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A. Bacterial Growth
Bacteria replicate by binary fission, a process by which one bacterium
splits into two.
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Bacteria increase their numbers by geometric progression.
1. Geometric progression refers to the population of bacteria
doubling every generation time as a result of dividing by binary
fission.
2. Generation time is the time it takes for a population of bacteria
to double in number.
 For many common bacteria, the generation time is quite short,
20-60 minutes under optimum conditions.
 For most common pathogens in the body, the generation time
is probably closer to 5-10 hours.
 Because bacteria grow by geometric progression and most
have a short generation time, they can astronomically increase
their number in a short period of time.
 The relationship between the numbers of bacteria in a
population at a given time (Nt).
 The original number of bacterial cells in the population (No)
 The number of divisions those bacteria have undergone during
that time (n) can be expressed by the following equation:
 Nt = No X 2n
For example, Escheichia coli, under optimum conditions, has a
generation time of 20 minutes.
a. If one started with only 10 E. coli (No = 10)
b. And allowed them to grow for 12 hours
c. (n = 36; with a generation time of 20 minutes they would divide
3 times in one hour and 36 times in 12 hours)
d. Then plugging the numbers in the formula, the number of
bacteria after 12 hours (Nt) would be10 X 236 = Nt
e. Or 687,194,767,360 E. coli
Bacterial division begins with replication of the bacterium's DNA.
1. During DNA replication, each strand of the replicating
bacterial DNA attaches to a different site on the cytoplasmic
membrane.
2. As the bacterium grows, the newly replicated chromosomes
become separated.
3. The cytoplasmic membrane then invaginates in the center
of the bacterium and synthesizes a peptidoglycan septum to
separate the two daughter cells
Fig.1: Prokaryotic Cell (Bacillus megaterium)
There are a number of factors the influence Bacterial Growth. We will
begin our discussion of chapter 5 by review those described as
Physical requirements.
I. Temperature
Bacteria have a minimum, optimum, and maximum temperature for
growth. Most species can be divided into 3 groups based on their
optimum growth temperature:
1. Psychrophiles are cold-loving bacteria.
a. Their optimum growth temperature is between -5C and
15C.
b. They are usually found in the Arctic and Antarctic regions
and in streams fed by glaciers.
2. Mesophiles are bacteria that grow best at moderate
temperatures.
a. Their optimum growth temperature is between 25C and
45C.
b. Most bacteria are mesophilic and include common soil
bacteria and bacteria that live in and on the body.
3. Thermophiles (def) are heat-loving bacteria.
a. Their optimum growth temperature is between 45C and
70C
b. These organisms are commonly found in hot springs and
in compost heaps.
4. d. Hyperthermophiles are bacteria that grow at very high
temperatures.
a. Their optimum growth temperature is between 70C and
110C.
b. They are usually members of the Archae and are found
growing near hydrothermal vents at great depths in the
ocean.
II. Oxygen requirements
Microorganisms show a great deal of variation in their requirements
for gaseous oxygen. Most can be placed in one of the following
groups:
1. Obligate aerobes are organisms that grow only in the presence
of oxygen. They obtain their energy through aerobic respiration.
2. Microaerophiles are organisms that require a low concentration
of oxygen (2% to 10%) for growth, but higher concentrations
are inhibitory. They obtain their energy through aerobic
respiration.
3. Obligate anaerobes are organisms that grow only in the
absence of oxygen and, in fact, are often inhibited or killed by
its presence.
a. They obtain their energy through anaerobic respiration or
fermentation.
b. They have an inability to break down hydrogen peroxide
4. Aerotolerant anaerobes, like obligate anaerobes, cannot use
oxygen to transform energy but can grow in its presence.
a. They obtain energy only by fermentation and are known
as obligate fermenters.
5. Facultative anaerobes are organisms that grow with or without
oxygen, but generally better with oxygen.
a. They obtain their energy through aerobic respiration if
oxygen is present,
b. but use fermentation or anaerobic respiration if it is
absent. Most bacteria are facultative anaerobes.
III. pH
Microorganisms can be placed in one of the following groups based
on their optimum pH requirements:
1. Neutrophiles grow best at a pH range of 5 to 8.
2. Acidophiles grow best at a pH below 5.5.
3. Allaliphiles grow best at a pH above 8.5.
IV. Osmotic pressure
Osmosis is the movement of water across a membrane from an area
of higher water concentration (lower solute concentration) to lower
water concentration (higher solute concentration).
1. No energy is required.
2. To understand osmosis, one must understand what is meant by
a solution.
a. In terms of osmosis, solute refers to all the molecules or
ions dissolved in the water (the solvent).
b. When a solute such as sugar dissolves in water, it forms
weak hydrogen bonds with water molecules.
c. While free, unbound water molecules are small enough to
pass through membrane pores, water molecules bound to
solute are not.
d. Therefore, the higher the solute concentration, the lower
the concentration of free water molecules capable of
passing through the membrane.
A cell can find itself in one of three environments: isotonic,
hypertonic, or hypotonic. (The prefixes iso-, hyper-, and hypo- refer to
the solute concentration).

Most bacteria require an isotonic environment or a hypotonic
environment for optimum growth.
1. Organisms that can grow at relatively high salt concentration
(up tp 10%) are said to be osmotolerant.
2. Those that require relatively high salt concentrations for growth,
like some of the Archea that require sodium chloride
concentrations of 20 % or higher halophiles.
Nutrient are any substance (compound or molecule) needed by an
organism for growth, maintenance, and reproduction. These
substances used in biosynthesis and energy production, and thus
required for microbial growth. All organisms need a source of energy
and carbon
There are 3 categories used to define microbial communities
1. Energy source
2. Carbon source
3. Electron source
Energy Sources
Microorganisms can be divided into two major categories based on
energy sources:
1. Phototrophs - these organisms that use light as their energy
source; capture radiant energy to phosphorylate ADP to ATP
 Green plants
 Algae
 cyanobacteria
2. Chemotrophs - most bacteria organisms that rely on oxidation
of organic or inorganic chemical compounds to supply energy
to phosphorylate ADP
Electron Sources
1. Lithotrophs – microbes that used reduced inorganic material as
their electron source
2. Organotrophs – microbes that use organic material as their
electron source
Subcategories of chemotrophs: (based on electron source)
(1) chemo- organotrophs - extract electrons (e-) from organic
compounds
(2) chemo- lithotrophs (i.e. rock-eaters) - use reduced inorganic
substances as energy sources
Carbon Sources
Two major divisions:
1. Autotrophs - those microorganisms that can use CO2 as their
sole or principal source of carbon. Reduction of CO2 is energy
expensive, so many microorganisms can not do this
a. photoautotroph (photolithotrophic autotroph) - obtain
energy from light and use CO2 as primary carbon source
i. Sulfur-oxidizing bacteria (purple & green)
ii. Eukaryotic algae
iii. Cyanobacteria
b. chemolithotrophic autotrophy - obtain energy from
oxidation of inorganic compounds and carbon from CO2
2. Heterotrophs - microorganisms that use reduced, preformed
(made by other organisms) organic molecule as carbon sources
a. chemoheterotroph (chemoorganotrophic heterotroph)
use organic compounds for energy, hydrogen, electrons,
and carbon (nonphotosynthetic)
b. photoheterotroph (photoorganotrophic heterotrophs)use organic compounds as carbon sources and radiant
energy for ATP synthesis
i. Purple non-sulfur bacteria
ii. Green non-sulfur bacteria
3. methylotrophs – These fastidious (selective feeders)
microorganisms grow on single carbon compounds, such as
methane (family Methylomonadaceae)
4. mixotrophs- These microorganisms grow using a combination
of chemolithotrophic and heterotrophic metabolic processes
a. Purple non-sulfur bacteria are photoorganotrophic
heterotrophs when O2 concentrations are low, but are
chemotrophs when O2 levels are normal.
b. Begiatoa use both inorganic and organic carbon sources