Many of us would agree that living together, although rewarding, can be challenging. A symbiotic relationship involves a long-term biological association between organisms from different species. Symbioses can be harmful or beneficial to the partners. In mutualism, both individuals benefit, such as the exchange between plants and their pollinators. However, in parasitism, one of the organisms benefits at the expense of the other, such as when viruses or bacteria infect humans.
In a microbial symbiosis, the host and symbiont living in the host exist along a parasitism-mutualism continuum, the dynamics of which are dependent upon numerous factors. One key factor is the transmission mode of viruses and bacteria. In vertical transmission, symbionts are inherited from host parent to offspring, which results in a more mutualistic interaction. In horizontal transmission, symbionts infect new host cells every round of infection. Since host survival is not important it often results in the death of the host.
My research at the University of Vienna involved the study of Parachlamydia, a member of the environmental chlamydiae, which are bacterial symbionts of single-celled organisms commonly found in water or soil.
These bacteria live inside their unicellular hosts and are dependent on host nutrients. Our lab set up an evolution experiment consisting of two treatments for 14 months.
In one treatment, the bacteria had to be transmitted horizontally to new host cells for survival, whereas in the other treatment the bacteria multiplied within the same host cell throughout. We then sought to identify any changes due to the different transmission modes. Like previous studies, bacteria became increasingly infectious when they were required to move from one host cell to another.
We went one step further and examined the genes of the bacteria and found that they differed in over 1,000 sites between the two treatments.
Even more crucial was the subsequent analysis of gene expression – namely, the use of the almost 2,500 genes during infection. We revealed that the increase in bacterial infectivity in the horizontal treatment was observed because genes that are crucial in (a) infecting new host cells, and (b) surviving outside the host cells, were highly active and effective.
Such studies are important as they improve our understanding of how environmental conditions affecting transmission mode can lead to the emergence of parasites from more benign associations.
This study has been published in PNAS: https://www.pnas.org/content/117/35/21658
Paul Herrera, PhD student
Did you know?
• The number of bacterial cells per human outnumbers the number of human cells.
• Corals live in a mutualistic symbiosis with microscopic algae that capture sunlight, providing the corals with most of their nutrients.
• The tiny organelles inside our cells that release energy from food, the mitochondria, were once free-living cells.
• Bobtail squid possess symbiotic bacteria within their bodies that light up and provide camouflage by mimicking moonlight.
• It takes roughly a year for the community of microorganisms in and on a newborn baby to mature to an adult status.
For more trivia see: www.um.edu.mt/think
Sound bites
• Wolbachia are a remarkable group of intracellular bacteria that occur naturally in 60 per cent of insects. Mosquitoes are known to carry viruses, such as dengue and Zika, transmitted between humans through mosquito bites. However, scientists discovered that when the mosquitoes carry Wolbachia, the bacteria compete with the viruses, making it harder for them to reproduce inside the insects. To reduce the likelihood of viruses spreading, researchers at the World Mosquito Program breed Wolbachia-carrying mosquitoes and then release them into areas affected by mosquito-borne diseases.
https://www.worldmosquitoprogram.org/en/work/wolbachia-method/how-it-works
• Not feeling great today? Well, maybe your gut bacteria are to blame! The mutualistic relationship between the gut microbiota (the microorganisms living in our digestive tracts) and humans is fascinating. A Belgian study of two European groups has discovered that various species of gut bacteria are lacking in people with depression. Researchers also demonstrated that gut bacteria were able to produce substances that may affect one’s mood. Resolving this microbiome-brain connection may eventually result in novel therapies for treating depression.
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