Image: M.J. Grimson & R.L. Blanton

The single-celled amoeba Dictyostelium discoideum has no brain, but its complicated social cycle enables farming.

Amoeba, single-cell, shape-shifters that eat bacteria and live in the dirt, don’t get much respect. When they run out of food, they gang up and move their sorry selves to greener pastures.

Pastures with edible bacteria, that is.

If ever a creature needed re-branding, this is it.

Could labeling amoeba as farmers boost their brand? In the human realm, farming gave rise to cities, writing, metallurgy and the computer in front of your face.

Amoeba don’t use the Internet. And although they do have a cell nucleus, nobody claims they have an ounce of smarts.

But now we know that some amoeba move “seeds” of bacteria to a new location and plant them as a food source. In other words, they farm.

Ants grow fungus. Termites and some saltwater snails do ditto. Damselfish grow algae. But until now, nobody has identified any life form that “farms” bacteria, and nobody has identified any single-celled farmers, says Debra Brock, a graduate student in ecology and evolutionary biology at Rice University.

Adds Brock, whose report on farming amoeba appears in Nature tomorrow, “Certainly there has never been an amoeba that’s known to farm.”

Bring on the rebranding!

Working with the well-studied amoeba Dictyostelium discoideum (“dicty” to you and me) Brock noticed that the fruiting bodies — reproductive structures that distribute the amoeba in new habitat — seemed to contain bacteria. That was odd, Brock admits. “To get anybody to believe me, I had to prove that the little spots were bacteria, and not an infection.”

When she spotted the sorus (mass of spores) on growth medium, colonies of bacteria grew on some of the plates — showing that about one dicty in three transports bacteria. The bacteria didn’t seem to be a harmful infection, since amoebas with and without bacteria grew similarly, she says.

She fed the shape-shifters antibiotic to kill their bacterial cargo, but when the amoebas resumed eating bacteria, some bacteria showed up in the sorus. Since this only happened with amoebas that had originally carried bacteria, Brock concluded that this was normal, healthy behavior for those amoeba, although she’s can’t yet say whether the bacteria are inside or alongside the amoeba spores.

Photo: Scott Solomon

Fruiting bodies of the amoeba Dictyostelium discoideum contain bacteria and spores of amoebas. Each sorus is attached to a single slug, comprised of about 100,000 individual amoebas.

Wild about amoeba

The project began when Brock was studying wild amoeba rather than a strain that had been living in labs since the 1930s, and she noticed that some clones consistently carried bacteria.

Brock says dictys are “social amoeba” because “they have a structured society, and can exist in two states.” Individual amoebas in the soil eat bacteria, divide and eat some more. So long as edible bacteria are available, “they are perfectly happy to do this,” says Brock. “But if they use up all the food, they start talking to each other with chemical signals: ‘Wow! There’s not enough food!’ And then approximately 100,000 come together to form a slug.”

The slug serves as a truck to haul amoeba to new territory, Brock says. “During the multi-cellular part of the life cycle, they are starving, and they want to go somewhere else.”

The slug eventually shoots up a stalk containing amoeba spores, and among the farmers, bacteria. When the sorus opens, the bacteria can plant themselves as amoeba food.

Reminds us of Johnny Appleseed…

The Darwinian decision

Why does the same species of dicty use two survival strategies? Why do some farm while others don’t? “It’s a smart evolutionary strategy,” says Brock. “It’s bet-hedging. If you happen to land in a patch without bacteria, farmers have a great advantage because they bring their food with them, which allows them to grow and divide and bear a huge number of progeny while the poor non-farmers have nothing to eat.”

But while the farmers quit eating before they remove all bacteria from their old location, non-farmers can eat all those bacteria, so non-farmers do benefit if the new home already contains edible bacteria.

Apparently, both strategies work, because both have survived the evolutionary gauntlet. Brock is exploring whether a “farmer gene” causes some amoeba to hoard bacteria…

It’s enough to give a person a new respect for protozoans, which offers a firm basis for rebranding. “From quite a long time ago, we’ve thought we are so special,” says Brock, “but you can’t imagine the number of genes the amoeba has that are just like human genes. It’s scary; it takes you down a notch or two.”

– David J. Tenenbaum

Even people who can’t distinguish the periodic table from a dining table know arsenic is poisonous, although few realize why. Arsenic is chemically akin to phosphorus, one of life’s essential elements. But it’s not identical, and when arsenic substitutes for phosphorus, it produce a toxic compound instead of a protein or chunk of DNA.

So we weren’t the only ones to be surprised by a study in today’s Science that identifies a bacterium that thrives on arsenic, at least in the lab, and incorporates this normally-poisonous element into proteins, fats and DNA.

Image: Wikipedia

A more typical reaction to arsenic comes from the elderly poisoning victims in the macabre comedy “Arsenic and Old Lace.” In that play and movie, two dotty spinsters spiked elderberry wine with arsenic, strychnine and cyanide for a freelance euthanasia project.

The new study is the first to show that it is possible to substitute for one of the elite elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur) that were thought to be found in all life, says Ariel Anbar, a professor of earth and space exploration at Arizona State University. “No one has previously shown that arsenic can be substituted, and I am not aware that anyone has found a substitution for any of the six essential elements. And that’s why this is a big deal.”

Arsenic: It’s what’s for dinner

Felisa Wolfe-Simon, a former post-doctoral fellow with Anbar, gathered sediment and water from salty, alkaline, arsenic-rich Mono Lake in California and placed them in cultures intended to replicate Mono Lake water.

Images courtesy of Science/AAAS

These bacteria, viewed under an electron microscope, metabolized arsenic as if it were phosphorus. Mouseover to see the same strain of bacteria growing with phosphorus but without arsenic.

“Over time we made serial dilutions, one in 10, one in 10,” always including a strain of lake microbes, says Wolfe-Simon.

Wolfe-Simon, who is now at the NASA Astrobiology Institute and U.S. Geological Survey, says the dilutions removed “essentially all” of the phosphorus. In some samples, she jacked up the arsenic roughly 2,000 times above the concentration in Mono Lake, which gets its arsenic from rocks and is already about 20,000 times above the Environmental Protection Agency standard for arsenic in drinking water.

“This is a huge amount of arsenic,” Wolfe-Simon says. “It’s surprising that they could grow, even with phosphorus, in that condition.”

Although the bacteria died in the absence of phosphorus and arsenic, they survived if only arsenic was available. “The arsenic seems to be substituting for phosphorus,” says Wolfe-Simon. “We have identified arsenic in cellular structures that are consistent with where we would expect to see phosphorus.”

Photo: ©2010 Henry Bortman

Mono Lake, California, is salty, alkaline conditions, and toxic to many organisms. The lake is ideal for the study of extremophiles, microbes that live under bizarre temperature or chemistry.

Stepping out of line

This elemental swaperoo could operate more broadly, since the elements in each column of the periodic table have chemical similarities. If one neighbor of phosphorus can sustain life without phosphorus, could the elements below carbon, nitrogen or oxygen do the same?

Photo: ©2010 Henry Bortman

“This is not just about arsenic or Mono Lake,” says Wolfe-Simon. Life on Earth and the rest of the universe will be limited if it always requires six elements, but “If microbes can use arsenic as they can use phosphorus, that opens the door. What else can life do that is not yet known?”

In searching for life in the universe, NASA has focused on liquid water, another prerequisite for known life, but Anbar asserts that a search for the chemistry of life should stay broad. “Felisa’s results say we should think harder about which elements we should follow. We don’t want to be too influenced by the particular example of life on Earth. We want to push the boundaries.”

Call it “Arsenic in a new place.” Roll cameras!

In Haiti, the body blows just keep coming. About 200,000 died in the January earthquake. Then the recovery was hampered by poverty, an ineffective and corrupt government, and a long tradition of class antagonism and social chaos.

And now Haiti is stricken by a cholera epidemic that has already killed about 1,300.

Cholera is a fast-moving bacterial disease that causes intense diarrhea and can kill within hours. Despite efforts to contain it, Haiti’s epidemic is spreading from its epicenter north of Port au Prince, the capital, and has reached the vast tent cities that still house earthquake survivors.

In cholera, the Vibrio cholerae bacterium multiplies in the intestines, forcing the patient to release vast quantities of highly infectious watery stool. Lacking proper disposal and treatment, the diarrhea can pollute drinking water and start new infections.
Cholera is vanishingly scarce in the developed world, and cholera thrives on poverty, disorganization and under-development.

Photo: PAHO

Drinking water contaminated with the cholera bacterium is the major cause of new infections. The Artibonite river, a source of drinking water for many Haitians, is suspected to be transmitting the cholera epidemic.

Haiti, where cholera had not been seen for a century, has been rocked by controversy about the source of the bacterium. Some angry Haitians blame United Nations peacekeeping troops for bringing it from Nepal, but at this point, treating patients and providing clean drinking water seems more pressing than doing genetic forensics to track the disease to its origin.

From the viewpoint of V. cholerae, chaotic, post-earthquake Haiti may be paradise, but outbreaks have also occurred in Latin America, Africa and India in recent years. The World Health Organization estimates that cholera annually infects three to five million people and kills 100,000 to 120,000.

Prompt treatment with electrolytes dissolved in clean water can prevent death in 99 percent of cases.

A violent announcement

Cholera announces itself with a sudden, violent outbreak of diarrhea – a “rice-water stool” named for its semblance of water used to cook rice. Diarrhea — and sometimes vomiting — can cause massive water loss and electrolyte imbalance. Muscles cramp and eyes recede into the skull.

Falling blood pressure and oxygen starvation cause a state of shock that can kill within minutes. A graphic description of cholera is mortifying: “A mid-nineteenth-century English newspaper report described cholera victims who were ‘one minute warm, palpitating, human organisms-the next a sort of galvanized corpse, with icy breath, stopped pulse, and blood congealed-blue, shriveled up, convulsed.’”

Saint Nicolas Hospital, St. Marc, Haiti; PAHO

Haiti’s hospitals, like this one north of Port au Prince, are being tested by the cholera outbreak, but most patients can be treated at an early stage with oral rehydration salts.

An incubation period as short as two hours is one reason for cholera’s dreadful reputation, but its efficient spread through contaminated water is another. As Haiti demonstrates, the conditions of poverty, filth and social chaos that help spread cholera also hinder prevention and treatment efforts.

At present, health organizations in Haiti are focusing on sanitation, clean water, hand washing, and other tactics to interrupt the chain of infection. Treatment is taking place in dedicated wards.

To restore the body’s electrolyte balance, patients with moderate to severe diarrhea need treatment with an oral rehydration mixture — essentially a medical-grade sports drink containing sodium and glucose dissolved in clean water. Treatment is simple and many patients need no hospitalization if treated promptly.

In severe cases, antibiotics are used to kill V. cholerae, although the main benefit is often a faster return to health and a reduction in the load of bacteria released in the feces.

Photo: PAHO

A patient in Saint Nicolas Hospital, St. Marc, Haiti, during the cholera outbreak. That hole in the bed accommodates the violent diarrhea that is cholera’s trademark.

Very versatile vermin

The cholera bacterium, like any self-respecting microbe, has evolved genetic tricks for optimizing its survival in changing circumstances. Once it passes through the human mouth, V. cholerae:

Photo: Dartmouth Electron Microscope Facility

The cholera culprit Vibrio cholerae infects its host quickly and spreads easily via diarrhea.

Cholera’s big gifts

Like an execution in the morning, fast-spreading cholera has served to concentrate the medical mind. Cholera was first seen for sure in 1817 in India; the disease then traveled with people and their commerce around the world and eventually gave humanity two durable gifts.

The first gift came when a mid-19th-century outbreak of cholera in London spawned the science of epidemiology — the study of epidemics. The story is often told of how, in 1854, physician John Snow marked where cholera cases lived, and realized that they all had gotten water from the same pump.

Even though the germ theory of disease was yet nascent, authorities removed the handle from the pump and the epidemic subsided. Although that removal is credited with ending the epidemic, it may have already been waning.

Images: Map; Snow: UCLA Department of Epidemiology

This map, drawn by Dr. John Snow (1813-1858), correlated London cholera cases (each marked by a dash) with drinking water from the Broad St. pump. Mouseover image for a photo of Snow, the founder of epidemiology.

Photo: Madison Metropolitan Sewerage District

Sewage treatment is essential for dozens of reasons, but many countries cannot afford expensive treatment systems.

Snow’s achievement is especially awesome considering that the bacteria that causes cholera would not be identified until 1883, by the great German microbiologist Robert Koch.
By correlating a disease with foul water, Snow showed that epidemics could be understood by analyzing the timing and location of the illnesses — two rudiments of epidemiology. And that led to a second gift: As epidemiologists realized that drinking feces was dangerous, not just disgusting, the health-giving revolution of sanitation got under way.

Virtuous vaccines?

Antibiotics kill cholera bacteria. But carpet-bombing with antibiotics (“mass chemoprophylaxis” in medico-lingo) is inadvisable because it stimulates bacteria to resist the drugs.

Vaccines must be given before an epidemic gets under way, and thus are most suitable in regions where cholera is endemic, like South Asia. But oral cholera vaccines are showing progress:

Breaking the chain

Infections are contained by interrupting the chain of infection; and no fundamental scientific or social hurdles prevent this from being done with cholera. Unlike HIV, cholera is not spread by sexual contact. Unlike tuberculosis or influenza, it is not spread by coughing.
Instead, cholera prevention requires attention to boring, even repulsive, topics like safe drinking water and sewage treatment. Granted, the technology can be expensive, but water and sanitation are also the primary defense against microbes, viruses and parasites that cause dozens of other waterborne diseases.

The United Nations’s Millennium Development Goals aim to raise the proportion of people getting clean water and adequate sanitation, but Unicef says progress is mixed: “Two and half billion people are still without access to improved sanitation – including 1.2 billion who have no facilities at all and are forced to engage in the hazardous and demeaning practice of open defecation. The news is better for water: the number of people without an improved source has dropped below one billion for the first time in history.”

Map: Unicef

Improved sanitation parallels national wealth. In Latin America and the Caribbean, 117 million people live without adequate sanitation.

In 2010, 884 million people have no access to “improved” drinking water, including 330 million in Sub-Saharan Africa, 222 million in Southern Asia and 151 million in Eastern Asia.
India and China account for the lion’s share of progress in both water and sanitation. Globally, city folks usually score higher in these basic barometers of human development.
So do rich people.

In terms of public health, clean water, clean air and sanitation are the big three environmental goals. By themselves, diarrhea diseases cause 4 percent of all time lost to illness, when measured by disability-adjusted life years.
The cholera question is scientifically straightforward, and is quickly solved when resources and social organization are available. Yet even if the victims of cholera are poor and powerless, the benefits of clean water and sanitation are so manifold that it’s hard to accept that these basic requisites for health are not for everybody.

But as the population soars, as people continue flooding into shantytowns around megacities, and as income inequality remains a fact of life, we anticipate this is not the last article you’ll read about such an avoidable epidemic.