1. (Environmental Factors /Lecture 9)
1-TEMPERATURE ADAPTATIONS
Microbial cells are unable to control their temperature and thereforeassume the
ambient temperature of their natural habitats. Their survivalis dependent on adapting
to whatever temperature variationsare encountered in that habitat. The range of
temperatures for microbialgrowth can be expressed as three cardinal temperatures.
A-Theminimum temperature is the lowest temperature that permits a
microbe’scontinued growth and metabolism; below this temperature,its activities are
inhibited.
B-The maximum temperature is the highesttemperature at which growth and
metabolism can proceed. If thetemperature rises slightly above maximum, growth will
stop, but ifit continues to rise beyond that point, the enzymes and nucleic acidswill
eventually become permanently inactivated and the cell willdie. This is why heat
works so well as an agent in microbial control.
C-The optimum temperature covers a small range, intermediate betweenthe
minimum and maximum, which promotes the fastest rate of growth and metabolism
(rarely is the optimum a single point).
a-Apsychrophile is a microorganism that has anoptimum temperature below 15°C
and is capable of growth at 0°C.It is obligate with respect to cold and generally
cannot grow above 20°C.
b-The majority of medically significant microorganisms aremesophiles, organisms
that grow at intermediatetemperatures. Although an individual species can grow at the
extremesof 10°C or 50°C, the optimum growth temperatures (optima)of most
mesophiles fall into the range of 20°–40°C. Organismsin this group inhabit animals
and plants as well as soil andwater in temperate
c-A thermophile is a microbe that grows optimallyat temperatures greater than
45°C. Such heat-loving microbeslive in soil and water associated with volcanic
activity andin habitats directly exposed to the sun. Thermophiles vary in
heatrequirements, with a general range of growth of 45°–80°C. Mosteucaryotic
forms cannot survive above 60°C, but a few thermophilicbacteria called
hyperthermophiles, grow between 80°Cand 110°C (currently thought to be the
temperature limit enduredby enzymes and cell structures). Strict thermophiles are so
heat-tolerant that researchers may use an autoclave to isolate them inculture.
2. THE POPULATION GROWTH CURVE
In reality, a population of bacteria does not maintain its potentialgrowth rate and does
not double endlessly, because in most systemsnumerous factors prevent the cells from
continuously dividing attheir maximum rate. Quantitative laboratory studies indicate
that apopulation typically displays a predictable pattern, or growthcurve, over time.
The method traditionally used to observe the populationgrowth pattern is a viable
count technique, in which the totalnumber of live cells is counted over a given time
period. In brief,this method entails (1) placing a tiny number of cells into a
sterileliquid medium; (2) incubating this culture over a period of severalhours; (3)
sampling the broth at regular intervals during incubation;(4) plating each sample onto
solid media; and (5) counting the numberof colonies present after incubation.
Microbits 7.6 gives the detailsof this process.
STAGES IN THE NORMAL GROWTH CURVE
The system of batch culturing is closed,meaning that nutrients and space are finite and
there is no mechanismfor the removal of waste products. Data from an entiregrowth
period of 3 to 4 days typically produce a curve with a seriesof phases termed the lag
phase, the exponential growth (log) phase, the stationary phase, and the death
phase.
I-The lag phase is a relatively ―flat‖ period on the graph whenthe population appears
not to be growing or is growing at less thanthe exponential rate. Growth lags
primarily because:
(1) The newlyinoculated cells require a period of adjustment, enlargement,
andsynthesis;
(2)the cells are not yet multiplying at their maximumrate;
(3)the population of cells is so sparse or dilute that thesampling misses them.
II-the length of the lag period varies somewhatfrom one population to another.The
cells reach the maximum rate of cell division during theexponential growth (log)
phase, a period during which the curveincreases geometrically. This phase will
continue as long as cellshave adequate nutrients and the environment is favorable.
III-At the stationary growth phase, the population enters a survivalmode in which
cells stop growing or grow slowly. The curvelevels off because the rate of cell
inhibition or death balances out the rate of multiplication.
IV- The decline in the growth rate is causedby depleted nutrients and oxygen,
excretion of organic acids andother biochemical pollutants into the growth medium,
and an increaseddensity of cells.As the limiting factors intensify, cells begin to die in
exponentialnumbers (literally perishing in their own wastes), and they areunable to
multiply. The curve now dips downward as the deathphase begins.
The speed with which death occurs depends on the relativeresistance of the species
and how toxic the conditions are, but itis usually slower than the exponential growth
phase. Viable cells oftenremain many weeks and months after this phase has begun.
In thelaboratory, refrigeration is used to slow the progression of the death phase so
that cultures will remain viable as long as possible.
3. (Microbial Genetics /Lecture10)
The basic unit of DNA structure is a nucleotide, a molecule composed of phosphate,
deoxyribose sugar, and a nitrogenous base. The nucleotides covalently bond to form
a sugar-phosphate linkage that becomes the backbone of each strand. Each sugar
attaches in a repetitive pattern to two phosphates. One of the bonds is to the number
5_ (read ―five prime‖) carbon on deoxyribose, and the other is to the 3_ carbon, which
confers a certain order and direction on each strand The nitrogenous bases, purines
and pyrimidines, attach by covalent bonds at the 1_ position of the sugar. They span
the center of the molecule and pair with appropriate complementary base from the
other side of the helix. The paired bases are so aligned as to be joined by hydrogen
bonds. Such weak bonds are easily broken, allowing the molecule to be ―unzipped‖
into its complementary strands.
Pairing of purines and pyrimidines is not random; it is dictated by the formation of
hydrogen bonds between certain bases. Thus, in DNA, the purine adenine (A) pairs
with the pyrimidine thymine (T), and the purine guanine (G) pairs with the
pyrimidine cytosine (C).
4. (Transcription& Translation /Lecture 11)
Transcription
The first step in gene expression is the production of anRNA copy of the DNA
sequence encoding the gene, aprocess called transcription. To understand the
mechanismbehind the transcription process, it is useful to focusfirst on RNA
polymerase, the remarkable enzyme responsiblefor carrying it out
Promoter
Transcription starts at RNA polymerase binding sitescalled promoters on the DNA
template strand. A promoteris a short sequence that is not itself transcribed bythe
polymerase that binds to it
5. .Lec 13/Antimicrobial chemotherapy.………….……..
DRUG, MICROBE, HOST—SOME BASICINTERACTIONS
(1) The drug is administeredto the host via a designated route. Delivery is
primarilyby oral, circulatory, muscular, and cutaneous routes.
(2) The drug isdissolved in body fluids.
(3) The drug is delivered to the infectedarea (extracellular or
intracellular).
(4) The drug destroys theinfectious agent or inhibits its growth.
(5) The drug is eventuallyexcreted or broken down by the host’s organs,
ideally withoutharming them.
6. MECHANISMS OF DRUG ACTION
Antimicrobial drugs function specifically in one of the following ways:
(1) They inhibit cell wall synthesis;
(2) they inhibit nucleic acid synthesis or function;
(3) they inhibitprotein synthesis;
(4) they interfere with the function of the cellmembrane.
7. OVERVIEW OF LABORATORY TECHNIQUES
The routes taken in specimen analysis are the following:
(1) directtests using microscopic, immunologic, or other specific methods that provide
immediate clues as to the identity of the microbe ormicrobes in the sample and
(2) cultivation, isolation, and identification of pathogens using a wide variety of
general and specific tests . Most test results fall into two categories:
presumptivedata, which place the isolated microbe (isolate) in a preliminarycategory
such as a genus, and more specific, confirmatorydata, which provide more definitive
evidence of a species
Biochemical Testing The physiological reactions of bacteria tonutrients and other
substrates provide excellent indirect evidence ofthe types of enzyme systems present
in a particular species
thepolymerase chain reaction(PCR). This method can amplify DNA present in
samples even intiny amounts, which greatly improves the sensitivity of the test PCR
tests are being used or developed for a wide varietyof bacteria, viruses, protozoa, and
fungi.
9. Microbiology (IMMUNITYAND IMMUNIZATION/Lecture 16)
Active immunity occurs when an individual receives animmune stimulus (antigen)
that activates the B and Tcells, causing the body to produce immune substancessuch
as antibodies. Active immunity is marked by several characteristics:
(1) It is an essential attribute of an immunocompetent individual; (2) it creates a
memory that renders the person ready for quick action upon reexposure to that same
antigen; (3) it requires several days to develop; and (4) it lasts for a relatively long
time, sometimes for life. Active immunity can be stimulated by natural or artificial
means.
Passive immunity occurs when an individual receives immune substances
(antibodies) that were produced actively in the body of another human or animal
donor. The recipient is protected for a time even though he or she has not had prior
exposure to the antigen. It is characterized by: (1) lack of memory for the original
antigen, (2) lack of production of new antibodies against that disease, (3) immediate
onset of protection, and (4) short-term effectiveness, because antibodies have a
limited period of function, and ultimately, the recipient’s body disposes of them.
Passive immunity can also be natural or artificial in origin.
Principles of Vaccine Preparation
A vaccine must be considered from the standpoints of antigen selection,
effectiveness, ease in administration, safety, and cost.In natural immunity, an
infectious agent stimulates appropriate B and T lymphocytes and creates
memory clones. In artificial active immunity, the objective is to obtain this same
response with a modified version of the microbe or its components.
A safe and effective vaccine should mimic the natural protective response, not
cause a serious infection or other disease, have long-lasting effects in a few doses,
and be easy to administer. Most vaccine preparations contain one of the following
antigenic stimulants (1) killed whole cells or inactivated viruses, (2) live, attenuated
cells or viruses, (3) antigenic components of cells or viruses, or (4) genetically
engineered microbes or microbial antigens. Large, complex antigens such as whole
cells or viruses are very effective immunogens. Depending on the vaccine, these are
either killed or attenuated.