~Neil deGrasse Tyson, Space Chronicles: Facing the Ultimate Frontier
Many of us recognize bacteria only as "germs," invisible nuisances that invade our bodies and make us sick. But bacteria have always coexisted with us, and perform an amazing array of helpful functions, such as making vitamins, breaking down garbage, and even maintaining our atmosphere. They are found inside and around all living creatures. Just think of them as the Force (well, midichlorians are probably a closer comparison).
Bacteria are tiny single-cell microorganisms, normally existing together in millions. They are the driving mechanism behind a vast percentage of natural processes, including decomposition, digestion, and regulation of the nutrient cycle in soils. A gram of soil typically contains 40-50 million bacteria (there are about 28 grams in an ounce). One-quarter teaspoon of fresh water holds about one million bacteria. Planet Earth is estimated to be home to at least five nonillion bacteria (5 nonillion = 5,000,000,000,000,000,000,000,000,000,000). In fact, the worldwide bacterial biomass exceeds that of all plants and animals on Earth.
Often, the first reaction people have to bacteria is eradication. We are clearly at a numbers disadvantage in this battle. Luckily, we don't have to annihilate all bacteria. True, there are some "bad" bacteria we simply can't have in our bodies, but there are also "good" bacteria in humans that are very important to everyday bodily functions. Many yogurts and supplements are fortified with good bacteria that alleviate digestive disorders and boost the immune system. Bacteria are used to preserve foods, enhance soil for crops, make cheese, and break down solid waste. These uses are beneficial to us. Many aspects of nutrient cycles depend on bacterial metabolism, such as nitrogen fixation from the Earth's atmosphere. Nitrogen makes up 78% of our atmosphere; it's pretty important.
Bacteria are members of the prokaryote group. Flashback to high school biology: a bacterial cell differs from the cell of a plant or animal (both eukaryotes) in that its nucleus and organelles are not bound by a membrane. Bacteria are divided into two major categories eubacteria and cyanobacteria and five clades (groupings that include a common ancestor and all the descendants, living and extinct, of that ancestor). While there are a number of recognized phyla, the dominant ones are Proteobacteria, Firmicutes, and Actinobacteria.
The study of bacteria is known as bacteriology, a sub-discipline of microbiology. Because bacteria are responsible for so many processes in our own bodies, as well the world at large, understanding how they function can only benefit us, whether we are protecting ourselves from the bad, or making our lives healthier with the good.
Modern bacteria's ancestors appeared on earth 3.5-4 billion years ago. They were the earliest forms of life on Earth. It's thought that their natural processes influenced the development of the planet's environment, sufficiently altering the atmosphere into a composition that enabled other, more complex life forms to develop. Cyanobacteria, a type of photosynthetic bacteria, paved the way. They grew in the water and produced oxygen (and still do today). More complex cells developed when the once free-living bacteria hung up their hats inside other cells, eventually becoming the organelles in modern complex cells. The mitochondria in our cells are one such example. This is also the first step towards the evolution of eukaryotes, organisms with membrane-bound nuclei.
Bacteria reproduce through a process called binary fission, which is essentially a period of growth followed by a division of the cell into two "daughter cells." In this way, most bacteria can replicate extremely quickly. The problem with binary fission is that every daughter cell is genetically identical to both its parent and its twin. This makes it harder for bacteria to adapt, in turn making them more susceptible to antibiotics. To mitigate this predicament, bacteria use a process called recombination. Bacterial recombination is achieved through three processes: conjugation, transformation, and transduction. Conjugation is simply the trading of gene fragments when two or more bacteria come in contact. There's a special tube for it and everything. Transformation is when bacteria grab DNA from the environment around them, often dead bacteria. They simply pull the DNA through their cell wall. (Wait - only plants have a cell wall! Well, yes and no. We'll cover that later.) Transduction is when a virus gets into the bacterium, steals secret gene files, and sells them to the next bacterium chump it hitches a ride with.
There are several nutritional classifications for bacteria. By this, I mean there are several ways they eat. First, either they are autotrophic or heterotrophic. Autotrophs make their own organic food from inorganic substances, such as turning carbon dioxide into a BLT. They can be further divided into photoautotrophs and chemoautotrophs. Photoautotrophs, such as the cyanobacteria, use light energy they soak up to create chemical energy. Chemoautotrophs collect inorganic compounds such as iron, hydrogen sulfide, and hydrogen gas and combine them with oxygen to create their energy (this process is known as oxidation).
Heterotrophs require pre-made organic substances for biosynthesis (the conversion of food into more complex products used to grow). They can also be divided into two major subgroups. Photoheterotrophs use organic carbon sources for biosynthesis but also engage in some photosynthesis on the side. Chemoheterotrophs use organic compounds, such as sugars, proteins, and lipids, as their source of energy. Vibrio cholerae are chemoheterotrophs. So are we.
Bacteria come in three main shapes. (1) Spiral: these are known are as spirilla. If their coil is very tight they are known as spirochetes. (2) Spherical: usually the simplest bacteria, called cocci. (3) Rod-shaped: designated as bacilli. Some bacillary bacteria are curved; these are known as vibrios.
Kingdom Eubacteria: true bacteria (separate from ancient bacteria, Kingdom Archaebacteria).
Phylum Proteobacteria: this major phylum incorporates a gamut of pathogens. Members are Gram-negative (a dye-based classification), with an outer membrane mainly composed of lipopolysaccharides (LPS important later, I promise).
Class Gamma Proteobacteria: the largest and most diverse subgroup of the proteobacteria.
Family Vibrionaceae: facultative anaerobes, meaning they can thrive in the presence or absence of oxygen (though given a choice, they prefer oxygen for respiration); capable of fermentation; containing oxidase (a special enzyme that reduces oxygen to water or hydrogen peroxide); and having one or more flagella. Most bioluminescent (light-emitting) bacteria belong to this family.
Members of the genus Vibrio consist of common, Gram-negative bacteria in aquatic environments, especially marine environments. Along with cell shape, Gram staining is a rapid diagnostic tool used to divide species into two basic groups. Vibrios are heterotrophic, obtaining nutrients from their mutualistic, parasitic, or pathogenic relationships with other organisms.
The bioluminescent, marine vibrios are usually symbionts of fish, squid, and other marine life. V. fischeri is well-known for mutualistic relationships, usually with squid. The host of this bacteria expels ninety percent of its stash of V. fischeri each day. This allows newly-hatched offspring to find the endosymbionts they need. Endosymbiosis is a type of symbiosis in which one organism lives inside the other, the two typically behaving as a single organism, and each usually benefiting from the other. Some endosymbiotic relationships are so intertwined that if the organisms were separated, they would die. Humans and mitochondria, for example, or plants and chloroplasts.
The most sensationalized vibrio is Vibrio cholerae, which causes epidemic or Asiatic cholera which, untreated, is one of the most rapidly fatal infectious diseases known. The cholera toxin, which is the classic model of a bacterial enterotoxin, is also produced by some strains of E. coli. An enterotoxin is a protein exotoxin released by a microorganism. An exotoxin is a toxin secreted by bacteria. It can cause damage to the host by causing cell death or disrupting normal cellular metabolism (basically, the cell life cycle). Exotoxins are highly potent and can cause major damage to the host. Exotoxins may be secreted during growth or released when the bacterial cell dies and breaks down.
You certainly wouldn't want to room with V. cholerae. But even though making you sick is a byproduct of its company, it's just the byproduct. Not the objective. Vibrio bacteria don't directly attack a person's body. Rather, they produce proteins, as a byproduct of feeding and growth, that are toxic to the human body (remember enterotoxins?). It's like being allergic to cat fur. The cat isn't actively clawing your eyes out, but she IS leaving her fur everywhere, making you miserable in the process. Still, it's a good reason to avoid cats.
Most bacteria secrete a covering for themselves which we call a cell wall. However, bacterial cell walls are not the same as the cell walls we talk about plants having. Plant walls contain cellulose, the compound that gives rigidity to the cells. Bacterial cell walls have no cellulose; they are composed mostly of a chemical called peptidoglycan.
Whether a bacteria has a thin or a thick cell wall determines what antibiotic will work against it. If you've ever waited for the results of a culture and sensitivity test, you may have heard the terms "Gram-positive" or "Gram-negative." This is the legacy of a cell-staining method developed by Christian Gram: bacteria with thick cell walls retain dye and are labeled Gram-positive; bacteria with thin walls do not and are labeled Gram-negative.
Some antibiotics, like penicillin, don't outright kill the bacteria; they just prevent the bacteria from making more cell wall, halting the bacteria's growth. That's why antibiotics typically need to be taken for several days. The bacteria, unable to grow, die of "old age." If a person stops taking the antibiotic too soon, any bacteria still living would resume growing and reproducing. Antibiotics tend to be more effective against thick-walled bacteria, the Gram-positives. Gram-negative bacteria are more resistant. That's why it is important to determine which of the two types of bacteria have moved in.
Thanks to improved sewage and water treatment, there is a low occurrence of V. cholerae in the United States. Over ninety percent of the cholera cases occurring in the U.S. are the result of travel to a country where this species is prevalent; recurring infections of cholera are rare.
Other headlining species of Vibrio include V. parahaemolyticus and V. vulnificus, known to cause seafood-borne illnesses such as septicemia and wound infections.
V. parahaemolyticus seems to be the more amiable of the two. Raw or undercooked seafood, usually oysters, are the primary culprits of the acute gastroenteritis caused by this bacterium. Wound infections can also occur in warm seawater but are less common than seafood-born maladies. Higher water temperatures breed higher levels of bacteria, so outbreaks are more common in summer and early fall. Symptoms typically occur in about 24 hours and include watery diarrhea, nausea, vomiting, abdominal cramps, and sometimes fever and chills. The illness usually resolves itself in three days, but it can persist for up to ten days in immunocompromised individuals. Treatment is unnecessary in most cases. Antibiotics don't seem to decrease the severity or the length of the illness anyway. Drinking plenty of liquids and electrolytes to replace fluids lost through diarrhea is the best course.
V. vulnificus is responsible for the majority of seafood-related deaths. Immunosuppressed individuals are eighty times more likely to contract Vibrio infections, though these bacteria are capable of harming anyone. However, most cases occur in males over the age of 50. Turns out, estrogen protects females against the V. vulnificus endotoxin. This is different from the entero- and exotoxins we already covered. Endotoxins, also known as lipopolysaccharides (LPS), are part of the outer membrane of the cell wall of Gram-negative bacteria. They are released when the bacterial cell disintegrates (whereas the other toxins are usually a byproduct of growth; endotoxins are a byproduct of death).
V. vulnificus primarily spreads through contaminated seafood; it does not alter the appearance, taste, or odor of oysters which, again, are the primary culprits. It can also be contracted by an open wound that is exposed to seawater. Among healthy people, ingestion of this bacteria can cause vomiting, diarrhea, and abdominal pain. In the immunocompromised, V. vulnificus can infect the bloodstream, causing septic shock and blistering skin lesions. These bloodstream infections are fatal about fifty percent of the time. If this bacteria is suspected, treatment should be initiated immediately because antibiotics improve survival. Luckily, V. vulnificus infections are rare.
These vibrios are potentially dangerous, but perhaps a little overdone in the media. Just take the correct precautions when preparing seafood, especially shellfish and don't swim in the bays with a significant open wound. It's not inconceivable that you'll have a rough experience with one, at some point in your life, but the probability is low.
Whew, vibrios are kind of a downer, so it's nice to know the human body contains huge amounts of friendly bacteria, even some that protect us from dangerous ones by occupying places in the body the pathogenic bacteria want attach to, and some that actively come to the rescue and attack the pathogens.
Also, lactic acid bacteria, such as Lactobacillus and Lactococcus are used with yeast and molds to make soy sauce, vinegar, yogurt, and pickles. In fact, humans have been using these bacteria to ferment foods for thousands of years. And, bacteria help clean up our oil spills!
So don't let one group of bacteria taint your worldview. After all, there's one in every family.
Where I learned about bacteria, and you can too!
Centers of Disease Control & Prevention
Vibrio parahaemolyticus: www.cdc.gov/vibrio/vibriop.html
Vibrio vulnificus: www.cdc.gov/vibrio/vibriov.html
National Institute of Allergy & Infectious Diseases
Todar's Online Textbook of Bacteriology
"Bacterial Protein Toxins": textbookofbacteriology.net/proteintoxins.html
"The Major Groups of Bacterial Pathogens": textbookofbacteriology.net/medical_2.html
"Vibrio vulnificus": textbookofbacteriology.net/V.vulnificus.html
Environmental & Public Health Consulting Group, Inc.
By Hans Gnter Schlegel
Oceans and Health: Pathogens in the Marine Environment
By Shimshon Belkin, Rita R. Colwell
Merck Animal Health
Medical News Today
UC - Clermont, College Biology
Encyclopedia of LIfe
What Are Bacteria?
Maryland Healthy Beaches