betsey's bacteria
The professor of biology publishes the tell-all text on her decades-old love affair with bacteria, which surround us in strange and wonderful ways.
By Betsey Dyer '75
I love studying and thinking about bacteria now, but this was not always the case.
When I was growing up, I was a serious amateur naturalist. Insects were my favorite organisms and I planned to become an entomologist. When I was 11, I got a microscope for Christmas and became quite enthusiastic about the microbial world, but not about bacteria. Many bacteria were below the limits of easy detection with my microscope and, even if I had seen them, I suspect that the tiny dots and dashes-the typical forms of bacteria-would not have held my interest. At Wheaton, my senior research project with John Kricher involved microscopy, but not of bacteria, which I avoided. Bacteria were too nondescript and I did not understand them. Likewise, I managed to avoid taking microbiology at Wheaton. The test tubes and plates of invisible organisms did not appeal to me at that time.
That changed for me in the summer of 1980, when I was fortunate to be on a field trip with Professor Stjepko Golubic of Boston University to the microbial mat The professor of biology publishes the tell-all text on her decades-old love affair with bacteria, which surround us in strange and wonderful ways. By Betsey Dexter Dyer '75 communities in Baja California. Stjepko had a wonderful little handheld microscope with which we looked at samples in the field. He explained all about the macroscopic evidence for bacteria-their nonmicroscopic field marks: felt-like blue green bacterial mats, pink layers of scum, black sediments, red-tinted salt crystals and bubbles rising up from murky water. Everywhere was a sulfur smell, an indication of a healthy microbial community in shallow, stagnant, salty waters. It occurred to me right then that a field guide to the macroscopic characteristics of bacteria would be possible.
On the following pages you'll find excerpts from that text I imagined more than 20 years ago, A Field Guide to Bacteria (Ithaca: Cornell University Press, 2003). It's aimed toward amateur naturalists who may or may not have access to microscopes, as well as biology teachers and professional biologists who might appreciate the accessibility of these otherwise obscure organisms. It's an untraditional approach to the study of nature's most-populous and least-visible living things.
Bacteria: Gram-positives such as Propionibacterium, Brevibacterium, Lactococcus, Lactobacillus and Leuconostoc
Common Object: Cheeses
Background: The first humans to look at a crock of coagulating spoiled milk were surely hungry as they took their initial tastes. It was fortunate for all of us that over the ages many humans from different cultures were adventurous enough to sample all sorts of dairy products that had "gone by." And we are many times fortunate that the French, in particular, experimented hundreds of times, producing some of our most exquisite examples of rotten milk, or cheese. Interestingly, cheese did not become part of the cuisine of China or the Americas. These cultures did, however, develop other fermented food, such as bean curds and fish sauces, using various gram-positive bacteria. Genetic differences in the abilities of adult humans to digest milk may have influenced these cuisines, although many people with an intolerance for lactose can tolerate lactic acid in fermented milk products.
Field Marks: Cheeses such as Swiss cheese with big eyes (formed as bubbles of carbon dioxide) are a result in part of propionic bacterial activity. Brevibacterium appears as an orangered covering on some strongly flavored surface-ripened (rind-washed) cheeses such as limburger, leiderkrantz, Bel Paese, Port l'Evêque, Port du Salut, Muenster and sometimes Stilton, which is also full of the blue fungus penicillin. Brevibacterium is also a cause of foot odor because it tends to break down proteins from flaking skin between the toes and form a smelly sulfur compound. Is there a connection between limburger and foot odor? Perhaps. After all, humans generally have some sort of contact with cheese during the production process. Brevibacteria simply fell into the vat. Lactococcus contributes its waste products, including lactic acid, to the flavors and aromas of eyeless or small-eyed cheeses such as cheddar, Camembert, Tilsit, cottage cheese and Gouda. Lactobacillus is a famous group of Gram-Positives featured in Emmenthaler, Gruyere, Gorgonzola, Mozzarella, Provolone and cheddar cheese, as well as Kefir and yogurt. Lactic acid, the waste product of these bacteria, confers a pleasant sour taste and acts as an inhibitor of other microorganisms.
Bacteria: Propionibacteria, Brevibacteria
Common Object: Human Skin
Background: Most of the microbes commonly found on the skin are gram-positives which settle in across the landscapes of the body seeking moist, nutrient-rich, protected crevices. We are carrying about 100 billion of them. For the most part, our skin bacteria are benign, taking advantage of substances discarded as wastes: sweat, oils, and dead skin cells. Only rarely are they opportunists, becoming harmful if they gain access to body cavities or if the immune system malfunctions. Generally, the day-to-day existence of skin bacteria goes unnoticed, with the exception of body odor, to which several of them eagerly contribute. However, it is not an easy life for our indigenous inhabitants. Everyday huge rafts of dead cells flake off, carrying bacteria. Fortunately (for them), washing ourselves in the everyday sense is not the sort of scrubbing process done by surgeons. Thus we rearrange our bacteria and make life challenging for them, but we do not clean our surfaces of them.
Field Marks: Propionibacteria thrive in the ducts of adolescent and adult sebaceous glands. They rejoice when their host reaches adolescence because then those glands can become a bit clogged with rich oils, making a very pleasant habitat for bacteria. Just as they do in making Swiss cheese, these bacteria produce propionic acid and carbon dioxide as waste products of their metabolism. Active sebaceous glands are their field marks. Between the toes, Brevibacteria are busy eating dead skin and converting an amino acid, methionine to methane thiol, the distinctive smell of socks and feet, as well as some delicious cheeses. Some mosquito species are attracted to the smells of brevibacterial products and use that to locate their hosts.
Bacteria: Cyanobacteria and others
Common Object: Works of Art or Architecture
Background: Objects made of marble, limestone, cement or plaster (gypsum), including statuary and walls (interior and exterior) of buildings, are susceptible to habitation by microorganisms, especially if there is some dampness to encourage their growth. Shadowy walkways and the dark, damp sides of buildings may have cyanobacterial growth. Cyanobacteria and other microbes can be a particular problem with some works of art. The identification of which microorganisms are on or in the work of art is a specialty of art conservation. The microbes may include fungi (which are not bacteria), actinomycetes and various nitrifying bacteria and cyanobacteria. If there is even a little light, including the ambient light within, for example, a shadowy cathedral, there is an opportunity for photosynthesizers to grow there, too.
Field Marks: Among the easier bacteria to identify are the cyanobacteria. Look for dimly lit, moist or transiently moist surfaces. A wet shadowy fountain provides a wonderful substrate. A bluegreen color is likely to be cyanobacteria. Brighter greens in more lit areas may be nonbacterial green algae. Dried cyanobacteria may form brown or black crusts or stains. However, do not expect to see cyanobacteria on any damp, lit art object or architecture that you happen to inspect. Art curators often go to great trouble and expense to keep that from happening.
Bacteria: Lactobacillus, Pediococcus
Common Object: Beer
Background: Our human ancestors must have been dismayed to find their dry stores of grain sometimes had become moist and sprouted and even worse, showing signs of musty fungal and bacterial activities. Hungry humans would not be too quick to toss out that grain; they would have discovered that immediately cooking up the mess would result in a reasonably palatable sprouted grain porridge, slightly sweetened by the sugars produced by germination and perhaps interestingly flavored by microbial byproducts. However, if a large store of grain had begun to sprout and decay, and the resulting vat of porridge was too large a quantity to eat quickly, a new set of microbes certainly would settle in. Some of our adventurous ancestors tasted the mix after it had been bubbling and frothing for a few days. Fermenting yeast would have been producing ethanol (grain alcohol) and various bacteria added acidity and complexity to the brew. Beer was born. Millennia later, many types of beer are well established, and they may be roughly divided according to how much bacterial activity is welcomed by the brewer. Lagers, porters, stouts and most ales are the result of yeast (fungal) fermentation and all bacteria are kept to a minimum or excluded completely. Brewing seems to have evolved differently in Belgium. Belgian lambic styles (including Lambic, Grieze, Taro and fruit beer), Belgian wheat beer and some Belgian ales (such as reds) are relatively acidic and full of complex flavors due to a host of gram-positive bacteria. The brewing process welcomes bacteria; windows and sometimes roofs of traditional breweries are kept open so that native bacteria can fall into the vats; cobwebs and dust (presumably full of bacteria) are allowed to adorn the walls and aged barrels (well inoculated with bacteria) are used to keep the beer, sometimes for years, as the complex flavors evolve. Beer drinkers accustomed to bitter lagers, stouts, and ales sometimes find Belgian-style brews to be an acquired taste.
Field Marks: The Belgian-style beers themselves are the field marks for Lactobacillus and Pediococcus. The rod-shaped lactobacillus bacteria are nearly ubiquitous in fermenting foods of all kinds; they convert sugars to lactic acid and give a pleasant acidic flavor to some beers. Brewers of lambics, Belgian wheats, Belgian red ales and Berlinerweisse either add lactobacillus or pedicoccus bacteria deliberately to the brew or allow them to tumble in naturally in open-air brew processes.
Bacteria: Unknown
Uncommon Object: Extraterrestrial Life
Background: In seeking life on other planets, it is wise to consider bacteria rather than animal-like organisms that somehow resemble or behave like us. Most life is bacterial and only bacteria have colonized Earth so diversely and with such versatility. Animals are an anomaly on our own planet, far outnumbered by microbes and relative latecomers in the history of life on Earth. We humans have occupied the planet only for a mere 50,000 generations. Some earthly bacteria are reasonable prototypes for the sorts of organisms that we might be looking or testing for on other planets. Indeed, when planetary geologists like Wheaton's Geoff Collins consider which planets and moons might be capable of supporting life, they are often thinking of life below the surface and most likely on a microscopic scale. "We used to think that life could only exist in environments like the surface of the Earth, but now we've found a huge mass of life on our planet that doesn't seem to have anything to do with the surface environment in volcanoes at the bottom of the sea and in the rocks miles beneath our feet," he reports. The question of whether bacterial life can survive on other planets is hotly debated among scientists, according to Wheaton professor of astronomy Tim Barker. Most planets or moons may be too cold or hot at the surface to have liquid water, but some may have molten interiors near which life might be hypothesized to dwell. For example, subsurface conditions may exist in which heat-loving bacterial-types, some of them chemoautotrophs, support a community of microbes (and possibly larger creatures) similar to that of some deep-sea hot springs on Earth.
Field Marks: None ... yet.
Betsey Dexter Dyer '75 is a professor of biology at Wheaton and a founding member of the college's genomics research team. This text is excerpted from Dyer's A Field Guide to Bacteria (Ithaca: Cornell University Press, 2003).
