 2D zimography followed by protein identification by MS allowed to detect Trypsin-1 and Chymotrypsin-1, which were not observed in diseased specimens. Further investigations are needed to clarify the involvement of these proteinases in the immune response of honeybee larvae and the mechanisms by which P. larvae inhibits protease production in its host.

Bacterial pathogens of bees

Pollination is an indispensable ecosystem service provided by many insects, especially by wild and managed bee species. Hence, reports on large scale honey bee colony losses and on population declines of many wild bees were alarming and resulted in increased awareness of the importance of bee health and increased interest in bee pathogens. To serve this interest, this review will give a comprehensive overview on bacterial bee pathogens by covering not only the famous pathogens (Paenibacillus larvae, Melissococcus plutonius), but also the orphan pathogens which have largely been neglected by the scientific community so far (spiroplasmas) and the pathogens which were only recently discovered as being pathogenic to bees (Serratia marcescens, Lysinibacillus sphaericus).

Drivers of colony losses

Over the past decade, in some regions of the world, honey bee (Apis mellifera L.) colonies have experienced rates of colony loss that are difficult for beekeepers to sustain. The reasons for losses are complex and interacting, with major drivers including Varroaand related viruses, pesticides, nutrition and beekeeper practices. In these endeavors it has also become apparent that defining a dead colony, and singling out the effects of specific drivers of loss, is not so straightforward. Using the class of neonicotinoid pesticides as an example we explain why quantifying risk factor impact at the colony level is at times elusive and in some cases unpractical. In this review, we discuss the caveats of defining and quantifying dead colonies. We also summarize the current leading drivers of colony losses, their interactions and the most recent research on their effects on colony mortality.

Bacteriophages as an alternative to conventional antibiotic use for the prevention or treatment of Paenibacillus larvae in honeybee hives

American Foulbrood (AFB) is an infectious disease caused by the bacteria, Paenibacillus larvae. P. larvae phages were isolated and tested to determine each phages' host range amongst 59 field isolate strains of P. larvae. Three phages were selected to create a phage cocktail for the treatment of AFB infections according to the combined phages' ability to lyse all tested strains of bacteria. Studies were performed to demonstrate the safety and efficacy of the phage cocktail treatment as a replacement for traditional antibiotics for the prevention of AFB and the treatment of active infections. Safety verification studies confirmed that the phage cocktail did not adversely affect the rate of bee death even when administered as an overdose. In a comparative study of healthy hives, traditional prophylactic antibiotic treatment experienced a 38±0.7% decrease in overall hive health, which was statistically lower than hive health observed in control hives. Hives treated with phage cocktail decreased 19±0.8%, which was not statistically different than control hives, which decreased by 10±1.0%. In a study of beehives at-risk for a natural infection, 100±0.5% of phage-treated hives were protected from AFB infection, while 80±0.5% of untreated controls became infected. AFB infected hives began with an average Hitchcock score of 2.25 out of 4 and 100±0.5% of the hives recovered completely within two weeks of treatment with phage cocktail. While the n numbers for the latter two studies are small, the results for both the phage protection rate and the phage cure rate were statistically significant (α=0.05). These studies demonstrate the powerful potential of using a phage cocktail against AFB and establish phage therapy as a feasible treatment.