Exercise 7 Physical Factors Affecting Growth

What is a niche?

The natural habitat of an organism.

What environmental factors affect bacterial growth?

Temperature, pH, O₂, osmotic pressure, CO₂, sunlight, and barometric pressure.

Mesophiles — Optimal temperature

20–40°C; moderate temperatures.

Thermophiles — Optimal temperature

60–80°C; high temperatures.

Extreme thermophiles — Temperature

Require temperatures as high as 120°C.

Psychrophiles — Temperature

Require temperatures below 20°C.

Psychrotrophs — Definition

Cold-tolerant mesophiles that grow in refrigerators.

Mesophiles — Growth below 15–20°C

Rarely grow at these temperatures.

Which organisms can grow in refrigerators?

Psychrotrophs.

Neutrophiles — Optimal pH

pH 6–8; neutral environments.

Acidophiles — Required pH

Below pH 4; acidic environments.

Alkalinophiles — Optimal pH

Above pH 8; basic environments.

Most bacteria — Preferred pH

pH 6–8; neutral environments.

Yeasts — Preferred pH

pH 4–6; slightly acidic.

Are yeasts classified as neutrophiles?

Yes, but they prefer slightly acidic conditions.

Obligate aerobes — Oxygen requirement

Require O₂ to survive.

Microaerophiles — Oxygen requirement

Require O₂ at reduced levels.

Facultative anaerobes — Oxygen requirement

Use O₂ when available; grow without it.

Aerotolerant anaerobes — Oxygen requirement

Do not use O₂ but tolerate it.

Obligate anaerobes — Oxygen requirement

Do not use O₂ and are killed by it.

Which bacteria switch metabolism when O₂ is limited?

Facultative anaerobes.

Which organisms need less O₂ than atmospheric levels?

Microaerophiles.

Which anaerobes tolerate O₂ without using it?

Aerotolerant anaerobes.

Which organisms cannot survive in O₂?

Obligate anaerobes.

Why do many bacteria not use free O₂?

They evolved before Earth's oxygen-rich atmosphere.

How do white blood cells use oxygen against pathogens?

They generate reactive oxygen forms to kill pathogens.

Why can oxygen be harmful to bacteria?

Reactive forms of oxygen can damage cells.

Hypertonic solution — Definition

Contains high concentrations of solutes.

Common solutes in hypertonic solutions

Salts and sugars.

What happens to cells in hypertonic solutions?

Water leaves the cells; they may die.

Most bacteria — Preferred osmotic environment

Isotonic environments.

Why does salt or sugar preserve food?

Draws water out of microbial cells.

Why must skin microbes tolerate salt?

Sweat makes the skin salty.

Osmotolerant organisms — Definition

Can grow at elevated osmotic pressure.

Halophiles — Definition

Require high osmotic pressure for growth.

Osmotolerant vs. Halophile

Osmotolerant = tolerates high pressure; halophile = requires it.