Chapter 4

Robert Koch

Studied disease-causing bacteria and developed methods of cultivating bacteria, with wife’s help, developed agar plate usage

How do prokaryotic cells divide?

Binary fission, which is exponential growth

Growth Calculation Formula

N_t = N₀ × 2ⁿ
N_t = Number of cells in population at time t
N₀ = initial number of cells
n = number of generations at that point

How do most microbes in nature live?

Attach to surfaces and live in polymer-encased communities termed biofilms

Biofilm Formation and Structure

Free cells adhere to a surface and multiply. They release polymers that other cells can attach to and grow. The extra polymeric substances (EPS) are what give the slimy appearance

Pure culture

population of cells derived from a single cell, allows study of a single species

Streak Plate Method

Reduces number of cells with each series of streaks in order to isolate species

Closed System Cultures

Closed System Cultures do not remove nutrients nor remove waste. Position of a single cell determines its environment. Edge of the colony thrives with little competition for nutrients/oxygen, center of colony dies easily with over-competition for nutrients/oxygen

Open System Culture

OSC continuously adds nutrients and removes waste product. This causes a relatively uniform growth pattern and a constant rate.

Major factors that influence microbial growth

temperature, atmosphere, pH, and water availability

Order of temperature ranges of species, lowest temperature to highest temperature

Psychrophiles, Psychrotrophs, Mesophiles, Thermophiles, Hyperthermophiles

How do most bacteria maintain a constant internal pH that is roughly neutral?

they push out protons in acidic environments and bring in protons in alkaline environments

Halotolerant bacteria

withstand up to 10% salinityH

Halophiles

require high salt levels

What kind of carbon do hetertrophs use?

Organic carbon

What type of carbon do autotrophs use?

inorganic carbon (CO2)

Nitrogen fixation

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

Where do phototrophs obtain energy from?

Sunlight

Where do chemotrophs get energy from?

Chemicals

Where do photoautotrophs get their energy/carbon from?

Sunlight and inorganic carbon from CO2

Where do photoheterotrophs get their energy and carbon from?

Sunlight and organic carbon

Where do chemolithoautotrophs get their energy and carbon from?

energy from inorganic compounds and inorganic carbon from CO2

Where do chemoorganoheterotrophs get their energy and carbon from?

energy and carbon from organic compounds

Complex Media

contains a variety of ingredients, exact composition is unknown

Chemically defined media

composed of exact amounts of pure chemicals

Selective Media

inhibits the growth of certain species in a mixed sample, while allowing growth of species of interest

What is MacConkey agar selective for?

Gram-negative rods, because it contains crystal violet that inhibits Gram-positive bacteria, and bile salts that inhibit most non-intestinal bacteria

Differential Media

Contains substance that microbes change in an identifiable way

How is blood agar differential?

Beta hemolysis produces a clear zone while alpha hemolysis produces zone of greenish partial clearing

Aerobic Species

Most obligate aerobes and facultative anaerobes can be incubated in air

Anaerobic Species

Obligate anaerobes are sensitive to O2

Enrichment Culture

Used to isolate organism present as only a small fraction of the mixed population. Provides conditions promoting growth of a specific species, thus the relative concentration of the chosen organism will increase

How to count cells on plate?

Single cells give rise to colonies, so the number of colonies reflects how many cells were in the sample used.

Membrane Filtration to Measure Microbial Growth

Known volume passes through sterile filter, which captures and concentrations microbes by filtration, then the filter is incubated on appropriate agar medium.

Spectrophotometer for measuring microbial growth

Standard tests correlate measured optical density to actual cell concentration.