Allies and Enemies: How the World Depends on Bacteria (17 page)

bacterium that exists only on sunlight for energy, carbon dioxide for

carbon, and very few other nutrients. In the evolution of life on Earth, photoautotrophs generated the first traces of oxygen in the atmosphere. Other photosynthetic bacteria followed, and they added more oxygen to the atmosphere, paving the way for the evolution of invertebrates, fish, mammals, and all other oxygen-requiring organisms.

Rubber-eating bacteria are not unusual. At least 100 different

rubber-degrading bacteria have been identified, and many more

unidentified strains exist. Both bacteria and fungi degrade the five—

carbon, eight-hydrogen isoprene units of natural rubber, such as the

rubber in latex gloves. In 2008, Mohit Gupta of Drexel University College of Medicine made the disturbing discovery of a Gordonia

polyisoprenivorans-caused pneumonia in a hospital patient. This bacterium normally grows in the stagnant water inside discarded tires, slowly eating away at the hard, black rubber. Perhaps the massive mountains of refuse tires throughout the United States will inspire a new science fiction thriller.

The scientists in The Andromeda Strain never found their magic bullet against the invader. The pathogen disappeared as many do by

mutating to a less virulent form and destroying too many of its hosts.

The book’s outcome should sound familiar: Medicine never defeated

the bubonic plague because it defeated itself.

Do bacteria devour art?

Who would guess that studying ancient artwork offers an excellent opportunity to learn about bacterial metabolism? Bacteria degrade art and historic treasures in the same way they decompose organic matter. Bacterial enzymes lipase and protease break down fats and peptides, respectively, in pigments and a variety of carbohydrate and fiber-degrading enzymes attack the canvas and wood. These are the

same enzymes animals use for digesting food. Different, more

92

allies and enemies

specialized bacteria get energy from chemical reactions involving inorganic salts. All of these microbial actions contribute to the slow decomposition of the world’s greatest works of art, principally because of bacteria acting in concert within a community (see Figure 4.1).

Figure 4.1 Biofilm bugs. (Courtesy of Center for Biofilm Engineering, Montana State University)

The decomposition of artwork is one small piece of bacterial

cycling of nutrients on Earth. Bacteria circulate the Earth’s elements

through the atmosphere, water, soil, and plant and animal life in processes called nutrient or biogeochemical cycles. These cycles take place in the seas, forests, and mountains. Nutrient cycling also occurs when bacteria decompose rubber, plastic water bottles, paint, and countless manmade items that were once believed to be indestructible. Bacteria corrode metal, stone, marble, and concrete, and they degrade paint, paper, canvas, leather, pigments, and wood. Chemical

chapter 4 · bacteria in popular culture

93

reactions driven by bacteria weaken modern infrastructure such as bridges, roadways, and oil tankers. In exactly the same way, bacteria have been steadily digesting the components found in art, whether these are made of metal, fiber, hide, or pigments.

Copper is one of the oldest metals used in civilization. In the Bronze Age (3000-1300 BCE) craftsmen took advantage of the

metal’s malleability to incorporate it into the alloys brass and bronze for tools, weapons, domestic items (bowls, plates, goblets, and so on), and jewelry. Microbiologists have learned just in the past several years that sulfate-reducing bacteria have been corroding bronzes such as Etruscan relics dating to the ninth century BCE. When a bacterium is said to “reduce” an element, it means a bacterial enzyme adds electrons to the element. In metal corrosion, sulfate-reducing bacteria convert sulfate, a sulfur atom with oxygens attached, to the element sulfur. When a biofilm forms on relics containing iron, the

anaerobic bacteria at the base of the film convert sulfur to pyrite, an iron atom attached to two sulfurs. Figure 4.2 illustrates the substantial biofilm growth that metal structures can support.

Figure 4.2 Biofilm corrosion. This pipe has been almost completely occluded by biofilm, which has dried and hardened. Few technologies exist for removing biofilm from living or nonliving surfaces. (Courtesy of Center for Biofilm Engineering, Montana State University) 94

allies and enemies

Sulfate-reducing Desulfovibrio and the iron-oxidizing Leptothrix work in concert to corrode iron; they are sometimes nicknamed

“iron-eating” bacteria. Leptothrix takes electrons away from iron atoms at the metal’s surface, and Desulfovibrio, hovering a few
ì m nearby, accepts the excess electrons. Even though iron corrodes when exposed to the air, the biofilm actually creates tiny anaerobic nooks called microenvironments in which these reactions take place.

Sergei Winogradsky discovered the general steps in iron-sulfur

metabolism between 1885 and 1889.

Bacterial deterioration of metal takes place every day 12,850 feet

at the bottom of the Atlantic. The H.M.S. Titanic has withstood the incredible hydrostatic pressures exerted on it for a century. The oxygenless deep waters have also allowed the ship to resist rusting. Yet the Titanic supports a ghostly collection of rusticles. These appendages from several inches to a few feet long and numbering in

the thousands hang from almost every part of the ship. Some are as

fragile as tissue; others hold their shape when research vessels pull

them to the surface. The rusticles demonstrate that the main cause

for the Titanic’s inexorable return to the Earth is bacteria.

The rusticles contain a mixture of bacteria able to thrive in the

cold and deep where the Titanic’s wreck settled on April 15, 1912.

The bacteria remove 0.3 gram of iron from every square centimeter

of the ship daily. The loss of iron causes about 300 kilograms of steel to detach from the wreck each day. Anaerobic “iron-eating” bacteria have been taking apart the Titanic one iron atom at a time and may cause the hull to cave in on itself within 100 years, perhaps as little as 40 years from now. The ship’s organic material, mostly wood paneling and fixtures, serve as the main nutrient source for the bacteria, but as the metals corrode more organic matter may be exposed. As a result, the deterioration of the Titanic will accelerate.

Stone and concrete undergo similar aboveground weathering.

The corrosion of ancient Greek and Roman stone statues illustrate bacteria’s role in the slowest of all biogeochemical cycles, the rock cycle or sediment cycle. Unlike nitrogen, which can migrate from soil to the atmosphere and return to plants within a day, the rock cycle

takes eons to complete. Bacteria begin by weakening the stone

through corrosion. Small pieces break off from the mass and make their way downhill with erosion. Erosion carries the matter to bodies

chapter 4 · bacteria in popular culture

95

of water where it sinks and becomes part of sediment. Sediments, especially under the ocean, compact under intense pressure. As tec—tonic plates shift, this sediment becomes part of metamorphic rock in

the Earth’s mantle and slowly pushes upward to the planet’s surface.

Some metamorphic rock sinks into the Earth’s interior where the planet’s molten core heats the sediment and turns it into magma.

Magma rushes to the surface all at once in volcanoes. New rock that

has either migrated up to the Earth’s surface gradually or exploded

toward the surface from a volcano becomes available for bacteria to

again begin degrading.

Molecular methods have now been applied to studies of how bacteria affect not only rock, but also prehistoric paintings in moist caves.

The 20,000-year-old cave paintings in Altamira, Spain, and Lascaux,

France, may be succumbing to the combined activities of bacteria that degrade the dyes as well as the underlying stone. Many of the offending bacteria have not yet been identified, but microbiologists

have noticed that Actinobacteria often dominate the microbial populations in the Spanish and French caves. Actinobacteria build tenta-cles called filaments that grow into the pores of rock surfaces, allowing the bacterial damage to occur in the rock’s subsurface. Molecular analyses of the cave paintings have also uncovered aerobes ( Pseudomonas) and anaerobes ( Thiovulum), sulfate- ( Desulfovibrio) and iron-users ( Shewanella), bacteria that use a wide variety of nutrients ( Clostridium), and species with narrow nutrient requirements ( Thiobacillus). Lascaux’s 600 paintings made of a mixture of mineral pigments and animal fat offers a banquet for the bacteria of the caves.

Art galleries have almost as difficult a time protecting treasures from bacterial decay, despite humidity and temperature-controlled environments. Fungi and the funguslike bacterium Actinomyces extend filaments into paintings’ surfaces to cause physical destruction.

Other microbes chemically decompose the pigments. Polymerase

chain reaction (PCR) technology, invented in the 1980s, has enabled

microbiologists to multiply bits of bacterial DNA recovered from painted frescoes on castles in Austria, Germany, and France to study the types and proportions of the bacteria. The analyses have so far revealed the presence of Clostridium, Frankia, and Halomonas on the ceiling painting in Castle Herberstein in Styria, Austria. Each genus contributes its own mode of destruction on the painting:

96

allies and enemies

·
Clostridium
—A spore-forming anaerobe that grows well on a wide variety of chemicals ·
Frankia
—A spore-forming genus that builds long, branching filaments that penetrate surfaces ·
Halomonas
—A versatile halophile that lives with or without oxygen and can degrade alcohols, acids, and organic solvents The constant streams of tourists who visit the world’s great works

of art accelerate decay. Human bodies and breath change the temperature and humidity in galleries and even in caves containing ancient wall paintings. The Lascaux caves had been in good condition when discovered in 1940. The rapid decay of the cave paintings that

took place after people started visiting then led to the closure of Lascaux in 1965 to prevent further deterioration.

On stone exposed to the weather, biofilms and cyanobacteria

each contribute in their own way to the deterioration of statues, buildings, and headstones. In some instances, bacterial growth on historical structures presents no more than a cosmetic problem due to

the discoloration of stone by bacterial pigments. In other cases, acids produced by bacteria in biofilms degrade the stone’s calcium carbonate as they degrade tooth enamel in dental caries. The actions of fungi, biofilm bacteria, and free bacteria with their different types of metabolism hasten the decomposition of historical structures and have already decimated most concrete structures, not only from ancient periods but since the 20th century.

Lichens form greenish black stains on old stone structures.