Physical requirements for growth • Prefixes and suffixes: • Bacteria are highly diverse in the types of conditions they can grow in. – Optimal or required conditions implied by “-phile” meaning “love” • Some bacteria prefer other conditions, but can tolerate extremes – Suffix “-tolerant” • Note the difference! 1 http://www.kodak.com/global/images/en/health/filmImaging/ thermometer.gif
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Physical requirements for growth Prefixes and suffixes: Bacteria are highly diverse in the types of conditions they can grow in. – Optimal or required.
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Physical requirements for growth
• Prefixes and suffixes:• Bacteria are highly diverse in the types of
conditions they can grow in. – Optimal or required conditions implied by “-
phile” meaning “love”
• Some bacteria prefer other conditions, but can tolerate extremes– Suffix “-tolerant”
• Most bacteria prefer a neutral pH– Many grow well from pH 6 to 8
• Some bacteria create their preferred conditions– Lactobacillus creates low pH environment in vagina
12Low water activity:halophiles, osmophiles, and xerotolerant
• Water is critical for life; remove some, and things can’t grow. (food preservation: jerky, etc.)
• Halophiles/halotolerant: relationship to high salt.– Marine bacteria; archaea and really high salt.
• Osmophiles: can stand hypertonic environments whether salt, sugar, or other dissolved solutes– Fungi very good at this; grandma’s wax over jelly.
• Xerotolerant: dry. Subject to desiccation. Fungi best– Bread, dry rot of wood– Survival of bacterial endospores.
13Bacterial growth defined
• Since individual cells double in size, then divide into two, the meaningful increase is in the population size.
• Binary fission: cell divides into two cells. No nucleus, so no mitosis.
• Cells do not always fully detach; produce pairs, clusters, chains, tetrads, sarcina, etc.
• “GROWTH” = increase in number of bacteria
14Mathematics of bacterial growth
• Because bacteria double in number at regular intervals, they grow exponentially:
• N = N0 x 2n where N is the number of cells after n number of doublings and N0
is the starting number of cells.
• Thus, a graph of the Log of the number of bacteria vs. time is a straight line.
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The Bacterial Growth Curve
• Bacteria provided with an abundant supply of nutrients will increase in number exponentially, but eventually run out of nutrients or poison themselves with waste products.
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1. Lag phase2. Exponential or
Log phase3. Stationary
phase4. Decline or
Deathphase.
16Growth curve (continued)
• Lag phase: growth lags; cells are acclimating to the medium, creating ribosomes prior to rapid growth.
• Log phase: cells doubling at regular intervals; linear graph when x-axis is logarithmic.
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•Stationary phase: no net increase in cell numbers, some divide, some die. Cells preparing for survival.•Decline phase: highly variable, depends on type of bacteria and conditions. Death may be slow and exponential.
17More about Growth
• The Growth curve is true under ideal conditions; in reality, bacteria are subject to starvation, competition, and rapidly changing conditions.
• Generation time: the length of time it takes for the population to double.
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1 2 3 4 5 6
•Growth of bacteria is nonsynchronous, not every bacterium is dividing at the same time.
•Instead of stepwise curve, smooth curve
Exponential growth
• “Balanced growth”– Numbers of bacteria are doubling at regular
intervals.– All components of bacteria are increasing in amount
at the same rate• 2x as many bacteria = 2x as much protein, 2x as
much peptidgolycan, 2x as much LPS, etc.– During exponential growth, bacteria are not limited
for any nutrients, i.e. they are not short of anything.
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19Measurement of cell numbers
• Direct methods: cells actually counted.– Petroff-Hausser
counting chamber (right), 3D grid. Count the cells, multiply by a conversion factor.
– Dry a drop of cells of known volume, stain, then count.
Coulter Counter20
Coulter-counter: single-file cells detected by change in electric current.
21Counting cells with plates
• Viable plate count– Relies on bacteria being alive,
multiplying and forming colonies.
– Spread plate: sample is spread on surface of agar.
– Pour plate: sample is mixed with melted agar; colonies form on surface and within agar.
– Does NOT provide an actual number unless a calibration curve (# of bacteria vs. O.D.) is created.• Indirect counting method
– Quick and convenient, shows relative change in the number of bacteria, useful for determining growth (increase in numbers).
– Does NOT distinguish between live and dead cells. To create a calibration curve, best to plot OD vs. number of cells determined with microscope (not plate count).
25Biomass: • Measure the total mass of cells or amount of any
component such as protein, PS, DNA, KDO.• Especially when cells are doubling,
the amounts of all the components of a cell are increasing at the same rate, so any could be measured.– Not so in stationary phase.
In this example, total biomass increases exponentially over time.