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Updated: Mar 9, 2026
Histology Basics and Cell Death Detection in Honeybee Tissue
Published on: July 7, 2022
Sepsis and Hemocyte Loss in Honey Bees (Apis mellifera) Infected with Serratia marcescens Strain Sicaria
Nancy L Burritt1, Nicole J Foss1, Eric C Neeno-Eckwall2
1Department of Biology, University of Wisconsin-Stout, Menomonie, WI, United States of America.
Abstract:
Global loss of honey bee colonies is threatening the human food supply. Diverse pathogens reduce honey bee hardiness needed to sustain colonies, especially in winter. We isolated a free-living Gram negative bacillus from hemolymph of worker honey bees (Apis mellifera) found separated from winter clusters. In some hives, greater than 90% of the dying bees detached from the winter cluster were found to contain this bacterium in their hemolymph. Throughout the year, the same organism was rarely found in bees engaged in normal hive activities, but was detected in about half of Varroa destructor mites obtained from colonies that housed the septic bees. Flow cytometry of hemolymph from septic bees showed a significant reduction of plasmatocytes and other types of hemocytes. Interpretation of the16S rRNA sequence of the bacterium indicated that it belongs to the Serratia genus of Gram-negative Gammaproteobacteria, which has not previously been implicated as a pathogen of adult honey bees. Complete genome sequence analysis of the bacterium supported its classification as a novel strain of Serratia marcescens, which was designated as S. marcescens strain sicaria (Ss1). When compared with other strains of S. marcescens, Ss1 demonstrated several phenotypic and genetic differences, including 65 genes not previously found in other Serratia genomes. Some of the unique genes we identified in Ss1 were related to those from bacterial insect pathogens and commensals. Recovery of this organism extends a complex pathosphere of agents which may contribute to failure of honey bee colonies.
Insights
A novel bacterium, Serratia marcescens strain sicaria (Ss1), was found in dying honey bees, potentially contributing to colony losses. This Gram-negative bacillus impacts bee hemocytes and may be spread by Varroa mites.
Area of Science:
- Apiculture
- Microbiology
- Insect Pathology
Background:
- Honey bee colony losses pose a significant threat to global food security.
- Various pathogens compromise honey bee health, particularly during winter months.
- The role of specific bacterial pathogens in adult honey bee mortality is not fully understood.
Purpose of the Study:
- To identify and characterize a novel bacterial pathogen associated with honey bee mortality.
- To investigate the impact of this bacterium on honey bee hemocytes.
- To determine the genetic makeup and potential virulence factors of the isolated bacterium.
Main Methods:
- Isolation and identification of bacteria from honey bee hemolymph.
- 16S rRNA sequencing and whole-genome analysis for bacterial classification.
- Flow cytometry to assess hemocyte populations in infected bees.
- Comparative genomic analysis to identify unique genes.
Main Results:
- A novel Gram-negative bacillus, identified as Serratia marcescens strain sicaria (Ss1), was isolated from the hemolymph of dying honey bees.
- Ss1 was found in over 90% of bees detached from winter clusters and in associated Varroa destructor mites.
- Septic bees exhibited a significant reduction in plasmatocytes and other hemocytes.
- Ss1 possesses unique genes, some related to known insect pathogens and commensals.
Conclusions:
- Serratia marcescens strain sicaria (Ss1) is a newly identified pathogen of adult honey bees.
- This bacterium may contribute to honey bee colony losses by impairing immune function.
- The association with Varroa mites suggests a potential vector for transmission.
