Microbiology for Nursing Students 2 PDF

Bacterial Chemotaxis Bacterial movement in response to chemicals. If a compound is a nutrient, it is an attractant, It causes cells to move toward it. ! If the compound is toxic, it is a repellent, ! It causes cells to move away. Forward movement called a run. the The change in...

Bacterial Chemotaxis Bacterial movement in response to chemicals. If a compound is a nutrient, it is an attractant, It causes cells to move toward it. ! If the compound is toxic, it is a repellent, ! It causes cells to move away. Forward movement called a run. the The change in direction causes the cell to stop and roll is called a tumble. Types: 1. Phototaxis: bacteria can respond to variations in light. 2. Aerotaxis: bacteria can respond to the concentration of oxygen. 3. Magnetotaxis: bacteria can react to the earth’s magnetic field.  Special structures 1. Lipopolysaccharide (LPS): Present in the cell wall of gram-negative bacteria. The LPS is composed of three distinct units: 1. A phospholipid called lipid A, (is responsible for the toxic effects of disease, such as fever and shock especially hypotension). 2. A core polysaccharide of five sugars linked to lipid A. 3. An outer polysaccharide consisting of up to 25 repeating units of three to five sugars, which is the somatic, or O antigen of G-ve bacteria The O antigen is used to identify bacteria in the clinical laboratory. 2. Teichoic acid: Special structures Fibers located in the outer layer of the gram-positive cell wall and extend from it. The medical importance of teichoic acids: ability to induce septic shock constitute major surface antigens of gram-positive bacteria. Teichoic acids help the attachment of Staphylococci to mucosal cells. Types: 1. wall teichoic acid (WTA), linked to peptidoglycan, 2. membrane teichoic acid, linked to cell membrane (lipoteichoic acids; LTA). WTA and LTA provides the elasticity, porosity, tensile strength, and electrostatic properties of the envelope. In the Streptococcus pneumoniae, the teichoic acids carry the Forssman antigen.  Special structures 3. Spores: Is highly resistant structures formed in response to adverse conditions Formed by Gram positive bacteria: the genus Bacillus, which includes the agent of anthrax the genus Clostridium, which includes the agents of tetanus and botulism. The spore forms inside the cell; and so, called endospores. Contains bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, very little water, and a thick, keratin-like coat The coat is responsible for the resistance of the spore to heat, dehydration, radiation, and chemicals.  Special structures By exposure to water and nutrients, specific enzymes degrade the coat, water and nutrients enter; and germination into a pathogenic bacterial cell occurs. Germination is not a means of reproduction, since one cell produces one spore that germinates into one cell. Factoring stimulating bacterial sporulation: 1. Unsuitable environmental conditions; high temperature, dryness.... 2. Depletion of nutrients as sources of carbon and nitrogen. 3. Accumulation of metabolic end-products. 4. pH changes. 5. Unsuitable gases; oxygen for anaerobic bacteria, absence of oxygen in aerobic bacteria  Special structures Classification of spores Function of spores: 1. According to position within the bacterial cells: 1. Protection of the bacteria from a. central physical and chemical agents. b. subterminal 2. Help in identification of the bacteria, c. Terminal e.g. Clostridium tetani: 2. According to the size in relation to the bacillus: a. Spore is pulging and terminal a. Same size; Genus Bacillus. b. Spore is rounded. b. Larger than Bacillus (pulging); Genus c. Drumstick in appearance. Clostridium. Special structures 4. Mesosomes: Special structures They are invaginations of the cytoplasmic membrane Important during cell division. Are the origin of the transverse septum that divides the cell in half Act as the binding site of the DNA of each daughter cell 5. Granules: Bacteria store reserve materials (nutrients) in the form of insoluble granules. These materials are accumulated when their precursors are in the medium. These are called inclusion bodies Function in: the storage of energy or as a reservoir of structural building blocks.  Special structures The most common inclusion bodies are: Poly-β-hydroxybutyric acid (PHB). Glycogen Sulfur Polyphosphate (volutin granules or metachromatic granules). Carboxysomes Magnetosomes Gas vesicles  Special structures 6. Cell Walls of Acid-Fast Bacteria: Mycobacteria, eg, Mycobacterium tuberculosis, have an unusual cell wall, resulting in their inability to be Gram-stained. These bacteria are acid-fast because they resist decolorization with acid-alcohol after staining with carbolfuchsin due to the high concentration in the cell wall of lipids called mycolic acids. 7. Slime layer: The glycocalyx is a polysaccharide coating secreted by some bacteria outside the cell wall Is removed easily and covers surfaces like a film allows the bacteria to adhere to various structures, eg, skin, heart valves, and catheters.  Bacterial Colonies Colony development on agar surfaces aids the microbiologist in identifying bacteria because individual species form colonies of characteristic size and appearance. A colony is composed of the progeny of one bacterial cell, and has the growth characteristics of this cell. The most rapid cell growth occurs at the colony edge. Growth is much slower in the center. cell autolysis takes place in the older central portions of some colonies. These differences in growth appear due to gradients of oxygen, nutrients, and toxic products within the colony  Bacterial growth Bacteria reproduce by binary fission (one parent cell divides to two progeny cells). The typical phases of a standard growth curve can be: 1. The lag phase: vigorous metabolic activity occurs but cells do not divide. This can last for a few minutes up to many hours. 2. The log {logarithmic) phase: is when rapid cell division occurs. Lactam drugs, such as penicillin, act during this phase because the drugs are effective when cells are making peptidoglycan, ie, when they are dividing. 3. The stationary phase: when nutrient depletion or toxic products cause growth to slow until the number of new cells produced equals the number of cells that die (a steady state). 4. The final (decline) phase is the death phase, marked by a decline in number of living bacteria. Bacterial growth  Biofilms Bacteria may live suspended in an aqueous environment, but many attaches to surfaces and live in a polysaccharide-encased community called a biofilm. Biofilms cause: The slippery nature of rocks in a stream bed, The slimy substance that coats kitchen drains, and The scum that gradually accumulates in toilet bowls. Biofilm formation begins when a bacterium adheres to a surface, it multiplies and synthesizes a loose glycocalyx to which unrelated cells attach and grow. Biofilms have open channels through which nutrients and waste materials can pass. Cells communicate with one another by synthesizing and responding to chemical signals.  Biofilms Thick biofilms, called microbial mats, are found in: Freshwater and marine environments. Plaque on teeth, which leads to tooth decay and gum disease. Persistent ear infections that resist antibiotic treatment and The complications of cystic fibrosis. Ocular diseases because Chlamydia, Staphylococcus, and other pathogens survive in contact lenses and in cleaning solutions Biofilms are particularly difficult because they protect organisms against harmful chemicals such as disinfectants. Bacteria encased in a biofilm may be hundreds of times more resistant to disinfectants.  Factors affecting the growth of bacteria 1-Temperature 3.Mesophiles, 1. Psychrophiles E. coli and most common pathogenic Grow at 0°C but have their optimum bacteria, between: 5°C and 15°C. Optimum temperature between 25°C and Cause spoilage of refrigerated food. 45°C. As Pseudomonas aerogenosa. Disease-causing bacteria adapted to 2. Psychrotrophs growth in the body, have an optimum Optimum temperature between 20°C and between 35°C and 40°C. 30°C, Mesophiles that inhabit soil, a colder Grow well between 0 and 7°C. environment, generally have a lower Important cause of food spoilage. optimum, close to 30°C.  Factors affecting the growth of bacteria 1-Temperature 5. Hyperthermophiles 4. Thermophiles Optimum temperature between 70°C Optimum temperature between 45°C and and 110°C. 70°C. Usually members of the Archaea. Occur in hot springs and water heaters. Effect of temperature on bacteria: Thermophiles differ from mesophiles in Dry heat: causes charring and having much more heat-stable enzymes oxidation. and protein synthesis systems able to Moist heat: causes melting of bacterial function at high temperatures. lipids, coagulation and denaturation of bacterial proteins.  Factors affecting the growth of bacteria 2. Oxygen (O2) 2. Obligate anaerobes Cannot multiply if any O2 is present; 1. Obligate aerobes they are killed by traces of O2 because of Have an absolute or obligate requirement its toxic derivatives. for oxygen. Transform energy by fermentation or by They use it to transform energy in the anaerobic respiration. process of aerobic respiration. Include members of the genus Bacteroides, Include members of the genus the major inhabitants of the large intestine. Pseudomonas, and Tubercle bacilli. Another obligate anaerobe is Clostridium botulinum, the causative agent of botulism.  Factors affecting the growth of bacteria 2. Oxygen (O2) 3. Facultative anaerobes Include E. coli, a common inhabitant of the Grow better if O2 is present, but can grow large intestine, and the yeast without it. Saccharomyces used to make bread and The organism is flexible, in its requirements alcoholic beverages. for O2. 4. Microaerophiles Use aerobic respiration if oxygen is Require small amounts of O2 (2% to 10%) available, for aerobic respiration; higher use fermentation or anaerobic respiration if concentrations are inhibitory. O2 absent. E.g. Helicobacter pylori, which causes gastric and duodenal ulcers.  Factors affecting the growth of bacteria 2. Oxygen (O2) 3. pH 5. Aerotolerant anaerobes Most bacteria can live and multiply within Are indifferent to O2. the range of pH 5 (acidic) to pH 8 (basic) They can grow in its presence, but they do and have a pH optimum near neutral (pH not use it to transform energy. 7). They do not use aerobic or anaerobic These bacteria are called 1- Neutrophiles. respiration, they are called obligate Preservation methods that acidify foods, fermenters. such as pickling, are intended to inhibit They include bacteria used in yogurt- these organisms. making, and Streptococcus pyogenes, which causes strep throat.  Factors affecting the growth of bacteria 3. pH Some neutrophiles have adapted special 2. Acidophiles mechanisms that enable them to grow at a Grow optimally at a pH below 5.5. very low pH. Helicobacter pylori grows in the 3. Alkalophiles stomach, where it can cause ulcers. Grow optimally at a pH above 8.5. To maintain the pH close to neutral in its For example, Bacillus alcalophilus grows immediate surroundings, H. pylori produces best at pH 10.5. the enzyme urease, which splits urea in the Alkalophiles often live in alkaline lakes stomach into carbon dioxide and ammonia. and soils. e.g. campylobacter species. The ammonia neutralizes the stomach acid in the bacterium’s immediate surroundings. Bacillus lactis is added to milk as milk starter.  Factors affecting the growth of bacteria 4. Osmotic pressure Many marine bacteria are mildly Bacteria that can tolerate high salt halophilic, requiring concentrations concentrations, up to approximately 10% of approximately 3% sodium NaCl, are called osmotolerant as chloride. Staphylococcus species. The growth-inhibiting effect of high Organisms that require high levels of concentrations of salt and sugars is sodium chloride to grow are called used in food preservation. halophiles Bacteria requiring high osmotic (halo means “salt” and phile means pressure care called osmophilic. “loving”).  Factors affecting the growth of bacteria 4. Osmotic pressure 5. Dryness and moisture Effect of solute concentration on Water is essential for the bacteria because it bacteria: takes its nutrient requirements in a soluble form Hypertonic solution causes plasmolysis by diffusion. as bacteria losses water resulting in salt Water constitutes about 70-90% of bacteria concentration followed by cell lysis. (vegetative from). Hypotonic solution causes Bacteria are very sensitive to dryness except plasmoptysis as water enter bacterial Tubercle bacilli which resists dryness. cell resulting in distention and Dryness leads to: loss of water and salts protrusion of the cell membrane at weak (plasmolysis), destruction of the enzyme areas. system and coagulation of proteins.  Factors affecting the growth of bacteria 5. Dryness and moisture Factors affecting death by dryness: 1. Species of bacteria: Tubercle bacilli are more resistant than other bacteria. 2. Form of bacteria: sporulated bacteria are more resistant than non-sporulated. 3. Capsule: capsulated bacteria are more resistant than non-capsulated. 4. Environment: high temperature increases the effect of dryness, sputum protects bacteria from dryness, proteineous substances protect bacteria from dryness.  Factors affecting the growth of bacteria 6. Carbon dioxide Those that use organic carbon as glucose are called heterotrophs. Autotrophs: Use inorganic carbon in the form of carbon dioxide as their carbon source. The amount of CO2 in the air is sufficient for the growth of most bacteria; but some need extra CO2. Brucella abortus needs 5-10% CO2 for primary isolation, but no extra CO2 is required for subculturing. Campylobacter needs 5-10% CO2 continuously.  Nutritional requirements for bacterial growth 1. Major elements: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, magnesium, calcium, and iron. They are the essential components of proteins, carbohydrates, lipids, and nucleic acids. 2. Trace elements: are required in very minute amounts by all cells. As cobalt, zinc, copper, molybdenum, and manganese, required for enzyme function.  Nutritional requirements for bacterial growth Type Energy source Carbon source Photoautotroph Sunlight CO2 Photoheterotroph Sunlight Organic compounds Inorganic chemicals (H2, Chemolithoautotroph CO2 NH3, NO2, Fe, H2S) Organic compounds (sugars, Chemoorganoheterotroph Organic compounds amino acids,…)