Snodgrassella alvi is a species of Gram-negative bacteria within the Neisseriaceae and was previously the only known species of the genus Snodgrassella. It was isolated and scientifically described in 2012 by Waldan K. Kwong and Nancy A. Moran, who named the bacteria after the American entomologist Robert Evans Snodgrass. Snodgrassella alvi lives symbiotically as part of the intestinal flora in the midgut of honeybees (Apis mellifera) and some social bumblebee species. In this section of the intestine, together with Gilliamella apicola, they are the dominant bacteria, with each representing up to nearly 40% of the microflora there. In the intestine, Snodgrassella alvi and Gilliamella apicola interact in the utilization of metabolic resources, using each other's metabolites, and accordingly, they colonize different areas of the intestinal wall. According to a study published in September 2018, Snodgrassella alvi is damaged by the use of the pesticide glyphosate, resulting in impairments of the gut microbiota. As a result, weakening of the bees' resistance to harmful bacteria and subsequent weakening of the animals was observed. This effect was subsequently discussed internationally in various media as a possible cause of the colony collapse disorder observed worldwide.
Characteristics
Appearance Snodgrassella alvi is a bacterial species and thus a unicellular organism without a nucleus (prokaryote). The individual cells are short and rod-shaped with a length of about 1.0 μm and a diameter of 0.4 μm. They are gram-negative and accordingly possess only a thin peptidoglycan envelope.
The bacteria are immobile and form colonies with other bacteria in the bays of the intestinal wall of the bee gut. The strains can grow on blood agar, trypticase soy agar (TSA), heart infusion agar (HIA), and lysogeny broth (LB), forming smooth, white, and round colonies about 1 millimeter or less in diameter after 2 days.
Growth and metabolism The species is microaerophilic, i.e. grows best at low oxygen levels. A 5% CO2 atmosphere at a temperature of 37 °C provides optimal growth conditions, while the bacteria show very weak or no growth in air or without oxygen (anaerobic). In the TSA, the growth range is approximately pH 6.0 to 6.5. The catalase test and the test for nitrate reductases are positive, while the oxidase test (detection of the enzyme cytochrome c oxidase) is negative. Tests of the strains for β-glucosidase, β-galactosidase, indole production, proteolysis of gelatin, and glucose fermentation are negative. They are not hemolytic and show variable reactions to urease and arginine dihydrolase. The bacterium can use citric and malic acid as its main carbon source.
Chemotaxonomic features The combined content of cytosine and guanine (GC-content) in DNA is 41 to 43 mol%. The main isoprenoid quinone is ubiquinone-10. The main components of the fatty acids produced are palmitic acid (C 16:0), cis-vaccenic acid (C 18:1ω7 c/C 18:1ω6 c) and lauric acid (C 12:0), by which Snodgrassella alvi can be distinguished from closely related species.
Genome The genome of Snodgrassella alvi has been completely sequenced. Out of 2,226 protein-coding genes, 519 genes are essential and 399 genes are involved in gut colonization of honeybees.
Lifestyle and physiology
Snodgrassella alvi lives in the gut of honey bees and other corbiculate (pollen-basket) bees. Strains comparable in genome were found in all subsequently studied species of the genera Apis (n=6) and Bombus (n=8) and in 9 out of 13 studied species of stingless bees (Meliponini); it is not detected in other species and outside their hosts. It is a keystone species in the honeybee gut and dominates this microbiome together with seven other species, some of which have not yet been finally identified: Lactobacillus spp. Firm-4, Lactobacillus spp. Firm-5 (phylum Firmicutes), Bifidobacterium spp. (phylum Actinobacteria), Gilliamella apicola, Frischella perrara, Bartonella apis, and Alpha 2.1 (phylum Proteobacteria). Together, these species represent 95% of the intestinal bacteria. Snodgrassella alvi, Gilliamella apicola, and Frischella perrara are considered species-specific keystone species. Snodgrassella alvi and the gammaproteobacterium Gilliamella apicola dominate the area of the ileum and colonize the inner wall of the intestine there. Only a few bacteria exist in the anterior region of the intestine, while Frischella perrara dominates the short region of the pylorus and is found almost exclusively there. In the rectum, Lactobacillus strains and Bifidobacterium are predominant. The transmission of the microbiome and especially of key species occurs in social insects within the hive via the transmission of saliva and food. Bee larvae and young workers are almost devoid of gut bacteria in their first days of life and acquire their normal gut microbial flora orally only later through social interactions with other workers and by transmission between individuals within a hive during mutual food transfer (trophallaxis) in their first days outside the combs and at the beginning of their life in the colony. Although larvae are also fed by workers, their intestines, whose anterior and posterior parts are not connected before pupation, are almost free of bacteria. This is mainly attributed to a strong immune defense of the larvae and a bacteriocidal effect of the saliva of the feeding bees. Only after pupation and in the presence of nurse bees or the feces of these bees do the young workers develop the intestinal flora typical of the hive. In contrast, when exposed only to the hive material such as honeycomb, honey and bee bread or only to the saliva of bees and trophallaxis of other bees, they form atypical intestinal flora. Within the intestine, the bacteria support the digestion of honey and pollen and probably also have a function in the immune defense against parasites and pathogenic bacteria. The protective effect of the natural bacterial composition has been demonstrated, among other things, against pathogens such as the protozoa Crithidia bombi or Nosema bombi, which infect various bumblebee species and lead to potentially fatal infections. However, excessive colonization of the bee gut by Snodgrassella alvi is likely to disrupt the gut flora and increase susceptibility to infection by the trypanosome Lotmaria passim.
Interaction with Gilliamella apicola
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