Ch 6

Microbial

Nutrition and

Growth

SLOs

• Define five terms used to express a microbe’s optimal growth temperature.

• Explain how microbes are classified on the basis of O2 needs.

• Identify 2 ways in which aerobes avoid damage by toxic forms of O2.

• Identify three important environmental factors (other than temperature and oxygen) with which microorganisms must cope.

• List and describe the five types of associations microbes can have with their hosts.

• Explain biofilms, describe their formation and their potential for causing infection.

• Summarize the steps of cell division used by most bacteria.

• Explain doubling time, and how it leads to exponential growth.

• Outline the 4 growth phases in a bacterial growth curve.

• Review some direct and indirect methods of measuring bacterial cell growth.

6.1 Microbial Nutrition

Review 6.1 in the textbook on your own if necessary (pages 142-148). Compare it to what

you have learned in BIO 30

6.2 Environmental Factors that Influence Microbes

1. Temperature

2. Gases

3. pH

4. Osmotic Pressure

5. Other Organisms

1. Temperature Minimum, optimum, and maximum growth temperatures

Five groups based on optimum growth temperature 1. Psychrophiles 2. Psychrotrophs 3. Mesophiles 4. Thermophiles 5. Extreme thermophiles

2. Gases

• O2 and CO2 influence microbial growth

• As O2 enters cellular reactions, it is transformed into several toxic products

• Microbes fall into one of three categories:

- Those that use oxygen and detoxify it.

- Those that can neither use oxygen nor detoxify it.

- Those that do not use oxygen but can detoxify it.

• Superoxide ion (also known as superoxide anion)

• Hydrogen Peroxide

Toxic Oxygen Products

·

2

• Superoxide ion (also known as superoxide anion)

• Hydrogen Peroxide

Oxygen Usage and Tolerance Patterns

Compare to Table 6.5

O2 requirements vary greatly

Capnophiles

• Aerobic bacteria that grow better in high ______ and low oxygen

• In human body, these conditions found in

– __________________

– __________________

Candle jar

Examples: Campylobacter jejuni, N. meningitides, S. pneumonieae

Use candle jar, CO2-generator packets, or CO2 incubators

3. pH and 4. Osmotic Pressure

Most bacteria, the so-called ______________ , grow best between pH 6.5 and 7.5.

Some bacteria prefer a pH range 1 to 5. These are the __________________ What bacterium lives in the stomach?

Some bacteria do well in hypertonic (?) environments: Obligate vs. facultative __________________

S. aureus can grow on NaCl media ranging from 0.1% to 20%.

What happens to a bacterium in a high salt environment if it is not salt tolerant?

5. Other Organisms

Microbes generally live in shared habitats. Major

types of microbial associations:

Symbioses Mutualism: Mutually beneficial relationship

Commensalism: Commensal benefits, partner doesn’t (but is not harmed)

Parasitism: Parasite benefits, host suffers. All microbes that cause disease.

Associations But Not Partnerships

Antagonism: Members of a community compete

• Antibiosis: Production of inhibitory compounds such as

antibiotics to inhibit or destroy another microbe in the same

habitat

• In soil, mixed communities compete for space and food

• In gut, normal microbiota prevents transient potential

pathogens to take hold

Synergism:

• Benefits but not necessary for survival

• Participants cooperate to produce result that none of them

could do alone

• E.g.: Mixed infections such as gum disease, dental caries, and

some bloodstream infections

Biofilm: Epitome of Synergy

• Mixed communities attached to each other and to a surface.

• Formation of a biofilm:

1. Attachment of planctonic bacteria (pioneer colonizers) to surface structures.

2. Other microbes attach to those bacteria or a polymeric sugar or protein substance secreted by the microbial colonizers.

3. Attached cells are stimulated to release chemicals as the cell population grows.

Bacteria communicate by chemicals via quorum sensing “The secret social lives of bacteria” TED talk by Bonnie Bassler

Bacteria in biofilm behave differently and are sheltered from harmful factors (disinfectants etc.)

Cause of most nosocomial infections, i.e.: Indwelling catheters

Fig 6.7

Study of Bacterial Growth

Binary fission – exponential grow

Generation time – time required for a complete fission cycle (also known as doubling time)

Ranges from 10 min (___________) to many days (_____________)

Average 30 – 60 mins.

Fig 6.8

Consider reproductive potential of E. coli!

Type of curve?

Compare to Fig 6.9

The Mathematics of Population Growth

• The size of a population can be calculated by the following equation:

Nt = (Ni)2n

- Nt is the total number of cells in the population, t denotes “at some point in time”

- Ni represents the starting number of cells

- The exponent n denotes the generation number.

- 2n represents the number of cells in that generation.

Fig 6.11

Chemostat

- Automatic growth chamber

- Admits steady stream of new nutrients

- Siphons off used media

and old bacterial cells

- Stabilizes growth rate

and cell number

- Used in research and

industrial applications

Practical Importance of Growth Curve

Direct Count Methods: Viable Plate Count

Fig 6.10

CFUs =__________________________

Additional Direct Counts

Direct microscopic count: Counting chambers (slides) for microscope

Fig 6.13

Coulter counter: electronically scans a fluid as it passes through a tiny pipette

Flow cytometer: similar to Coulter counter, but can measure cell size and differentiate between live and dead cells

Fig 6.14

Indirect Count: Spectrophotometry measures turbidity as an indicator of growth

OD (Absorbance) is function of cell number

Compare to Fig 6.12

Case File: Wound Care

Who will present?

Inside the Clinic: Fever – To treat or not to treat?