Questions and Answers
Which bacterium would theoretically be more likely to grow at refrigerator temperatures: a human intestinal pathogen or a soilborne plant pathogen?
Soilborne plant pathogen
Why is osmotic pressure an important factor in microbial growth?
Osmotic pressure affects microbial growth by regulating water movement into or out of cells.
Given a shallow pan and a deep pot with the same volume, which would cool faster? Why?
Shallow pan; because it has a higher surface area to volume ratio.
How would one determine whether a microbe is a strict anaerobe?
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What is the most obvious way for a microbe to avoid damage from oxygen?
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Why is prevention of biofilms important in a health care environment?
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What is a culture medium?
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What is the known record for bacterial growth and replication at high temperatures?
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Why do most bacteria grow best in a narrow pH range near neutrality?
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What is the primary producers at the ocean floor in deep-ocean areas where no light penetrates?
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What advice would you give to a patient who is alarmed because hydrogen peroxide caused her wound to bubble?
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What are the three primary groups microorganisms are classified into based on their preferred range of temperature?
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Molecular oxygen has always been abundantly present in Earth's atmosphere.
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___________ reproduce best at their optimum growth temperature.
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Psychrophiles are classified as heat-loving microbes.
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What are some elements important for microorganisms?
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Obligate aerobes can only grow in the absence of oxygen.
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What is the role of catalase in bacteria?
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Microorganisms use nitrogen primarily to form the ______ group of amino acids.
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Match the following elements with their importance for microorganisms:
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What is an inoculum in microbiology?
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What are the criteria that the culture medium must meet to inhibit the formation of biofilms?
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What purpose does lactoferrin serve in inhibiting biofilm formation?
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What is the technical name for bacteria that require a higher-than-atmospheric-concentration of CO2 for growth?
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In what way would an anaerobic chamber resemble the Space Special Culture Techniques?
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What term is used for microbes that grow better at high CO2 concentrations?
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What does an enrichment culture provide to favor the growth of a specific microbe?
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What type of medium contains red blood cells and is used to identify bacterial species that destroy these cells? Blood _____ agar.
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Hemolysis causes a clear ring around bacterial colonies on blood agar.
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Study Notes
Microbial Growth
- Microbial growth refers to the increase in the number of cells, not the size of the cells.
- Microbes can grow slowly in nutrient-poor environments by forming biofilms.
- Biofilms are frequently sources of healthcare-associated infections.
The Requirements for Growth
- The requirements for microbial growth can be divided into two main categories: physical and chemical.
- Physical aspects include temperature, pH, and osmotic pressure.
- Chemical requirements include sources of carbon, nitrogen, sulfur, phosphorus, oxygen, trace elements, and organic growth factors.
Temperature
- Most microorganisms grow well at temperatures that humans favor.
- Microorganisms are classified into three primary groups based on their preferred range of temperature: psychrophiles (cold-loving microbes), mesophiles (moderate-temperature-loving microbes), and thermophiles (heat-loving microbes).
- Each bacterial species grows at a particular minimum, optimum, and maximum temperature.
- The optimum growth temperature is the temperature at which the species grows best.
- The maximum growth temperature is the highest temperature at which growth is possible.
pH
- The pH of culture media is controlled to support microbial growth.
- pH affects the activity of enzymes and the availability of nutrients.
Osmotic Pressure
- Osmotic pressure is an important factor in microbial growth.
- Microorganisms need to maintain a balance between the concentration of solutes inside and outside the cell.
- Plasmolysis occurs when the concentration of solutes outside the cell is higher than inside, causing the cell to shrink.
Oxygen Requirements
- Microorganisms can be classified based on their oxygen requirements: aerobes, anaerobes, and facultative anaerobes.
- Aerobes require oxygen for growth and can avoid damage by toxic forms of oxygen.
- Anaerobes do not require oxygen for growth and can be damaged by oxygen.
- Facultative anaerobes can grow in the presence or absence of oxygen.
Case Studies
- Clinical Case: Glowing in the Dark - Pseudomonas fluorescens bloodstream infections in patients after the recall of an intravenous heparin solution.
- Microorganisms can grow in a very short time and form biofilms on medical devices, such as urinary catheters and contact lenses.
Microbial Growth and Food Preservation
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Refrigeration is the most common method of preserving household food supplies.
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Microbial growth rates decrease at low temperatures, making refrigeration effective in preventing the growth of spoilage and disease organisms.
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Psychrotrophs, which can grow at low temperatures, can cause food spoilage.
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The importance of low temperatures for preventing the growth of spoilage and disease organisms is illustrated in Figure 6.2.### Osmotic Pressure
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Osmotic pressure affects microbial growth, as it can cause water to enter or leave the cell.
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Microorganisms obtain nutrients from surrounding water, requiring 80-90% water in their composition.
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High osmotic pressure can remove necessary water from a cell, causing plasmolysis (shrinkage of cytoplasm).
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Isotonic solution: solute concentration in the cell is equivalent to the surrounding environment (e.g., 0.85% NaCl).
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Hypertonic solution: solute concentration in the surrounding environment is higher than in the cell (e.g., 10% NaCl).
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Hypotonic solution: solute concentration in the surrounding environment is lower than in the cell.
pH and Temperature
- Most bacteria grow best in a narrow pH range near neutrality (pH 6.5-7.5).
- pH affects microbial growth, with acidophiles tolerant of acidity and alkalinity inhibiting growth.
- Thermophiles: microorganisms capable of growth at high temperatures (e.g., 50-60°C).
- Hyperthermophiles: microorganisms capable of growth at extremely high temperatures (e.g., 80°C or higher).
- Thermophiles are important in organic compost piles and in hot springs.
Halophiles
- Halophiles: microorganisms adapted to high salt concentrations.
- Obligate halophiles: require high salt concentrations for growth.
- Facultative halophiles: can grow in high salt concentrations but don't require them.
Chemical Requirements
- Carbon: the structural backbone of living matter, needed for all organic compounds.
- Nitrogen: required for protein synthesis, makes up 14% of dry weight of a bacterial cell.
- Sulfur: required for sulfur-containing amino acids and vitamins.
- Phosphorus: essential for nucleic acid synthesis, phospholipid cell membranes, and ATP energy bonds.
Oxygen Requirements
- Obligate aerobes: require oxygen to live, use oxygen for aerobic respiration.
- Facultative anaerobes: can use oxygen when present but can continue growth without oxygen using fermentation or anaerobic respiration.
- Obligate anaerobes: cannot use oxygen, are harmed by it, and use alternative electron acceptors.
Other Elements
- Potassium, magnesium, and calcium: required as cofactors for enzymes.
- Trace elements: small amounts of other mineral elements (e.g., iron, copper, molybdenum, zinc) required for certain enzymes.### Microbial Growth
The Effect of Oxygen on Bacterial Growth
- Obligate aerobes: require oxygen for growth, can only grow in aerobic conditions
- Facultative anaerobes: can grow in both aerobic and anaerobic conditions, but grow better in aerobic conditions
- Obligate anaerobes: cannot grow in aerobic conditions, require anaerobic conditions
- Aerotolerant anaerobes: cannot use oxygen for growth, but can tolerate its presence
- Microaerophiles: require oxygen for growth, but only in low concentrations
Toxic Forms of Oxygen
- Singlet oxygen: highly reactive, formed when molecular oxygen is boosted to a higher energy state
- Superoxide radicals: formed during normal respiration, toxic to cellular components
- Hydrogen peroxide: produced during normal aerobic respiration, contains peroxide anion and is toxic
- Hydroxyl radical: highly reactive, formed in cellular cytoplasm by ionizing radiation
Enzymes to Neutralize Toxic Forms of Oxygen
- Catalase: converts hydrogen peroxide into water and oxygen
- Superoxide dismutase (SOD): converts superoxide radicals into molecular oxygen and hydrogen peroxide
- Peroxidase: breaks down hydrogen peroxide into water and oxygen
Biofilms
- Thin, slimy layer of bacteria that adheres to a surface
- Formed through cell-to-cell chemical communication (quorum sensing)
- Provide benefits to bacteria, such as protection from harmful factors and sharing of nutrients
- Can be composed of single species or diverse groups of microorganisms
- Can take various forms, such as filamentous streamers or pillar-like structures
Biofilm Formation and Importance
- Begins when a free-swimming bacterium attaches to a surface
- Can be a problem in pipes and tubing, impeding circulation
- Important in human health, with 70% of human bacterial infections involving biofilms
- Can form on medical devices, such as catheters and mechanical heart valves
Organic Growth Factors
- Essential organic compounds that an organism cannot synthesize
- Examples: vitamins, amino acids, purines, and pyrimidines
- Required for growth and function of certain enzymes
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