Changes in chemical cues of Melissococcus plutonius infected honey bee larvae

Abstract

European foulbrood (EFB), caused by Melissococcus plutonius, is a globally distributed bacterial brood disease affecting Apis mellifera larvae. There is some evidence, even if under debate, that spreading of the disease within the colony is prevented by worker bees performing hygienic behaviour, including detection and removal of infected larvae. Olfactory cues (brood pheromones, signature mixtures, diagnostic substances) emitted by infected individuals may play a central role for hygienic bees to initiate the disease-specific behaviour. However, the mechanisms of cue detection and brood removal, causing hygienic behaviour in EFB affected colonies, are poorly understood. Here, coupled gas chromatography-mass spectrometry (GC–MS) was used to detect disease-specific substances, changes in cuticular hydrocarbon (CHC) profiles, and brood ester pheromones (BEPs) of honey bee larvae artificially infected with M. plutonius. Although no diagnostic substances were found in significant quantities, discriminant analysis revealed specific differences in CHC and BEP profiles of infected and healthy larvae. β-Ocimene, a volatile brood pheromone related to starvation and hygienic behaviour, was present in all larvae with highest quantities in healthy young larvae; whereas oleic acid, a non-volatile necromone, was present only in old infected larvae. Furthermore, γ-octalactone (newly discovered in A. mellifera in this study) was detectable in trace amounts only in infected larvae. We propose that the deviation from the olfactory profile of healthy brood is supposed to trigger hygienic behaviour in worker bees. To confirm the relevance of change in the chemical bouquet (CHCs, BEPs, γ-octalactone, etc.), a field colony bioassay is needed, using healthy brood and hygienic bees to determine if bouquet changes elicit hygienic behaviour.

Evaluating approved and alternative treatments against an oxytetracycline-resistant bacterium responsible for European foulbrood disease in honey bees

“…1 a, b). The average weight of healthy larva that survived the rearing experiment was 130 ± 20 mg (n = 517), in line with previous studies 26 , 40 . Experiments were conducted with a range of distinct genetic lineages with larva sourced from different colonies (Table 2 ).…”

In the battle of the disease: a transcriptomic analysis of European foulbrood-diseased larvae of the Western honey bee (Apis mellifera)

“…Although, different mechanisms of resistance and implicated factors have been discussed in former studies, the most critical factor seems to be the pathogen multiplication time until pupation with a potential clearance of the pathogen and a lag of bacterial toxin release [8,10]. Former studies reported presumably EFB-resistant (or rather tolerant) larvae found in naturally as well as experimentally infected colonies, an observation we also regularly made in our in vitro studies [6][7][8][11][12][13]. These larvae survive pathogen infestation, succeed to pupate (after defecation) and finally emerge as adult workers.…”

“…In M. plutonius infections we usually observe diseased larvae with a strong reduction in body weight and cessation of feeding [ 6 , 7 ]. Hence, we were interested in the expression of candidate genes related to feeding behaviour.…”

“…Besides an urgency of developing efficient programs and treatment methods to prevent and counteract new outbreaks, several features of the Gram-positive bacterium Melissococcus plutonius, the causative agent of EFB, make it an interesting and promising model pathogen to illuminate general pathogenic mechanisms and counteracting host immune responses ranging from the basic cellular to a behavioural colony level [ 5 , 6 ]. Its features are the wide range of virulence diversity of different strains [ 7 , 8 ] and regional enzootic states [ 9 ], its resilience to desiccation, high sugar concentrations, and spore-independent viability for months as well as a brood specialization [ 2 ].…”

Hygiene-Eliciting Brood Semiochemicals as a Tool for Assaying Honey Bee (Hymenoptera: Apidae) Colony Resistance to Varroa (Mesostigmata: Varroidae)

“…In honey bees, hygienic behavior is regulated by chemical communication between developing brood and adult nurse bees, where quantitative ( Salvy et al 2001 , Nazzi et al 2004 , Schöning et al 2012 , McAfee et al 2017 , Wagoner et al 2019 , Liendo et al 2021 ) and qualitative ( Kathe et al 2021 , Mondet et al 2021 ) changes of brood odor profiles can signal health status and trigger hygienic uncapping and removal of brood. Of particular interest are cuticular hydrocarbons (CHCs), which form a waxy layer on insect cuticles to prevent desiccation ( Jackson and Baker 1970 , Blomquist et al 1987 ) and facilitate communication, including nestmate recognition ( Howard and Blomquist 2005 , van Zweden and d’Ettorre 2010 ) and task performance ( Greene and Gordon 2003 ).…”