Download Microbial nutrition

Medical biology –Lec.4
Lecturer – Dr. Hanan S.
Microbial nutrition
Growth of a living organism, may be defined as an increase in mass or size
in any direction accompanied by synthesis of macromolecules, leading to the
production of a newly organized structure. Growth when applied to bacteria
normally refers to an increase in number of individual cells and so , is a
measure of population density from original inoculums. Bacterial growth can
be very rapid for ex. culture of Escherichia coli can double in size in 20 minutes
in a rich medium . so , to obtain energy and construct new cellular components,
organisms must have a supply of raw materials or nutrients.
Nutrients : are substances used in biosynthesis and energy production ,
therefore are required for microbial growth.
Common nutrient requirements
Microbial cell require a basic nutrient elements for growth and replication
as fallow :
1- Microbial cell composition is made up of few major elements like
carbon , oxygen , hydrogen, nitrogen , sulfur , phosphorus , potassium ,
calcium, magnesium and iron are called macro elements because they are
required in large amounts as components of carbohydrate , lipid , protein ,
nucleic acids synthesis.
2- Microorganisms require several micronutrients like manganese , zinc ,
cobalt , nickel and copper are available in nature , have many roles in the
cells as a part of enzymes , cofactors that make to catalysis of reaction and
maintenance of protein structure .
3- Growth factors are organic compounds require as an essential cell
components and can not synthesis by M.O. There are three major
classes of growth factor :
a- purines and pyrimidines for nucleic acid synthesis .
b- amino acids are needed for protein synthesis .
c- vitamins are small organic molecules that usually make –up all or
part enzymes and cofactors that are needing in very small amounts to
sustain growth such as folic acid , biotin , vitamins B12 and K etc .
Some bacteria for ex. E. coli do not require any growth factors , they can
synthesize all essential purines, pyrimidines, amino acids and vitamins, starting
with their carbon source, as part of their own intermediary metabolism, while
other bacteria like Lactobacillus requires purines, pyrimidines, vitamins and
several amino acids in order to grow.
1
Aspects of bacterial growth
Cell multiplication requires a source of energy and raw materials for
synthesis of cell components . Microorganisms can be classified as either
heterotrophs or autotrophs with respect to their source of carbon & for
phototrophs , chemotrophs with respect to their source of energy as fallow:
1- Photoautotroph :- Which use light energy and CO2 as a source of
carbon.
2- Photoheterotrophs :- Which use light energy and organic
compounds as a source of carbon.
3- Chemoautotrophs :- Which use chemical energy and CO2 as a
source of carbon.
4- Chemoheterotrophs :-Which use chemical energy and organic
compounds as a source of carbon.
Physical requirements for bacterial growth
There are some factors influencing microbial growth include :1- Temperature
Environmental temp. affects on M.O like other organisms because they are
usually unicellular and their temp. varies with that of the external environment
. M.O can be placed in fours classes based on their temp. range for growth:
a- Psychrophiles:- M.O grow well at 0c° and have an optimum growth
temp. between( 15-20) c° such as Vibrio.
b- Mesophiles:- M.O grow optimally around (20-45)c° such as all human
pathogen for ex. Staphylococcus aureus , Enterococcus.
c- Thermophiles:- M.O they can grow at temp of 55c° or higher for ex.
Bacillus , Sulfolobus .
d- Extreme thermophiles:- M.O grow optimally above 80c° for ex.
Pyrolobus fumarri.
2- Oxygen
a- Obligate aerobes have an absolute requirement for oxygen such as
Pseudomonas .
b-Obligate anaerobic : they can not multiply if oxygen is present and it's
kill bacteria if present such as Clostridium .
c- Facultative anaerobes :- can utilize oxygen if it's available but they can
grow in it's absence such as Staphylococcus .
d- Microaerophilic organisms :- require small amounts of oxygen (210)%, but higher concentrations are toxic for example Campylobacter .
e- Aero tolerant organisms :- grow in the presence or absence of oxygen
but they derive no benefit from the oxygen such as Enterococcus.
2
3- PH
a- Neutrophiles :- most species of bacteria grow best in a medium with a
pH 7.2 such as E. coli.
b- Acidophile :- include bacteria tolerate highly acidic condition pH
between (0-5) such as Lactobacilli
c- Alkalophiles :- Grow optimally under alkaline condition PH about
(8.5-11.5) for example Vibrio.
Reproduction in bacteria
In bacteria the genetic material is organized in a continuous strand of DNA , this
circle of DNA is localized in an area called nucleoide , bacterial cell may include one
or more plasmid that can replicate independently and not responsible for
reproduction of the microorganisms .
Bacteria can reproduce through :
1-Binary fission
It is occurs normally under favorable conditions , the cell elongate and becomes
constricted at middle, then the nucleus divided and the constriction increases until
the cell is split into two cells , each receiving one nucleus in generation time that
meaning the time needed for cell to divide into two cells and differs according to
species and prevailing condition, may be range between 20 minute to 6 hours .
Fig. binary fission in bacteria
2- Conidia
It occurs in members of certain families belonging to Actinomycetes e.g.,
Streptomyces , the ends of the filaments become septated forming a chain of
conidia similar to those produced by true fungi .
3
Exchange of genetic material
In this process , part of the genetic material of a donor cell is transferred
to a recipient cell by one of the fallowing mechanisms :
a-Transformation
It is uptake of the naked free DNA of the donor bacteria by the recipient
bacteria . The taken fragment is incorporated in chromosomal DNA and
expressed as a new character in the recipient cell as seen in the figure.
b- Transduction
It is transfer of part of DNA of the donor bacteria to recipient bacteria by
bacteriophage as seen in the figure below .
c-Conjugation
Conjugation occurs through mating of two bacteria cells one of them carries
plasmid ( F + ) and the other is plasmid (F-) . Conjugation is the most common
method occur in nature and it is a major phenomenon that is responsible for
transfer o f antibiotic resistant in bacteria (fig.) .
4
Robert Koch was a pioneer in medical microbiology and worked on
cholera , anthrax and tuberculosis , finally he proved the germ theory , in
Koch's postulates , he set out criteria to proved if an organism is the cause
of a disease and can be summarized as follows :
1- M.O must be present in every case of the disease but absent from
healthy organisms.
2- The suspected M.O must be isolated and grown in a pure culture.
3- The same disease must result when the isolated M.O is inoculated
into a healthy host.
4- The same M.O must be isolated again from the diseased host.
Mechanisms of bacterial pathogen
The steps when the pathogenic agents try to infect the host :
1-Adherence
2- Colonization
3- Evasion of host defenses
4- Causing disease
5- Exit from the host
Determinants of virulence
Pathogenic bacteria are able to produce disease because they
possess certain structural biochemical or genetic traits that render
them pathogenic or virulent such as :
5
1-flagella and pilli
2- capsule
3- enzymes such as hyaluronidase , streptokinase
4- toxin ( exotoxin & endotoxin)
5- lipopolysaccharid in outer membrane of G-ve bacteria .
Some of important definitions in medical microbiology
Pathogen : is a microorganism that is able to cause disease .
Pathogenicity : is the ability of M.O to produce disease in a host
organism.
Virulence : refers to the degree of pathogenicity of the microbe.
Opportunistic pathogen
Some of the normal bacteria flora that live in a commensal
relationship in a certain host without producing disease such as
Staphylococcus aureus , Haemophilus influenza may causes disease if
there is some compromise or weakness in the host's immunity .
Infection
The invasion of a host by M.O with subsequent establishment
and multiplication of the agent ,an infection may or may not lead to
cause disease .
Infectious disease
Any change from a state of health in which part or all of the
host's body can't carry on it's normal functions because of the
presence of an infectious agent or its product
6