Phenotypic and genotypic characterization of Paenibacillus larvae isolates

The Application of MALDI-TOF MS for a Variability Study of Paenibacillus larvae

Simple Summary

An understanding of Paenibacillus larvae, the etiological agent of American foulbrood disease, species diversity is crucial for disease epidemiology investigations. Our data indicate that the protein fingerprinting-based MALDI-TOF method provides a much more thorough insight into P. larvae diversity compared to the DNA fingerprinting methods used at present.

Abstract

In recent decades, the significant deterioration of the health status of honey bees has been observed throughout the world. One of the most severe factors affecting the health of bee colonies worldwide is American foulbrood disease. This devastating disease, with no known cure, is caused by the Gram-positive spore-forming bacteria of Paenibacillus larvae species. At present, DNA-based methods are being used for P. larvae identification and typing. In our study, we compare two of the most advanced DNA-based technologies (rep-PCR and 16S rRNA analyses) with MALDI-TOF MS fingerprinting to evaluate P. larvae variability in Central Europe. While 16S rRNA analysis presents a very limited variation among the strains, MALDI-TOF MS is observed to be more efficient at differentiating P. larvae. Remarkably, no clear correlation is observed between whole-genome rep-PCR fingerprinting and MALDI-TOF MS-based typing. Our data indicate that MALDI-TOF protein profiling provides accurate and cost-effective methods for the rapid identification of P. larvae strains and provides novel perspectives on strain diversity compared to conventional DNA-based genotyping approaches. The current study provides a good foundation for future studies.

Molecular diversity of Paenibacillus larvae strains isolated from Lithuanian apiaries

Paenibacillus larvae bacterium is known to be the causative agent of American foulbrood (AFB), a widespread, highly contagious and fatal disease in honey bees (Apis mellifera). There are four genotypes of Paenibacillus larvae that are named after their enterobacterial repetitive consensus (ERIC), and a fifth ERIC genotype has recently been found. In this study, a total of 108 independent P. larvae isolates from different geographical regions in Lithuania collected between 2011 and 2021 were investigated by molecular methods. The aims of this study were to detect which enterobacterial repetitive intergenic consensus (ERIC) genotype is the most common in Lithuania apiaries, identify and differentiate subtypes of the defined genotype by using multiple-locus variable number of tandem-repeat analysis (MLVA), and review how bacterial molecular diversity has changed over time in different parts of Lithuania. The obtained molecular analysis results showed that 100% of P. larvae bacterial isolates from Lithuania belong to the ERIC I genotype and can be differentiated to nine different subtypes by using the MLVA and capillary electrophoresis methods.

American Foulbrood in the Czech Republic: ERIC II Genotype of Paenibacillus Larvae Is Prevalent

American foulbrood (AFB) is a dangerous disease of honeybees (Apis mellifera) caused by the spore-forming bacterium Paenibacillus larvae. According to the ERIC (enterobacterial repetitive intergenic consensus) classification, five genotypes are distinguished, i.e., I, II, III, IV, and V, which differ in their virulence and prevalence in colonies. In the Czech Republic, AFB prevalence is monitored by the State Veterinary Administration; however, the occurrence of specific P. larvae genotypes within the country remains unknown. In this study, our aim was to genotype field P. larvae strains collected in the Czech Republic according to the ERIC classification. In total, 102 field isolates from colonies with AFB clinical symptoms were collected from various locations in the Czech Republic, and the PCR genotypization was performed using ERIC primers. We confirmed the presence of both ERIC I and II genotypes, while ERIC III, IV, and V were not detected. The majority of samples (n = 82, 80.4%) were identified as ERIC II, while the ERIC I genotype was confirmed only in 20 samples (19.6%). In contrast to other European countries, the ERIC II genotype is predominant in Czech honeybee colonies. The ERIC I genotype was mostly detected in border regions close to Poland, Slovakia, and Austria.

Phenotypic, molecular and pathological characterization of motile aeromonads isolated from diseased fishes cultured in Uruguay

Information about motile aeromonads from aquaculture systems of the Neotropical region is scarce. The aim of this study was to characterize motile Aeromonas isolated from ornamental and consumable fishes cultured in Uruguay. Biochemical and molecular methods were used for species identification. Antimicrobial susceptibility and the presence of virulence genes were evaluated. Genetic diversity was analysed by rep-PCR, and virulence of the most representative isolates was determined by calculating the fifty lethal dose in experimentally challenged fish (Australoheros facetus). Aeromonas hydrophila and A. veronii were the most prevalent identified species (38.2% and 32.4%, respectively), whereas A. allosacharophila, A. bestiarium, A. caviae and A. punctata were less prevalent. This study constitutes the first report of these last four species in Uruguay. All isolates were resistant to at least three antimicrobials, and 82.3% of them showed multidrug resistance. Virulence genotypes were correlated with the Aeromonas species and haemolytic activity. The genotype act+/alt+/ast+/ela+/lip+ was the most prevalent (26.5%). A correlation between virulence genotypes and Aeromonas species was found. A. punctata showed a clonal structure according to rep-PCR analysis, whereas other species showed high genetic diversity. The number of virulence genes of the isolates was related with virulence according to the experimental challenge assays.

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).

Lactobacillus kunkeei strains decreased the infection by honey bee pathogens Paenibacillus larvae and Nosema ceranae

Due to their social behaviour, honey bees can be infected by a wide range of pathogens including the microsporidia Nosema ceranae and the bacteria Paenibacillus larvae. The use of probiotics as food additives for the control or prevention of infectious diseases is a widely used approach to improve human and animal health. In this work, we generated a mixture of four Lactobacillus kunkeei strains isolated from the gut microbial community of bees, and evaluated its potential beneficial effect on larvae and adult bees. Its administration in controlled laboratory models was safe for larvae and bees; it did not affect the expression of immune-related genes and it was able to decrease the mortality associated to P. larvae infection in larvae and the counts of N. ceranae spores from adult honey bees. These promising results suggest that this beneficial microorganism's mixture may be an attractive strategy to improve bee health. Field studies are being carried out to evaluate its effect in naturally infected colonies.

Biocidal properties of maltose reduced silver nanoparticles against American foulbrood diseases pathogens

Bee disease caused by spore-forming Paenibacillus larvae and Paenibacillus alvei is a serious problem for honey production. Thus, there is an ongoing effort to find an effective agent that shows broad biocidal activity with minimal environmental hazard. In this study, the biocidal effect of maltose reduced silver nanoparticles (AgNPs) is evaluated against American foulbrood and European foulbrood pathogens. The results demonstrate that the maltose reduced AgNPs are excellent short and long-term biocides against P. larvae isolates. The long-term effect suggests that the Ag⁺ ions are released from the AgNPs with increasing time in a controlled manner.

Multilocus sequence typing, biochemical and antibiotic resistance characterizations reveal diversity of North American strains of the honey bee pathogen Paenibacillus larvae

Paenibacillus larvae is a Gram positive bacterium and the causative agent of the most widespread fatal brood disease of honey bees, American foulbrood (AFB). A total of thirty-three independent Paenibacillus larvae isolates from various geographical origins in North America and five reference strains were investigated for genetic diversity using multilocus sequence typing (MLST). This technique is regarded to be a powerful tool for epidemiological studies of pathogenic bacteria and is widely used in genotyping assays. For MLST, seven housekeeping gene loci, ilvD (dihydroxy-acid dyhydrogenase), tri (triosephosphate isomerase), purH (phospharibosyl-aminoimidazolecarboxamide), recF (DNA replication and repair protein), pyrE (orotate phosphoribosyltransferase), sucC (succinyl coenzyme A synthetase β subunit) and glpF (glycerol uptake facilitator protein) were studied and applied for primer designs. Previously, ERIC type DNA fingerprinting was applied to these same isolates and the data showed that almost all represented the ERIC I type, whereas using BOX-PCR gave an indication of more diversity. All isolates were screened for resistance to four antibiotics used by U.S. beekeepers, showing extensive resistance to tetracycline and the first records of resistance to tylosin and lincomycin. Our data highlight the intraspecies relationships of P. larvae and the potential application of MLST methods in enhancing our understanding of epidemiological relationships among bacterial isolates of different origins.

Multiple Locus Variable number of tandem repeat Analysis: A molecular genotyping tool for Paenibacillus larvae

American Foulbrood, caused by Paenibacillus larvae, is the most severe bacterial disease of honey bees (Apis mellifera). To perform genotyping of P. larvae in an epidemiological context, there is a need of a fast and cheap method with a high resolution. Here, we propose Multiple Locus Variable number of tandem repeat Analysis (MLVA). MLVA has been used for typing a collection of 209 P. larvae strains from which 23 different MLVA types could be identified. Moreover, the developed methodology not only permits the identification of the four Enterobacterial Repetitive Intergenic Consensus (ERIC) genotypes, but allows also a discriminatory subdivision of the most dominant ERIC type I and ERIC type II genotypes. A biogeographical study has been conducted showing a significant correlation between MLVA genotype and the geographical region where it was isolated.

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

The gram-positive bacterium Paenibacillus larvae is the etiological agent of American Foulbrood of honey bees, a notifiable disease in many countries. Hence, P. larvae can be considered as an entomopathogen of considerable relevance in veterinary medicine. P. larvae is a highly specialized pathogen with only one established host, the honey bee larva. No other natural environment supporting germination and proliferation of P. larvae is known. Over the last decade, tremendous progress in the understanding of P. larvae and its interactions with honey bee larvae at a molecular level has been made. In this review, we will present the recent highlights and developments in P. larvae research and discuss the impact of some of the findings in a broader context to demonstrate what we can learn from studying "exotic" pathogens.

Seasonal Variation of Honeybee Pathogens and its Association with Pollen Diversity in Uruguay

Honeybees are susceptible to a wide range of pathogens, which have been related to the occurrence of colony loss episodes reported mainly in north hemisphere countries. Their ability to resist those infections is compromised if they are malnourished or exposed to pesticides. The aim of the present study was to carry out an epidemiological study in Uruguay, South America, in order to evaluate the dynamics and interaction of honeybee pathogens and evaluate their association with the presence of external stress factors such as restricted pollen diversity and presence of agrochemicals. We monitored 40 colonies in two apiaries over 24 months, regularly quantifying colony strength, parasite and pathogen status, and pollen diversity. Chlorinated pesticides, phosphorus, pyrethroid, fipronil, or sulfas were not found in stored pollen in any colony or season. Varroa destructor was widespread in March (end of summer-beginning of autumn), decreasing after acaricide treatments. Viruses ABPV, DWV, and SBV presented a similar trend, while IAPV and KBV were not detected. Nosema ceranae was detected along the year while Nosema apis was detected only in one sample. Fifteen percent of the colonies died, being associated to high V. destructor mite load in March and high N. ceranae spore loads in September. Although similar results have been reported in north hemisphere countries, this is the first study of these characteristics in Uruguay, highlighting the regional importance. On the other side, colonies with pollen of diverse botanical origins showed reduced viral infection levels, suggesting that an adequate nutrition is important for the development of healthy colonies.

Comparative analysis of Paenibacillus larvae genotypes isolated in Connecticut

Ninety-six strains of Paenibacillus larvae, causative agent of American foulbrood in honey bee (Apis mellifera) larvae, collected from Connecticut, USA (CT), honey bees, and 12 P. larvae strains not from CT, were genotyped via ERIC-PCR and XbaI-RFLP analysis. All CT-isolates, five strains isolated in South America, three strains from North America (not CT), and one strain isolated in Australia grouped into the ERIC I genotype. Three P. larvae formerly subsp. pulvifaciens strains grouped into ERIC III and IV genotypes. XbaI-RFLP genotyping showed three genotypes within the CT-isolates, and two were identified as XbaI-RFLP Type I and III. The third XbaI-RFLP genotype (Type Ib) represented one of four new XbaI-RFLP genotypes identified. Comparison of genotype results for the P. larvae strains tested was used to develop a correlation between ERIC-PCR genotyping and XbaI-RFLP genotyping. Sixteen CT-isolates were tetracycline-resistant and demonstrated PCR amplification using oligonucleotide primers for tetL. All 16 isolates grouped within XbaI-RFLP Type Ib, suggesting limited introduction of a tetracycline-resistant strain into CT.

Biogeography of Paenibacillus larvae, the causative agent of American foulbrood, using a new multilocus sequence typing scheme

American foulbrood is the most destructive brood disease of honeybees (Apis mellifera) globally. The absence of a repeatable, universal typing scheme for the causative bacterium Paenibacillus larvae has restricted our understanding of disease epidemiology. We have created the first multilocus sequence typing scheme (MLST) for P. larvae, which largely confirms the previous enterobacterial repetitive intergenic consensus (ERIC)-polymerase chain reaction (PCR)-based typing scheme's divisions while providing added resolution and improved repeatability. We have used the new scheme to determine the distribution and biogeography of 294 samples of P. larvae from across six continents. We found that of the two most epidemiologically important ERIC types, ERIC I was more diverse than ERIC II. Analysis of the fixation index (FST ) by distance suggested a significant relationship between genetic and geographic distance, suggesting that population structure exists in populations of P. larvae. Interestingly, this effect was only observed within the native range of the host and was absent in areas where international trade has moved honeybees and their disease. Correspondence analysis demonstrated similar sequence type (ST) distributions between native and non-native countries and that ERIC I and II STs mainly have differing distributions. The new typing scheme facilitates epidemiological study of this costly disease of a key pollinator.

Metalloprotease production by Paenibacillus larvae during the infection of honeybee larvae

American foulbrood is a bacterial disease of worldwide distribution that affects larvae of the honeybee Apis mellifera. The causative agent is the Gram-positive, spore-forming bacterium Paenibacillus larvae. Several authors have proposed that P. larvae secretes metalloproteases that are involved in the larval degradation that occurs after infection. The aim of the present work was to evaluate the production of a metalloprotease by P. larvae during larval infection. First, the complete gene encoding a metalloprotease was identified in the P. larvae genome and its distribution was evaluated by PCR in a collection of P. larvae isolates from different geographical regions. Then, the complete gene was amplified, cloned and overexpressed, and the recombinant metalloprotease was purified and used to generate anti-metalloprotease antibodies. Metalloprotease production was evaluated by immunofluorescence and fluorescence in situ hybridization. The gene encoding a P. larvae metalloprotease was widely distributed in isolates from different geographical origins in Uruguay and Argentina. Metalloprotease was detected inside P. larvae vegetative cells, on the surface of P. larvae spores and secreted to the external growth medium. Its production was also confirmed in vivo, during the infection of honeybee larvae. This protein was able to hydrolyse milk proteins as described for P. larvae, suggesting that could be involved in larval degradation. This work contributes to the knowledge of the pathogenicity mechanisms of a bacterium of great economic significance and is one step in the characterization of potential P. larvae virulence factors.

Paenibacillus larvae enolase as a virulence factor in honeybee larvae infection

Paenibacillus larvae is a gram-positive spore-forming bacteria, causative agent of American Foulbrood (AFB), a severe disease affecting larvae of the honeybee Apis mellifera. In an attempt to detect potential virulence factors secreted by P. larvae, we identified an enolase among different secreted proteins. Although this protein is a cytosolic enzyme involved in glycolytic pathways, it has been related to virulence. The aim of the present work was to evaluate its role during the infection of honeybee larvae. Toxicity assays showed that enolase was highly toxic and immunogenic to honeybee larvae. Its production was detected inside P. larvae vegetative cells, on the surface of P. larvae spores and secreted to the external growth medium. P. larvae enolase production was also confirmed in vivo, during the infection of honeybee larvae. This protein was able to hydrolyze milk proteins as described for P. larvae, suggesting that could be involved in larval degradation, maybe through the plasmin(ogen) system. These results suggest that P. larvae enolase may have a role in virulence and could contribute to a general insight about insect-pathogen interaction mechanisms.

American Foulbrood in honeybees and its causative agent, Paenibacillus larvae

After more than a century of American Foulbrood (AFB) research, this fatal brood infection is still among the most deleterious bee diseases. Its etiological agent is the Gram-positive, spore-forming bacterium Paenibacillus larvae. Huge progress has been made, especially in the last 20 years, in the understanding of the disease and of the underlying host-pathogen interactions. This review will place these recent developments in the study of American Foulbrood and of P. larvae into the general context of AFB research.

Use of suppression subtractive hybridization to identify genetic differences between differentially virulent genotypes of Paenibacillus larvae, the etiological agent of American Foulbrood of honeybees

Paenibacillus larvae is the causative agent of American Foulbrood of honeybees, a fatal brood disease not only killing infected larvae but also lethal to infected colonies. Recently four different genotypes of P. larvae (enterobacterial repetitive intergenic consensus I-IV) have been described and it was shown that these genotypes also differ in phenotype, especially in virulence. To unravel the genetic differences between these four genotypes, suppression subtractive hybridization was used. From 106 analysed clones, 92 represented genotype-specific sequences, whereas 14 sequences turned out to be specific only for the particular strain used as tester in the subtraction. Nearly half of the sequences (46%) could only be annotated based on poorly characterized sequences. The remaining sequences corresponded to categories related to metabolism, especially secondary metabolite biosynthesis, transport and catabolism, to information storage and processing, and to cellular processes. In particular, we could show that the P. larvae genome contains genes and/or giant gene clusters coding for antibiotics, and we identified the first P. larvae toxin, a member of the family of adenosine diphosphate-ribosyltransferases.

Characterization of secreted proteases of Paenibacillus larvae, potential virulence factors involved in honeybee larval infection

Paenibacillus larvae is the causative agent of American Foulbrood (AFB), the most severe bacterial disease that affects honeybee larvae. AFB causes a significant decrease in the honeybee population affecting the beekeeping industry and agricultural production. After infection of larvae, P. larvae secretes proteases that could be involved in the pathogenicity. In the present article, we present the secretion of different proteases by P. larvae. Inhibition assays confirmed the presence of metalloproteases. Two different proteases patterns (PP1 and PP2) were identified in a collection of P. larvae isolates from different geographic origin. Forty nine percent of P. larvae isolates showed pattern PP1 while 51% exhibited pattern PP2. Most isolates belonging to genotype ERIC I - BOX A presented PP2, most isolates belonging to ERIC I - BOX C presented PP1 although relations were not significant. Isolates belonging to genotypes ERIC II and ERIC III presented PP2. No correlation was observed between the secreted proteases patterns and geographic distribution, since both patterns are widely distributed in Uruguay. According to exposure bioassays, isolates showing PP2 are more virulent than those showing PP1, suggesting that difference in pathogenicity could be related to the secretion of proteases.

Negative correlation between individual-insect-level virulence and colony-level virulence of Paenibacillus larvae, the etiological agent of American foulbrood of honeybees

Paenibacillus larvae is the etiological agent of American foulbrood (AFB) in honeybees. Recently, different genotypes of P. larvae (ERIC I to ERIC IV) were defined, and it was shown that these genotypes differ inter alia in their virulence on the larval level. On the colony level, bees mitigate AFB through the hygienic behavior of nurse bees. Therefore, we investigated how the hygienic behavior shapes P. larvae virulence on the colony level. Our results indicate that P. larvae virulence on the larval level and that on the colony level are negatively correlated.