Archive

Archive for September, 2012

The Bacteria That Make Insects Eat Their Own Brains

September 5, 2012 Leave a comment

As far as bacteria are concerned, other living creatures are just another niche to exploit, which means that pretty much every animal and plant has a set of bacterial pathogens that come along with it. These bacteria have made the animal in question their speciality, and are highly adapted to live inside their hosts. While these bacteria often make the host ill, or less fit, or sometimes dead, the longer they live with their host, overall, the less they damage it. After all, it’s no help to the bacteria if their home drops down dead right after they’ve moved in.

A great example of this appeared in PLoS Pathogens this month (reference 1), concerning the bacteria Wolbachia. These bacteria infect insects and other arthropods and are much beloved of journalists (well, compared to other insect bacteria at least) because one of their effects is to stop insects producing male offspring (so only female survive to pass on the bacterial genome), which gives journalists the opportunity to write silly headlines.

An electron micrograph of an insect cell, with three Wolbachia bacteria inside (the large circular blobs with white lines surrounding them). Image from reference 2. (up)

As well as passing from females onto their offspring, Wolbachia can also be transmitted horizontally, that is between insects in the same generation. In its normal host the Wolbachia is not hugely damaging (apart from removing all males from the population) but when transmitted to a new species it causes various unpleasant nervous system complications, often leading to death. Clearly, the bacteria are more virulent when they encounter a new species. However when the bacterial infection was closely examined, it was found that infected individuals of both species contained the same number of bacteria. It wasn’t just that the new species couldn’t respond to the infection, it was in fact the way they responded that was doing the damage.

As it turns out, the reason Wolbachia are more dangerous in new species isn’t because the bacteria go wild in the unexplored territory, rather it’s because the new host doesn’t know how to deal with them. The insects that are used to dealing with the presence of the bacteria have developed ways to contain the infection, or just tolerate it. New species however, tend to panic, particularly as the bacteria tend to congregate in the gonads (sex organs) and the central nervous system, which even insects understand are bad places to have bacteria.

As the bacteria are found inside cells, the best way for an insect immune system to get rid of them, is by destroying the cells that house the bacteria. Which, as previously mentioned, are mainly the gonads and the central nervous system. When the Wolbachia get into a new species, the first response of the insect is to quickly and efficiently destroy any cells which have bacteria inside them. As a consequence the unfortunate insect basically destroys its own brain, leading to various unpleasant symptoms and death.

The carpenter ant, Camponotus pennsylvanicus. Many species of Camponotus are infected with Wolbachia. Image from reference 3. (up)

Even in insects, the immune system is vital to defend animals from bacterial, fungal, and viral attacks. However it’s fascinating to see the cases where the immune system (even ‘primitave’ immune systems that consist of nothing more than infected cells quickly being removed) can lead to issues by over-reacting to a threat. The best response to the Wolbachia is for the insects to learn to deal with it, rather than to attempt to launch counter-attacks which can be damaging for the animal as a whole.

Reference:

Adopted from: Rat Lab blog

1: Le Clec’h W, Braquart-Varnier C, Raimond M, Ferdy JB, Bouchon D, & Sicard M (2012). High virulence of wolbachia after host switching: when autophagy hurts. PLoS pathogens, 8 (8) PMID22876183

2: (2004) Genome Sequence of the Intracellular Bacterium Wolbachia. PLoS Biol 2(3): e76. doi:10.1371/journal.pbio.0020076

3: Wernegreen JJ (2004) Endosymbiosis: Lessons in Conflict Resolution. PLoS Biol 2(3): e68. doi:10.1371/journal.pbio.0020068

 

 

 

Advertisements

Common Parasite May Trigger Suicide Attempts

September 5, 2012 Leave a comment

A parasite thought to be harmless and found in many people may actually be causing subtle changes in the brain, leading to suicide attempts.

New research appearing in the August issue of The Journal of Clinical Psychiatry adds to the growing work linking an infection caused by the Toxoplasma gondii parasite to suicide attempts. Michigan State University’s Lena Brundin was one of the lead researchers on the team.

About 10-20 percent of people in the United States have Toxoplasma gondii, or T. gondii, in their bodies, but in most it was thought to lie dormant, said Brundin, an associate professor of experimental psychiatry in MSU’s College of Human Medicine. In fact, it appears the parasite can cause inflammation over time, which produces harmful metabolites that can damage brain cells.

“Previous research has found signs of inflammation in the brains of suicide victims and people battling depression, and there also are previous reports linking Toxoplasma gondii to suicide attempts,” she said. “In our study we found that if you are positive for the parasite, you are seven times more likely to attempt suicide.”

The work by Brundin and colleagues is the first to measure scores on a suicide assessment scale from people infected with the parasite, some of whom had attempted suicide.

The Toxoplasma gondii parasite has been linked to inflammation in the brain, damaging cells. Image adapted from MSU press release image. (up)

The results found those infected with T. gondii scored significantly higher on the scale, indicative of a more severe disease and greater risk for future suicide attempts. However, Brundin stresses the majority of those infected with the parasite will not attempt suicide: “Some individuals may for some reason be more susceptible to develop symptoms,” she said.

“Suicide is major health problem,” said Brundin, noting the 36,909 deaths in 2009 in America, or one every 14 minutes. “It is estimated 90 percent of people who attempt suicide have a diagnosed psychiatric disorder. If we could identify those people infected with this parasite, it could help us predict who is at a higher risk.”

T. gondii is a parasite found in cells that reproduces in its primary host, any member of the cat family. It is transmitted to humans primarily through ingesting water and food contaminated with the eggs of the parasite, or, since the parasite can be present in other mammals as well, through consuming undercooked raw meat or food.

Brundin has been looking at the link between depression and inflammation in the brain for a decade, beginning with work she did on Parkinson’s disease. Typically, a class of antidepressants called selective serotonin re-uptake inhibitors, or SSRIs, have been the preferred treatment for depression. SSRIs are believed to increase the level of a neurotransmitter called serotonin but are effective in only about half of depressed patients.

Brundin’s research indicates a reduction in the brain’s serotonin might be a symptom rather than the root cause of depression. Inflammation, possibly from an infection or a parasite, likely causes changes in the brain’s chemistry, leading to depression and, in some cases, thoughts of suicide, she said.

“I think it’s very positive that we are finding biological changes in suicidal patients,” she said. “It means we can develop new treatments to prevent suicides, and patients can feel hope that maybe we can help them.

“It’s a great opportunity to develop new treatments tailored at specific biological mechanisms.”

References:

Source: Michigan State University press release
Image Source: T. gondii image adapted from Michigan State University press release image
Original Research: Abstract and full paper from MSU (PDF file) for “Toxoplasma gondii Immunoglobulin G Antibodies and Nonfatal Suicidal Self-Directed Violence” by Yuanfen Zhang, MD, PhD; Lil Träskman-Bendz, MD, PhD; Shorena Janelidze, PhD; Patricia Langenberg, PhD; Ahmed Saleh, PhD; Niel Constantine, PhD; Olaoluwa Okusaga, MD; Cecilie Bay-Richter, PhD; Lena Brundin, MD, PhD; and Teodor T. Postolache, MD in Journal of Clinical Psychiatry online July 2012 73(8):1069–1076 doi: 10.4088/JCP.11m07532