BIOL 2460 Chapter 9: Microbial Growth

Most common form of bacterial reproduction

Binary fission

4 Basic Steps of Binary Fission

1. Growth of cell size and increase in cell components

2. Replication of DNA/chromosomes

3. Division of cytoplasm (cytokinesis)

4. Septum formation and division of daughter cells

Cytokinesis is directed by the ___________ protein.

FtsZ

Where is the Z ring assembled during binary fission?

On the cytoplasmic membrane

Divisome activates the production of __________________ and ____________.

Peptidoglycan; septum

Generation Time

Also known as doubling time; the time it takes to double the bacterial population (or one round of binary fission).

Generation Time for E. Coli

20 minutes

Generation time for B. subtilis

120 minutes

Generation time for M. tuberculosis

15-20 hours

Generation time for S. aureus

30 minutes

Bacterial population size calculation

𝑁𝑛=𝑁02𝑛

The Z ring is anchored by what?

FtsZ binding proteins; also helps define the division plane between daughter cells.

𝑁𝑛 is what?

The number of cells at generation n.

n is what?

The number of generations.

N0 is what?

The initial number of cells.

Growth curve

Microorganisms grown in closed culture, in which no nutrients are added and waste is not removed, follow a reproducible growth pattern.

Culture Density

The number of cells per unit volume.

Within a closed culture, it can also refer to a measure of the number of cells in the population.

Phases of the Growth Curve

1. Lag phase

2. Log phase

3. Stationary phase

4. Death phase

Lag Phase

Period where inoculum are gearing up for the next phase of growth; the number of cells doesn't change, however, they grow larger and are metabolically active.

If any cells were damaged during the transfer, they will be repaired during this phase.

Inoculum

Microbes that are introduced into a culture medium to initiate growth.

Culture Medium

Nutritional broth that supports microbe growth.

The duration of the lag phase is determined by what factors?

Species and genetic makeup of cells

Media composition

Initial size of inoculum

Log Phase

Sometimes called the exponential growth phase; cells are actively undergoing binary fission and cell replication is greater than cell death; cells are constantly growing and are most susceptible to disinfectants and antibiotics that target proteins, DNA, and cell-wall synthesis at this point.

Intrinsic Growth Rate

Generation time under specific conditions is genetically determined.

Stationary Phase

Waste products accumulate and nutrients get used up, so the growth rate slows; oxygen depletion limits aerobic cell growth and population growth starts to stall; the number of cells dying is equal to the number of cells replicating.

Cells are less susceptible to antibiotics during this phase and certain pathogenic bacteria express virulence factors.

What pathogenic bacteria expresses virulence during the stationary phase of the growth curve?

S. aureus; during the stationary phase, it initiates the production of enzymes that help break down human tissue to enable spread to new tissues, where nutrients are more abundant.

Death Phase

The number of dying cells exceeds the number of cells replicating at this point; cells are lysing and releasing nutrients into the medium, allowing any existing cells to maintain viability and form endospores; persistent cells have slow metabolic rates and do not respond to antibiotic treatments.

During which phase would you see the most endospores?

Death phase.

What is a chemostat used for?

To maintain a microbial population in which nutrients and air are supplied at a steady rate and dead cells are waste are simultaneously removed.

The number of bacteria in a clinical sample serves as an indication of what?

The extent of infection.

Bacterial count

Estimating the number of bacterial cells in a sample.

What approaches are used to measure cell numbers?

Direct and indirect methods.

Types of direct measuring methods:

Microscopic cell count

Fluorescent staining (for alive and dead cells)

Coulter count

Viable cell count

Direct Cell Count

Refers to counting the cells in a liquid culture or colonies on a plate.

Direct Microscopic Cell Count

Transferring a known volume of a culture to a calibrated slide and counting the cells under a light microscope.