What is the structure of bacteria? How does bacteria grow?
Bacteria are single-celled organisms that reside in every habitat, including the human body. Bacteria are a necessary part of the normal flora of the human body; very few species actually cause illness, and many are beneficial. Bacteria are the smallest known organisms that can reproduce independently.
Bacteria are the most common life-form on Earth. These single-celled organisms come in a variety of shapes and sizes. The millions of known species of bacteria live in a wide range of environments, from vents deep in the ocean floor to the recesses of the human digestive tract. The vast majority of bacteria are harmless to humans; some are actually helpful and necessary for human health, while a small fraction are pathogenic. Despite these diverse features, all types of bacteria have fundamental characteristics in common.
Bacteria have a simpler structure than plant and animal cells, which are higher life-forms called eukaryotes. Eukaryotes have cells that are divided into smaller compartments by membranes. Each compartment, or organelle, carries out specialized functions. Bacteria are prokaryotes, which have no organelles. They consist of just one compartment that is separated from the outside world by a cell membrane and a cell wall. The interior of the cell, called cytoplasm, contains a solution of sugars, salts, vitamins, enzymes, and other substances dissolved in water. Suspended in the cytoplasm are large numbers of ribosomes and a nucleoid made of DNA (deoxyribonucleic acid).
The cell membrane is a semipermeable barrier that separates the inside of the cell from the outside. This thin structure is vital to the survival of the cell. The membrane is created by the assembly of phospholipids and proteins into a bilayer. The inner and outer surfaces of the bilayer are charged and, thus, are attracted to the water molecules inside and outside the cell. The center layer of this structure is composed of fatty acids, which repel water. These chemical properties of the cell membrane ensure that the watery contents of the cell cannot leak through.
The structure of cell membranes also allows for the selective passage of certain molecules. This important feature ensures that necessary nutrients are allowed to enter the cell and that waste products are allowed to exit. While some substances cross the membrane through passive diffusion, most are transported actively by processes that require energy. The active transport of molecules across the membrane is mediated by proteins that are embedded in the cell membrane.
The cell membrane also serves as a site for the attachment of proteins involved in essential biochemical reactions. One example is the electron transport system, which generates adenosine triphosphate (ATP), the cell’s energy currency. In bacteria, ATP is generated by a chain of proteins bound to the inner side of the cell membrane. In eukaryotes, this process occurs on the inner membranes of mitochondria. The bacterial cell membrane thus provides some of the functions carried out by organelles in eukaryotes.
The cell wall is a tough network of fibers that encloses and protects the bacterial cell. The substance that makes up the cell wall is a unique polymer called peptidoglycan, which is not found in eukaryotes. Peptidoglycan is made of long sugar molecules that are connected to each other by short peptides. Bacteria can be divided into two major groups based on the structure of their cell walls. Gram-positive bacteria have a thicker peptidoglycan cell wall that will turn purple when treated with a Gram’s stain. The cell walls of gram-negative bacteria are surrounded by an outer membrane, which prevents the adhesion of a Gram’s stain. The extra protection provided by the more complex cell wall of gram-negative bacteria makes them less sensitive to some antibiotics, which can penetrate the cell walls of only the gram-positive bacteria.
Several classes of antibiotics target the cell walls of bacteria. Penicillins, cephalosporins, and vancomycin interfere with cell-wall construction, causing the bacteria to rupture and die. The goal in treating bacterial infections with antibiotics is to kill the intended organisms without damaging the cells of the host. Because human and animal cells lack cell walls, they are not affected by such drugs.
The internal components of bacteria use nutrients in the environment to allow the organisms to grow and reproduce. The bacterial cytoplasm is rich with ribosomes. As in eukaryotic cells, bacterial ribosomes carry out protein synthesis and are made of ribonucleic acid (RNA). Slight differences in the structure of eukaryotic and prokaryotic ribosomes make the ribosome a target for antibiotic action. Multiple classes of antibiotics, including streptomycin (and its relatives), tetracycline, and erythromycin, disrupt protein synthesis in bacteria but not in the cells of the host.
Bacterial DNA is organized into one large ring-shaped chromosome. In contrast to eukaryotes, the bacterial chromosome is not encased in a nucleus. The bacterial chromosome contains all the information needed to provide for the basic functions of the organism. Bacteria may also contain circular DNA structures called plasmids. The genes on plasmids are not usually necessary for survival, but they may become so in certain environments; plasmids can carry genes for antibiotic resistance, allowing the host bacteria to survive in the presence of a drug that is normally deadly to its species.
The variety of specialized features found in bacteria reflects their adaptation to the broadest range of environments of any organism on Earth. Bacteria are diverse in their size and morphology. Although the average size of a bacterial cell is 1 to 5 micrometers (m) in diameter, they range in size from 0.1 to 750 m in diameter. One of the most distinguishing features of bacterial cells is their shapes, which can be used diagnostically. The most common shapes are spheres (cocci), rods (bacilli), comma shapes (vibrios), and spirals (spirochetes and spirillum).
Many bacteria have developed specialized structures that allow them to move in their environment. Some have flagella, which are long filaments that protrude from the cell wall and are used to produce a swimming motion. The arrangement of flagella on the bacterial cell depends on the species. A cell can have a single flagellum or multiple flagella, either clumped at one end of the cell or spread over the entire surface. Some bacteria exhibit a gliding motion, which is created by structures known as pili. These cell surface projections can extend and retract, causing the bacteria to move. Bacteria also use pili to attach to surfaces and to each other. Some aquatic bacteria use gas vesicles to adjust their position in their environment. Gas vesicles are hollow structures made of protein. When present, they increase the buoyancy of the organism, making it rise to the water surface. Gas vesicles disintegrate and reassemble according to the concentration of nutrients in the cell.
Capsules are specialized structures that add an extra layer of protection to the exterior of some bacterial cells. The capsule is made of a polysaccharide-containing material that forms rigid layers on the cell wall’s exterior. Species that have capsules are extremely resistant to the action of phagocytes, cells of the host immune system that engulf and kill bacteria. Capsule-bearing strains of Streptococcus pneumoniae, for example, cause a particularly invasive and dangerous form of pneumonia.
Some species of bacteria can survive harsh conditions by forming endospores, which allow the bacteria to become dormant. Endospores, small cells that develop within bacterial cells, contain DNA and a portion of the cytoplasm. A strong wall surrounds and protects the endospore. Once the bacteria die, the endospores are released into the environment, where they can survive indefinitely. These tough structures are resistant to heat, radiation, chemicals, and desiccation. When environmental conditions improve, the endospore rapidly germinates and develops into a bacterial cell. Endospore-forming bacteria include Bacillus anthracis, which causes anthrax, and Clostridium botulinum, responsible for a serious form of food poisoning called botulism.
Bacteria possess all the machinery necessary to grow and reproduce independently of other cells. They are the smallest creatures on Earth that have this capacity. While they may use a host organism as a habitat, nearly all bacteria can reproduce without invading host cells. This feature sets them apart from viruses, which carry their own genetic material but require host-cell components for reproduction. The small size and relatively simple structure of bacteria allow them to grow and reproduce much faster than eukaryotic cells.
Bacteria reproduce asexually by dividing in half, in a process called binary fission. Individual bacterial cells grow continuously, making copies of their components and duplicating their DNA. The two copies of the chromosome move toward opposite ends of the cell, ensuring that each “daughter” cell will receive this essential DNA. When enough new material is present to sustain two cells, the cell membrane begins to pinch inward at the center. A cell wall grows to form a partition that divides the cell into two daughter cells. Because bacterial reproduction is asexual, each daughter will be identical to the parent cell.
Populations of bacteria grow at a rate determined by the time it takes individual cells to grow and divide, creating the next generation. The population doubles in size with each generation. The time required for a population of cells to double is known as the doubling time. Bacterial doubling times vary with the species, ranging from a few minutes to several hours. The nearly explosive growth rate of bacteria is about one hundred times faster than that of eukaryotic cells. Rapid binary fission allows bacteria to become extremely numerous in a short amount of time. If one bacterium with a doubling time of twenty minutes were allowed to grow for forty-four hours, the resulting mass of bacteria produced would equal the mass of the earth.
The actual occurrence of exponential bacterial growth is greatly limited by environmental factors, both in natural habitats and in laboratories. Long before a bacterial population could grow to match the earth’s mass, the supply of nutrients in the environment for the bacteria would be depleted. Bacterial growth rates are highly dependent on many factors, including temperature, the availability of nutrients, pH (acidity), and oxygen concentrations. Measures that reduce the rate of bacterial growth can be used to prevent illnesses caused by bacteria; most pathogenic bacteria must be present in large numbers to cause illness.
The optimal temperature for bacterial growth depends upon the species. Bacteria that live inside humans, including those of medical significance, thrive at an optimal temperature of about 98.6° Fahrenheit (37° Celsius). They can survive at temperatures generally ranging from 50° to 118.4° F (10° to 48° C), but their growth rates will be significantly reduced at lower temperatures. Their ability to survive below the optimal temperature may allow them to live outside a host for short periods until they enter a new host. This temperature tolerance facilitates the spread of bacteria from one host to another.
Bacterial growth rates can be reduced by controlling the temperature of the environment. Refrigeration of food slows the growth of bacteria, keeping their numbers low enough to prevent illness. Aqueous solutions heated to boiling 212° F (100° C) for thirty minutes will kill all bacteria in the solution. Medical instruments and solutions can be sterilized in an autoclave by heating above 248° F (120° C), which kills bacteria and heat-tolerant endospores.
Bacteria take in nutrients from their environment. Specific nutrients will vary depending on the habitat of a given species. General nutritional requirements of most bacteria include a carbon-source for energy, such as sugar; a nitrogen source, such as ammonia or nitrate; a variety of minerals and salts; vitamins; and other growth factors.
Bacteria are sensitive to the pH of their environment and can live only within a relatively narrow pH range. Most species of bacteria grow optimally in neutral environments, with a pH level between 6 and 8. Some species are specially adapted to live in extremely acidic or basic environments. The optimal pH of a given species will determine where it thrives, even within the human body. The stomach, with a pH of 2, is home to low numbers of acid-tolerant species of lactobacilli and streptococci. The large intestine, with a neutral pH of 7, is a much more popular residence; enormous numbers of bacteria from a minimum of ten different species live in the large intestine. The sensitivity of most bacteria to low pH can be used to inhibit bacterial growth, as occurs when foods are pickled in vinegar.
The presence of oxygen in the environment is another factor that affects bacterial growth. Most species, the aerobes, require oxygen for growth. For these species, low oxygen will cause a decrease in growth rate; if oxygen levels fall too low, they will not survive. For other species, the anaerobes, oxygen is not necessary for growth. Oxygen is toxic to some species; these obligate anaerobes cannot survive in environments where oxygen is present. Oxygen tolerance is an attribute used to identify bacterial species.
Bacteria are ubiquitous, and they will remain so. They have developed diverse traits that allow them to thrive in an amazing variety of habitats, including unimaginably harsh conditions. Their demonstrated adaptability should give pause and guide future scientific and medical strategies for preventing and treating bacterial illnesses.
Braude, Abraham I., Charles E. Davis, and Joshua Fierer. Infectious Diseases and Medical Microbiology. 2d ed. Philadelphia: W. B. Saunders, 1986. Microbiology from a medical perspective, designed for medical students. Provides a systematic approach, with highly detailed information about pathogens.
Brooker, Robert J., et al. Biology. New York: McGraw-Hill Higher Education, 2008. A standard biology textbook for undergraduate college students. Bacterial structure and reproduction covered in a concise manner, with excellent photographs.
Koch, Arthur L. The Bacteria: Their Origin, Structure, Function, and Antibiosis. Bloomington, Ind.: Springer, 2006. Evolutionary history of bacterial structures.Focuses on how the evolution of the cell-wall structure led to diversification of bacterial species. Covers the mechanism of action of cell-wall antibiotics and presents an evolutionary perspective on antibiotic resistance.
Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. A standard microbiology textbook for undergraduate students, with detailed descriptions of cell structures and clear illustrations. Includes evolutionary perspectives and covers pathogenesis.