Each person carries around at least 10,000 times more bacteria than there are people on earth. Each adult human also plays host to about 1,500 different microbes, but, somewhat comforting, is the fact that only about 100 are potentially dangerous. In 1980, The International Committee on Systematic Bacteriology agreed to reduce the accepted number of named species of bacteria from more than 30,000 to about 2,500 species. Yet, without these organisms, life would cease to exist.
To most, bacteria are known as “germs” – nasty things that must be eliminated. However, the vast array of their talents is most impressive. Bacteria can be more deadly than snake venom or strychnine, but the majority are actually beneficial. Many are used in the production of antibiotics, enzymes for detergents, for leaching out metals from low grade ores, in the making of foods, and for the convertion of milk sugar (lactose) into lactic acid. Vinegar is produced through bacterial action. Bacteria are even used in the manufacture of cocoa and coffee.
Bacteria have such unusual characteristics as the targeting of specific organs of the body or attaching themselves to tissues to prevent being washed away. Others can change their protein coats to avoid detection by the immune system. Some bacteria convert nitrogen and carbon dioxide into usable forms, while others are responsible for fermentation. Bacteria have the ability to produce organic matter from inorganic material by means of chemical energy (chemo synthesize). Some can photosynthesize just like plants. Bacteria can be the “power plants” in many of the nutrient cycles that support the food chain.
As stated, bacteria can be organ and tissue specific. For example, H. pylori will ignore cells from other areas of the body and stick only to stomach epithelial cells, where they firmly attach themselves. There they begin to replicate, and, as they multiply, they produce and spread their toxins. Attachment results from a special communication between a microbe and a body cell.E. coli, for example, uses at least two different types of adhesions to attach themselves to the intestinal cells. Each type of adhesion sticks to a different kind of receptor on the cell. When the bacterium attaches with the first adhesion, a signal releases calcium inside the body cell. The increased calcium affects filaments of actin, a protein involved in cell structure. Actin filaments begin to accumulate under the bacteria. Next, a pedestal begins to grow out of the cell allowing the bacterium to firmly attach itself to the second adhesion. Only then will the bacterium begin to multiply.
Bacteria are literally found everywhere. They live in the salty Dead Sea, in the tremendous heat of a desert or jungle, in the depths of the ocean, and in subfreezing temperatures of the Arctic Circle. Some live only when there is oxygen present, while others would die in that environment. Still others do not care if there is oxygen or not. They will grow anyway.
Bacteria (singular bacterium) come in all shapes and sizes.
- Round (spherical) cells are called ‘cocci’ (singular coccus). They may be true spheres (eg. staphylococci), helmet-shaped (eg. pneumococci), or kidney-shaped (eg. Neisseriae). Cocci may occur alone, in pairs, or in groups. If found in pairs they are called diplococci, threes are a triad, etc. Other group types include: Tetracoccus (groups of four), Streptococcus (chains), Sarcina (cubes of eight), and Staphylococcus (irregular clusters). Diseases caused by cocci include the following: pneumonia, tonsillitis, bacterial heart disease, meningitis, septicemia (blood poisoning), and various skin diseases.
- Elongated or rod-shaped cells are called ‘bacilli” (singular bacillus). NOTE that the shape of the bacteria can be confused with the name of its genera. A capital “B” indicates the genus “Bacillus” while a small “b” indicates the shape (bacillus). Bacillus is a large genus with many endospore producing species that are ever present. Some bacteria are not truly elongated, but can be kidney-shaped, curved (vibrios), or taper at each end (fusiform bacilli). These do not have the same arrangements as the cocci, but may be found in pairs (diplobacilli), or chains (streptobacilli). Bacilli may be single or adhere end to end to form chains. Some produce spores and some have flagella for locomotion. Certain species form a picket-fence pattern, which may look like a stack of wood (Corynebacterium diphtheriae). Diseases caused by bacilli-shaped bacterium include the following: tuberculosis (TB), whooping cough, tetanus, typhoid fever, diphtheria, salmonellosis, shigellosis, legionnaires’ disease, and botulism.
- Ovoid cells are something in between cocci and bacilli. These are known as ‘coccobacilli’ (singular coccobacillus).
- Spiral shaped cells can be one of two types: either “rigid” called ‘spirilla’ (singular spirillum) or “flexible” called ‘spirochaetes’ (singular spirochaete). There are two types of spirilla – short rods that are slightly bent and rigid spirals. Spirilla are solitary bacteria, rarely found in chains or clusters. Spiral-shaped bacteria are distinguished by their length, the number and size of the spirals, and direction of the coil. Short segments or incomplete spirals are common, as the comma-shaped Vibrios. The spirochetes of syphilis are typical spiral bacteria. Diseases caused by spirochaetes include the following: syphilis, yaws, leptosporosis, and Lyme disease.
- Square bacteria are flat and box-like, but can vary in their angular shape.
- There are also fungal bacteria, known as “actinomycetes,” which grow like fine threads, called ‘hyphae’ (singular hypha). A mass or group of these is known as ‘mycelium’. One example is Actinomyces scabies, which resembles fungal mycelia. Specialized reproductive elements produce conidia (functioning similar to spores) that are eventually released into the air.
Bacteria are classified in a hierarchy of categories which include families, genera, species, etc. Special characteristics then determine a further breakdown in their categories including shape, size, structure, chemical activity, types of nutrients needed, the energy used, the environment needed for growth, and their reaction to certain dyes. However, this may not necessarily indicate any evolutionary relationships of bacteria.
The bacterial kingdom is named Monera (Monerans) and the subkingdom is Eubacteria, which are considered to be “true” bacteria. All Monerans are prokaryotes (simple cells, no nucleus). All other forms of life are eukaryotes (have a membrane-bound nucleus). Eukaryon cells contain a highly organized nucleus surrounded by a nuclear membrane and contains chromosomes that divide by mitosis. Eukaryotic cells also contain membrane-bound organelles – mitochondria, chloroplasts, lysosomes, and the Golgi apparatus. Plants, animals, protozoa, lichens, fungi, viruses and subviral agents, and algae (except blue-green) are eukaryotes. Therefore, it is safe to say that all bacteria are microorganisms, but not all microorganisms are bacteria.
In 1884, a Danish bacteriologist, Hans Christian Joachim Gram developed a dye system to label all bacteria. When a bacterial coating rejects this dye, it does not become stained and is then labelled as a Gram-negative bacterium. If the bacteria accepts the dye, or turns color, they are referred to as Gram-positive bacteria.
The Gram stain involves spreading bacteria on a glass slide, then waving it over a bunsen burner flame for a few seconds. This causes the bacteria to stick to the surface of the glass. Then, crystal violet and dilute iodine solutions are dripped onto the slide. After a few moments, the slide is washed with alcohol. Gram-positive bacteria become stained a deep blue-black, while Gram-negative bacteria are left colorless. Why this happens is anyone’s guess, but the best one is that it has something to do with the bacterium coat.
Gram-positive and Gram-negative cell walls may consist of up to forty layers of peptidoglycan found only in bacteria and made up mainly of acetylated sugars comprising about 90% of the cell wall. Gram-negative cell walls are more complex than the Gram-positive in that they contain only a few layers of peptidoglycan (5-20% of the total cell wall). There are other differences in structure, though. Three genera of Gram-positive bacilli (Bacillus, Clostridium, and Sporosarcina) are able to form spores (endospores) inside the bacterial cell.
The following examples gives an idea of which bacteria fit under the Gram stain label:
- Gram-positive cocci: Enterococcus, Staphylococcus, Streptococcus
- Gram-positive bacilli: Bacillus, Clostridium, Corynebacterium, Listeria
- Gram-negative cocci: Branhamella, Neisseria
- Gram-negative bacilli: Bordetella, Brucella, Enterobacteria, Haemophilus, Pseudomonas
- Cocci that do not stain: Mycoplasma, Rickettsia
- Bacilli that do not stain: Leptospira, Mycobacterium, Treponema
The smallest bacteria is the size of the largest virus, while many are a hundred times larger. Bacteria come with their own DNA and are able to live independently. All they require is a hospitable environment in which to grow, and most do so without utilizing any chemical means from the host. This is opposite to the requirements of a virus. Heterotrophic bacteria absorb such organic molecules as carbohydrates, or can break down dead or decaying matter for their food. Autotrophic bacteria manufacture their own food.
What bacteria need for growth are as follows:
- A supply of suitable nutrients (sugars and other carbohydrates, amino acids, sterols, alcohols, hydrocarbons, methane, inorganic salts, and carbon dioxide). However, no individual bacterium has the capacity to deal with all of these compounds, and most only require simple forms of a few nutrients.
- A source of energy to complete its chemical reactions. Some bacteria require light, while others derive what they need from their environment.
- Water. Since bacterial cells are made up of about 80% water.
- An appropriate temperature for each species. There is a maximum and a minimum temperature where growth will take place and varies according to the species of bacteria. For example, thermophilic bacteria require temperatures greater than 45°C (113°F) and grow in compost, hot springs, etc. Thermoduric bacteria can survive in temperatures that would kill most other organisms, as in the case of pasteurization. Mesophilic bacteria grow at temperatures between 15°C (59°F) and 45°C (113°F). These include pathogens in man and other animals. Psychrophilic bacteria grow at or below 15°C (59°F) and do not grow above 20°C (68°F). These occur around the polar seas. Psychrotrophic bacteria can grow at low temperatures, but they prefer above 15°C (59°F) with an upper limit of 20°C (68°F) or more.
- An appropriate pH level. However, most bacteria grow best at or near pH7 (neutral). The majority of bacteria cannot grow under strong acid or alkaline conditions. There are some, however, called ‘acidophiles’ that prefer the more acidic conditions of mine drainage or certain hot springs.
- Appropriate levels of oxygen. Some bacteria require oxygen in order to grow (aerobes). Other bacteria cannot grow if oxygen is present (anaerobes). Microaerophilic bacteria generally grow best when oxygen concentrations are lower than air.
The following are examples of bacteria responsible for some common outbreaks caused by animal sources.
- Bacillus cerus
- mainly from pork, but also barbecued meats and sausages.
- Campylobacter jejuni
- from poultry, but also beef, cornish hens, goat’s milk, and raw cow’s milk.
- Clostridium botulinum
- from various food sources, including venison jerky and improperly canned foods.
- Clostridium perfringens
- from beef, corned beef, chicken, and turkey.
- E. coli 0157:H7
- from beef, hamburgers, raw milk, semi-soft cheeses.
- Listeria monocytogenes
- dairy products (cottage cheese, ice cream, raw milk, soft cheeses), and pork tongue.
- Salmonella enteritidis
- eggs, egg nog, hollandaise sauce, homemade ice cream, raw milk, yogurt, barbecued meats, infant formula, powdered milk.
- Staphylococcus aureus
- from various sources, including corned beef, ham, and raw milk.
- Trichinella spiralis
- mainly from pork, but also bear meat and homemade sausages.
- Yersinia enterocolitica
- mainly from pork, but also chitterlings, chocolate milk, pasteurized milk, raw milk, and sausage.