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Welcome to "Bacteria and Infection."
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Course Details
Course Length: Approximately 2 hours. Your time may vary based on modem speed, prerequisite knowledge and other factors.
Date Published: September 2005
This course focuses on the microorganisms called bacteria and on a process termed infection, in which bacteria first invade the body and then multiply within the tissues. Bacteria are responsible for numerous infectious diseases, those resulting from infection, in humans. Bacteria can damage human tissues, thereby affecting the way the body functions. Infectious diseases can occur in and harm virtually any part of the body, the skin, eyes, and the nervous, cardiovascular, respiratory, gastrointestinal, and urogenital systems. This course, Bacteria and Infection, is divided into these major sections:
The first section, OVERVIEW OF BACTERIA, describes the structure and normal functioning of bacteria.
The second section, BACTERIAL CLASSIFICATION, discusses different types of bacteria, focusing on particular actions and characteristics of some common pathogenic bacteria.
The third section, THE PROCESS OF INFECTION, describes how bacteria cause infection, and provides a list of common types of bacterial infection in humans.
The fourth section, HOST DEFENSES AGAINST INFECTION, explains the passive and active immune responses that are mounted by the host in response to bacterial infection.
Bacteria are microorganisms, or living things, that are neither plant nor animal. Since each bacterium is composed of only one cell, bacteria are unicellular. These single cells exist in a vast array of shapes and sizes, and although their internal cell structure is simple, bacteria are the most diverse life forms on earth in terms of the chemical processes by which they live. This section provides an overview of bacteria, including their structure, reproduction, and requirements for survival and growth.
These are the objectives of this section. You will be tested on these objectives in the final assessment.
After you finish this section, you should be able to:
A cell is the simplest structural unit of life that is capable of functioning on its own. Because bacteria are unicellular, we are describing the entire microorganism when we discuss their cell structure. This lesson describes structures and functions of basic components of all bacterial cells.
The basic components of bacterial cells are:
The Cell wall
The Plasma membrane
The Cytoplasm
The Chromosome
The Ribosomes
The outermost structure of bacteria is the cell wall, a semi-rigid envelope that maintains the integrity of the cell in the same way that the skin maintains the integrity of the human body. The cell wall helps protect the cell against environmental changes - e.g., heat, cold, drought - that would otherwise damage or destroy it, but allows most molecules to pass through it. This layer is composed of molecules called peptidoglycans, and so often this is called the peptidoglycan layer. We will return to the detailed structure of peptidoglycans and how they are made by the bacterial cell later.
Just inside the cell wall is a second, less rigid envelope, the plasma membrane, which encloses the cell contents. In bacterial cells, the plasma membrane has two primary functions. First, it serves as a selective barrier to molecules that have penetrated the cell wall, allowing some (such as water and oxygen) to flow easily into the cell interior, and restricting the passage of others (such as proteins).
Second, the plasma membrane contains enzymes (proteins that cause chemical reactions to occur) that are vital to the life functions of the cell.
Bacteria can be divided into two main groups based on differences in their cell wall structure. These differences were first noticed as differences in staining with a dye called Gram’s stain. Gram positive organisms have the structure defined so far, but Gram negative bacteria have an additional membrane outside the peptidoglycan layer called the outer membrane which contains openings, called channels (some of them formed by proteins called porins), which allow nutrients, waste products, and fluid to flow into and out of the cell. Some of these channels are nonspecific - they allow any molecule up to a certain size to flow through them.
Other channels are specific - only certain particular molecules can pass through them. By prohibiting the entry of other molecules - including antibiotic drugs - into the cell, the cell wall channels protect the cell contents and enhance the likelihood of the cell’s survival.
Cytoplasm is the viscid (thick, sticky) substance that serves as the matrix for all of the interior contents of the cell. Cytoplasm is made up primarily of water, accounting for 80% of the total content. It also contains enzymes, nutrients, such as carbohydrates and lipids (or fats), and other molecules.
In most bacteria, the genetic material is contained in one circular chromosome, and contains all the hereditary information required to form a new bacterium. The chromosome consists of genes, which are made up of deoxyribonucleic acid (DNA). DNA is a material that is composed of long, thin, twisted strands of compounds - called nucleotides - arranged in a double-helix pattern. DNA carries the codes for reproducing specific proteins, which determine characteristics or functions of the organism.
In higher organisms, such as humans and plants, chromosomes are enclosed in the cell nucleus. Therefore, these organisms are called eukaryotes. In contrast, bacterial cells are prokaryotes, since they lack the cell nucleus.
In addition to a single chromosome, many bacterial cells contain circular molecules of DNA called plasmids. Plasmids may carry genes that code for resistance to antibiotics and for production of toxins, both of which are important to bacterial survival. However, the DNA contained in them is not necessary for reproduction or other basic cell functions.
There is more than one type of RNA. Ribosomal RNA and other proteins complex to form the Ribosome. Ribosomes are structures that serve as protein “factories” for a cell. Because a cell cannot survive without proteins, ribosomes are vital to cell life. Like DNA, RNA is a component of all living cells and is composed of nucleotides and arranged in long strands; but unlike the double-stranded DNA, RNA is a single strand of nucleic acid. The primary function of RNA is protein synthesis. The genetic code contained in DNA is transcribed into messenger RNA, which then travels to the Ribosome. The genetic code is then translated into proteins.
Some bacteria consist entirely of the structures that have just been defined. Others have specialised external structures that enhance their chances of survival. This lesson describes these additional components, which include capsules, flagella, fimbriae, and pili.
A capsule is a gel-like compound that surrounds the exterior of some bacterial cells. The gel compound is produced inside the bacterial cell and is excreted. Outside, it attaches firmly to the cell exterior. In some bacteria, this material is not organised and is more loosely attached; in this case, it is called a slime layer.
A capsule can help the bacterium survive harmful environmental changes and provide effective protection against an attack by the host’s defense system. In addition, the sticky capsule helps the bacterium adhere to surfaces inside the host’s tissues or in other environments - such as streams or plants - for survival.
Flagella are whip-like tails that are specialised extensions of bacterial cell walls. A bacterium may have one, several, or numerous flagella. Flagella can be located anywhere on the bacterium, and give bacteria motility - the ability to move on their own. Motility allows the bacterium to move into more favourable environments.
The response of sensors located in the cell wall to specific stimuli produces taxis - movement toward or away from the stimulus. The major responses of motile bacteria are chemotaxis, the response to chemical stimuli, and phototaxis, the response to light. These responses control the speed and direction of the flagellum rotation to create different movement patterns. The cells can move ahead at a constant rate, or they can tumble, turn, and continue on another path.
The cell walls of some bacteria have hair-like appendages called fimbrae and pili. These appendages are shorter and thinner than flagella - less whip-like and more like bristles and function in cellular attachment, rather than cell movement.
Fimbriae are present in multiple numbers and are found either at each end of the cell or distributed evenly over the entire cell surface. Like the bristles on a burr, fimbriae help the bacterium adhere to tissue surfaces within the host.
Pili are longer than fimbriae, and they are present either singly or in pairs on a single bacterium. Their purpose is to enable one bacterium to adhere to another during a specialised process called conjugation, in which DNA is transferred between cells.
A single bacterium does not live long. Since it either dies or reproduces within minutes or hours, the survival of bacteria depend upon their reproduction. Higher organisms, such as humans, reproduce by the joining of male and female reproductive cells during sexual reproduction, and human growth refers to the development of an individual organism. In contrast, bacteria reproduce by a simpler process, called asexual reproduction, in which a cell reproduces by dividing, rather than by joining with another cell. Moreover, bacterial growth implies an increase in the number of individual bacteria. When hundreds of bacteria have accumulated, the mass is called a clump, thousands of bacteria are called a colony, and billions of bacteria are called a population. As the number of bacteria multiplies, the likelihood of infection and disease increases. This lesson covers mechanisms of bacterial reproduction, and describes various phases of bacterial growth.
Most bacterial cells reproduce by splitting in two, in a process called binary fission. During the first step of binary fission, the entire chromosome makes an identical copy of itself in a process called replication. Once there are two complete chromosomes, the bacterial cell begins to elongate and pinch off in the centre. The two chromosomes migrate to opposite sides of the cell, and the cell wall fills in the gap at the splitting point between the two new cells. At the end of the process, two identical cells exist in place of the original parent cell.
Some bacteria, such as the filamentous types, reproduce via a process called fragmentation. During one form of fragmentation, bacteria form bulges at the ends of their cells, followed by the splitting off from the parent to form new cells. In another form of fragmentation, the parent cell breaks up into pieces, each of which becomes a new cell.
Bacteria have both physical and chemical requirements for growth and reproduction. The physical requirements include specific temperatures, and water and acid balances within the environment.
For pathogenic bacteria to survive, they must be able to thrive at the body temperature of the host. Thus, bacteria that cause disease in humans generally grow well at human body temperature (37°C).
Since most bacteria live in a liquid environment and the cytoplasm is mostly water, salt concentration has an important impact on bacterial growth. If the salt concentration of the liquid outside the bacterium is higher than the concentration inside, water in the bacterial cell diffuses (flows passively) to the outside liquid, and the cell shrinks. On the other hand, if the salt concentration inside the bacterial cell is higher than that in the surrounding solution, water diffuses into the cell, causing it to expand. If the bacterium has a weak cell wall, this diffusion of water into the cell will lyse the bacterium.
Besides temperature and salt concentration, bacteria are affected by the acidity of their environment. Most bacteria grow best in environments with a neutral pH - neither acid nor alkaline (about pH 7.0). However, pathogenic bacteria that cause food poisoning are able to survive the extreme acidity of the stomach and the alkalinity of the intestines.
In addition to physical factors, growth and reproduction of bacteria depend on chemical requirements, which include nutrients and oxygen. The nutritional requirements of most bacteria are similar to those of any living cell - carbon, nitrogen, and smaller amounts of sulfur, phosphorus, and trace elements. These elements are required for synthesis of the basic cell components. Most bacteria obtain these necessary elements from their host environments.
Different types of bacteria have different oxygen requirements. For example, aerobes, which are bacteria that use oxygen, produce energy more efficiently than anerobes, those that do not use oxygen. However, anerobic bacteria are able to survive and grow in oxygen-poor host environments in which aerobes cannot survive.
Some bacteria have the ability to respond to adverse environmental conditions by forming endospores via a process called sporulation. During this process, the bacterial cell first replicates its DNA, followed by its isolation of the replicated DNA and some cytoplasm at one end of the cell. A portion of the plasma membrane then grows inward and separates the DNA and cytoplasm from the rest of the cell, eventually encircling it completely. The resulting spore gradually loses most of its water and develops a thick protein coat. When the endospore is complete, the original cell wall dissolves, and the cell dies, thereby releasing the endospore. Thus, although DNA replication is involved, sporulation is a survival mechanism, not a type of reproduction that produces two identical cells. The released endospores can withstand adverse environmental conditions, such as extreme heat, lack of nutrients or water, or harsh chemicals, and can survive for thousands of years as a dormant cell. When environmental conditions improve, enzymes within the endospore dissolve its coat, water enters the cell, and the cell resumes normal functions.
In a new environment, the growth of bacterial colonies follows a pattern of four distinct phases. The lag phase is an initial adaptive period in which little or no reproduction and growth occurs. During this time, which can range from hours to days, the bacteria are preparing to adapt to their new environment by producing new vital proteins. The bacterial cells begin to reproduce rapidly in the second phase, called the log (logarithmic), or exponential phase. During this period, binary fission achieves a constant rate and the number of bacteria increases logarithmically. When nutrient supplies exhaust and waste products accumulate, exerting negative changes in acidity or temperature, the reproduction rate slows and the number of bacterial cells stabilises during the stationary phase. Unlike the rapidly dividing bacteria that are usually highly susceptible to conditions that threaten their survival (such as the presence of antibiotics) during the log phase, bacteria in the stationary phase are often less vulnerable to antibiotics due to their slower metabolism. This can be because the bacteria are not at a stage in their cell-cycle where the antibiotic can cause harm, or because the antibiotic targets proteins necessary for the replication process. However, even without the presence of antibiotics, as nutrients are used up over time, individual cells start to die. This final phase is called “death” or “decline.” Eventually, the number of dead cells exceeds the number of live (viable) bacteria, and the population is either drastically diminished or completely destroyed.
This “drag and drop” progress check will test your knowledge of the information presented in this section. Because your score will not be recorded, you may take the progress check as many times as you would like.
Please feel free to review the lessons as often as required to successfully complete the progress check.
Use the terms to label the diagram. Not all terms are used. Use the “rewind” button to reset.
Match the terms with the correct definitions. Not all terms are used. Use the “rewind” button to reset.
Use the terms to fill in the blanks. Not all terms are used. Use the “rewind” button to reset.