Characteristics of Microorganisms

CYTOPLASM: The cytoplasm is contained within the cytoplasmic membrane and is a viscous material consisting of water, enzymes and other proteins, carbohydrates, lipids, nucleic acids, essential nutrients, oxygen, and waste products. Embedded in the cytoplasm is the bacterial nucleoid, which consists of a single, long chromosome of DNA that contains most of the genes (approximately 2500) controlling cell activities. The nucleoid usually is diffused throughout the cytoplasm rather than well defined as in mammalian cells. Some bacteria also have extrachromosomal DNA in small units in the cytoplasm called plasmids. Plasmids frequently carry genes that express special activities such as resistance to chemical and physical agents and antibiotics.

The cytoplasm also contains submicroscopic particles called ribosomes that serve as physical sites for the synthesis of proteins from amino acids. Some bacteria may have granules in the cytoplasm that consist of stored substances such as starch, lipids, or iron.

CYTOPLASMIC MEMBRANE: The cytoplasmic membrane surrounds the cytoplasm and is composed of lipids and protein substances. Its structure is similar to that of mammalian cells, and its functions include regulating the entrance and exit of nutrient materials and waste products, maintaining proper pressure within the cell to keep it from bursting, containing the enzymes responsible for synthesizing the outer cell wall, serving as the physical site for attachment and allocation of newly formed chromosomal DNA during cell division, controlling release of certain extracellular enzymes, and serving as the site of many metabolic reactions through which the cell gains energy for growth.

Mesosomes, present mostly in gram-positive bacteria, are inward foldings of the cytoplasmic membrane. Mesosomes house many hydrolytic enzymes that are released into the extracellular environment and break down macromolecules into their subunits, which are taken into the cell as food.

Antimicrobial agents in many disinfectants, handwashing agents, and mouth rinses kill or inhibit the growth of bacteria by acting on the cytoplasmic membrane. Chlorhexidine gluconate, present in handwashing products and mouth rinses, is thought to affect the cell membrane and interfere with energy metabolism and uptake of nutrients by the cells.

CELL WALL: The bacterial cell wall is a rigid structure that gives the cell its characteristic shape (coccus, rod, spirillum) and is not present in mammalian cells. The cell wall is a complex structure in which its basic components (peptidoglycan) form a tight-knit “net” over the entire surface of the cell. Peptidoglycan (peptide, small protein; glycan, polysaccharide) consists of long polysaccharide chains with short side chains of peptides. The peptides of one polysaccharide chain are linked to the peptides of other polysaccharide chains, forming a large continuous macromolecule. Besides giving the cell its characteristic shape, the peptidoglycan also protects the cell from mechanical crushing. Gram-positive bacteria have several layers of peptidoglycan in their cell walls and are much more resistant to external physical forces than gram-negative bacteria, because the latter have only a few layers of peptidoglycan.

The peptidoglycan of the cell wall is the site of action of several antimicrobial agents. Lysozyme is an enzyme present in saliva, tears, nasal secretions, and other body secretions and is present inside white blood cells (e.g., phagocytes, which are described later) that destroy bacteria. This enzyme lyses susceptible bacteria by breaking the bond between the subunits of the polysaccharide chain in peptidoglycan. The antibiotic penicillin and its many derivations act by preventing cross-linking of the peptidoglycan units as the cell wall is being synthesized during cell division. Other antibiotics such as vancomycin and the cephalosporins also prevent cell wall synthesis. The action of all of these antimicrobial agents results in “holes” forming in the peptidoglyan that cause a loss of the cytoplasm into the external environment (lysis) and cell death.

OUTER MEMBRANE: Gram-negative (but not gram-positive) bacteria have an outer membrane just external to the cell wall. This membrane covers the entire cell surface, and its basic structure and composition are like the cytoplasmic membrane. The outer membrane, however, contains an important component called endotoxin that is composed of lipid, polysaccharide, and protein (lipopolysaccharide-protein complex). When endotoxin is released from bacteria present in the body (after cell death and lysis, or in some instances during its synthesis), it can cause damage to nearby body cells and stimulate several reactions in the body, including fever, inflammation, bone destruction, hemorrhage, and vomiting. The action of endotoxin is thought to play a role in many infectious diseases, including periodontal diseases, dysentery, meningitis, typhoid fever, gonorrhea, and cholera.

CAPSULE: Some gram-positive and gram-negative bacteria contain another structure called the capsule, which also is referred to as a slime layer or glycocalyx. The capsule covers the entire outer surface external to the cell wall in some gram-positive bacteria or external to the outer membrane in some gram-negative bacteria. The capsule is produced by the cytoplasmic membrane and secreted through the cell wall and remains associated with the cell surface in the form of a gelatinous covering. Usually the capsule consists of polysaccharide, but in a few instances it may contain proteins. Some species may have large capsules, such as Streptococcus pneumoniae (the cause of lobar pneumonia and middle ear infections); others may have only a thin layer of capsular material, such as S. mutans (a cause of dental caries).

Capsules contain a large amount of water (hydrated) and may help bacteria survive in dry environments. The presence of a capsule also influences how bacteria interact with cells and other surfaces in the human body. Surface polysaccharides in the microcapsule of S. mutans are involved in sucrose-induced plaque formation on tooth surfaces (see Chapter 5). The presence of capsules also reduces the ability of white blood cells to surround, engulf, and destroy the bacterium through a process called phagocytosis (eating cells) and digestion (see Chapter 3). Thus bacteria with capsules tend to escape these early body defense mechanisms against bacterial disease agents. Bacteria without capsules are engulfed and destroyed more easily.

FLAGELLA: Some bacteria have long, threadlike appendages called flagella (singular flagellum). These protein structures are attached to the cytoplasmic membrane extending through the cell wall and, if present, the outer membrane and capsule. They have a whiplike motion and allow the bacterium to move through fluids. The number of flagella per bacterial cell may range from one to many, exhibiting different arrangements over the cell surface.

FIMBRIAE AND PILI: Fimbriae and pili are hairlike protein appendages projecting from the cytoplasmic membrane into the external environment. They are much shorter than flagella and have two major functions. Sex pili, found mainly on gram-negative bacteria, serve as a tube through which DNA can be passed directly from a donor cell to a recipient cell during a process called conjugation. This process permits properties expressed by the DNA genes of one cell to be transferred to and expressed by another cell. Fimbriae, also called pili, serve as mechanisms by which cells can attach to other cells or environmental surfaces. The fimbriae act as a bridge between the cell and other surfaces. Attachment fimbriae provide an important virulence property. Almost all bacteria that cause harmful infections on or through mucous membranes must first attach to the epithelial cells of the mucous membrane, or they will be washed away by secretions of the body. Attachment fimbriae provide the mechanisms of attachment in diseases such as “strep throat,” scarlet fever, gonorrhea, and diphtheria and cause some bacteria to attach to each other during the formation of dental plaque.

ENDOSPORES: Some bacteria have developed a defense mechanism against death caused by adverse environmental conditions. When available nutrients become depleted, cells of Bacillus, Geobacillus, and Clostridium form a large internal structure that contains the DNA and other substances surrounded by several coats of protein. This dense, thick-walled structure is called a spore or endospore and is one of the most resistant forms of life against heat, drying, and chemicals (Figure 2-4). Once formed, the spore can remain dormant for years and then, when exposed to the proper nutrients and other growth requirements, it can germinate into an actively dividing (vegetative) cell. Only one endospore forms within a bacterial cell, and it germinates into only one vegetative bacterial cell. This is in contrast to fungal spores, in which several spores may form a single cell. Bacterial spores are particularly important in the field of infection control because of their high resistance to heat and antimicrobial chemicals. Spores of Geobacillus stearothermophilus and Bacillus atrophaeus are used biologically to monitor the use and functioning of heat or ethylene oxide gas sterilizers (see Chapter 11). Also, spores of B. atrophaeus and Clostridium sporogenes are used to test the antimicrobial effectiveness of liquid sterilants used at room temperature.

TEMPERATURE: Bacteria can be divided into three groups based on the temperature required for optimal growth. Thermophiles grow best at 56° C (132.8° F) with a range of 45° C to 70° C (113° F to 158° F). Such bacteria were found in the hot waters of the “Old Faithful” geyser in Yellowstone National Park. More importantly, the spore-forming thermophilic bacterium G. stearothermophilus was selected many years ago to test the use and functioning of heat sterilizers because of its resistance to heat. Mesophiles grow at temperatures ranging from 22° C to 45° C (71.6° F to 113° F) with optimal growth at body temperature, 37° C (98.6° F). Most of the bacteria that grow and survive in the human body, including those that cause infectious diseases (such as dental caries, periodontal diseases, tuberculosis, bacterial pneumonia, tetanus, and many others), are mesophiles. Psychrophiles grow between temperatures of 1° C and 22° C (33.8° F and 71.6° F), with optimal growth at the typical refrigerator temperature, 7° C (44.6° F). The bacteria present in the oceans, as well as many of those that spoil food stored in a refrigerator, are psychrophiles. Refrigeration has long been used to maintain the freshness of food or extend the storage life of many items that may be contaminated with microorganisms. Refrigeration slows down or prevents the growth of many bacteria and molds, but refrigeration cannot be considered a means of killing microorganisms. If psychrophiles are present, they will continue to grow at normal rates at these low temperatures. If fresh pasteurized milk or high-quality hamburger is purchased at the food market and stored unopened in the refrigerator, it will eventually spoil because microorganisms, some of which may be psychrophiles, will grow using the nutrients in the milk or meat and will produce physical and chemical changes associated with food spoilage.

Thus, if an item or solution becomes contaminated with microorganisms, placing it in the refrigerator or freezer cannot be relied on to kill the microorganisms to make it “safe” for use.

ACIDITY: pH is a measure of acidity, and the pH scale ranges from 0 (high acidity) to 14 (high alkalinity). Values from 0 to 7 are acidic, and values from 7 to 14 are basic or alkaline. The neutral pH of 7 indicates an equal amount of acid and base or no acid or base present. Examples of acidic solutions are stomach acid (pH 1.5) and orange juice (pH 2.9). Examples of basic or alkaline solutions are household ammonia cleaners (pH 11.9) and human blood (about pH 7.4). Pure water has a pH of 7.0.

Most bacteria that survive in the human body grow over a pH range of 5.5 to 8.5, with optimal growth at pH 7.0. Bacteria that produce acids during growth are called acidogenic; those that survive and grow in an acidic environment (usually below pH 5.5) are called aciduric. Acidogenic and aciduric bacteria are important in the initiation and progression of dental caries, as described in Chapter 5.

NUTRIENTS: Bacteria, like mammalian cells, must synthesize all of the macromolecules of protein, polysaccharides, lipids, and nucleic acids (DNA and RNA) needed to grow. These macromolecules are synthesized from the building blocks of amino acids, monosaccharides, fatty acids, and purines/pyrimidines, respectively. Bacteria also must use a variety of smaller molecules such as vitamins and inorganic substances (e.g., sodium, potassium, iron, chlorine, manganese, magnesium, sulfur, phosphorus, calcium, and trace elements). If a bacterium cannot synthesize a given building block such as an amino acid or a vitamin such as niacin, these substances must be available from the environment for the bacterium to survive and grow. Just as we require “eight essential vitamins,” bacteria also require nutrients for growth.

Different bacteria have different nutritional requirements. For example, S. mutans may require six amino acids, five vitamins, all four purines and pyrimidines, and other small molecules plus an energy source, nitrogen, oxygen, inorganic salts, and trace elements from its environment. Conversely, some types of E. coli need only carbon dioxide, a nitrogen source, and trace elements and from this meager “diet” can synthesize all of the macromolecules needed for growth.

Many organic nutrients available for growth may not be in a form that can be used by the bacteria. For example, amino acids are the building blocks for proteins, and many bacteria require certain amino acids to synthesize their own proteins needed for growth. In many instances, proteins rather than the amino acid building blocks are present in the environment, but the entire protein molecules are too large to get into bacterial cells. Thus many bacteria make extracellular proteases, which are enzymes released into the environment that break down proteins into amino acids that can enter the cell. Bacteria also may produce extracellular enzymes that degrade polysaccharides into monosaccharides or lipids into fatty acids, which then can be taken into the cell and be used for growth.

Unfortunately, when these extracellular bacterial enzymes are produced in the human body during a bacterial infection, they may degrade proteins, polysaccharides, or lipids that are important components of body cells, causing damage to these cells. This is one of the mechanisms by which bacteria cause disease.

OXYGEN METABOLISM: Bacteria are divided into four groups based on their requirement for oxygen:

Almost all bacteria (regardless of their growth requirement for oxygen) have enzymes that produce toxic substances (e.g., superoxide and hydrogen peroxide) from oxygen that can kill the bacteria. These toxic substances also are produced by white blood cells and help these cells kill bacteria after phagocytosis (see Chapter 3). Some bacteria that produce these toxic substances in the presence of oxygen have mechanisms to remove them to prevent death. One mechanism involves the enzyme superoxide dismutase that converts superoxide to hydrogen peroxide and molecular oxygen. Hydrogen peroxide is converted to water and oxygen by the enzyme catalase.

Obligate aerobes, microaerophiles, and facultative anaerobes, all of which can grow in the presence of oxygen, have superoxide dismutase and catalase to detoxify the oxygen metabolites. Obligate anaerobes do not have these enzymes and therefore cannot tolerate the presence of oxygen. About 99% of the bacteria that live in the mouth are obligate anaerobes or facultative anaerobes.

WATER: All life forms require water to dissolve nutrients and to permit nutrient entrance or transport into cells. Also, the water molecule is required in several enzymatic reactions to break down (e.g., hydrolyze) certain substances.

ENZYMES: Metabolism occurs through the action of enzymes, which are catalysts made of protein. Catalysts speed up chemical reactions and are used over and over because they are not changed during the reaction. Action of some enzymes requires small molecules such as magnesium or calcium, called cofactors; other enzymes require organic molecules such as vitamins, called coenzymes. An enzyme (E) molecule binds to a specific molecule referred to as the substrate (S), and the enzyme binds to the part of the substrate molecules that will be changed. This binding causes the breaking of chemical bonds or the formation of new chemical bonds in the substrate molecule, changing the substrate to a new substance called the product (P). This reaction is written as follows:

A bacterial cell has about 1000 different enzymes, and the enzyme names all end with the suffix -ase (e.g., protease and hydrolase). Some enzymes break down extracellular substrates into subunits (products) that then can be taken into the cell and used for growth. Other enzyme reactions may change one small molecule into another small molecule. In the example given, pyruvic acid (produced in the cells from the metabolism of sugar) is changed into lactic acid by the enzyme lactate dehydrogenase. Lactic acid produced by streptococci and lactobacilli in dental plaque leaves the cell as a waste product and can cause demineralization of the tooth surface, producing dental caries (see Chapter 5).

CATABOLISM: Bacterial multiplication requires the synthesis of new cell components, and these anabolic reactions require energy. Catabolism provides some of the building blocks for the macromolecules and the energy needed to drive anabolism. All bacteria require nutrients that can serve as sources of energy, and the energy is released from the nutrients as they are processed through catabolic reactions. Two major catabolic pathways that generate energy in bacteria are fermentation and respiration. The pathway used depends on the types of enzymes and metabolic machinery present in the bacterial cell.

ENERGY PRODUCTION: A common energy source for many bacteria is glucose. After glucose is transported into the cells, it is processed through one of two major catabolic pathways known as fermentation or respiration (described later). These reactions are associated with the release of energy from the glucose, but the energy cannot be used to drive anabolic reactions unless it is transferred to special molecules such as adenosine triphosphate (ATP). The ATP then participates in the energy-requiring anabolic reactions such as the synthesis of polysaccharides from monosaccharides and proteins from amino acids.

FERMENTATION: Fermentation is an anaerobic process that usually involves the breakdown of sugars (called glycolysis) with end products of organic acids or alcohols. During fermentation of sugars, hydrogen atoms (also referred to as electrons) are released from the sugar breakdown products. These electrons then bind to organic molecules, changing their structure. In respiration, these electrons bind to oxygen or inorganic substances (as described later). During the fermentation of glucose, pyruvic acid usually accepts the released electrons, changing to lactic acid as a final end product. This is the metabolic pathway that is responsible for dental caries formation, and it is active in caries-conducive oral bacteria such as S. mutans and Lactobacillus spp. The metabolic machinery present in these oral bacteria gives the cells only one metabolic choice, the generation of building blocks and energy for anabolism through the fermentation of sugar mainly to lactic acid. Other bacteria may produce other products from fermentation, depending on the species and the nutrients available.

RESPIRATION: Respiration by infectious bacteria also involves glycolysis of sugars to pyruvic acid as described for fermentation. The electrons released from the sugar breakdown products, however, are transferred to oxygen in aerobic respiration through a series of molecular processes called the electron transport chain. The pyruvic acid, rather than being changed to an end product lactic acid as in fermentation, is processed further through a series of reactions that yield considerably more energy in the form of ATP.

PREVENTING GROWTH: Prevention of growth can be achieved by changing or eliminating a physical or nutritional requirement for growth or by using a chemical agent that interferes with cell division. Agents or conditions that prevent bacterial growth without killing them are called bacteriostatic.

Storing items in the refrigerator stops or slows down bacterial growth, unless the bacteria are psychrophiles. Freezing bacteria stops their growth and, because of formation of ice crystals inside the cell, can rupture the cell membrane. Usually, some cells survive, however, and may begin to grow when thawed.

Eliminating the availability of oxygen prevents the growth of aerobes. Adding oxygen prevents growth of anaerobes. Changing the pH beyond optimal values stops growth except for bacteria that can survive extreme pH values, such as aciduric bacteria in the mouth. Treating bacterial diseases with antibiotics can result in inhibiting growth or in killing the cells, depending on the bacterium and the antibiotic used. Antibiotics are chemicals that interfere with some metabolic activity of the bacterium but usually do not affect the metabolic activity of human body cells. This effect is referred to as selective toxicity. For example, penicillins prevent the formation of the cell wall in certain bacteria, which prevents their growth and usually kills the bacterium. Penicillins do not affect the growth of human body cells because our cells do not have cell walls.

KILLING BACTERIA: Killing of bacteria is accomplished by physical or chemical means and is an important aspect of disease prevention and infection control. Agents or conditions that kill microorganisms rather than just prevent their growth are called bactericidal, virucidal, or fungicidal agents or conditions. Probably the surest way to kill bacteria (or any other type of microorganisms) in the shortest amount of time is to expose them to high temperatures such as those achieved in a steam, dry heat, or unsaturated chemical vapor sterilizer (see Chapter 11). The heat may destroy proteins, break down DNA and RNA, or cause structural damage to the cell membrane.

Several chemicals can kill bacteria and other microorganisms, and the type of chemical used frequently is determined by location of the microorganisms to be killed. Strong chemicals cannot be used to kill microorganisms on or in the body, whereas inanimate objects, such as operatory surfaces, can withstand treatment by such agents. Chemicals kill microorganisms by several mechanisms, depending on the properties of the chemical.

a. prion

b. bacterium

c. virus

d. yeast

a. DNA viruses

b. RNA viruses

c. chlamydia

d. regular bacteria

a. polysaccharides

b. DNA and RNA

c. proteins

d. fats

a. bacillus

b. coccus

c. spirillum

a. a human red blood cell

b. a bacterium

c. a virus

a. cell membrane

b. cell wall

c. capsule

d. fimbriae

a. vegetative cell

b. virus

c. ricketsium

d. spore

a. aerobic

b. anaerobic

c. aciduric

d. acidogenic

a. aerobic

b. anaerobic

c. aciduric

d. acidogenic

a. aerobic

b. anaerobic

c. fermentative

d. acidogenic

a. flagella

b. ribosomes

c. cocci

d. fimbriae

a. nucleus

b. capsule

c. cytoplasmic membrane

d. cell wall

a. Streptococcus

b. mutans

a. grows best at cold temperatures such as those in a refrigerator

b. produces acids during growth

c. is gram-negative

d. needs oxygen for growth

a. a chemical substance produced by a microorganism that can inhibit the growth of or destroy some other microorganisms

b. a protein that is produced by the body in response to an antigen and is capable of binding specifically to that antigen

c. a chemical agent used to kill microorganisms on contaminated operatory surfaces

d. a negatively charged ion