Using whole-genome sequencing to assess the diversity of Paenibacillus larvae within an outbreak and a beekeeping operation

The spore-forming bacterium Paenibacillus larvae is the causative agent of American foulbrood (AFB), a devastating disease of honeybees (Apis mellifera). In the present study, we used whole-genome sequencing (WGS) to investigate an extensive outbreak of AFB in northwestern Slovenia in 2019. A total of 59 P . larvae isolates underwent WGS, of which 40 originated from a single beekeeping operation, to assess the diversity of P. larvae within the beekeeping operation, apiary and colony. By applying a case-specific single-linkage threshold of 34 allele differences (AD), whole-genome multilocus sequence typing (wgMLST) identified two outbreak clusters represented by ERIC II-ST11 clones. All isolates from a single beekeeping operation fell within cluster 1 and the median pairwise AD between them was 10 (range=1–22). The median pairwise AD for apiaries of the same beekeeping operation ranged from 8 to 11 (min.=1, max.=22). For colonies of the same apiary and honey samples from these colonies, the median pairwise AD ranged from 8 to 14 (min.=1, max.=20). The maximum within-cluster distance was 33 pairwise AD for cluster 1 and 44 for cluster 2 isolates. The minimum distance between the outbreak-related and non-related isolates was 37 AD, confirming the importance of associated epidemiological data for delineating outbreak clusters. The observed transmission events could be explained by the activities of honeybees and beekeepers. The present study provides insight into the genetic diversity of P. larvae at different levels and thus provides information for future AFB surveillance.

Novel TaqMan PCR Assay for the Quantification of Paenibacillus larvae Spores in Bee-Related Samples

Paenibacillus larvae is the causative agent of American foulbrood (AFB), a devastating disease of honeybees. P. larvae spore counts in bee-related samples correlate with the presence of AFB symptoms and may, therefore, be used to identify at-risk colonies. Here, we constructed a TaqMan-based real-time PCR (qPCR) assay targeting a single-copy chromosomal metalloproteinase gene for reliable quantification of P. larvae. The assay was calibrated using digital PCR (dPCR) to allow absolute quantification of P. larvae spores in honey and hive debris samples. The limits of detection and quantification were 8 and 58 spores/g for honey and 188 and 707 spores/mL for hive debris, respectively. To assess the association between AFB clinical symptoms and spore counts, we quantified spores in honey and hive debris samples originating from honeybee colonies with known severity of clinical symptoms. Spore counts in AFB-positive colonies were significantly higher than those in asymptomatic colonies but did not differ significantly with regard to the severity of clinical symptoms. For honey, the average spore germination rate was 0.52% (range = 0.04–6.05%), indicating poor and inconsistent in vitro germination. The newly developed qPCR assay allows reliable detection and quantification of P. larvae in honey and hive debris samples but can also be extended to other sample types.

American foulbrood and its causative agent, Paenibacillus larvae, in New Zealand’s registered hives and apiaries

<p>Though the honey bee (Apis mellifera) is exposed to an extensive diversity of parasites and pathogens from multiple kingdoms, few are as devastating as American foulbrood. American foulbrood is a highly contagious bacterial disease, of which the causative agent (bacterium Paenibacillus larvae) infects honey bee brood through the ingestion of its spores, ultimately leading to the death of the infected larva and the collapse of the infected hive. Paenibacillus larvae’s genotypes (ERIC I-IV) exhibit differing ‘killing time’ of infected larvae, resulting in different larval and colony level virulence of the disease within hives. American foulbrood is found in New Zealand’s registered hives, and poses a threat to the country’s apiculture industry. The first objective of this thesis was to perform a genetic analysis on New Zealand’s P. larvae field strains using the well-established methodology of rep-PCR with MBO REP1 primers. A total of 172 bacteria isolates were gathered from registered hives from 2011 to 2014 and examined. The MBO REP1 primer identifies the ‘beta’ genetic subgroups of P. larvae. By identifying beta subgroups, the ERIC genotypes that are present in New Zealand can also be concluded. The genetic analysis of P. larvae using rep-PCR is a first for New Zealand, and appears to be a first for Australasia. The second objective of this thesis was to conduct a temporal and geographical statistical analysis on American foulbrood infection rate trends in New Zealand’s national and regional, divided into seven regions, registered hives and apiaries from 1994 to 2013. The genetic analysis of P. larvae detected three ‘beta’ genotypic subgroups: B, b, and Б. From these findings it was concluded that ERIC I and ERIC II are present in New Zealand. Previous to my findings, subgroup B and Б and ERIC II genotype had not been recorded outside of Europe. The statistical analysis reported that American foulbrood infection rates were significantly decreasing nationally. Results also reported that four of the seven regions’ infection rates were significantly decreasing, whilst three regions were significantly increasing. Conclusions on the subgroups and genotypes present in New Zealand gives the first insight to the virulence and occurrence of P. larvae strains. Additionally, the use of rep-PCR for the genetic analysis of P. larvae enables this thesis to contribute to the increasing knowledge on American foulbrood. By examining the temporal and geographic dynamics of American foulbrood, the results allow for the evaluation of current management strategies and the most recent understanding on the national and regional infection rates of the disease.</p>

American foulbrood and its causative agent, Paenibacillus larvae, in New Zealand’s registered hives and apiaries

<p>Though the honey bee (Apis mellifera) is exposed to an extensive diversity of parasites and pathogens from multiple kingdoms, few are as devastating as American foulbrood. American foulbrood is a highly contagious bacterial disease, of which the causative agent (bacterium Paenibacillus larvae) infects honey bee brood through the ingestion of its spores, ultimately leading to the death of the infected larva and the collapse of the infected hive. Paenibacillus larvae’s genotypes (ERIC I-IV) exhibit differing ‘killing time’ of infected larvae, resulting in different larval and colony level virulence of the disease within hives. American foulbrood is found in New Zealand’s registered hives, and poses a threat to the country’s apiculture industry. The first objective of this thesis was to perform a genetic analysis on New Zealand’s P. larvae field strains using the well-established methodology of rep-PCR with MBO REP1 primers. A total of 172 bacteria isolates were gathered from registered hives from 2011 to 2014 and examined. The MBO REP1 primer identifies the ‘beta’ genetic subgroups of P. larvae. By identifying beta subgroups, the ERIC genotypes that are present in New Zealand can also be concluded. The genetic analysis of P. larvae using rep-PCR is a first for New Zealand, and appears to be a first for Australasia. The second objective of this thesis was to conduct a temporal and geographical statistical analysis on American foulbrood infection rate trends in New Zealand’s national and regional, divided into seven regions, registered hives and apiaries from 1994 to 2013. The genetic analysis of P. larvae detected three ‘beta’ genotypic subgroups: B, b, and Б. From these findings it was concluded that ERIC I and ERIC II are present in New Zealand. Previous to my findings, subgroup B and Б and ERIC II genotype had not been recorded outside of Europe. The statistical analysis reported that American foulbrood infection rates were significantly decreasing nationally. Results also reported that four of the seven regions’ infection rates were significantly decreasing, whilst three regions were significantly increasing. Conclusions on the subgroups and genotypes present in New Zealand gives the first insight to the virulence and occurrence of P. larvae strains. Additionally, the use of rep-PCR for the genetic analysis of P. larvae enables this thesis to contribute to the increasing knowledge on American foulbrood. By examining the temporal and geographic dynamics of American foulbrood, the results allow for the evaluation of current management strategies and the most recent understanding on the national and regional infection rates of the disease.</p>

Anti-Virulence Strategy against the Honey Bee Pathogenic Bacterium Paenibacillus larvae via Small Molecule Inhibitors of the Bacterial Toxin Plx2A

American Foulbrood, caused by Paenibacillus larvae, is the most devastating bacterial honey bee brood disease. Finding a treatment against American Foulbrood would be a huge breakthrough in the battle against the disease. Recently, small molecule inhibitors against virulence factors have been suggested as candidates for the development of anti-virulence strategies against bacterial infections. We therefore screened an in-house library of synthetic small molecules and a library of flavonoid natural products, identifying the synthetic compound M3 and two natural, plant-derived small molecules, Acacetin and Baicalein, as putative inhibitors of the recently identified P. larvae toxin Plx2A. All three inhibitors were potent in in vitro enzyme activity assays and two compounds were shown to protect insect cells against Plx2A intoxication. However, when tested in exposure bioassays with honey bee larvae, no effect on mortality could be observed for the synthetic or the plant-derived inhibitors, thus suggesting that the pathogenesis strategies of P. larvae are likely to be too complex to be disarmed in an anti-virulence strategy aimed at a single virulence factor. Our study also underscores the importance of not only testing substances in in vitro or cell culture assays, but also testing the compounds in P. larvae-infected honey bee larvae.