• Skip to main content
  • Skip to primary sidebar
virology blog

virology blog

About viruses and viral disease

How viruses infect a cell within a cell

11 November 2016

Eucyclops agilisEndosymbionts, organisms that may live inside of another cell, can be infected with viruses. An example is Wolbachia, which lives inside the cells of insects and nematodes, and is infected with Wolbachia phage WO. It’s always been a puzzle how viruses of endosymbionts pass through the host cell to infect their host. A study of the giant chloroviruses, which infect endosymbiotic algae, provides some answers (link to article).

Chloroviruses are well known because they are big: at one time they had the largest known dsDNA genome, up to 370,000 bases in length (they have since been eclipsed by viruses with far larger genomes). These viruses infect green algae (called zoochlorellae) which are endosymbionts of a variety of fresh water hosts, including paramecium, hydra, and coral.

While inside a paramecium, zoochlorellae can’t be infected with chloroviruses – the viruses can attach to the cell surface but cannot enter the host. If the zoochlorellae are mechanically released from the paramecium, they are readily infected with chloroviruses.

Chloroviruses are certainly found in the same environments as zoochlorellae – which begs the question, how do they infect their hosts in nature when they are shielded by paramecium? The answer is foraging.

Copepods, which are crustaceans found in nearly every freshwater habitat, include Eucyclops agilis (pictured; image credit) which eats paramecia. These protozoans pass through the Eucyclops digestive tract partially disrupted, exposing the zoochlorellae to the external environment. The zoochlorellae can then be infected with chloroviruses in the water.

Here we have a solution to an age old question in virology which involves copepods not fully digesting their food. A line from the movie Matilda comes to mind: Chew your food!

Modeling predator-prey cycles can also explain the observation that the abundance of chloroviruses in lakes fluctuate throughout the year, peaking in late spring and late fall. Rising levels of prey (paramecium) are followed by rising numbers of predators (Eucyclops), exposing zoochlorellae and leading to a burst of virus production.

The traditional view of virus-host interactions involves random collisions between virus and cells that lead to infection. The observation that predation can expose cells to virus infection places a new variable into this process. Whether foraging can explain how viruses of other endosymbionts access their cell hosts remains to be seen. I can imagine, for example, that bacteriophage WO might reach its host cell after an insect containig the endosymbiont is eaten by a predator, releasing free Wolbachia.

Primary Sidebar

by Vincent Racaniello

Earth’s virology Professor
Questions? virology@virology.ws

With David Tuller and
Gertrud U. Rey

Follow

Facebook, Twitter, YouTube, Instagram
Get updates by RSS or Email

Contents

Table of Contents
ME/CFS
Inside a BSL-4
The Wall of Polio
Microbe Art
Interviews With Virologists

Earth’s Virology Course

Virology Live
Columbia U
Virologia en Español
Virology 101
Influenza 101

Podcasts

This Week in Virology
This Week in Microbiology
This Week in Parasitism
This Week in Evolution
Immune
This Week in Neuroscience
All at MicrobeTV

Useful Resources

Lecturio Online Courses
HealthMap
Polio eradication
Promed-Mail
Small Things Considered
ViralZone
Virus Particle Explorer
The Living River
Parasites Without Borders

paulhasmeasles

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.