Chapter 4: Dynamics of Microbial Growth

Facultative anaerobe

Grow better with O2 present but can grow without it

Ubiquitous

Found everywhere

Fastidious

Strict nutritional requirements

Most Probable Number (MPN) Method

Estimates cell concentration using dilution series; sets of tubes are incubated; results are recorded and compared to table to give statistical estimate of cell concentration

Growth factors

organic molecules that an organism cannot synthesize; must be present in the environment

Photoautotrophs

energy from sunlight; carbon from CO2

Photoheterotrophs

energy from sunlight; carbon from organic compounds

Chemolithoautotrophs

energy from inorganic compounds; carbon from CO2

Chemoorganoheterotrophs

energy and carbon from organic compounds

Nitrogen fixation

converting N2 gas to ammonia and then incorporating it into organic compounds

Mixed microbial community interactions

The metabolic waste of one microbe can serve as nutrients for another

Microbes will often compete for nutrients, some utilizing the method of toxin synthesis to inhibit competitors

The creation of low O2 microenvironments allows for the consumption of O2 through aerobic growth

Solid media vs liquid culture 

Solid media create spatial gradients of nutrients and oxygen, causing cells in a colony to exist in multiple growth phases simultaneously, whereas liquid culture provides a uniform environment where cells are more likely to grow synchronously in the same phase.

Liquid batch culture

aerated to support the growth of aerobic microbes by growing them in tubes that are shaken continuously

Continuous culture 

a method used to maintain microbial populations in a state of constant growth by continuously supplying fresh nutrients and removing waste and excess cells

Batch culture 

a closed system where microbes grow in a fixed volume of nutrient medium without replenishment; it progresses through all growth phases (lag, log, stationary, death) and ends when nutrients are exhausted or waste accumulates

Initial cell number x 2^generation number

Doubling time equation; calculates the number of cells at a certain time

Obligate anaerobe

Cannot multiply with O2 present, killed by air exposure

Obligate aerobe

Absolutely needs O2 to survive

Microaerophile

Require small amounts of O2 for aerobic respiration

Capnophiles

thrive in elevated CO2

Aerotolerant anaerobe

Indifferent to O2

Psychrophiles

cold-loving

Psychrotrophs

moderate temperatures, but can tolerate cold; responsible for spoilage

Mesophiles

optimal/body temperatures; human pathogens

Thermophiles

heat-loving

Hyperthermophiles

extreme heat-loving

Acidophile

acidic pH-loving

Neutrophile

neutral pH-loving

Alkaliphile

basic pH-loving

Halophilic

salt-loving

Extreme halophiles

saturated salt-loving

Osmotolerant

can grow in salty or dry environments 

Binary fission

Asexual reproduction where one cell divides into two identical daughter cells

Generation time

Time required for a population to double; varies by species and conditions

Exponential growth

Population doubles at regular intervals; plotted as a straight line on a log scale

Lag phase

The growth phase in which cells metabolically active but not dividing; adapting to environment

Log Phase

The growth phase with rapid cell division; most sensitive to antibiotics

Stationary phase

The growth phase in which nutrients deplete, waste accumulates; growth rate = death rate

Death phase

The growth phase in which cells die faster than they divide

Prolonged decline 

The phase in which some cells adapt and persist; slow decline in viable cells

Complex media

Contains unknown exact composition

Chemically defined media

Precise chemical composition known

Selective media

Inhibits unwanted microbes, promotes target growth

Differential media

Distinguishes microbes by biochemical reactions

Enrichment media

Enhances growth of rare microbes

Streak plate method (Isolation technique)

A technique where a microbial sample is spread across agar in successive streaks to dilute and isolate individual colonies for pure culture

Spread plate method (Isolation technique)

A diluted sample is evenly distributed over the surface of solid agar using a sterile spreader to grow and count surface colonies

Pour plate method (Isolation technique)

A diluted sample is mixed with molten agar and poured into a Petri dish, allowing colonies to grow both on the surface and within the medium

Enrichment culture (Isolation technique)

A method that uses selective nutrients or conditions to favor the growth of a specific microbe from a mixed population, enhancing its isolation

Microscopic count

A direct method that uses a counting chamber to directly count cells under a microscope, including both live and dead organisms.

Viable plate count

Direct method that involves serial dilution and plating to count only live cells capable of multiplying, forming colonies (CFUs)

Membrane filtration 

A direct method that filters a liquid sample through a membrane, then places the membrane on agar to count colonies from low-density populations.

Turbidity (Spectrophotometry)

An indirect method that measures cloudiness of a culture using light absorbance, which correlates with cell density but includes dead cells.

Dry weight

An indirect method that measures microbial biomass by filtering, drying, and weighing cells; useful for filamentous fungi or large cultures where direct counting is impractical. Measures total mass, not cell number

Metabolic activity

An indirect method that estimates growth by measuring byproducts like CO₂, acid, or ATP, reflecting overall cellular activity.

Reducing media 

Specialized culture media containing agents that remove oxygen to support the growth of obligate anaerobes

Biofilms

structured microbial communities that adhere to surfaces, resist antibiotics, and communicate via quorum sensing, often causing persistent infections; form the slime layer/capsules

Selective media

contain agents that inhibit unwanted microbes while promoting the growth of specific target organisms, useful for isolating pathogens from mixed samples

Differential media

include indicators that reveal metabolic differences between microbes, often through color changes, allowing visual distinction of species or traits