(2) Chapter 6

Microbial Growth

What is osmotic pressure?

The pressure exerted by water moving across a selectively permeable membrane.

What is an isotonic environment?

An environment where the solute concentration inside and outside the cell is equal.

What happens to a bacterial cell in an isotonic solution?

There is no net movement of water, and the cell remains normal.

What is a hypertonic environment?

An environment with a higher solute concentration outside the cell than inside.

What happens to a bacterial cell in a hypertonic solution?

Water leaves the cell, causing plasmolysis.

What is plasmolysis?

Shrinkage of the cytoplasmic membrane away from the cell wall due to water loss.

Why do high concentrations of salt or sugar preserve food?

They create a hypertonic environment that inhibits microbial growth.

What is a hypotonic environment?

An environment with a lower solute concentration outside the cell than inside.

What happens to bacteria in a hypotonic solution?

Water enters the cell, but the cell wall prevents bursting.

What are halophiles?

Microorganisms that require high salt concentrations for growth.

What are facultative halophiles?

Microorganisms that tolerate high salt but do not require it.

Which foods commonly contain halophiles?

Salted fish, cured meats, and salty foods.

What is an obligate aerobe?

A microorganism that requires oxygen for growth.

Why do obligate aerobes require oxygen?

Oxygen is the final electron acceptor in aerobic respiration.

What is an obligate anaerobe?

A microorganism that cannot grow in the presence of oxygen.

Why is oxygen toxic to obligate anaerobes?

They lack enzymes that detoxify reactive oxygen species.

What is a facultative anaerobe?

A microorganism that grows with or without oxygen but grows better when oxygen is present.

Give an example of a facultative anaerobe.

Escherichia coli (E. coli).

What is an aerotolerant anaerobe?

A microorganism that does not use oxygen but tolerates its presence.

What is a microaerophile?

A microorganism that requires oxygen but at lower concentrations than atmospheric oxygen.

Why can't microaerophiles tolerate atmospheric oxygen levels?

High oxygen concentrations damage their cells.

What are reactive oxygen species (ROS)?

Toxic oxygen-containing molecules produced during aerobic metabolism.

Give examples of reactive oxygen species.

Superoxide radicals, hydrogen peroxide, and hydroxyl radicals.

What enzyme converts superoxide radicals into hydrogen peroxide?

Superoxide dismutase (SOD).

What enzyme breaks hydrogen peroxide into water and oxygen?

Catalase.

Which enzyme also detoxifies hydrogen peroxide but does not release oxygen gas?

Peroxidase.

Which microorganisms typically have catalase?

Obligate aerobes and facultative anaerobes.

Why do obligate anaerobes die in oxygen?

They lack enzymes such as catalase and superoxide dismutase.

What are the four phases of the bacterial growth curve?

Lag, log (exponential), stationary, and death.

What happens during the lag phase?

Cells adapt to the environment and prepare for division.

Does the population increase during the lag phase?

No.

Which phase has the fastest rate of cell division?

The log (exponential) phase.

During which phase are bacteria most metabolically active?

Log phase.

Which phase is most susceptible to antibiotics?

Log (exponential) phase.

What happens during the stationary phase?

The rate of cell division equals the rate of cell death.

Why do bacteria enter the stationary phase?

Nutrients become limited and waste products accumulate.

What happens during the death phase?

Cells die faster than they reproduce.

Why is the death phase called the decline phase?

The total number of living cells decreases.

What is a biofilm?

A community of microorganisms attached to a surface and enclosed in a protective matrix.

Why are biofilms difficult to eliminate?

The protective matrix reduces the effectiveness of antibiotics and disinfectants.

Give examples of biofilms.

Dental plaque, catheter infections, and slime on rocks.

What is direct microscopic count?

Counting cells under a microscope.

What is a limitation of direct microscopic counts?

They cannot distinguish living from dead cells.

What is a viable plate count?

A method that counts only living cells capable of forming colonies.

What does CFU stand for?

Colony-forming unit.

Why are serial dilutions used before plating bacteria?

To obtain a countable number of colonies.

What is turbidity?

A measure of cloudiness used to estimate bacterial growth.

Which instrument measures turbidity?

A spectrophotometer.

What is membrane filtration used for?

Concentrating microorganisms from liquids before culturing.

Which counting method is fastest?

Direct microscopic count.

Which counting method measures only living bacteria?

Viable plate count.

Which growth measurement is indirect?

Turbidity measurement.

Why is turbidity considered an indirect measurement?

It estimates cell density rather than counting individual cells.