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Questions and Answers
What characterizes exponential growth in bacterial populations?
Which of the following accurately describes generation time?
In binary fission, how does the parent cell reproduce?
Which phase of the bacterial growth cycle is characterized by metabolic activity but little increase in cell number?
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What occurs during the stationary phase of bacterial growth?
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Which of the following statements about budding is correct?
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During which phase does the population experience its maximum growth?
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Which of the following statements about the growth rate of bacteria is true?
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What is the main role of catalase in cells?
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What do biofilms commonly form on in the medical industry?
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Which of the following methods involves counting colonies on agar plates?
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What impact do biofilms have on antibiotic resistance?
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What is a common characteristic of bacteria within biofilms?
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Which indirect method measures cell growth by assessing cloudiness?
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What is one consequence of chronic infections due to biofilms?
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Which method uses a filter to collect bacteria for counting?
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What role does quorum sensing play in biofilms?
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What characterizes the formation of biofilms?
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At what temperature range do mesophiles typically grow?
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What adaptation classification includes organisms that require extremely high salt concentrations?
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Why is the pH of culture media controlled using buffers?
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What is the effect of a hypertonic environment on bacterial cells?
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Which of the following types of microbes cannot survive in the presence of oxygen?
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What organisms are classified as acidophiles?
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What role does superoxide dismutase (SOD) play in aerobic organisms?
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Which condition is most likely to cause plasmolysis in bacterial cells?
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How is nitrogen primarily acquired by most bacteria?
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What class of microbes can grow in environments under extreme pressure?
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Study Notes
Microbial Growth
- Linear growth occurs at a constant rate, adding the same number of cells per unit of time.
- Exponential growth occurs by multiplication, where the population doubles at regular intervals.
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Generation time is the time it takes for a bacterial population to double in number.
- Most bacteria have a generation time of 1-2 hours.
- The time can range from 20 minutes to 24 hours.
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Theoretical growth calculation: Nt = N0 x 2n
- Nt: number of cells after a certain time
- N0: initial number of cells
- n: number of generations (calculated by dividing the total time by the generation time)
Bacterial Reproduction
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Binary fission: the most common method of bacterial reproduction.
- The parent cell duplicates its DNA and divides into two identical daughter cells.
- The division is symmetrical, meaning the two daughter cells are the same size.
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Budding: less common in bacteria but seen in some species.
- A small outgrowth, or bud, forms on the parent cell.
- The bud grows until it detaches, becoming a new, smaller daughter cell.
- The division is asymmetrical, meaning the daughter cell is smaller than the parent cell.
Bacterial Growth Cycle
- Consists of distinct phases that represent changes in the growth rate of a population over time.
- Occurs in a closed system, like lab culture, where nutrients are limited.
- Lag phase: a "flat" period of adjustment, adapting to the environment. Cells are metabolically active, preparing for division. Little growth occurs.
- Log (exponential) phase: period of maximum growth, continues as long as cells have adequate nutrients and a favorable environment. The population doubles at regular intervals, and resources are abundant.
- Stationary phase: the rate of cell growth equals the rate of cell death caused by depleted nutrients, oxygen, and excretion of organic acids and pollutants. Growth slows down as nutrients become depleted, and waste products accumulate.
- Death (decline) phase: as limiting factors intensify, cell death begins to occur. Cells begin to die at an exponential rate due to the complete depletion of nutrients and the accumulation of toxic waste products.
Physical Growth Requirements
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Temperature adaptation classifications:
- Psychrophiles: cold-loving, grow between -20°C and -10°C, found in arctic and Antarctic regions.
- Mesophiles: moderate-temperature-loving, grow between 20°C and 40°C, include many human pathogens, as body temperature is around 37°C.
- Thermophiles: heat-loving, optimum growth >45°C, found in hot springs and compost heaps.
- Hyperthermophiles: thrive at extremely high temperatures, optimum growth temperature >70°C, found in environments like hydrothermal vents.
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pH adaptation classifications:
- Acidophiles: grow in acidic environments (pH 8).
- Neutrophiles: grow best at neutral pH (pH 6.5 to 7.5). Most bacteria fall into this category.
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Osmotic adaptation classifications:
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Halophiles: live in salty environments.
- Extreme (obligate) halophiles: require extremely high levels of salt for growth (high osmotic pressure).
- Halotolerant (facultative): can tolerate high salt but do not require high salt to grow.
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Osmotic pressure: the difference in solute concentration inside vs outside the cell.
- Hypertonic environment: higher in solutes than inside the cell, plasmolysis occurs due to loss of water.
- Bacterial plasmolysis: water leaves the cell, growth is inhibited.
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Halophiles: live in salty environments.
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Atmospheric pressure adaptation classification:
- Barophiles: can survive in environments under extreme pressure, such as deep-sea organisms. Will rupture if exposed to normal atmospheric pressure.
Controlling pH of Culture Media
- The pH of culture media is controlled using buffers to maintain a stable environment for microbial growth.
- Microbes produce acidic or basic metabolic byproducts, which can alter the pH of the media, inhibiting growth.
- Buffers help to neutralize these byproducts, ensuring the pH remains suitable for the microbe being cultured.
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Preferred pH for most bacterial species vs fungal species:
- Most bacteria grow between pH 6.5 and 7.5.
- Molds and yeast grow between pH 5 and 6.
Importance of Osmotic Pressure to Microbial Growth
- Osmotic pressure refers to the balance of water between the inside of a cell and its external environment.
- Hypertonic: water will leave the cell, causing it to shrink (plasmolysis), which can inhibit growth.
- Hypotonic: water enters the cell, which can cause the cell to swell and potentially burst.
- Maintaining proper osmotic pressure is essential for cells to prevent water loss or gain, which affects their ability to grow and survive.
Chemical Growth Requirements
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Four main elements needed in large amounts for microbial growth:
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Carbon: structural backbone of all organic molecules, required for all life forms.
- Heterotrophs: use organic molecules as energy (sugar, protein, lipids).
- Autotrophs: use CO2 to make their own organic molecules.
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Nitrogen: component of amino acids, DNA, RNA, and ATP.
- Most bacteria decompose protein material for their nitrogen source.
- A few bacteria use N2 in nitrogen fixation.
- Some bacteria use N2+ or NO3- from organic material.
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Sulfur: important for the synthesis of some acids (like cysteine) and vitamins. Required for protein structure and enzyme function.
- Some bacteria use SO42- or H2S.
- Most bacteria decompose protein for their sulfur source.
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Phosphorus: essential for the formation of DNA, RNA, ATP, and phospholipids.
- Found in membranes – phospholipid bilayer.
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Carbon: structural backbone of all organic molecules, required for all life forms.
Acquiring Chemical Growth Requirements
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Carbon:
- Heterotrophs: get carbon from organic compounds (like glucose).
- Autotrophs: fix carbon from CO2 in the atmosphere.
- Nitrogen: acquired from ammonia (NH3), nitrates (NO3-) or atmospheric nitrogen (N2) in nitrogen-fixing bacteria.
- Sulfur: obtained from sulfate (SO42-), hydrogen sulfide (H2S), or organic sulfur-containing compounds.
- Phosphorus: taken up as phosphate ions (PO43-) from the environment.
Microbial Oxygen Requirements
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Classification based on oxygen requirements:
- Obligate aerobes: require oxygen for growth and cannot survive without it.
- Obligate anaerobes: cannot survive in the presence of oxygen, as it is toxic to them.
- Facultative anaerobes: can grow with or without oxygen.
- Aerotolerant anaerobes: tolerate oxygen but cannot use it.
- Microaerophiles: require oxygen lower than air.
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Three enzymes that help aerobes avoid damage by toxic forms of oxygen:
- Superoxide dismutase (SOD): converts the toxic superoxide radicals (O2-) into hydrogen peroxide (H2O2) and oxygen (O2).
- Catalase: converts hydrogen peroxide (H2O2), which is still toxic to cells, into water (H2O) and oxygen (O2).
- Peroxidase (hydrogen peroxidase): breaks down hydrogen peroxide (H2O2), but instead of producing oxygen, it produces water by using reducing agents like NADH or glutathione.
Biofilms
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Biofilms: communities of microorganisms (bacteria, fungi, or other microbes) that are embedded in a self-produced matrix of extracellular polymeric substances (EPS), such as proteins.
- They form slime or hydrogels that adhere to surfaces.
- Bacteria communicate cell-to-cell via quorum sensing, share nutrients, and shelter bacteria from harmful environmental factors.
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Formation:
- Microbes attach to a surface.
- They secrete EPS, which protects them and allows them to stick together.
- As the biofilm grows, different layers of microbes form, and the biofilm becomes highly structured.
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Potential for causing infection:
- Biofilms protect bacteria from environmental threats, including antibiotics and the immune system, making them much harder to eradicate.
- They can form on medical devices like catheters, heart valves, joint implants, and other surfaces inside the human body, leading to chronic infections.
- Bacteria within biofilms are up to 1,000 times more resistant to antibiotics compared to free-floating bacteria.
Why Biofilms Are of Special Concern in the Medical Industry
- Antibiotic resistance: creates a protective environment for bacteria, making them highly resistant to antibiotics. This complicates treatment, as higher doses or more potent drugs may be required, but even then, they may fail to fully penetrate the biofilm.
- Chronic infections: can lead to persistent, chronic infections. They are difficult to eliminate and often cause infections to reoccur, such as in cases of UTI or chronic wounds.
- Medical devices: commonly form on medical devices. Once formed, they can seed infections into the bloodstream or surrounding tissues, causing life-threatening infections like sepsis or endocarditis.
- Increased healthcare costs: biofilm-related infections often require longer hospital stays, repeated surgeries, and additional treatments, which raises the overall cost of care and increases the burden of healthcare systems.
Methods of Measuring Cell Growth
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Direct methods of measuring cell growth: counting microbial cells
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Plate count: count colonies on plates that have 30 to 300 CFUs.
- To ensure the right number of colonies, the original inoculum must be diluted via serial dilution.
- Spread plate method: bacteria diluted in liquid broth and spread directly on the surface of an agar plate.
- Pour plate method: bacteria diluted into a soft agar and poured onto an agar plate.
- Filtration: solution passed through a filter that collects bacteria. The filter is transferred to a Petri dish and grows as colonies on the surface.
- Most probable number (MPN) method: multiple tubes inoculated with different sample volumes. Count positive tubes and compare with a statistical table.
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Direct microscopic count: a volume of a bacterial suspension is placed on a slide with a grid. The average number of bacteria per viewing field is calculated. Uses a special Petroff-Hausser cell counter.
- Number of bacteria/ml = (number of cells counted) / (volume of area counted).
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Plate count: count colonies on plates that have 30 to 300 CFUs.
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Indirect methods of measuring cell growth: estimate cell growth by measuring factors related to cell activity rather than directly counting cells.
- Turbidity: measurement of cloudiness with a spectrophotometer.
- Metabolic activity: the amount of metabolic product (color change) is proportional to the number of bacteria.
- Dry weight: bacteria are filtered, dried, and weighed, used for filamentous organisms.
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Description
This quiz explores the concepts of microbial growth, including linear and exponential growth rates and generation times. It also covers bacterial reproduction methods like binary fission and budding. Test your understanding of these crucial microbiological processes.
