Microbial growth Typically refers to an increase in population rather than in size.

transcript

Microbial growth

Typically refers to an increase in population rather than in size

Growth curves

Carried out using batch cultures or a closed system (no fresh media added)

Characterized by several phases

Lag phase

Occurs when cells are placed into fresh media

Likely due to the cells’ need to synthesize new components before reproducing

Lag phase

Can vary depending on:

1. Type of media

2. Condition of the cells

Exponential phase

Cells are growing at the maximum rate possible under given conditions

Rate of growth is constant

Population most uniform

Stationary phase

Bacteria in stationary phase are usually at a concentration of 109 cells per ml

Balance between cell division and cell death or cells cease to divide

Stationary phase

Due to:

Nutrient depletion

Toxic waste accumulation

Critical cell density reached

Stationary phase

Bacteria subjected to starvation may become resistant to killing

Some pathogens may become more virulent when starved

Death phase

Decline in viable cells due to toxic wastes and nutrient depletion

Death may be at a constant rate (logarithmic)

Death rate may decrease after majority of population has died (resistant cells)

Mathematics of growth

Cells dividing at a constant rate during exponential growth

Generation time/doubling time = time it takes for population to double

Mathematics of growth

More convenient to graph as log10 of cell number vs. time

Generation time

Determining generation time

Measurement of microbial growth

Measurement of cell number

Measurement of cell mass

Measurement of culture turbidity

Measurement of cell number

Counting chambers

Coulter counters

Plating techniques

Membrane filter techniques

Petroff-Hauser chamber

Used for counting prokaryotic cells

Use of stains or fluorescent or phase-contrast microscopes make counting easier

Using a Petroff-Hauser chamber

Chamber is of known depth and has grid etched into bottom

25 squares cover an area of 1 mm2

Determining average number per square and multiplying by 25 gives total number of cells in chamber

Using a Petroff-Hauser chamber

280 cells in 10 squares

280/10 = 28/square

28 x 25 = 700 cells/ mm2

Chamber is 0.02 mm deep

700/0.02 = 700 x 50

= 3.5 x 104 cells/mm3

= 3.5 x 107 cells/cm3

Coulter counter

Cells forced through small opening with electrodes on either side

Passage of cell will cause resistance to increase and cell is counted

More useful for counting eukaryotes

Counting chambers and Coulter counters

Neither can distinguish between living and dead cells

Plating techniques

Diluted sample spread over the surface of agar plate

Number of cells can be calculated by multiplying colony number by dilution factor

Useful for measuring number of cells in aquatic samples

Sample passed through filter with small pore size

Filters placed on agar plates to allow growth of colonies

Measurement of dry weight

Cells collected by centrifugation, washed and dried in an oven and weighed

Most useful for fungi

Measurement of turbidity

Degree of light scattering induced by a culture is indirectly related to the cell number

Spectrophotometers measure amount of light scattering

Can measure transmittance or absorption of light

Continuous culture of microorganisms

Two most common systems

Chemostat

Turbidostat

Chemostat

Sterile media fed into vessel at same rate that media containing bacteria are removed

Final cell density is dependant on the conc. of a limiting nutrient

Turbidostat

Makes use of a photocell to measure turbidity of culture

Flow rate of media is regulated to maintain a constant cell density

Influence of environmental factors on growth

Acidophiles

Neutrophiles

Alkalophiles

Influence of environmental factors on growth

Quorum sensing

Bacteria can communicate via quorum sensing or autoinduction

Cell senses concentration of signal

When threshold is reached, cell begins expressing sets of certain genes

Quorum sensing

Most common signal molecules in gram-negative bacteria are acyl homoserine lactones (HSLs)

Gram-positives often use an oligopeptide signal molecule

Important in pathogenicity and biofilm formation

Microbial growth Typically refers to an increase in population rather than in size.

Documents