Читать книгу Honey Bee Medicine for the Veterinary Practitioner - Группа авторов - Страница 27

An oft‐overlooked aspect of the bee environment that is essential to the good lifestyle of honey bees is their microbiome, that is, the community of specialized microbes (bacteria and yeasts) that have coevolved to live inside the bees and in their nests (e.g. in their pollen stores). We again return to the tenet of our chapter: the need to learn about the honey bee's natural biome to understand its biology, including its relationships with its pathogens. The honey bee microbiome is remarkable in that it is nearly consistent across thousands of individuals from hive to hive and even across continents. The honey bee's microbiome is similar to that of humans in that both feature specialized bacteria that have coevolved with their host and are socially transmitted (Engel et al. 2012; Zheng et al. 2018). Honey bees are first inoculated with bacteria in the larval stage, presumably through the food provided by nurse bees. However, during pupation, when bees undergo the final phase of metamorphosis, a bee's exoskeleton (including the gut lining and any associated bacteria) is shed in a process known as ecdysis. Therefore, honey bees emerge as young adults without a gut flora, except for those microorganisms they pick up when chewing through the wax cappings of their cells. The characteristic microflora of a worker bee is, therefore, developed mainly following emergence and through direct social interactions with conspecific worker bees. By four to six days of age, the population of a worker bee's gut flora stabilizes at 10⁸–10⁹ bacterial cells.

Although both wild honey bees and those living in apiaries possess complex microbiomes, some beekeeping practices – such as feeding pollen substitutes and treating with antibiotics – can alter the microflora of honey bees (Fleming et al. 2015; Maes et al. 2016). Dysbiosis, or unhealthy shifts in gut microflora, was observed in bees consuming aged pollen or pollen substitutes and was linked to impaired larval development, increased bee mortality and infection with pathogens such as Nosema and Frischella. Raymann et al. (2017) observed considerable changes in the gut microbial community composition and size following treatment with tetracycline, the most commonly used antibiotic in beekeeping operations globally. The authors concluded that decreased survival in honey bees was directly attributed to increased susceptibility to infection by opportunistic pathogens that colonized the gut after antibiotic use. The honey bee microbiome is thought to promote bee health and development in several ways. Gut microbes are required for normal bee weight gain, an effect which can be attributed to regulation of endocrine signaling of important bee hormones. The microbiome increases the levels of vitellogenin and juvenile hormone in worker bees, and these regulate the nutritional status and the development of their social behaviors, so it is likely that the state of the bees' microbiomes affects the health of the whole colony. Bee microbes are also implicated in modulating the worker bee's immune system (Zheng et al. 2018).

Alterations in the microbiota of the bee gut have been linked to disease and reduced fitness of the bee host. The use of tetracycline – an antibiotic commonly used to treat American foulbrood and European foulbrood, and often given prophylactically – reduces both the number and the composition of normal bacteria in the bee gut. Raymann and colleagues (2018) found that Serratia marcescens, a known pathogen of honey bees and other insects, normally inhabits the bee gut without eliciting a host immune response. However, bee disease occurs when this pathogen is inoculated into a bee's hemolymph through the bite of a Varroa mite or when the gut microbiome is disturbed with antibiotic use. Researchers studying Colony Collapse Disorder observed a shift in gut pathogen abundance and diversity, and proposed that such shifts within diseased honey bees may be a biomarker for collapsing colonies (Cornman et al. 2012). See Chapter 9 for more details on the bee microbiome.