By Justin Bauer
Insect pollinators maintain natural terrestrial ecosystems and are vital to agricultural ecosystems. Data suggests that while no shortage of insect pollinators has been reported yet, dependence on insects as pollinators has grown immensely (Aizen, 2008). Amongst these pollinators, bees are at the forefront. The Western Honey Bee (A. Mellifera) for example, is used in nearly 90% of commercial pollination. As pollination needs increase across the world, so does the number of managed honey bee colonies. However, the demand for domesticated honey bees is not being met (Aizer, 2009). In addition to that, there are several pathogens that are threatening to wreak havoc on bee species – both domestic and wild. Not only are there many potential economic downsides, but the environmental impact would be huge if these pathogens are not brought under control.
P. larvae (American Fouldbrood) is the most destructive bacterial disease of honey bees (A. Mellifera). Eastern honey bees are technically susceptible to P.larvae as well, but no clinical signs of disease are commonly observed (Chen, 2000). P.larvae is gram positive and aerobic to microaerophilic bacterium. It is also peritrichously flagellated and is able to form endospores under rough conditions (Genersch 2006). Only its spores are infectious and so far, only young honey bee larvae can host this pathogen (Hoage, 1966). It is an obligate killer, as after the infected larva dies, the entire larval cadaver is digested by P.larvae before spores are transmitted to healthy larvae. Much research has been performed on this disease and the key virulent factor has been determined to be a chitin-binding and degrading enzyme: PICBP49 (Gonzalez, 2014). There also many secondary metabolites which help outcompete microbial competitors in larval guts so that there is a pure culture of P. larvae to degrade the larval cadaver (Müller 2015).
M. Plutonius is another concern to bees. It is the causative agent of a similar deadly bacterial disease to bee larvae; the European Foulbrood (EFB). In contrast to AFB is is gram positive and non-spore forming, but it is also microaerophilic to anaerobic. The non-motile cocci are to blame for infection. By ingestion of larval food Larvae become infected. Larvae at the age of up to 48hrs are the most susceptible to the disease but older larvae can also become infected. In this case larval decomposition is achieved by secondary invaders such as saprophytes. M. Plutonius requires microaerophilic to anaerobic conditions, added potassium phosphate to the culture media, and carbon dioxide for growth (Bailey, 1982). These difficult growth requirements, along with lack of interest and funding from the public, led to the pathogenesis of M. plutonius being rarely researched.
L. Sphaericus, a gram-positive and rod-shaped soil bacterium is another important pathogen. Its strains, produce insecticidal protein toxins. One of those is sphaericolysin (active against cockroaches and caterpillars), another is a binary toxin produced during sporulation, and the third is a different mosquitocidal Mtx or Cry toxins (Berry, 2012). Colonies affected by this bacterium seem to lack a queen and exhibit a reduced adult bee population, with little entrance activity. Originally, L. spaericus was assessed as not being pathogenic for bees, but the tests did not consider any effects on longevity of the exposed bees and brood production (Davidson, 1977).
S. marcescens is another well-known insect bacterial pathogen. It is rod-shaped, gram-negative and belong to the family Enterobacteriaceae. It is primarily found in water as a saprophyte, but it has also been isolated from plants and even hospitalized human patients where it can cause infections in both the respiratory and urinary tract (Hejazi, 1997). The bacterium is most likely taken up from plants, after which the hemocoel is penetrated and the bacterium proliferates in the hemolymph and leads to lethal septicemia. After penetrating the chitin-rich peritrophic matrix, the pathogen is now protected from the insect midgut epithelium (Kolstad, 2013). Colonies affected by this, seemingly collapsed over winter and symptomatic infected honey bees seemed to be immobilized and distanced themselves from other bees. In addition to that, symptomatic worker bees were only found in the winter season, whereas symptomatic drones were found in warm seasons (Burritt, 2016). Both diseased honey bee larvae and adult honey bees have been found, indicating that it is important to research more about this disease as it seems able to target bees in any form of development.
Whilst insect lives may seem insignificant to some, it is vital to remember the important impact bees have on our daily life. There are many more diseases affecting bees than the ones mentioned above, and if we want to maintain a high enough population of bees, both wild and domestic, more research on possible cures needs to be completed. There is no denying the large role that bees play in human agriculture and the faster the pathogens threatening them can be defeated, the better.
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