Microbiology Chapter 6 PDF

CHAPTER 6 Bacterial Growth, Nutrition, and Differentiation Copyright © 2021 W. W. Norton & Company, Inc. Bacterial Growth, Nutrition, and Differentiation Chapter Objectives ▪ Explain how nutrition and the environment can impact microbial growth and differentiation. ▪ Discuss microbial clas...

CHAPTER 6 Bacterial Growth, Nutrition, and Differentiation Copyright © 2021 W. W. Norton & Company, Inc. Bacterial Growth, Nutrition, and Differentiation Chapter Objectives ▪ Explain how nutrition and the environment can impact microbial growth and differentiation. ▪ Discuss microbial classifications based on nutritional needs and environmental limits. ▪ Describe how understanding microbial growth helps identify disease-causing pathogens. ▪ Discuss biofilms and their importance to infectious diseases. 2 6.1 Culturing and Counting Bacteria Section Objectives ▪ Explain how pure cultures are obtained and why they are important in medicine. ▪ Distinguish among synthetic, complex, selective, and differential media and their use in clinical microbiology. ▪ Describe the ways bacterial growth is measured, and explain the advantages and disadvantages of each method. 3 Growing Bacteria in Culture ▪ For detailed laboratory studies microbes must be grown separately in pure culture—that is, as a single species. ▪ Bacterial culture media can be liquid or solid. Liquid media allows bacteria to move freely. Solid media is useful to separate mixtures of different organisms. 4 Obtaining Pure Cultures – 1 ▪ Solid media Bacterial cells form colonies on solid media with agar added to make a firm surface. Isolation streaking allows for separation of colonies into pure cultures. 5 Obtaining Pure Cultures – 2 ▪ Pure colonies can also be isolated using the spread plate technique. 6 Selective and Differential Media ▪ Selective media Compounds in the media prevent some types of bacteria from growing, favoring the growth of one specific type. ▪ Differential media Species grow equally well but compounds in the media are metabolized differently, often distinguished by a color indicator. ▪ Example: MacConkey medium Selective agents are bile salts and crystal violet, which prevent growth of bacteria other than Gram-negative enteric bacteria. Differential agent is lactose, which some bacteria ferment (pink) and others do not. 8 6.2 The Growth Cycle – 1 Section Objectives ▪ Understand the phases of a typical bacterial growth curve. ▪ Explain how bacterial growth correlates to disease. ▪ Describe the purpose of continuous culture and how it correlates to the human digestive tract. 9 6.2 The Growth Cycle – 2 ▪ Bacterial growth is measured at the population level. ▪ Most bacteria reproduce by binary fission (one parent cell divides and forms two offspring cells). Binary fission may be symmetrical or asymmetrical. ▪ Eukaryotic microbes divide by mitosis. 10 Exponential Growth ▪ Growth in which population size doubles at a fixed rate (say, every 20 minutes) is called exponential growth. For example two cells divide to become four cells, then four cells divide to become eight. ▪ In an environment with few bacteria but plenty of resources, bacteria will divide at a constant interval called the generation time (also called doubling time). ▪ Starting with any number of organisms at time zero (N0), the number of organisms after n generations will be N0 × 2n. 11 Phases of Growth ▪ Bacterial growth curve, showing the change in growth rate over time 12 6.3 Microbial Nutrition Section Objectives ▪ Describe the importance of the nitrogen and carbon cycles and the role of microbes in their maintenance. ▪ Discuss biofilms and their relevance to infectious diseases. 13 Nutrients and Environmental Niches – 1 ▪ Essential nutrients are ▪ Elements those compounds a Carbon (C) microbe cannot make Nitrogen (N) itself but must gather Phosphorus (P) from its immediate Hydrogen (H) environment if the cell is Oxygen (O) to grow and divide. Sulfur (S) Magnesium (Mg2+) Iron (Fe2+) Potassium (K+) Trace elements such as cobalt, copper, and zinc 14 Obtaining Nitrogen – 1 ▪ Nitrogen is needed by cells to make proteins and nucleic acids. The nitrogen cycle converts nitrogen to various forms. ▪ (The following examples are in bacteria outside of the body). Some bacteria perform nitrogen fixation to convert nitrogen gas (N2)to ammonium ions (NH4+), which is a form that can be used for biosynthesis. Other bacteria transform ammonia to nitrate NO3 (nitrification) and then convert nitrate to N2 (denitrification). 15 ▪ In the context of UTIs, nitrate reduction is a relevant concept: ▪ Nitrate Reduction: Some bacteria, particularly Escherichia coli (E. coli), can reduce nitrates (NO₃⁻) to nitrites (NO₂⁻) in the urine. ▪ In the gut, dietary proteins are broken down into amino acids, which are further deaminated to release ammonia. ▪ This ammonia can be absorbed into the bloodstream → the liver → Urea → excreted by the kidneys. ▪ Microbial Role: Some gut bacteria may influence ammonia levels.  The gut microbiota and hepatic encephalopathy ▪ Hepatic encephalopathy: a brain disorder caused by toxin buildup in the brain that occurs in individuals with severe liver disease (cirrhosis) ▪ The gut microbiota in cirrhosis is characterized by overgrowth of potential pathogenic bacteria (Dysbiosis) ▪ Increased pathogenic bacteria → rise of bacterial metabolites → decreased liver function → bacterial products enter the brain thus causing disease (ex. Ammonia) 18 6.4 Environmental Limits on Microbial Growth – 1 Section Objectives ▪ List different classes of microbes based on their preferred environmental niches (pH, temperature, and salt). 19 6.4 Environmental Limits on Microbial Growth – 2 ▪ In addition to food, bacterial growth considerations include: Temperature and pressure Osmotic balance pH level 20  Normophiles (Mesophiles): – Thrive in moderate environments: – Temperature: Around 37°C (human body temperature). – pH: Neutral to slightly alkaline (around pH 7.4). – Oxygen: Aerobic, anaerobic, or facultative anaerobic. o Pathogenic Examples: o Escherichia coli (E. coli) o Staphylococcus aureus o Salmonella species o Streptococcus species ▪ Extremophiles: Thrive in extreme environments: Thermophiles: High temperatures (e.g., hot springs). Halophiles: High salt concentrations. Acidophiles: Very acidic conditions. Rarely pathogenic: Human body doesn’t provide extreme conditions needed for survival. 6.4 Environmental Limits on Microbial Growth – 3 23 Variations in Temperature – 1 ▪ Temperature 24 6.5 Living with Oxygen Section Objectives ▪ Differentiate anaerobes from aerobes and describe how each are cultured. ▪ Explain how both aerobes and anaerobes can cause disease. ▪ Discuss the basic differences between respiration and fermentation and how this impacts where an organism grows. 25 Aerobes versus Anaerobes – 1 ▪ Strict aerobes Require oxygen for energy metabolism Successfully detoxify reactive oxygen species (ROS) Survive only in environments with oxygen ▪ Strict anaerobes Do not require oxygen for energy metabolism Generally unable to detoxify ROS, making oxygen toxic Survive only in environments without oxygen 26 Aerobes versus Anaerobes – 2 27 Aerobes versus Anaerobes – 3 ▪ Microaerophiles Aerobic, but ROS can be toxic Survive in environments with lower oxygen concentration ▪ Aerotolerant anaerobes Anaerobic but less susceptible to ROS, and usually lack catalase Prefer anaerobic conditions Cannot use oxygen but tolerates it's presence ▪ Facultative anaerobes Aerobic AND anaerobic Can use oxygen if it is present but can grow without it 28 Catalase ▪ Catalase is an enzyme produced by bacteria that catalyzes the decomposition of hydrogen peroxide into oxygen and water. o This prevents the accumulation of hydrogen peroxide which can cause potential damage to cellular organelles or tissues o Essentially, the catalase enzyme is an antioxidant Catalase test ▪ In a clinical setting, the catalase test can be used to distinguish between different bacteria, as not all bacteria express catalase Ex. Catalase positive Micrococcaceae from catalase- negative Streptococcaceae ▪ In the catalase test, bacteria are exposed to hydrogen peroxide o Bacteria capable of synthesizing catalase will contain bubbles as a result of the reaction 6.6 Microbial Communities and Cell Differentiation Section Objectives ▪ Discuss how biofilms develop and the role of quorum sensing in the process. ▪ Explain the importance of biofilms to infection. ▪ Describe the process of sporulation, and explain how spores impact certain infections. 32 Biofilms: Multicellular Microbes? – 1 ▪ A biofilm is a mass of bacteria that stick to and multiply on a solid surface. Can include a single species or multiple collaborating species Can grow on organic or inorganic surfaces 33 Biofilms: Multicellular Microbes? – 2 ▪ Cells communicate and coordinate actions through quorum sensing. (Individual signaling → Autoinducer → Cells can ‘sense’. ▪ Bacteria in biofilms are very resistant to destruction. 34 Case History: Death by Biofilm ▪ Cystic fibrosis (CF) is an inherited disease in which chloride ion (Cl–) transport is compromised in many organ systems. The lungs are especially affected by the production of thick, sticky mucous buildup. 35 Case History: Death by Biofilm ▪ In 1951, two women with CF developed acute respiratory distress. From their chest X-rays, they were diagnosed as having an infectious bronchopneumonia. Treatment at the time consisted of intramuscular administration of the antibiotics streptomycin and penicillin. Case History: Death by Biofilm ▪ Repeated bronchial lavage (washing) was attempted in a futile bid to clear airway secretions. Tragically, the two women did not respond to therapy and died within hours. Pseudomonas aeruginosa, a Gram-negative bacillus common in soil and water, was subsequently isolated from both victims. This was the first report of Pseudomonas infection in CF patients. Endospores: Time Capsules for Bacteria – ▪ Endospores Certain Gram-positive bacteria produce destruction-resistant endospores. Bacteria in endospore form are dormant and require no nutrition or energy. – Bacillus, such as B. anthracis – Clostridium, such as C. botulinum and C. tetani Endospore state can last for decades or centuries before reactivation may occur. 38 Clicker Question 1 You are interested in determining the total number of viable cells in a bacterial culture. Which method of counting would you choose for the most accurate results? a. spread plate dilutions b. direct cell counts using a microscope c. flow cytometry d. turbidity with a spectrophotometer 40 Clicker Question 2 A culture of bacteria is growing exponentially under ideal conditions. Its generation time is 20 minutes. If the culture starts with two bacteria, how many bacteria will you find after 1 hour of growth? a. 2 b. 4 c. 8 d. 16 e. 32 41 Clicker Question 3 How would you classify a bacterial species that grows only in the highlighted area of the culture tube? a. aerotolerant b. strict anaerobe c. strict aerobe d. facultative anaerobe e. microaerophile 42