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Transcript
Dr. Hams H. Hashem
Medical Microbiology
2nd Class. 1st Semester
Bacterial physiology
The biochemical reactions that together enable bacteria to live, grow, and
reproduce. Strictly speaking, metabolism describes the total chemical
reactions that take place in a cell, while physiology describes the role of
metabolic reactions in the life processes of a bacterium.
Cell Metabolism
Cell metabolism is the total energy released and consumed by a cell.
Metabolism describes all of the chemical reactions that are happening in
the cell. Some reactions, called anabolic reactions, create’ needed
products. Other reactions, called catabolic reactions, break down
products. Your body is, performing both anabolic and catabolic reactions
at the same time and around the clock, twenty four hours a day, to keep
your body alive and functioning. Even while you ‘sleep, your cells are
busy metabolizing.
• Catabolism:
The energy releasing process in which a chemical or food is used (broken
down) by degradation into smaller pieces.
• Anabolism:
Anabolism is just the opposite of catabolism. In this portion of
metabolism, the cell consumes energy to produce larger molecules via
smaller ones. ATP is the currency of the cell. When the cell needs to use
energy such as when it needs to move substances across the cell
membrane via the active transport system, it “pays” with molecules of
ATP.
Dr. Hams H. Hashem
Medical Microbiology
2nd Class. 1st Semester
Physical and chemical growth determinate:
Main Requirements:
Many food-poisoning bacteria have to multiply to high numbers in food
before they are likely to cause illness. The four main requirements for
bacterial growth are nutrition, moisture, warmth and time.
Nutritional Requirements
1-Basic nutritional requirements for growth:
A-Carbon
building
blocks
of
cell
components
b-Nitrogen
production
of
proteins,
nucleic
acids
c-Hydrogen
occur
in
organic
compounds
d-Oxygen
involved
in
the
production
of
energy
e-Minerals, Trace Elements - required in small amount
2-Special metabolites (growth factors)
a-Substances required for growth that the cell cannot produce ,
( Ex. : vitamins, amino acids, carbohydrates, blood factors )
b- Organisms may be described as being fastidious.
Dr. Hams H. Hashem
Medical Microbiology
2nd Class. 1st Semester
Two types organisms based on source of nutrients :
1-Autotrophs - utilize inorganic compounds
( C - CO2, carbonates; N - NH4, N2, NO3 )
2-Heterotrophs - utilize organic compounds
( C - CHO, lipids; N - proteins )
a- Saprophytes - nonliving organic material
b- Parasites - viable (living) organic material
Moisture
Most foods naturally contain sufficient moisture to provide bacteria with
the water they need in order to grow. Where moisture has been
deliberately removed (e.g. in dehydrated foods such as milk powder, soup
mixes, etc.), then bacteria will not grow whilst the food remains dry, but
once water is added then bacterial growth may occur once more.
Warmth / Temperature
Bacteria have varying requirements in terms of the range of temperatures
in which they will grow. Those which grow at low temperatures (usually
below 20°C) are called psychrophiles and at high temperatures (above
45°C) are thermophiles. Some spoilage bacteria fall into these categories.
Most pathogens, however, like warmth and are known as mesophiles.
They will grow at temperatures between 5°C and 63°C, commonly
referred to as the growth or danger zone and have an optimum
temperature for growth of about 37°C.
Dr. Hams H. Hashem
Medical Microbiology
2nd Class. 1st Semester
Time
In ideal conditions (i.e. in moist foods at 37°C) bacteria will grow and
multiply
by
dividing
into
two
every
20
minutes.
Other Factors Affecting Growth
PH Level
The acidity or alkalinity of foods will affect bacterial growth. Most
bacteria
like
neutral
conditions
(pH
value
of
7).
Concentration
of
H+,
OHions
1
----------------------------------7
Acid
Neutral
------------------------------14
Basic
.
. Divided into four groups according to pH range:
*Neutrophiles
- 5 to 8 *Optimum pH 7.0
- 7.2
*Alkalinophiles - 8 to 12 *Acidophiles - 0 to 5
Oxygen
Pathogens vary in their oxygen requirements. Those which require
Dr. Hams H. Hashem
Medical Microbiology
2nd Class. 1st Semester
oxygen are called aerobes, e.g. Bacillus cereus. Those which do not need
oxygen are called anaerobes, e.g. Clostridium perfringens .Those which
will grow or survive with or without oxygen are known as facultative
anaerobes and include Salmonella species and Staphylococcus aureus.
Microaerophilic - require the presence of small amounts of oxygen (2% 10%).
Osmotic pressure
ab(
c(
d-
Exerted
by
solutes
in
water
Increase
o.p.
outside
cell
water
leaves
cell
very
high
o.p.
dehydrates
cell
)
Decreased
o.p.
outside
cell
water
enters
cell
very
low
o.p.
lysis
of
cell
)
Halophiles
require
the
presence
of
3%
NaCl
( extreme halophiles - 20 to 30% NaCl )
LIGHT ( RADIATION )
1-Very small group photosynthetic bacteria (cyanobacteria) - require UV
light.
2- Nonphotosynthetic bacteria (eubacteria) - UV light is lethal (causes
mutations).
Competition
Where there are a number of different bacteria present in food, they
compete for the same nutrients. Pathogens are often not as competitive as
spoilage bacteria and unless present in high numbers, will usually die.