The spectrum of microbe-host interactions
Many microbes have adapted to live in association with another organism, which we refer to as their host. Every animal and plant is colonised by a huge number of different microbes, collectively known as microbiota, that form a community we refer to as their microbiome. Within a microbiome, each species of microbe has different effects on the host.
From the host perspective, the outcome of a microbe-host interaction lies on a spectrum. At one end of the spectrum are mutualists, which provide a benefit to their host. At the other end of the spectrum are pathogens, which damage their host and cause the symptoms we recognise as infectious diseases. Right in the middle of the spectrum are the commensals which have neither a negative nor positive effect on their host. The outcome of a microbe-host interaction can also be influenced by environmental conditions. Opportunistic pathogens only cause disease in immunocompromised (ie their immune system is not working properly) or wounded hosts.
We’ll cover pathogens and disease in more detail later in the course, but let’s first explore an example of mutualistic microbes and learn how they can benefit their hosts.
The bacterium Vibrio fischeri is an example of a mutualist. It lives in seawater in a free-living, non-host associated form but it can also form a mutualistic association with the Hawaiian bobtail squid, Euprymna scolopes (Figure 1: Left). The bacteria are bioluminescent (they glow in the dark) because they have a set of genes called the lux operon which encodes the enzyme luciferase. The bobtail squid allows Vibrio fischeri to make a home within a specialised structure called the light organ. The squid provide the bacteria with nutrients and a stable, predictable environment and in return, the bacteria help to camouflage the squid. At night, when the squid is feeding there is a high number of bacteria in the light organ and they make the squid glow from underneath with their blue light. This prevents the squid from casting a shadow in the moonlight (counter illumination) and hides it from predators looking up from the seabed (Figure 2: Right).
Figure 1: Left: The Hawaiian Bobtail squid (‘Euprymna scolopes’). © Chris Frazee and Margaret McFall-Ngai CC BY 4.0. Right: Counter-illumination can help marine animals to hide from predators swimming below them by reducing the shadow they cast in the moonlight © Chiswick chap CC BY 4.0.
Every morning, when the squid goes to sleep, it expels most of the bacteria from the light organ into the sea. The bacteria that are left in the light organ switch off the lux operon and stop producing light. During the day, the bacteria replicate within the light organ: each cell produces a small amount of signalling chemicals called autoinducers. As the number of bacteria rise, the concentration of autoinducers increases and when it reaches a certain threshold, the autoinducers activate the production of luciferase and they light up. This form of bacterial cell-to-cell communication is called quorum sensing.
You can see two more fascinating examples of mutualist microbe-host interactions in this additional resource PDF.
It is clear that many viruses are pathogens, but surprisingly not all virus-host interactions fall at this end of the spectrum. Some, such as herpes viruses hide from the immune system in an inactive (latent) state and could help protect their animal host from bacterial pathogens and even fight cancerous cells. Some plant viruses make their plant hosts more resistant to drought or cold stress or protect them from bacterial pathogens. White clover cryptic virus (WCCV) prevents its host plants from forming a mutualistic association with nitrogen-fixing Rhizobium bacteria if the soil contains enough nitrogen. This saves the plant from wasting energy producing root nodules and donating sugar to the bacteria when it doesn’t need nitrogen.
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