Pasteuria ramosa

Parasitic infections can severely debilitate the host in many ways, sometimes this manifests itself as the loss of some, or even all, of the host's reproductive ability. Evolutionary speaking, an organism that cannot reproduce is as good as dead. However, it's not entirely clear who (if anyone) is benefiting from this outcome - is it; (1) a survival strategy by the host to temporarily free up resources to compensate for the parasite's presence? Or is it (2) an adaptive strategy by the parasites to divert as many resources as possible to themselves without compromising the host's ability for self-maintenance and survival? Or is it (3) merely an unintended side-effect of infection? Of course, (1) and (2) are not mutually exclusive, and in the case of (3), even if it had started out as an unintended side-effect of infection, if host castration resulted in higher reproductive fitness for the parasite, then that trait will be positively selected for and become part of the its repertoire of host-exploitation strategies.

Waterfleas (Daphnia) are infected by all manner of parasites (we featured one of them during the early days of the blog: Caullerya mesnili) ; most of them are pretty nasty - they often end up castrating and/or killing the host. Pasteuria ramosa is no different - it is a spore-forming bacterium that infects waterfleas, makes them bloated, darkening their body (see the right waterflea in the photo) and castrates them in the process. While it was previously thought that any waterfleas infected by P. ramosa were permanently castrated, it turns out that some lucky Daphnia can actually recover from their infection.

So do these little crustaceans adjust their reproductive output in response to the parasites and is castration a way for them to compensate for a (potentially) temporary hiccup in their baby-making ability? To find out, a team of scientists from Norway set out to see just who benefits the most from host castration. Their logic is that if it is an adaptive strategy by the parasite, then we should see higher spore output from a permanently castrated host. Whereas, if castration is an adaptive coping mechanism by the waterflea, then there should be a jump in reproduction upon the onset of infection as the waterflea tries to make as many baby Daphnia as possible before P. ramosa put a stop to it, then store up reserves during the infection to "wait it out".

To correct for any potential sex differences (there are many documented case of sex-bias in parasitism), these scientists used only female waterfleas for the experiment. During the course of the study, about half the waterfleas they infected with P. ramosa managed to regain their reproductive capacity. In those lucky ones, the parasite produced many fewer spores than in waterfleas that had been permanently castrated. So evidently, P. ramosa benefits from having permanently castrated hosts. But what about the waterfleas themselves? Were they able to compensate by adjusting their reproductive output in the parasite's presence?

The scientists found that by far, the strongest predictor for the lifetime reproductive output of a parasitised waterflea is the age at which it becomes infected - the later that it became infected, the more time it had to churn out babies before it came down with a severe case of P. ramosa. So it's pretty much a case of "use it or lose it". They did not find evidence to suggest the waterfleas made any effort to increase their reproductive output before they are castrated by their parasites. This is unlike other systems where parasite-castration occurs. In trematode-snail systems, the infected snails are less likely to recover from their infection. The strategy which has evolved among snails in areas with high parasite prevalence is to reach sexual maturity as quickly as possible (For example: see this study) so they can eke as many baby snails as they can before they inevitably become infected and be taken over by squirming body snatchers.

It should be noted that the waterfleas used in the experiment were from Southern Finland, whereas the parasites were isolated from a pond in Northern Germany. So perhaps the reproductive strategy of the Daphnia population used in that experiment has evolved in response to their local parasite(s) population instead. Other studies have found waterfleas to be locked in a close evolutionary race with their parasites across space and time, so the outcome of any host-parasite interaction will be dependent on the genetic identity of both host and parasite.

Image credit: Jensen et al./PLoS Biology

Reference:
Magerøy, J.H., Grepperud, E.J. and Jensen, K.H. (2011) Who benefits from reduced reproduction in parasitized hosts? An experimental test using the Pasteuria ramosa-Daphnia magna system. Parasitology 138: 1910-1915

Caenorhabditis briggsae (KT0001)

Today's parasite is in the same genus as the famous and well-studied model lab nematode worm Caenorhabditis elegans. Caenorhabditis briggsae is a relative of C. elegans and is often used in comparative studies with its more famous counterpart because many of the tools developed for C. elegans can also be used on C. briggsae. While C. elegans is the darling lab worm due to its usefulness in studying genetics and developmental biology, until very recently, very little is known about its natural ecology.

Worms in the genus Caenorhabditis are often associated with invertebrates, hitching a ride on them as a way of traveling between food sources, or even opportunistically feeding on their ride if it happens to drop dead for whatever reason. In a paper published last year, a group of researchers reported on a strain of C. briggsae (KT0001) from South Africa displaying an ability not previously known for any Caenorhabditis species - it is capable of infecting and killing wax moth larvae. This strain of C. briggsae was found to be in a symbiosis with the pathogenic bacteria Serratia which presumably allows C. briggsae (KT0001) to become a parasitic killer.

Furthermore, when the researchers tested 10 wild strains of Caenorhabditis species which had not previously displayed any ability to infect insects - including a strain of C. elegans - and cultured them with Serratia, all but one strain gained the ability to infect, kill, and reproduce in insects, including the famous C. elegans. It seems that Serratia gives Caenorhabditis a license to kill - upon forming a partnership with the bacteria, these worms turn from mere passengers into deadly killers.

Reference:
Abebe, E., Jumba, M., Bonner, K., Gray, V., Morris, K., Thomas, W.K. (2010) An entomopathogenic Caenorhabditis briggsae. Journal of Experimental Biology 213: 3223-3229.

December 5 - Haemophilus influenzae

Did you get a flu shot this year? Good. But, it's not going to protect you against this pathogen, Haemophilus influenzae. This gamma proteobacterium, a member of the Pasteurellaceae, was mistakenly thought to be the agent responsible for influenza until the 1930's when the actual culprit, viruses, were found. That said, these bacteria can still cause a whole slew of illnesses such as lower respiratory tract infections, pneumonia, ear infections, and meningitis. H. influenzae also holds another important distinction - it was the first bacterium to have its entire genome sequenced - this was published in 1995.

October 11 - Treponema pallidum

On Columbus Day, it seemed appropriate to feature Treponema pallidum, the bacterium that causes syphilis and other diseases such as bejel and yaws. Why? Because Columbus had syphilis? No...at least not that I know of. The reason stems from another book by Robert Desowitz called "Who Gave Pinta to the Santa Maria?" In this book, Desowitz ponders the exchange of diseases between the Old World and the New World when exploration and colonization by Europeans began. One of these potential continent swaps was Treponema pallidum. Some contend that Europeans brought the bacteria to the New World where it infected people there and mutated to cause not syphilis, but pinta, which is more of a skin affliction. Others argue the opposite- that the voyages to the Americas resulted in the introduction of T. pallidum to Europe, where, because of the colder climate inspiring people to wear more clothing, the bacteria became spread via sexual contact instead. The bacteria found in each of the diseases are almost identical in morphology, serology and even genetically, so it's still somewhat of an open question as to why they sometimes produce one pathology instead of another. Currently, though, most agree that the bacterium that causes pinta is a different species, now known as Treponema carateum, with the others classified as subspecies of T. pallidum.

September 2 - Bdellovibrio bacteriovorus

When I was teaching microbiology, Bdellovibrio was always one of my favorite things to lecture about. These bacteria are essentially intracellular parasites of other bacteria. Bdellovibrio has a rather unusual mode of entering its hosts - it uses its flagellum to crash into them at amazing speeds - 160 um/second (ok, that doesn't sound very fast, but when you're only about 1 micron long, that's hauling!) Once inside its host cell, the Bdellovibrio consumes the nutrients inside it, growing longer and longer the whole time. Eventually, when the host cell has run out of nutrients, the long filament will separate into about 3 to 6 individual cells, lyse the membrane of their former host and go off and infect new victims.

June 14 - Chlamydophila psittaci

There are many reasons not to keep pet parrots. They're long-lived birds that require years, if not decades, of care. They can be messy and destructive. And, above all, many species are in danger of extinction and quelching the market for them is one helpful thing we can do. But, today's parasite is another reason. Chlamydophila psittaci is a species of bacteria that causes a very serious disease in both birds and mammals, including humans, known as psittacosis. These bacteria have a pretty unusual life cycle - they alternate between being intracellular bacteria in the lungs of their hosts and very resistant stages known as "elementary bodies." When engulfed by phagocytosis and attacked with a lysosome, the elementary bodies say "Ha!" and just begin to replicate instead - even going so far as to use some of the host cell's own organelles. Eventually they kill the host cell and become elementary bodies again, ready to reinfect this host or be spread to another one. In 1929, a major outbreak of psittacosis occurred in the U.S. The result of that was the drafting of more strict regulations concerning the trade of parrots and other pet birds but this epidemic was also partly responsible for the creation of the National Institutes of Health. You can read more about that in this article from The New Yorker that was published last year.

The image comes from this site.

May 31 - Rickettsia prowazekii

On Memorial Day, we honor the many soldiers who fought for our country. While bullets and other forms of arms are certainly the primary concern for soldiers at war, diseases that are spread under the conditions of warfare have taken their share of casualties as well. One of the most important ones, from the times of the Peloponnesian Wars in ancient Greece up through World War II, was epidemic typhus, caused by the bacterium, Rickettsia prowazekii. The bacteria are transmitted from person to person by the human body louse (Pediculus humanus humanus) and causes high fevers, rashes, headaches. muscle pains and delirium. Typhus has played a major role in history: the disease killed more of Napoleon's soldiers than the Russians did and then in World War I, over 3 million Russians died from typhus. The disease was rampant in Nazi concentration camps - both Anne Frank and her sister died of it.

The photo, from Wikipedia, shows two soldiers demonstrating a DDT gun, which was used to kill lice.

April 30 - Coxiella burnetii

Previously we met Quahog Parasite Unknown or QPX. Today’s parasite also had a mystery name for a while. In the 1930’s slaughterhouse workers in Australia began to come down with a combination of many symptoms that resembled flu, but sometimes progressed to pneumonia and if still left untreated, went on to induce endocarditis – an inflammation around the heart. At first it was unknown what the etiological agent was, so they simply called it “Q fever” with the “Q” standing for “query.” The actual pathogen, a bacterium that was isolated and pegged as the guilty party. The history of its naming is rather convoluted as well. A Japanese researcher in the 1920’s had found a bacterium in ticks that was capable of passing through filters and he published it, but since his samples did not survive, when the Australians Derrick and Burnet and Americans Cox and Davis found similar organisms, they could not be sure if it was the same organism or not (this is why one must have type species for new descriptions!) Thinking it was a close relative of Rickettsia, the Americans proposed the name Rickettsia daiporica, because if they named it after either of the discoverers, that name would be sunk if the Japanese one were to be found to be the same species. The Australians, not so worried about losing out on having their names on a bacterial species, proposed Rickettsia burnetii. More investigation revealed that this bug was not a Rickettsia and so first a subgenus and then a whole new genus, Coxiella, came into play, honoring one American and one Australian microbiologist.

C. burnetii can be found just about everywhere in the world, but the symptoms can differ geographically. Livestock animals are believed to serve as reservoirs and it has been demonstrated that several species of ticks can transmit the bacteria from one animal to another. Humans can become infected when they inhale the bacteria in dust, ingest meat or milk from infected animals or come into contact with blood or other fluids.

In the 1950’s, the U.S. developed weaponized C. burnetii , partly because it takes such a small inoculum to begin an infection and because it is extremely resistant to heat, dessication, and even disinfectants. (The U.S. ended this bioweapons program in 1969.)

April 7 - Brucella abortus

Brucella abortus is a gram-negative bacterium that uses cattle and other ruminants as its primary host. It can however, also be transmitted to humans and will produce a disease known as "Malta fever" or "undulant fever", characterized by sweating and joint pain and the latter name specifically referring to the waves of fever that a patient can show. Brucellosis is a major health threat to cattle because it causes a high degree of spontaneous abortions, thus all cattle in the U.S. (and many other countries, for that matter) must be vaccinated against this pathogen. Recently farmers in the U.S. West have become worried that wild ungulates such as elk and bison might transfer the bacteria to their herds. Humans can acquire the bacteria either from coming into physical contact with infected cattle or even by drinking milk or eating meat from infected animals. The genomes of two strains of B. abortus have been completely sequenced and these bacteria are somewhat unusual in having two separate chromosomes.

Image from this page.

March 14 – Bacillus anthracis

In the days and weeks after September 11, 2001, panic and suspicion seemed to be everywhere. After the terrorist attacks with airplanes, new threats in the form of letters sent to such famous people as newscaster Tom Brokaw and Senators Patrick Leahy and Tom Daschle were found to contain purified spores of Bacillus anthracis. B. anthracis is a Gram-positive bacterium that is usually found in the soil. When the spores of the bacteria are inhaled or ingested, they reproduce in the animal or human’s body, and can cause the disease anthrax, characterized by a multitude of health problems ranging from skin lesions (cutaneous anthrax) to gastrointestinal and pulmonary problems. B. anthracis is also notable as a parasite because it was the organism that the German microbiologist, Robert Koch used to formulate Koch’s postulates, the set of criteria that are necessary to define that an organism is the causative agent of a disease.

Image is from the CDC Public Health Image Library.

February 16 - Holospora undulata

So, according to the news, one of the big trends in fashion this year is that prints are back – especially paisley. And one cannot think of paisley (if one is a biologist, anyway), without picturing Paramecium. But, of course, paramecium is not a parasite – they are free-living unicellular organisms that zip around with their cilia and gobble up bacteria. But they have parasites, including the alphaproteobacterium, Holospora undulata. Paramecium have two kinds of nuclei – there is one macronucleus, which contains the genes for all the proteins involved in being a Paramecium on a day-to-day basis, and one or more micronuclei, which contain the genes involved in reproduction and essentially represent its “germ line.” Holospora undulata bacteria live in the micronuclei of their hosts. (A different species, Holospora obtusa, lives in the macronucleus.) These bacteria exist in two different forms – short rods, which are the reproductive forms, busily dividing in the nucleus, and a longer infectious form, which leave the host cell and enter the environment. Paramecia become infected when they ingest H. undulata along with the other bacteria that they’re munching up. The Holospora escape from the phagosome and move to the nuclear membrane where they bind to receptors and then enter inside. Because these bacteria infect one of biology’s favorite lab organisms, not surprisingly some very nice research on infectious disease and the ecology of hosts and parasites have been done using this system as a model.

Photo from the Encyclopedia of Life.

February 7 - Legionella pneumophila

In 1976, the nation celebrated the American bicentennial. One event, a convention of members of the American Legion in Philadelphia, had tragic results that led to the discovery of a new pathogenic bacterium. In the weeks following the convention, over 200 people became ill and 34 of them died, triggering public health officials to track down the unknown culprit. The guilty party was Legionella pneumophila, a Gram-negative, rod-shaped bacterium that is both an intracellular parasite of protists (like the ciliate shown here - the long red bits that look like yarn are chains of bacteria inside these cells), and also free-living in the environment, particularly warm, wet places like air-conditioners at convention centers and saunas. People acquire the bacteria when they breathe in vapor containing them, but the disease cannot be spread from person to person. For some, particularly elderly people, people with weak immune systems, and smokers, the bacteria can present a very serious health problem in the form of pneumonia. For others, a milder infection produces what has come to be called "Pontiac fever", named for a similar outbreak that occurred (in all places, amongst county public health department employees) in Pontiac, Michigan eight years before the Legionnaire's convention of 1976. At that point, they could not find the pathogen, but were later able to tie those cases to Legionella pneumophila.

Image from the CDC Public Health Image Library.

January 27 - Borrelia burgdorferi

Blame Borrelia burgdorferi for one of the most common vector-borne diseases in the United States: Lyme disease. Discovered in 1982 by NIAID zoologist Willy Burgdorfer, this species belongs to a phylum of corkscrew-shaped bacteria known as spirochetes. Spirochetes are quite at home in the guts of humans and other mammals, bivalves, and insects. In the United States, black-legged ticks (also known as deer ticks) ingest Borrelia burgdorferi from an infected animal during a blood meal. When the ticks bite other suitable hosts, they pass along the spirochete through their saliva.

Learn more about wildlife hosts of B. burgdorferi in this new Science Bulletins video from the American Museum of Natural History. A second video highlights new imaging techniques that can detect B. burgdorferi's influences on the human brain.

Contributed by Laura Allen at Science Bulletins, AMNH

Image by Jeffrey Nelson, North Park University

January 12 - Wolbachia pipientis

Talk about manipulative little buggers - Wolbachia pipientis are alpha proteobacteria that infect a very wide variety of insects and other arthropods. Like mitochondria, they are vertically inherited from mothers to offspring. But, they get very cranky if they don't get their way. In some cases, they kill off infected males, in some they feminize males, in some they allow females to reproduce parthenogenically, and then there are several documented cases of more complicated interactions in the form of cytoplasmic incompatibilities. Some are now investigating their potential use in biocontrol of malaria vectors and other insect pests, taking advantage of these manipulative tendencies.

Thanks to Mike Charleston for this nomination.