Microbial Growth

For microorganisms, growth is measured by increase in cell number, due to their limited increase in cell size.

Bacteria are classified by:

• Temperature Range• pH Range• Osmotic Pressure• Need for Oxygen

Other Classifications

• Facultative: Microorganisms that can grow in more than one environment

• Obligate: Microorganisms that have specific environments strictly needed for growth

Temperature Range

• Psychrophile: 0-30 degrees C range, 15 C optimum growth temperature

• Mesophile: 25-40 degrees C range, 37 C optimum growth temperature. These have most pathogens, optimized for body temperature.

• Thermophile: 40-110 degrees C range, 55 C optimum growth temperature.

pH Range

• Acidophiles: Grow in acidic environment, pH less than 6.5. Linked to stomach ulcers.

• Neutrophiles: Grow in neutral environment, pH 6.5-7.5. Most pathogens are neutrophiles, as your body is mostly within this pH range.

• Alkaliphiles: Grow in basic environments, pH greater than 7.5.

pH

• PH a factor in food spoilage: Canned food high in botulism

• Acid rain effect the environment• Bacteria often produce acids that eventually

interfere w/ their own growth.

Osmotic Pressure Range

• Low osmotic pressure kills many prokaryotes.• Halophiles: microbes that can survive in salty

environments.• Obligate Halophiles: microbes that need salty

environments to survive.

Nutritional Requirements for Growth

• Carbon: All living things need a carbon source• Nitrogen: used in protein and nucleic acid

synthesis• Sulfur: used in protein synthesis• Phosphorus: Used in ATP and nucleic acids• Trace Elements: (Fe, Zn, Cu, etc.) Co-factors

and co-enzymes to activate enzymes.

Nutritional Requirements Cont.

• Trace Elements: (Fe, Zn, Cu, etc.) Co-factors and co-enzymes to activate enzymes. Only in small amounts.

• Organic Growth Factors are organic compounds that are essential to the organism but the organism is unable to synthesize it itself (ex. Vitamins, amino acids, nucleotides).

Classifications for Oxygen Need

• Obligate Aerobes: Must have free oxygen for aerobic respiraiton.

• Obligate Anaerobes: Killed by free oxygen. Growth only occurs where there is no oxygen.

Oxygen Needs Cont.

• Facultative Anaerobes: Have both aerobic and anaerobic growth. Growth is greater in the presence of oxygen because full respiration occurs.

• Aerotolerant anaerobes: Only anaerobic growth, but can tolerate the presence of oxygen. Growth occurs evenly throughout media as oxygen has no effect.

Oxygen Needs Cont.

• Microaerophiles: Only aerobic growth. Oxygen is required in low concentrations. Growth occurs only where a low concentration of oxygen has diffused into the medium.

Bacteria classified according to their Oxygen needs

• Obligate aerobes– Must have free oxygen for

aerobic respiration– Grow only occurs on top of

media where there is high concentration of oxygen

Bacteria classified according to their Oxygen needs

• Falcutative Anaerobes– Both aerobic and anaerobic

growth– Growth is greater in the

presence of oxygen .– Growth occurs throughout

media.

Bacteria classified according to their Oxygen needs

• Obligate anaerobes– killed by free oxygen– Growth occurs only

where there is no oxygen

Bacteria classified according to their Oxygen needs

• Aerotolerant Anaerobes– Only anaerobic growth; but

can tolerate the presence of oxygen

– Growth occurs evenly throughout media as oxygen has no effect

Bacteria classified according to their Oxygen needs

• Microaerophiles– Only aerobic growth– oxygen is required in low

concentrations.– Growth occurs only

where a low concentration of oxygen has diffused into medium

Culture, Colony and Culture Media

• Culture: The actual bacteria being grown.• Colony: A group of the same bacteria growing

together.• Culture Media: What the bacteria are growing

on or in.

Culture Media

Two types of culture media• Broth: A liquid nutrient media without agar• Agar: a common solidifying agent for a culture

medium.

Agar

• It is added to nutrient medium to solidify it in order to bacterial grow colonies.

• It is made form seaweed. It has long been used in various foods from jellies, soups, and ice cream.

• Few microbes can break down agar.• Different freezing/melting points. Solid to liquid

at 94 C, liquid to solid at 42 C. Labs keep it liquid at 50 C

Culture Media Cont.

• Chemically Defined: Medium in which the exact chemical composition is known. Often used to grow fastidious organisms. Expensive to buy.

• Complex: Used to grow most of the common bacteria and fungi that would be used in an introductory lab. Made of nutrients such as extracts from yeast, meat and plants.

• Anaerobic Growth Media/Reducing Media: No free oxygen and used to grown obligate anaerobes.

Culture Media Cont.

• Selective: Media that is designed to suppress the growth of unwanted bacteria and encourage the growth of wanted bacteria.

• Differential: Makes it easier to distinguish colonies of the desired organism from other colonies growing on the same plate.

• Enrichment: Used to encourage the growth of a particular organism in a mixed culture. Example soil, fecal samples etc...