GROWTH & REPLICATION OF
BACTERIA
Assist Prof Dr.
Syed Yousaf Kazmi
OBJECTIVES
1. Explain growth requirements of bacteria
2. Relate bacterial growth curve to survival and
spread of bacteria
3. Identify bacterial methods of replication and
its clinical significance
GROWTH REQUIREMENTS
• Bacteria reproduce by binary
fission
• Despite rapid growth time in
many bacteria e.g. 10 min
for E coli, the population of
microorganisms in the
biosphere is roughly
constant
• Growth is counterbalanced
by death -depletion of
nutrients/ accumulation of
toxic comp
Binary Fission In Bacteria
GROWTH REQUIREMENTS
A. PHYSICAL REQUIREMENTS
1. Temperature
• Psychrophiles cold-loving bacteria- Grow between -5o to 15o C e.g.
Listeria monocytogenes, Flavobacterium spp.
• Thermophiles heat-loving bacteria-Grow between 45oC and 70oC e.g.
Bacillus stereothermophilus
• Mesophiles optimum growth temperature between 25oC and 45oC
Most pathogenic bacteria
2. pH
•
Neutrophiles grow best at a pH range of 5 to 8. Most bacteria
•
•
Alkalophiles grow best at a pH above 8.5 e.g. Vibrio cholerae
Acidophiles grow best at a pH below 5.5. e.g. Lactobacillus
acidophilus
MCQ
• A lady brought soft cheese from Panda store at Majmaah
and placed it in refrigerator. Five days later, she consumed
soft cheese and developed diarrhea and vomiting. Given
that the contamination of the pathogen occurred at home,
which organism will most likely be the pathogen?
a. Escherichia coli
b. Listeria monocytogenes
c. Bacteroides fragilis
d. Streptococcus pyogenes
e. Clostridium tetani
Correct Ans: b
GROWTH REQUIREMENTS
3. Oxygen requirements
• Obligate aerobes grow only in the presence of oxygen e.g. M.
tuberculosis
• Obligate anaerobes grow only in the absence of oxygen e.g.
Bacteriodes fragilis
• Facultative anaerobes grow with or without oxygen e.g. E. coli
• Microaerophiles require a low conc of O2 (2% to 10%) for growth
e.g. Campylobacter spp., Helicobacter spp.
• Aerotolerant anaerobe are Anaerobic, but tolerates exposure to
oxygen e.g. Clostridium perfringens
• Capnophilic prefers increased carbon dioxide levels e.g. Neisseria
spp.
Two enzymes superoxide dismutase and catalase required to combat
free radicals generated during resp
2O2+2H+ →H202+O2, 2H202 →2H2O +O2
MCQ
A microbiology student noticed that Thioglycollate
culture broth tube was very turbid at the surface
and clear in the rest of the tube. She can conclude
that the
1. Broth is sterile.
2. Organisms cannot tolerate oxygen.
3. Organisms are aerobes
4. Organisms should be put in a candle jar.
5. Organisms are facultative anaerobes
Correct Ans: 3
GROWTH REQUIREMENTS
4. Tonicity
•
•
•
Isotonic Growth at isotonic atmosphere: Most bacteria
Osmo-tolerant Tolerate growth at high salt conc: e.g. Staph aureus,
Halophiles Grow at very high salt conc e.g. some Vibrio spp
B. Nutritional requirements
•
•
•
•
•
Carbon source Autotroph, Heterotroph
Minerals e.g. Iron very imp for bacterial survival, utilize
siderophores to chelate iron
Water
Trace elements
Growth factors like heme, nicotinamide, purines etc. required
for fastidious org e.g. Hemophilus spp, Group B strep etc.
BACTERIAL GROWTH CURVE
• Bacteria are said to undergo exponential growth (logarithmic
growth)
1, 2, 4, 8, 16, 32
20 , 2 1 , 2 2 , 2 3 , 2 4 , 25
• 1 bacterium will produce 16 bacteria after 4 generations
• The doubling time for Escherichia coli is 20 min, while it is 24 hrs
for Mycobacterium tuberculosis
• One E. coli will produce over 106 org in 7 hrs
• Doesn't happen due to toxic metabolites/ loss of nutrients
BACTERIAL GROWTH CURVE
BACTERIAL GROWTH CURVE
1. Lag phase
 Vigorous metabolic activity occurs but
cells do not divide
 Few minutes up to many hours
2. Log (logarithmic) phase
•
•
Rapid cell division occurs
Lactam drugs act during this phase when
cells are making peptidoglycan
3. Stationary phase
•
Nutrient depletion or toxic products growth slows till number of new cells
produced = the number of cells that die
4. Death phase
•
Decline in the number of viable bacteria
MCQ
• A student wish to study the effect of an
antimicrobial that inhibits peptidoglycan
synthesis of bacteria. In which phase of the
growth curve should he observe the maximum
effect of the antimicrobial?
1.
2.
3.
4.
Lag phase
Log phase
Stationary phase
Death phase
MCQ
• A butcher developed a non healing ulcer on his
left forearm after lacerated wound during his
work at shop. The ulcer had erythematous and
edematous edges with black necrotic central
scab. The infective form of this disease developed
in which stage of the growth curve of this
pathogen?
1.
2.
3.
4.
Lag phase
Log phase
Stationary phase
Death phase
CLINICAL IMPLICATIONS OF GROWTH
CURVE OF BACTERIA
• Cholera is a disease that can
kill within 12 h, whereas
tuberculosis takes months to
develop
• Result of C/S for V cholerae in
48 hrs while it takes weeks for
M tuberculosis
• Length of the lag phase and
rates of exponential growth in
different circumstances make
predictions and contribute to
safety standards for storage in
the food industry