Allies and Enemies: How the World Depends on Bacteria (7 page)

For carbon to make a similar cycle through the Earth’s organic and inorganic matter, the bacteria of decay must help decompose the

planet’s fallen trees, plants, and animals. The common soil inhabitant

Bacillus
breaks down proteins, fats, and carbohydrates by excreting the enzymes protease, lipase, and amylase, respectively. Thousands of other species break down organic matter in similar ways. For example,
Cellulomonas
bacteria produce the enzyme cellulase—rare for bacteria—that digests plant cellulose fibers. Bacteria emit carbon dioxide as an end product, which enters the atmosphere. A massive

population of photosynthetic bacteria in the Earth’s surface waters then captures this gas and inserts the carbon into a new food chain of

bacterial cells, protozoa, invertebrates, and so on until the carbon ends up in tuna sashimi on a restaurant menu.

If clouds begin to form while a person lunches on sashimi, bacteria have a part in that, too. Photosynthetic marine bacteria and algae

produce dimethyl sulfide gas as a waste product of their normal metabolism; they emit 50 million tons annually. When the gas rises

and enters the atmosphere, it chemically rearranges into sulfate, which attracts water vapor. The vapor turns to droplets and forms clouds. On a global scale clouds inhibit the photosynthetic bacteria and less dimethyl sulfide forms. When the clouds thin, the cycle begins again.

Albert Kluyver of the Technical School of Delft—the town where

van Leeuwenhoek discovered bacteria in 1677—praised the wonderful “unity and diversity” of microorganisms, a perfect description for

34

allies and enemies

dissimilar organisms that share more than 95 percent of their genes.

The human body possesses its own unity and diversity of microbes that in most situations keep the body’s metabolism working at its best. Pathogens more than good bacteria gain the attention of researchers and doctors. For this reason, epidemics have expanded our knowledge of bacteria. Many of the discoveries in microbiology

came about from a blend of genius and serendipity, a fair description

of all science.

2

Bacteria in history

Other than infectious disease, humanity’s early dealings with bacteria

involved mainly the production of foods. Wheaton College biologist

Betsey Dexter Dyer once noted that a meal can be assembled completely from bacteria-produced foods, such as the following items.

·
Cheeses
—Swiss from
Propionibacterium
and limburger from Brevibacterium ·
Olives
—Leuconostoc
, Lactobacillus , and Pediococcus ·
Dry sausages — Pediococcus ·
Sourdough bread
—Various lactic acid-producing bacteria

·
Butter
— Lactobacillus

·
Cottage cheese
— Streptococcus

A steak or a glass of milk results from the digestion of grasses by

anaerobic bacteria in the rumen of cattle. The rumen fermentations

are oxygen-free conversions of sugar into microbial energy with acid

or alcohol as a by-product.

Olives may be the oldest food fermented specifically to make a

new food. The Phoenicians brought olives throughout the Greek isles

by 1600 BCE. The production of acids in the fermentation process

helped preserve the product during long sea voyages. No one knows

who made this discovery, but food historians assume that fermented

foods were discovered by accident or perhaps by necessity by explorers who had already eaten all other food supplies.

Bacterial food spoilage takes the form of acid production, protein

curdling, gas or toxin production, or decomposition. The latter two cause foodborne illness or a loss of food’s nutritional value, respectively, 35

36

allies and enemies

and render the food unusable. Properly controlled acid production, however, preserves fresh vegetables, fruits, and juices and retains most nutrients, while protein curdling does the same in dairy products.

Evidence of winemaking from alcohol-producing bacteria dates

to 6000 BCE Mesopotamia and no doubt started earlier. Over the next two millennia, Hebrew, Chinese, and Inca cultures perfected yeast fermentations for wines and beers, but retained bacteria for fermenting crops to make sauerkraut, pickles, wine, soy sauce, silage, and other foods that lasted longer with an acid preservative than in the fresh form. The names of brave souls who tasted spoiled foods have been lost to history, but either by necessity or a sense of adven—ture, they invented food preservation.

Bacteria-made dairy products date to before 3000 BCE using milk

from cows, yaks, goats, sheep, horses, camels, and even reindeer. Fermented milk products, the “mere white curd of ass’s milk” as described by 18th century poet Alexander Pope likely originated in more than

one place. Traders used pouches made of cleaned animal entrails for

carrying milk between villages and would not have realized that the

stomach enzyme rennin (also called chymosin) remained active in the

pouch lining. This enzyme helps nursing infants digest milk by curdling the milk proteins and thus slowing their passage through the digestive tract. In a pouch slung over a horse’s rump, the rennin made cheese.

Lactic acid-producing Lactobacillus, Lactococcus, Streptococcus, and Leuconostoc make up the main bacteria used in cheeses, yogurt, butter, buttermilk, and sour cream today as they did centuries ago.

Manufacturers of salad dressings, coleslaw mixes, and mayonnaise now encourage the growth of lactic acid bacteria to produce an acidic tangy flavor and preserve the food.

When bacterial contaminants did not produce a tasty, edible

product, the ancients froze, smoked, or dried the food or added salt,

sugar, or honey. These preservation methods inhibit bacteria’s growth

by making water molecules unavailable for cellular reactions. Food producers still use these ancient methods, but they now also use chemicals to inhibit the growth of microbes in food.

Bacteria have ploys for escaping physical injury from lack of

water or harm from chemicals. Many bacteria enter a state of

chapter 2 · bacteria in history

37

dormancy when water becomes scarce and grow again when

water returns to their environment. The normal soil inhabitants Clostridium
and
Bacillus have evolved an adaptation that protects better than dormancy: the formation of endospores. More than any other type of cell in biology, endospores resist freezing, heating, boiling, chemicals, and irradiation. A microbiologist need only dilute a small amount of soil in nutrient broth and then incubate it to make the endospores germinate into actively growing cells. (Sometimes stubborn endospores need to be heat-shocked at 130°F for five minutes before they will germinate.) In 1993, American microbiologists Raúl Cano and Monica

Borucki found endospores resembling
Bacillus sphaericus
in an extinct bee that had been preserved in amber estimated at 25 to 40

million years old. As is customary in science when radically new discoveries are made, skeptics came forward suggesting the bacteria were contaminants from a later period. The critics charged that no living organism can survive that long. But in 2000, biologist Russell Vreeland found
Bacillus
endospores buried in 250-million-year-old

salt deposits and showed they remained viable by growing the cells in

his laboratory. Vreeland and his team then completed 16S rRNA

analysis on the microbe and identified it as an ancestor of modern Bacillus. Perhaps expecting the same skepticism Cano had met with, Vreeland also calculated the chances of a contaminant invading the sterilized equipment or breeching his aseptic techniques at one in one billion. Assuming these bacteria are not contaminants, research

like this demonstrates the astonishing durability of bacterial endospores and also hints at the challenges of protecting food from spore-forming pathogens.

The ancients

Paleopathology is the investigation of ancient artifacts for clues on history’s diseases (see Figure 2.1). Paleopathologists use fiber optics, X-ray imaging, and computerized tomography to see inside caskets without disturbing the contents. Only when they find evidence of damaged tissue do they open the casket and salvage DNA from a bit of tissue, bone, or tooth pulp. By comparing the ancient DNA with

38

allies and enemies

that of present-day pathogens, scientists have identified the main bacterial diseases that have haunted society for millennia: anthrax, bubonic plague (
Yersinia pestis ), cholera (
Vibrio cholera ), diphtheria (
Corynebacterium diphtheria ), leprosy (
Mycobacterium leprae ), syphilis (
Treponema pallidum ), tuberculosis (TB) (
M. tuberculosis ), and typhoid fever (
Salmonella typhi ). Facts gleaned from ancient writings have supplemented the technology of paleopathology. Pliny the Younger wrote of Roman society from 79 to 109 CE and in one

essay described an illness affecting a close friend:

She has continued fever, her cough gets worse day by day, she

is very thin and weak. Still she is mentally alert, and her spirit

does not flag, a spirit worthy of her husband Helvidius....In

everything else she is failing to such an extent that I not only

fear but grieve.

Figure 2.1 Leprosy.
Mycobacterium leprae
preferentially attacks the cooler extremities of the body, mainly skin and peripheral nerves. The disease erodes the skeleton, such as these feet dated to a leprosy sufferer from c. 1350.

(Courtesy of Science and Society Picture Library, Science Museum, London) The mention of coughing and weakness without reference to

fever or delirium suggested to medical historians that Pliny wrote of

tuberculosis. Studies on the emergence of diseases have been aided

by the knowledge that cancer and heart disease were rare in antiquity;

most deaths from disease can be attributed to infectious diseases.

Some people sensed that hygiene affected quality of life 1,000

years before microbiologists connected bacteria with disease.

Mesopotamia’s Sargon I decreed the construction of privies for the

chapter 2 · bacteria in history

39

ruling class in the 3rd century BCE, and the Greeks and Egyptians

devised similar toiletlike receptacles to protect drinking water and food from human waste. The Roman Empire’s largest cities established a model for sanitation infrastructure with freshwater aque-ducts, public baths, and sewers for the wealthy. (Rome’s poor endured squalid conditions that led to chronic infections and short lives.) Romans sprinkled spices and herb oils into bath waters for fra—grance. These substances are now known to kill bacteria when used in

low concentrations.

Hygiene practices changed when the Roman Empire declined.

The Roman Catholic Church took a bigger role in influencing public

opinion as well as science, teaching that disease came from God as

punishment for evil; some present-day clergy continue to embrace this belief. Human behavior certainly influences disease transmission, but evil has nothing to do with it.

The legacy of bacterial pathogens

During World War II, scientists in Germany and Great Britain raced

to find a “magic bullet.” They sought not a weapon but a drug to stop