Low-Molecular-Weight Metabolites Secreted by Paenibacillus larvae as Potential Virulence Factors of American Foulbrood

Abstract

The spore-forming bacterium Paenibacillus larvae causes a severe and highly infective bee disease, American foulbrood (AFB). Despite the large economic losses induced by AFB, the virulence factors produced by P. larvae are as yet unknown. To identify such virulence factors, we experimentally infected young, susceptible larvae of the honeybee, Apis mellifera carnica, with different P. larvae isolates. Honeybee larvae were reared in vitro in 24-well plates in the laboratory after isolation from the brood comb. We identified genotype-specific differences in the etiopathology of AFB between the tested isolates of P. larvae, which were revealed by differences in the median lethal times. Furthermore, we confirmed that extracts of P. larvae cultures contain low-molecular-weight compounds, which are toxic to honeybee larvae. Our data indicate that P. larvae secretes metabolites into the medium with a potent honeybee toxic activity pointing to a novel pathogenic factor(s) of P. larvae. Genome mining of P. larvae subsp. larvae BRL-230010 led to the identification of several biosynthesis gene clusters putatively involved in natural product biosynthesis, highlighting the potential of P. larvae to produce such compounds.

Molecular pathogenesis of American Foulbrood: how Paenibacillus larvae kills honey bee larvae

“…Based on larval feeding assays it has been suggested that low molecular weight metabolites present in P. larvae secretomes may possess larval toxicity although no further identification of the proposed cytotoxin was provided [43]. However, it was speculated that NRP or NRP/PK hybrid molecules are responsible for the observed effect [43].…”

“…In another study, the lipopeptides paenilarvins exhibited some larvicidal activity in feeding assays [32 ]. In both studies control groups did not receive any control peptides but were fed normal larval diet [32 ,43] and all secretome fractions were toxic to larvae [43]. Therefore, it cannot be ruled out that the observed larvicidal effects were just due to the added peptides per se rather than to a specific peptide because larvae are very sensitive to changes in the larval diet during artificial rearing [44].…”

“…However, it was speculated that NRP or NRP/PK hybrid molecules are responsible for the observed effect [43]. In another study, the lipopeptides paenilarvins exhibited some larvicidal activity in feeding assays [32 ].…”

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

“…A third toxin, (C3 larvin) was identified via a bioinformatics strategy . Moreover, P. larvae produces further unidentified low molecular mass metabolites toxic for honeybee larvae .…”

Biological effects of paenilamicin, a secondary metabolite antibiotic produced by the honey bee pathogenic bacterium Paenibacillus larvae

“…; Schild et al. ) including an NRPS gene cluster leading to the production of a novel nonribosomal peptide antibiotic, sevadicin (Garcia‐Gonzalez et al. ).…”