Sepsis and Hemocyte Loss in Honey Bees (Apis mellifera) Infected with Serratia marcescens Strain Sicaria

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.

Morphological and functional characterization of honey bee, Apis mellifera, hemocyte cell communities

“…For example, particles which we refer to as permeabilized cells resembled granulocytes and plasmatocytes undergoing cell lysis, cell death, or degranulation and were consistently observed in both adults and larvae (Figure 1f, g). Such cellular deterioration could be the product of our methodological handling of samples, though permeabilized cells have similarly been found in flow cytometric studies (Marringa et al 2014;Burritt et al 2016). Hemolymph particles resembling large permeable nuclei were also observed in larvae (Figure 1c).…”

“…Negri et al (2014b) made considerable progress toward understanding and elucidating the types of hemocytes present in honey bees using traditional approaches based on bright-field microscopy. However, the characterization of honey bee hemocytes is a nascent field and technological advancements, including the use of immunohistochemistry (Gardiner and Strand 1999;Bryant and Michel 2016), RNA sequencing (Irving et al 2005;Pinto et al 2009), and flow cytometry (Marringa et al 2014;Burritt et al 2016;Hystad et al 2017), will ultimately be needed to more conclusively distinguish, separate, and interrogate honey bee hemocyte types. The past hemocytespecific monoclonal antibody works of Strand and Johnson (1996) and Pech and Strand (2000) provide clear evidence of the potential utility of such efforts.…”

Genetic Engineering of Bee Gut Microbiome Bacteria with a Toolkit for Modular Assembly of Broad-Host-Range Plasmids

“…Previous studies have examined how S. alvi colonizes the honey bee gut, 31,49 but colonization by G. apicola , B. apis , and S. marcescens has not been investigated. 26,34 …”

The role of the gut microbiome in health and disease of adult honey bee workers

“…Commonly sampled groups include species of Enterobacteriaceae, including Hafnia alvi , and species of Enterobacter , Klebsiella , and Serratia . Serratia marcescens strains can be pathogenic, causing sepsis and death [23 • ]. Strains isolated from hives can cause mortality when administered orally to workers in the laboratory [24 •• ].…”

“…Strains isolated from hives can cause mortality when administered orally to workers in the laboratory [24 •• ]. Potentially, these Enterobacteriaceae pathogens are under-recognized as causes of bee mortality, since infected bees usually leave the hive to die; they are more likely to accumulate in wintering hives [23 • ]. …”