Microbial Nutrition and Growth

• Microbial nutrition and growth is vital for understanding microbial behavior
• Microbial growth refers to an increase in population size, not individual cell size
• This growth results from the reproduction of individual microbial cells

Discrete Colonies and Biofilms

• Discrete colonies are aggregations of cells arising from a single parent cell
• Biofilms are collections of microbes living on a surface in a complex community

Essential Nutrients

• Organisms use nutrients for energy and to build organic molecules and cellular structures
• Common, necessary elements include carbon, oxygen, nitrogen, and hydrogen

Autotrophs and Heterotrophs

• Autotrophs get carbon from inorganic sources, such as CO2
• Heterotrophs get carbon from organic sources

Chemotrophs and Phototrophs

• Chemotrophs get energy from organic or inorganic molecules
• Phototrophs get energy from light

Organotrophs and Lithotrophs

• Organotrophs obtain electrons from organic molecules and are also heterotrophs
• Lithotrophs get electrons from inorganic molecules like H2 and NO2

Oxygen Requirements

• Oxygen is essential for obligate aerobes
• Oxygen is deadly for obligate anaerobes because toxic forms of oxygen are highly reactive oxidizing agents
• Resulting oxidation causes irreparable damage to cells

Aerobes and Anaerobes

• Obligate aerobes must have oxygen
• Obligate anaerobes must not have oxygen
• Facultative anaerobes grow in low oxygen, preferring high oxygen
• Aerotolerant anaerobes do not need oxygen, but can live around oxygen
• Microaerophiles live in very low oxygen levels

Nitrogen Requirements

• Anabolism often ceases due to insufficient nitrogen
• Nitrogen can be acquired from organic and inorganic nutrients All cells recycle nitrogen for amino acids and nucleotides
• Nitrogen fixation is essential to life
• Nitrogen fixation reduces nitrogen gas (N2) into ammonia (NH3)

Other Chemical Requirements

• Other chemical requirements include phosphorus, sulfur, and trace elements
• Trace elements are only required in small amounts
• Growth factors are necessary organic chemicals that cannot be synthesized by certain organisms

Temperature Effects

• Temperature affects the three-dimensional structure of proteins
• Lipid-containing membranes are also temperature sensitive
• Membranes become rigid and fragile if too cold
• Membranes become too fluid if too hot

Temperature Classifications of Microbes

• Hyperthermophiles thrive in extremely high temperatures
• Psychrophiles thrive in extremely cold temperatures

pH sensitivity

• Organisms are sensitive to changes in acidity
• Hydrogen ions interfere with hydrogen bonding
• Neutrophiles grow best in a narrow range around neutral pH
• Acidophiles grow best in acidic habitats
• Alkalinophiles live in alkaline soils and water

Water Needs

• Microbes need water to dissolve enzymes and nutrients
• Water is an important reactant in many metabolic reactions
• Most cells die in absence of water
• Some cells have cell walls that retain water
• Endospores and cysts cease most metabolic activity

Osmotic Pressure

• Osmotic pressure is the pressure exerted on a semipermeable membrane due to solute concentration
• Hypotonic solutions have lower solute concentrations
• Cells in hypotonic solutions swell
• Hypertonic solutions have greater solute concentrations
• Cells in hypertonic solutions shrivel
• Osmotic pressure restricts organisms to certain environments
• Obligate and facultative halophiles thrive in high salt concentrations

Hydrostatic Pressure

• Water exerts pressure in proportion to its depth
• Barophiles live under extreme pressure
• Their membranes and enzymes depend on pressure to maintain their three-dimensional functional shape

Associations Between Organisms

• Organisms live in different types of associations with different species
• These include antagonistic, synergistic, and symbiotic relationships

Biofilms

• Biofilms have complex relationships among numerous microorganisms
• They form on surfaces, medical devices, and mucous membranes
• Biofilms form via quorum sensing
• Quorum sensing is when the organisms sense the density of other microbes and change behaviors
• Scientists are seeking ways to prevent biofilm formation

Inoculum and Culturing

• Inoculum is introduced into a medium to create a culture
• Cultures can be environmental, clinical, or stored specimens
• Culture is the act of cultivating microorganisms or the microorganisms that are cultivated

Pure Cultures

• Pure cultures composed of cells arise from a single progenitor; termed a colony-forming unit (CFU)
• Aseptic technique prevents contamination of sterile substances
• Two common isolation techniques are streak plates and pour plates

Other Isolation Techniques

• Some fungi are isolated using streak and pour plates
• Protozoa and motile unicellular algae can be isolated through the dilution of broth cultures
• Single-cell isolation can establish a culture from large microorganisms

Culture Media Types

• The majority of prokaryotes cannot be grown in lab culture
• Culture media is available in liquid (broth) and solid forms (agar)
• 6 different types of culture media exist: defined, complex, selective, differential, anaerobic, and transport media

Defined Media

• Defined media has a precisely known chemical composition

Complex Media

• Complex media exact chemical composition is unknown
• Commonly derived from the breakdown of yeast, beef, soy, and proteins
• Complex media supports a wide variety of microorganisms
• Complex media is useful when the nutritional needs of an organism are unknown

Selective Media

• Selective media contains substances that favor or inhibit growth of particular microorganisms
• Selective media used to increase small numbers of a microbe to observable levels through enrichment culture
• Cold enrichment can enrich a culture with cold-tolerant species

Differential Media

• Blood agar can differentiate the various degrees of hemolysis
• Carbohydrate utilization tubes can differentiate carbohydrate metabolism

Anaerobic and Transport Media

• Anaerobic media supports the growth of obligate anaerobes
• Transport media is used to ensure clinical specimens are not contaminated and to protect people from infection
• Rapid sample transport is important

Special Culture Techniques

• Animal and cell culture uses living cells instead of artificial media
• Viruses require growth within living cells
• Carbon dioxide incubators mimic the environment of certain body tissues
• Candle jars create an environment with low oxygen and high CO2 levels

Culture Preservation

• Refrigeration - short periods of time
• Deep-freezing - years
• Lyophilization - decades

Binary Fission

• Most microorganisms reproduce by binary fission
• Involves one cell dividing in half to produce two daughter cells

Generation Time

• Generation time refers to the time required for a bacterial cell to grow and divide
• It depends on chemical and physical conditions

Measuring Microbial Reproduction

• Estimating the number of microorganisms is useful to:
• Determine severity of certain infections
• Determine effectiveness of food preservation techniques
• Measure the degree of contamination of water supplies
• Evaluate disinfectants and antibiotics
• Measuring microbial growth can be done directly or indirectly
• Direct methods do not require incubation: microscopic counts
• Electronic counters like the Coulter counter count cells as they interrupt an electrical current
• Flow cytometry detects changes in light transmission as cells pass a detector

Direct Methods

• Direct methods requiring incubation include serial dilution and viable plate counts
• Membrane filtration counts colonies on a membrane after filtration

Indirect Methods include measurement of:

• Turbidity where the more turbid a culture, the greater the population
• Metabolic activity
• Dry weight
• Molecular methods using DNA such as Polymerase Chain Reaction (PCR)