Double damage: When working together isn't always a good thing.

Although many symbionts help their hosts, others work together to harm their hosts. In some cases pathogens such as viruses and bacteria work together to double the damage to their host. One example of this the parasitic mite Varroa destructor and the deformed wing virus of Apis mellifera honey bees.

Varroa destructor feeds of a single hole on a honey bee which its young will also feed from, it is still unclear whether the mites also stimulate immune suppression (Di Prisco et al. 2016). The deformed wing virus suppresses the immune system of their host by enhancing the transcription of genes which downregulate the bee’s immune response. DWV also causes deformation of wings, disrupts bee behaviour and reduces lifespan (Stamets et al. 2018). 

A Varroa destructor mite feeding off a honeybee pupa (Anderson 2017).

Scientists have found that these two pathogens have a mutualistic relationship whereby the mite acts as a vector for the virus and both actively weaken their host making it easier for both to infect and reproduce (Di Prisco et al. 2016). Both pathogens reproduce more when they infect a bee together and it is believed that this mutualism plays a key role in the downfall of honeybee colonies (Di Prisco et al. 2016).

Fortunately, scientists have found a promising symbiotic solution. Leonard et al. (2020) successfully inoculated A. mellifera bees with an engineered version of the symbiotic bee gut bacteria Snodgrassella alvi which continuously produces dsRNA that upregulates immune gene expression in the bees. This increases the survival of infected bees and reduces viral and mite loads (Leonard et al. 2020). Another study from 2018 found that feeding the bees polyphore mushroom extract reduced viral load of DWV by 74% (Stamets et al. 2018). Although feeding them fungi is not a symbiosis, it is promising news for an insect that we rely so heavily on for the pollination of plants, flowers, and crops (Stamets et al. 2018).

Aedes aegyptii - A powerful vector for disease (Gathany 2006)

Similarly, Pan et al. (2017) was able to introduce a symbiotic Wolbachia bacteria into the mosquito Aedes aegypti to enhance their immune system and reduce their vector capabilities for dengue and zika virus. Wolbachia bacteria share symbiotic relationships with many insects including the mosquito Aedes albopictus, Pan et al. (2017) found that if the bacteria Wolbachia AlbB is introduced at the right life cycle of the Aedes aegypti mosquito it will be passed on paternally through each generation. Pan et al. (2017) found that this bacteria uses the innate immune system of their host to colonize their host and stimulate the immune system to protect against fungi and bacteria.

Although the exact mechanisms behind interactions between viruses, fungi and bacteria are still unclear, it is very common (Jagdale & Joshi 2018).

Thankyou for reading! Check back next week for more 😊