Abstract
American foulbrood (AFB) is a highly virulent disease afflicting honey bees (Apis mellifera). The causative organism, Paenibacillus larvae, attacks honey bee brood and renders entire hives dysfunctional during active disease states, but more commonly resides in hives asymptomatically as inactive spores that elude even vigilant beekeepers. The mechanism of this pathogenic transition is not fully understood, and no cure exists for AFB. Here, we evaluated how hive supplementation with probiotic lactobacilli (delivered through a nutrient patty; BioPatty) affected colony resistance towards a naturally occurring AFB outbreak. Results demonstrated a significantly lower pathogen load and proteolytic activity of honey bee larvae from BioPatty-treated hives. Interestingly, a distinctive shift in the microbiota composition of adult nurse bees occurred irrespective of treatment group during the monitoring period, but only vehicle-supplemented nurse bees exhibited higher P. larvae loads. In vitro experiments utilizing laboratory-reared honey bee larvae showed Lactobacillus plantarum Lp39, Lactobacillus rhamnosus GR-1, and Lactobacillus kunkeei BR-1 (contained in the BioPatty) could reduce pathogen load, upregulate expression of key immune genes, and improve survival during P. larvae infection. These findings suggest the usage of a lactobacilli-containing hive supplement, which is practical and affordable for beekeepers, may be effective for reducing enzootic pathogen-related hive losses.
Highlights
Managed honey bees (Apis mellifera) perform critical pollination services to many agricultural crops and contribute an estimated $225 billion USD annually to the global economy [1]
Lactobacillus plantarum Lp39 (Lp39; American Type Culture Collect (ATCC), number 14917), Lactobacillus rhamnosus GR-1 (LGR-1; ATCC number 55826), and Lactobacillus kunkeei BR-1 (LkBR-1; previously isolated from a healthy honey bee hive) were routinely cultured anaerobically at 37 °C using de Man, Rogosa, and Sharpe broth or agar supplemented with 10 g/L D-fructose, unless otherwise stated
A significant increase was found in proteolytic activity of homogenized larvae from vehicle pollen pattysupplemented hives compared to no-treatment control (NTC) hives (Fig. 1c)— despite no differences in pathogen load (Fig. 1b)
Summary
Managed honey bees (Apis mellifera) perform critical pollination services to many agricultural crops and contribute an estimated $225 billion USD annually to the global economy [1]. One well-known pathogen afflicting honey bee brood is the spore-forming bacterium Paenibacillus larvae, which causes American foulbrood (AFB). This highly adapted pathogen infects A. mellifera during early development and can kill brood through secretion of secondary metabolites (that have antimicrobial properties to counter microbial competitors) and chitin-degrading enzymes (enabling degradation of the peritrophic matrix) that allow breaching of the midgut epithelium, invasion of the haemocoel, and decomposition of the larva to a ropy mass [6]. AFB is a notifiable disease in many countries and requires the destruction of clinically infected hives [9]. It has been suggested that P. larvae may exist as a pathobiont in the native microbiota of adult worker bees, from where it is passively and constitutively transmitted throughout the hive to fresh brood cells [12]
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