Microbial Nutrition, Ecology, And Growth

Microbiology: A Systems Approach, 2nd ed. Chapter 7: Microbial Nutrition, Ecology, and Growth

7.1 Microbial Nutrition

• Nutrition: a process by which chemical substances (nutrients) are acquired from the environment and used in cellular activities • All living things require a source of elements such as C, H, O, P, K, N, S, Ca, Fe, Na, Cl, Mg- but the relative amounts vary depending on the microbe • Essential Nutrient: any substances that must be provided to an organism – Macronutrients: Required in relatively large quantities, play principal roles in cell structure and metabolism (ex. C, H, O) – Micronutrients: aka trace elements, present in smaller amounts and involved in enzyme function and maintenance of protein structure (ex. Mn, Zn, Ni)

• Nutrients are processed and transformed into the chemicals of the cell after absorption • Can also categorize nutrients according to C content – Inorganic nutrients: A combination of atoms other than C and H – Organic nutrients: Contain C and H, usually the products

Chemical Analysis of Microbial Cytoplasm

Sources of Essential Nutrients • • • • • • •

Carbon sources Nitrogen sources Oxygen sources Hydrogen sources Phosphorus sources Sulfur sources Others

Carbon Sources • The majority of C compounds involved in normal structure and metabolism of all cells are organic • Heterotroph: Must obtain C in organic form (nutritionally dependent on other living things) • Autotroph: Uses inorganic CO2 as its carbon source (not nutritionally dependent on other living things)

Nitrogen Sources • Main reservoir- N2 • Primary nitrogen source for heterotrophs- proteins, DNA, RNA • Some bacteria and algae utilize inorganic nitrogenous nutrients • Small number can transform N2 into usable compounds through nitrogen fixation • Regardless of the initial form, must be converted to NH3 (the only form that can be directly combined with C to synthesize amino acids and other compounds)

Oxygen Sources • O is a major component of organic compounds • Also a common component of inorganic salts • O2 makes up 20% of the atmosphere

Hydrogen Sources • H is a major element in all organic and several inorganic compounds • Performs overlapping roles in the biochemistry of cells: – Maintaining pH – Forming hydrogen bonds between molecules – Serving as the source of free energy in oxidation-reduction reactions of respiration

Phosphorus (Phosphate) Sources • Main inorganic source of phosphorus is phosphate (PO43-) – Derived from phosphoric acid – Found in rocks and oceanic mineral deposits

• Key component in nucleic acids • Also found in ATP • Phospholipids in cell membranes and coenzymes

Sulfur Sources • Widely distributed throughout the environment in mineral form • Essential component of some vitamins • Amino acids- methionine and cysteine

Other Nutrients Important in microbial Metabolism • Potassium- protein synthesis and membrane function • Sodium- certain types of cell transport • Calcium- stabilizer of cell walls and endospores • Magnesium- component of chlorophyll and stabilizer of membranes and ribosomes • Iron- important component of cytochrome proteins • Zinc- essential regulatory element for eukaryotic genetics, and binding factors for enzymes • Cooper, cobalt, nickel, molybdenum, manganese, silicon, iodine, and boron-

Growth Factors: Essential Organic Nutrients • Growth factor: An organic compound such as an amino acid, nitrogenous base, or vitamin that cannot be synthesized by an organism and must be provided as a nutrient • For example, many cells cannot synthesize all 20 amino acids so they must obtain them from food (essential amino acids)

How Microbes Feed: Nutritional Types

Main Determinants of Nutritional Type • Sources of carbon and energy • Phototrophs- Microbes that photosynthesize • Chemotrophs- Microbes that gain energy from chemical compounds

Autotrophs and Their Energy Sources • Photoautotrophs – Photosynthetic – Form the basis for most food webs

• Chemoautotrophs – Chemoorganic autotrophs- use organic compounds for energy and inorganic compounds as a carbon source – Lithoautotrophs- rely totally on inorganic minerals – Methanogens- produce methane from hydrogen gas and carbon dioxide • Archae • Some live in extreme habitats

Figure 7.1

Heterotrophs and Their Energy Sources – Majority are chemoheterotrophs that derive both carbon and energy from organic compounds • Saprobes – Free-living microorganisms – Feed primarily on organic detritus from dead organisms – Decomposers of plant litter, animal matter, and dead microbes – Most have rigid cell wall, so they release enzymes to the extracellular environment and digest food particles into smaller molecules » Obligate saprobes- exist strictly on dead organic matter in soil and water » Facultative parasite- when a saprobe infects a host, usually when the host is compromised (opportunistic pathogen)

Figure 7.2

Other Chemoheterotrophs • Parasites – Derive nutrients from the cells or tissues of a host – Also called pathogens because they cause damage to tissues or even death – Ectoparasites- live on the body – Endoparasites- live in organs and tissues – Intracellular parasites- live within cells – Obligate parasites- unable to grow outside of a living host

Transport Mechanisms for Nutrient Absorption • Cells must take nutrients in and transport waste out • Transport occurs across the cell membrane, even in organisms with cell walls

The Movement of Water: Osmosis • Osmosis: Diffusion of water through a selectively permeable membrane • The membrane is selectively permeable- having passageways that allow free diffusion of water but can block certain other dissolved molecules • When the membrane is between solutions of differing concentrations and the solute is not diffusible, water will diffuse at a fast rate from the side that has more water to the side that has less water

Figure 7.3

Osmotic Relationships • •

The osmotic relationship between cells and their environment is determined by the relative concentrations of the solutions on either side of the cell membrane Isotonic: The environment is equal in solute concentration to the cell’s internal environment – No net change in cell volume – Generally the most stable environment for cells

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Hypotonic: The solute concentration of the external environment is lower than that of the cell’s internal environment – Net direction of osmosis is from the hypotonic solution into the cell – Cells without cell walls swell and can burst

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Hypertonic: The environment has a higher solute concentration than the cytoplasm – Will force water to diffuse out of a cell – Said to have high osmotic pressure

Figure 7.4

Adaptations to Osmotic Variations in the Environment • Example: fresh pond waterhypotonic conditions – Bacteria- cell wall protects them from bursting – Amoeba- a water (or contractile) vacuole that moves excess water out of the cell

• Example: high-salt environmenthypertonic conditions – Halobacteria living in the Great Salt

The Movement of Molecules: Diffusion and Transport • Diffusion: When atoms or molecules move in a gradient from an area of higher density or concentration to an area of lower density or concentration – Random thermal movement of molecules will eventually distribute the molecules from an area of higher concentration to an area of lower concentration – Evenly distributes the molecules – Diffusion of molecules across the cell membrane is largely determined by the concentration gradient and permeability of the substance – Simple or passive diffusion is limited to small nonpolar molecules or lipid soluble molecules

Figure 7.5

Facilitated Diffusion • Utilizes a carrier protein that binds a specific substance, changes the conformation of the carrier protein, and the substance is moved across the membrane • Once the substance is transported, the carrier protein resumes its original shape • Carrier proteins exhibit specificity • Saturation: The rate of a substance is limited by the number of binding sites on the transport proteins • Competition: When two molecules of similar shape can bind to the same binding site on a carrier protein

Figure 7.6

Active Transport • Nutrients are transported against the diffusion gradient or in the same direction as the natural gradient but at a rate faster than by diffusion alone • Requires the presence of specific membrane proteins (permeases and pumps) • Requires the expenditure of energy • Items that require active transport: monosaccharides, amino acids, organic acids, phosphates, and metal ions • Specialized pumps- an important type of active transport • Group translocation: couples the transport of a nutrient with its conversion to a substance that is immediately useful inside the cell

Figure 7.7

Endocytosis: Eating and Drinking by Cells • A form of active transport • Transporting large molecules, particles, lipids, or other cells • Occurs in some eukaryotic cells • The cell encloses the substance in its cell membrane, simultaneously forming a vacuole and engulfing it • Phagocytosis- amoebas and certain white blood cells; ingesting whole cells or large solid matter • Pinocytosis- Transport of liquids such as oils or molecules in solution

7.2 Environmental Factors that Influence Microbes • Temperature Adaptations – Microbial cells cannot control their temperature, so they assume the ambient temperature of their natural habitat – The range of temperatures for the growth of a given microbial species can be expressed as three cardinal temperatures: • Minimum temperature: the lowest temperature that permits a microbe’s continued growth and metabolism • Maximum temperature: The highest temperature at which growth and metabolism can proceed • Optimum temperature: A small range, intermediate between the minimum and maximum, which promotes the fast rate of growth and metabolism

– Some microbes have a narrow cardinal range while others have a broad one – Another way to express temperature adaptation- to describe whether an organism grows optimally in a cold, moderate, or hot temperature range

Psychophile • A microorganism that has an optimum temperature below 15°C and is capable of growth at 0°C. • True psychrophiles are obligate with respect to cold and cannot grow above 20°C. • Psychrotrophs or facultative psychrophiles- grow slowly in cold but have an optimum temperature above 20°C.

Figure 7.8

Figure 7.9

Mesophile • An organism that grows at intermediate temperatures • Optimum growth temperature of most: 20°C to 40°C • Temperate, subtropical, and tropical regions • Most human pathogens have optima between 30°C and 40°C

Thermophile • A microbe that grows optimally at temperatures greater than 45°C • Vary in heat requirements • General range of growth of 45°C to 80°C • Hyperthermophiles- grow between 80°C and 120°C

Gas Requirements • Atmospheric gases that most influence microbial growth- O2 and CO2 • Oxygen gas has the greatest impact on microbial growth • As oxygen enters into cellular reactions, it is transformed into several toxic products – Most cells have developed enzymes that go about scavenging and neutralizing these chemicals • Superoxide dismutase • Catalase

– Essential for aerobic organisms

Several General Categories of Oxygen Requirements • Aerobe: can use gaseous oxygen in its metabolism and possesses the enzymes needed to process toxic oxygen products • Obligate aerobe: cannot grow without oxygen • Facultative anaerobe: an aerobe that does not require oxygen for its metabolism and is capable of growth in the absence of it • Microaerophile: does not grow at normal atmospheric concentrations of oxygen but requires a small amount of it in metabolism • Anaerobe: lacks the metabolic enzyme systems for using oxygen in respiration • Strict, or obligate, anaerobes: also lack the enzymes for processing toxic oxygen and cannot tolerate any free oxygen in the immediate environment and will die if exposed to it. • Aerotolerant anaerobes: do not utilize oxygen but can survive and grow to a limited extent in its

Figure 7.10

Figure 7.11

Carbon Dioxide • All microbes require some carbon dioxide in their metabolism • Capnophiles grow best at a higher CO2 tension than is normally present in the atmosphere

Effects of pH • Majority of organisms live or grow in habitats between pH 6 and 8 • Obligate acidophiles – Euglena mutabilis- alga that grows between 0 and 1.0 pH – Thermoplasma- archae that lives in hot coal piles at a pH of 1 to 2, and would lyse if exposed to pH 7

Osmotic Pressure • Most microbes live either under hypotonic or isotonic conditions • Osmophiles- live in habitats with a high solute concentration • Halophiles- prefer high concentrations of salt • Obligate halophiles- grow optimally in solutions of 25% NaCl but require at least 9% NaCl for growth

Miscellaneous Environmental Factors • Nonphotosynthetic microbes tend to be damaged by the toxic oxygen products produced by contact with light – Some produce yellow carotenoid pigments to protect against the damaging effects of light by absorbing and dismantling toxic oxygen

• Other types of radiation that can damage microbes are ultraviolet and ionizing rays • Barophiles: deep-sea microbes that exist under hydrostatic pressures ranging from a few times to over 1,000 times the pressure of the atmosphere • All cells require water- only dormant, dehydrated cell stages tolerate extreme drying

Ecological Associations Among Microorganisms • Most microbes live in shared habitats • Interactions can have beneficial, harmful, or no particular effects on the organisms involved • They can be obligatory or nonobligatory to the members • They often involve nutritional interactions

Symbiosis • A general term used to denote a situation in which two organisms live together in a close partnership – Members are termed symbionts – Three main types of symbionts • Mutualism: when organisms live in an obligatory but mutually beneficial relationship • Commensalism: the member called the commensal receives benefits, while its coinhabitant is neither harmed nor benefited – Satellitism: when one member provides nutritional or protective factors needed by the other

• Parasitism: a relationship in which the host organism provides the parasitic microbe with nutrients and a habitat

Figure 7.12

Nonsymbiotic Relationship • Organisms are free-living and relationships are not required for survival – Synergism: an interrelationship between two or more free-living organisms that benefits them but is not necessary for their survival – Antagonism: an association between free-living species that arrises when members of a community compete • One microbe secretes chemical substances into the surrounding environment that inhibit or destroy another microbe in the

Interrelationships Between microbes and Humans • Normal microbiotia: microbes that normally live on the skin, in the alimentary tract, and in other sites in humans • Can be commensal, parasitic, and synergistic relationships

7.3 The Study of Microbial Growth • Growth takes place on two levels – Cell synthesizes new cell components and increases in size – The numer of cells in the population increases

• The Basis of Population Growth: Binary Fission

Figure 7.13

Figure 7.14

The Rate of Population Growth

– Generation or doubling time: The time required for a complete fission cycle – Each new fission cycle or generation increases the population by a factor of 2 – As long as the environment is favorable, the doubling effect continues at a constant rate – The length of the generation time- a measure of the growth rate of an organism • Average generation time- 30 to 60 minutes under optimum conditions • Can be as short as 10 to 12 minutes

– This growth pattern is termed exponential

Graphing Bacterial Growth • The data from growing bacterial populations are graphed by plotting the number of cells as a function of time – If plotted logarithmically- a straight line – If plotted arithmetically- a constantly curved slope

• To calculate thesize of a population over time: Nf = (Ni)2n – Nf is the total number of cells in the population at some point in the growth phase – Ni is the starting number – N denotes the generation number

The Population Growth Curve • A population of bacteria does not maintain its potential growth rate and double endlessly • A population displays a predictable pattern called a growth curve • The method to observe the population growth pattern: – Place a tiny number of cells in a sterile liquid medium – Incubate this culture over a period of several hours – Sampling the browth at regulat intervals during incubation

Stages in the Normal Growth Curve • Data from an entire growth period typically produce a curve with a series of phases • Lag Phase • Exponential Growth Phase • Stationary Growth Phase • Death Phase

Lag Phase • Relatively “flat” period • Newly inoculated cells require a period of adjustment, enlargement, and synthesis • The cells are not yet multiplying at their maximum rate • The population of cells is so sparse that the sampling misses them • Length of lag period varies from one population to another

Exponential Growth (Logarithmic or log) Phase • When the growth curve increases geometrically • Cells reach the maximum rate of cell division • Will continue as long as cells have adequate nutrients and the environment is favorable

Stationary Growth Phase • The population enters a survival mode in which cells stop growing or grow slowly – The rate of cell inhibition or death balances out the rate of multiplication – Depleted nutrients and oxygen – Excretion of organic acids and other biochemical pollutants into the growth medium

Death Phase • The curve dips downward • Cells begin to die at an exponential rate

Figure 7.15

Potential Importance of the Growth Curve • Implications in microbial control, infection, food microbiology, and culture technology • Growth patterns in microorganisms can account for the stages of infection • Understanding the stages of cell growth is crucial for working with cultures • In some applications, closed batch culturing is inefficient, and instead,

Other Methods of Analyzing Population Growth – Turbidometry- a tube of clear nutrient solution becomes turbid as microbes grow in it

Figure 7.16

Enumeration of Bacteria • Direct or total cell count- counting the number of cells in a sample microscopically – Uses a special microscope slide (cytometer) – Used to estimate the total number of cells in a larger sample

Figure 7.17

Automated Counting • Coulter counter- electronically scans a culture as it passes through a tiny pipette • Flow cytometer also measures cell size and differentiates between live and dead cells • Real-time PCR allows scientists to quantify bacteria and other microorganisms that are present in environmental or tissue samples

Figure 7.18