Part 1. Is Nosema a disease or a symptom of dysbiosis of the bee gut microbiota?
A smoking gun is often used to describe something that serves as conclusive evidence or proof. In science it is often related to data that best fits a hypothesis and/or theory. When it comes to honeybee decline there are a number of smoking guns. Habitat loss, land use, pesticides, herbicides, climate change, viruses, bacterial diseases, fungal diseases, Varroa destructor and Nosema. So much so that we often think in terms of multiple stress factors producing a perfect storm leading to the decline of our most valued pollinator, it’s amazing that they have survived for so long.
In this post I shall be looking at another smoking gun which has gone under the radar. A new hypothesis which, if tested and the data turns out to have some significance, can add to our understanding of honeybees decline.
Advances in technology has help us understand much more about complex natural systems which exist in the microscopic world. All organisms with a gut will have a specialised microbiota composed of bacteria, fungi, and other micro-organisms specialised in breaking down dietary intake and providing compounds essential for switching on and off genes, development of organs, a healthy immune system, and biochemical processes.
The hypothesis is centered around the breakdown of pollen by the gut microbiota of bees. The microbiota of bees consists of bacteria and fungi essential for the breakdown of pollen walls and pollen contents. When one component of the microbiota disappears the health of the bee declines and in honeybees the entire colony can become weak and can suddenly die without any indication to the cause of death.
New Hypothesis: Fungicide use may be a principle cause of honeybees decline.
The hypothesis can best be illustrated by considering Nosema and changing the belief that Nosema is a disease to Nosema being part of the bee gut microbiota.
Nosema as a disease (Nosemosis).
Nosema was first discovered in the early 1900. Nosema is currently thought of as a fungal parasite and pathogen of the bee gut which affects all bees including bumble bees and solitary bees. Nosemosis (Nosema disease) has been stated as one of the most serious and prevalent adult honeybee diseases worldwide. A cure has not been found for Nosemosis.
Once Nosema has invaded they multiply rapidly and, within a short period of time, large number of spores accumulates in the gut which are excreted by the bee. It has been observed that other bees ingest excreta containing spores becoming infected and therefore completing the life cycle of Nosema.
The hypothesis that Nosema is a disease has persisted for over a century to present day. Traditional treatment for Nosema are antibiotics. Unfortunately, this has not lead to a reduction or limit to the spread of Nosemosis.
Diagnosis of Nosemosis are the presence of Nosema spores found in the gut of bees as indicated in Figure 1.
Nosema as a part of the bee gut microbiota.
Based on the new hypothesis; Nosema is part of the bee gut microbiota and is valuable to bees. The living Nosema fungi is capable of producing spores as part of its life cycle and also when living conditions become detrimental to its existence. Therefore, in the presence of fungicide Nosema spores are produced. If in the presence of high concentrations of fungicides, Nosema spores are absence which is an indication that the living organism is killed before being capable of producing spores.
Spores which are excreted from bees, infected by fungicides, are ingested by healthy bees. Once present in the gut of healthy bees; integrate into the gut microbiota.
Dysbiosis is the term used to describe an imbalance of the gut microbiota which has consequences relating to health issues.
In honeybees dysbiosis is prevented by eliminating exposure to fungicides.
The new hypothesis stems from recent research on the human gut microbiota. Diseases and medical conditions produced from changes to the gut microbiota includes diabetes, obesity and mental health conditions.
Over the last decade there has been an an explosion of studies relating microbiota to health and disease. This has been possible with advances in methods and techniques used for DNA analyses.
Gut microbiota can be defined as indigenous microbial communities consisting of bacteria, fungi, viruses, archaea and protozoans. These communities are commensal and are beneficial to the host organism. Studies on the human gut microbiome (the ecosystem which includes microbiota, the environment which they inhabit and genetics) have indicated the following;
- The human microbiota is made up of trillions of cells – including bacteria, viruses and fungi and they outnumber our own cells tenfold.
- The largest populations of microbes reside in our gut – the gut microbiota. Other habitats include the skin.
- The microbial cells – and their genetic material, the microbiome – live with us in an innate relationship that is vital to normal health, although some species are also pathogens and an overgrowth can cause disease.
- The microorganisms living inside the gastrointestinal tract – also known as the gut flora – amount to as much as 1.8 Kg of biomass, with every individual having a unique mix of species.
- The microbiota is important in nutrition, immunity and effects on the brain and behaviour. It is implicated in numerous diseases when the normal individual balance of microbes is disturbed. This is called dysbiosis.
All organism with a digestive tract have a gut microbiota. Based on information gained from human gut microbiota studies we may predict that the bee gut microbiota is unique and contains bacteria and fungi that are specialised in breaking down pollen walls and pollen contents which contributes to the health and development of the bee including the development of a healthy immune system.
Fungicides and antibiotics therefore can affect fungal and bacterial communities in the bee microbiota leading to dysbiosis. Under these conditions bees are more vulnerable to diseases and have a shortened life expectancy.
Sources & pathways of fungicide in bees.
Generally there are two main categories of pesticides, contact pesticides and systemic pesticides. Contact pesticides are normally sprayed onto plant foliage whereas systemic pesticides move into plants through roots and seed coated with pesticides. Systemic fungicides are active within plants and are translocated to all parts of the plants including anthers where pollen are undergoing development. Small amounts of fungicide end up within pollen content of each individual pollen grains. This process is called Bioaccumulation.
Bees collect large numbers of pollen grains. Foraging honeybees collect an extraordinary amount of pollen for colonies which averages 40,000 bees per colony. If a honeybee colony is located in an area where systemic fungicides are used than it is more than like that a significant percentage of pollen collected contain systemic fungicides.
Newly emerged bees and young bees consume large amounts of pollen during their first feeds. If a high percentage of pollen grains contain systemic fungicides is consumed the total concentration of fungicide could become significant to affect the gut microbiota of the bee. A process called Biomagnification is the total sum of all the accumulated amounts of fungicide from individual pollen grains.
At low level of fungicide Nosema spores are more likely to be seen, at high levels Nosema may be killed before producing spores. Dysbiosis caused by fungicides can block developmental pathways and shorten the life expectancy of infected bees.
Systemic fungicides have been used to coat seeds since 1969 and over a short period of time have developed in effectiveness in preventing fungal growth on plants. Compounds which make up fungicides can persist in the environment for a long time as there are no natural means to breakdown these compounds. Today almost all seeds used in agriculture and most seeds used in horticulture are coated with systemic fungicides.
Data and testing the hypothesis.
Two very different hypotheses are presented. One which has been used for over a century and the other developed from recent research on human gut microbiota. Data already exist which associate Nosema to fungicide levels found in honeybees, however, that data was evaluated on Nosema being a disease. If data is reevaluated in the context of Nosema being part of the bee gut microbiota there maybe a better fit. Another way of testing the hypothesis is to look for the presence of Nosema in healthy gut microbiota, its association with other fungi and bacteria and its function. All of these things are possible using new methods and techniques developed for DNA analyses.
In the second part of this post I shall discuss another fungal group, Chytrids that break down pollen, which are vital for the health and development of the honeybee immune system.