How do Bacteria Grow ? | Organisms | Microbiology

A typical growth curve for a population of bacterial cells, illustrates some of the dynamics that affect the population over the course of time. The population’s history may begin when several bacteria enter the human respiratory tract or are transferred to a tube of growth medium in the laboratory phases of the curve are recognized as follows:

The lag phase encompasses the first few hours of the curve. During this time bacteria adapt to their new environment. In the respiratory tract scavenging white blood cells may engulf and destroy some bacteria: in growth medium some organisms may die from the shock of transfer.

However, the biochemical activity in the remaining bacteria is intense as they store nutrients, synthesize enzymes, and prepare for binary fission. The curve remains at a plateau, balanced by early reproduction in some cells and death in others.

The population then enters an active stage of growth called the logarithmic phase (often shortened to “log phase”). The mass of each cell increases rapidly and reproduction follows. As each generation time passes, the number of bacteria doubles and the graph rises in a straight line if logarithms (powers of 10) of the actual numbers are used for the curve.

In humans, disease symptoms usually develop during the log phase because the bacterial population has reached a high enough level to cause tissue damage. Coughing or fever may occur, and fluid may enter the lungs if the air sacs are damaged. If the bacteria produce toxins, tissue destruction may become apparent.

In the laboratory, the population growth may be so vigorous that visible colonies appear on solid media, each colony consisting of millions of organisms. Broth media may become cloudy with growth. Because the population is at its biochemical optimum, research experiments are generally performed during the log phase.

After some hours or days, the vigor of the population changes and, as the reproductive and death rates equalize, the population enters another plateau, the stationary phase. In the respiratory tract, antibodies from the immune system have begun to attack the bacteria and phagocytosis by white blood cells adds to their destruction.

Perhaps the person was given an antibiotic to supplement the body’s defensive measures. In the culture tube, nutrients have become scarce, waste products have accumulated, and factors such as oxygen or water are in short supply.

If these conditions continue, the external environment will exert its limiting powers on the population and the decline phase will ensue. Now the number of cells dying exceeds the number, of new cells formed. A bacterial capsule may forestall death by acting as a buffer to the environment, and flagella may allow organisms to move to a new location.

If the organism is a species of Bacillus or Clostridium, the vegetative cells will have reverted to spores by this time and the stationary phase may be extended for months or years. For many species of bacteria, though, the history of the population soon comes to an end with the death of the last cell.

i. Temperature:

Bacteria inhabit almost every environment on Earth because different species can tolerate the myriad conditions found on the planet. For example, different bacterial species grow at different temperatures.

Certain bacteria have their shortest generation times at temperatures in the range of 0°C to 20°C; these bacteria are called psychrophiles. Other bacteria, the mesophiles, thrive at the middle range of 20°C to 40°C: and thermophiles multiply best at high temperatures of 40°C to 90°C or higher.

Most bacteria appear to be mesophiles. This is especially true of pathogenic bacteria growing in the human body, where the temperature is 37°C. It should be noted that pathogenic bacteria usually grow over a 35°C to 42°C range. Thus, when the body temperature rises to 40°C (104°F), there is a negligible “cooking effect’ on bacteria. The common laboratory incubator is set at 37°C to provide the proper environment for mesophiles.

Some mesophiles can grow at temperatures substantially below their normal range. Certain species, for instance, grow in refrigerated foods at 5°C. Here they may produce waste products and cause food spoilage. For example, staphylococci deposit their toxins in cold cuts, salads, and various leftovers.

When such foods are consumed without heating, the toxins may cause mild food poisoning. Other examples of mesophiles growing in the cold include Salmonella species, which can be infectious, and Proteus vulgaris, which causes blackening of eggs with a characteristic rotten odor.

Since these organisms are not truly psychrophilic, some microbiologists prefer to describe them as psychrotrophic. True psychrophiles live in the ocean depths and in Arctic and Antarctic regions where no other form of life is known to exist.

Thermophiles are present in compost heaps and are important contaminants in dairy products because they survive pasteurization temperatures. However, thermophiles pose little threat to human health because they do not grow well at cooler body temperature. In the 1980s scientist’s isolated thermophilic bacteria from seawater brought up from hot water vents along rifts on the floor of the Pacific Ocean. Using high pressure to keep the water from boiling, they found that the bacteria grew at an astonishing 250°C (482°F).

ii. Oxygen:

The growth of many bacteria also depends on a plentiful supply of oxygen and, in this respect, the aerobic bacteria are similar to more complex organisms. Anaerobic bacteria, by contrast, must have an oxygen-free environment in order to survive. Some anaerobic bacteria actually die if oxygen is present, while others simply fail to grow and multiply.

Certain anaerobic bacteria use sulfur in their chemistry instead of oxygen, and therefore they produce hydrogen sulfide (H₂S) rather than water (H₂O) as a waste product of their metabolism. Others synthesize considerable amounts of methane (CH₄), often called swamp gas. Both of these gases give putrid odors to marshes, swamps, and landfills. Petroleum is a product of anaerobic metabolism.

Certain anaerobic bacteria cause disease in humans. For example, the Clostridium species that cause tetanus and gas gangrene multiply in the dead, anaerobic tissue damage. Another species of Clostridium multiplies in the oxygen-free environment of a vacuum-sealed can of food, where it produces the lethal toxin of botulism.

In one bizarre incident, a restaurant owner died of botulism after tasting a piece of fish marinating under a layer of oil. Anaerobic conditions had apparently developed under the oil giving the Clostridium an opportunity to grow and produce its toxin.

Anaerobic conditions may be established in the laboratory by a number of methods. One method involves using thioglycollic acid to bind oxygen in thioglycollate medium. Another employs a mixture of pyrogallic acid and sodium hydroxide to absorb oxygen from the environment. Among the most widely used methods is the GasPak system, in which hydrogen reacts with oxygen in the presence of a catalyst to form water, thereby, creating an oxygen free atmosphere.

Some bacteria are neither aerobic nor anaerobic, but facultative. Facultative bacteria grow either in the presence or absence of oxygen. This group includes many staphylococci and streptococci, as well as members of the genus Bacillus and a variety of intestinal rods, among them E. coli.

Some microbiologists believe that a majority of bacteria may be facultative organisms. A facultative aerobe prefers aerobic conditions (but grows anaerobically), while a facultative anaerobe prefers oxygen-free conditions (but grows aerobically).

A final group is the microaerophilic bacteria. These organisms require a low concentration of oxygen for growth. In the body, certain microaerophiles cause disease of the oral cavity, urinary tract, and gastrointestinal tract. Certain species of bacteria, said to be capnophilic, require an atmosphere low in oxygen but rich in carbon dioxide. The CO₂ content can be increased in the laboratory by using a gas-generating apparatus or by burning a candle in a closed jar with the bacteria.

iii. Acidity/Alkalinity:

Because the internal environment of most bacteria has a pH of about 7.0, the majority of species grow best under neutral pH conditions. Although most growth media for laboratory cultivation are set at pH 7.0, bacteria tolerate acidic conditions as low as pH 6.5 and alkaline conditions as high as pH 7.5. Human blood and tissues, with a pH of approximately 7.2 to 7.4, provide a suitable environment for the proliferation of disease-causing bacteria.

Certain acid-tolerant bacteria called acidophiles are valuable in the food and dairy industries. For example, Lactobacillus and Streptococcus produce the acid that converts milk to buttermilk, cream to sour cream, and milk curds to cheese. These organisms pose no threat to good health even when consumed in large amounts. The “active cultures” in a cup of yogurt are actually acidtolerant (or, acidophilic) bacteria.

The vast majority of bacteria, however, do not grow well under acidic conditions. Thus the acidic environment of the stomach helps deter disease in this organ while posing a natural barrier to the organs beyond. In addition, you may have noted that certain acidic foods are hardly ever contaminated with bacteria.

Examples include lemons, oranges, and other citrus fruits, as well as vegetables such as cabbage and rhubarb. Traditionally, tomatoes were too acidic to support bacterial growth, but modern technologists have developed the “neutral tomato” and with it a bevy of new problems for consumers, especially those who grow and can their own tomatoes.

iv. Patterns of Nutrition:

Bacteria must meet certain nutritional requirements in order to grow. Most bacteria have relatively simple requirements, with water an absolute necessity. In addition, bacteria need foods that can serve as energy sources and raw materials for the synthesis of cell components. These foods generally include proteins for structural compounds and enzymes, carbohydrates for energy, and a series of vitamins, minerals, and inorganic salts.

Two different patterns exist for satisfying an organism’s nutritional needs. These patterns are called autotrophy and heterotrophy. They are primarily based on the source of carbon used for making cell components.

Organisms that practice autotrophy are able to synthesize their own foods from simple carbon sources. The organisms are said to be autotrophic (literally “self-feeding”). Autotrophs obtain their carbon from inorganic compounds such as carbon dioxide and ions such as carbonate, nitrate, and sulfate. Energy for food synthesis may come from the sun or from chemical reactions taking place in the cytoplasm as explains.

The second pattern, heterotrophy, is employed by heterotrophic organisms (literally “other-feeders”). These organisms obtain preformed organic molecules from the environment and use them for structural components and energy. The heterotrophic bacteria that feed exclusively on dead organic matter such as rotting wood are commonly called saprobes.

For many years these organisms were known as saprophytes, from Greek stems meaning “rotten” and “plant,” but the name has been changed to saprobes to reflect feeding on both plants and animals. Heterotrophs that feed on living organic matter such as human tissues are commonly known as parasites. The word pathogen (from the Greek pathos for “suffering”) is used if the parasite causes disease in its host organism.

v. Bacterial Cultivation:

Since the time of Pasteur and Koch, microbiologists have used media such as beef broth for the laboratory cultivation of bacteria. The modern form of this liquid medium, called nutrient broth, consists of water, beef extract, and peptone, a protein supplement from plant or animal sources.

When agar is added to solidify the medium, the product is called nutrient agar Agar is a poly-saccharide derived from marine algae. It adds no nutrients to the medium but only serves to make it solid so that bacteria can be cultivated on the surface. Sometimes it is valuable to use a semisolid medium, such as when testing bacterial motility. In this case, a small portion of agar is added to the medium to make it stiff but not as solid as nutrient agar.

Most common bacteria grow well in nutrient broth and nutrient agar, but certain fastidious bacteria may require specially enriched media. For example, the streptococci that cause strep throat and scarlet fever grow well when whole blood is added to the nutrient medium. In this instance, the medium is called blood agar.

To encourage the growth of Neisseria species, blood agar is heated before solidification, a process that disrupts the red blood cells and releases the hemoglobin. The medium is now termed chocolate agar because of its charred brown appearance.

Selective media are those containing ingredients to inhibit the growth of certain bacteria in a mixture while permitting the growth of others. For example, staphylococci are cultivated on mannitol salt agar. This medium contains mannitol, an alcoholic carbohydrate fermented by staphylococci, as well as a high salt concentration that inhibits most other bacteria.

Another example is eosin methylene blue (EMB) agar. This selective medium has carbohydrates fermented by E. coli and other Gram-negative bacteria, but it also contains eosin and methylene blue, two dyes that inhibit Gram-positive bacteria.

Another type of medium is the differential medium. This medium makes it easy to distinguish colonies of one organism from colonies of other organisms on the same plate. MacConkey agar is typical. It contains the dyes neutral red and crystal violet as well as the carbohydrate lactose.

Those bacteria that ferment the lactose take up the dyes and form red colonies; other bacteria show up as colourless colonies. In addition, MacConkey agar contains bile salts that inhibit the growth of Gram- positive bacteria. This medium is thus selective as well as differential.

The media just described represent non-chemically defined or natural media. This is because one cannot be certain of the exact components or their quantity. Another type of medium is the chemically defined or synthetic medium. Here the nature and amount of each component are known.

Such a medium might contain glucose, ammonium phosphate, potassium phosphate, magnesium sulfate, and sodium chloride. The glucose supplies energy to the cell; the ammonium ions are a source of nitrogen for amino acid and nucleic acid formation; the phosphate is used in DNA and RNA synthesis; sulfur from magnesium sulfate is valuable for enzyme formation; and sodium chloride maintains a stable internal environment in the cytoplasm.

Bacteria rarely occur in nature alone. In virtually all cases, they are mixed with species of other bacteria, a so-called mixed culture (a swab from the gum area is an example). But to work with bacteria, the laboratory technologist must use a pure culture, that is, a population of only one species of bacteria. This is particularly important if an identification of a pathogen is to be made.

When isolating bacteria from mixed cultures, two standard methods are available. The first method, called the streak plate isolation method, uses a single plate of bacteriological medium. An inoculum is taken with a loop or needle, and a series of streaks is made in one area of the plate. The instrument is flamed, touched to the first area, and a second series is made in a second area.

Similarly, streaks are made in the third and fourth areas, thereby spreading out the different bacteria so they can form discrete colonies on incubation. The second method is the pour plate isolation method. Here, a sample of bacteria is diluted in several tubes of melted agar medium.

The agar is then poured into sterile Petri dishes and permitted to harden. On incubation, the bacteria will form discrete colonies where they have been diluted the most. The technologist can then pick samples of the colonies for further testing.

Various types of culture methods are available to the research microbio­logist. For example, bacteria can be culti­vated in a large batch method, then removed from the container for further study; or they may be cultivated by a conti­nuous method where nutrients are regularly added to the container and waste products are drawn off. Bacteria remain in the log phase in this method, and they can be used to pro­duce useful industrial products such as vita­mins and enzymes.

On occasion, it is desirable to have all bacteria at the same stage of population growth (e.g., the stationary phase). A physical treatment such as heat is used in this so-called synchronous method of cultivation.

To measure the amount of bacterial growth in a medium, there are numerous methods. For example, the cloudiness, or turbidity, of a liquid culture may be used to indicate the number of bacteria it contains.

It is also possible to perform a direct microscopic count using a known sample of the culture on a specially prepared slide (a Petroff-Hauser counting chamber) containing a counting grid. The dry weight of the bacteria gives an indication of the cell mass, and the oxygen uptake in metabolism can be measured as an indication of the bacterial number.

It is also possible to perform a most probable number test or a standard plate count procedure. In the former test, samples of bacteria are added to numerous lactose broth tubes and the presence or absence of gas formed in fermentation gives a rough estimation of the bacterial number.

In the latter test, a bacterial culture is diluted, and samples of dilutions are placed in agar plates. The number of colonies appearing after incubation reflects the number of bacteria originally present. This test is desirable because it gives the viable count of bacteria (the living bacteria only) as compared to a microscopic count or dry weight test that gives the total bacterial count (the living as well as dead bacteria).

Some bacteria are impossible to cultivate on artificial laboratory media but, instead, require a living tissue medium. Most rickettsiae and chlamydiae are examples of such bacteria. They must be grown in fertilized eggs, tissue cultures, animals, or other environments where living cells are found. The difficulty in cultivation often makes detection and study of these organisms burdensome.

vi. Intermicrobial Relationships:

Closely allied to the nutritional needs of bacteria is their relationship with other organisms. The term symbiosis (literally, “living together”) is applied to the relationship. Symbiosis implies a situation in which two populations of organisms interact in a close and permanent association. The benefits obtained through this interaction may involve food, protection, support, or other life sustaining factors.

If a symbiosis between two populations of organisms benefits both populations, the relationship is further defined as mutualism. For example, bacteria live on the roots of pod-bearing plants such as peas and beans where they trap nitrogen from the atmosphere and convert it to ammonium ions. The plant then uses the ammonium ions to synthesize amino acids. The plant, in turn, provides a stable environment for the bacteria and supplies them with essential growth factors.

Another type of symbiosis is commensalism. This relationship occurs when one population receives benefit from the relationship while the other is neither benefited nor harmed. An example is found in the populations of bacteria inhabiting the human skin. Such bacteria are called commensals.

Another commensal is Escherichia coli, the Gram-negative rod that thrives in the human intestine but usually causes no damage. To some investigators this relationship is more symbolic of mutualism, because research suggests that E. coli provides certain vitamins for human nutrition and breaks down otherwise indigestible foodstuffs.

A third type of symbiosis is synergism. In this case, two populations or organisms live together and accomplish what neither population could accomplish alone. In trench mouth, for example, at least two populations of bacteria must be present for infection of the oral cavity to occur. Usually one population consists of rods, the other of spirochetes.

When a symbiosis is beneficial to one organism but harmful to the other, the result is parasitism. In this situation, the organism that benefits is the parasite, and the one that suffers injury is the host. The bacteria of human disease are typical parasites. Many microbiologists believe that injury to the host is probably not in the best interest of the parasite, because if the host were to die, the parasite might also die.