(Environmental Factors /Lecture 9)1-TEMPERATURE ADAPTATIONSMicrobial cells are unable to control their temperature and thereforeassume theambient temperature of their natural habitats. Their survivalis dependent on adaptingto whatever temperature variationsare encountered in that habitat. The range oftemperatures for microbialgrowth can be expressed as three cardinal temperatures.A-Theminimum temperature is the lowest temperature that permits amicrobe’scontinued growth and metabolism; below this temperature,its activities areinhibited.B-The maximum temperature is the highesttemperature at which growth andmetabolism can proceed. If thetemperature rises slightly above maximum, growth willstop, but ifit continues to rise beyond that point, the enzymes and nucleic acidswilleventually become permanently inactivated and the cell willdie. This is why heatworks so well as an agent in microbial control.C-The optimum temperature covers a small range, intermediate betweentheminimum 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°Cand is capable of growth at 0°C.It is obligate with respect to cold and generallycannot grow above 20°C.b-The majority of medically significant microorganisms aremesophiles, organismsthat grow at intermediatetemperatures. Although an individual species can grow at theextremesof 10°C or 50°C, the optimum growth temperatures (optima)of mostmesophiles fall into the range of 20°–40°C. Organismsin this group inhabit animalsand plants as well as soil andwater in temperatec-A thermophile is a microbe that grows optimallyat temperatures greater than45°C. Such heat-loving microbeslive in soil and water associated with volcanicactivity andin habitats directly exposed to the sun. Thermophiles vary inheatrequirements, with a general range of growth of 45°–80°C. Mosteucaryoticforms cannot survive above 60°C, but a few thermophilicbacteria calledhyperthermophiles, grow between 80°Cand 110°C (currently thought to be thetemperature limit enduredby enzymes and cell structures). Strict thermophiles are soheat-tolerant that researchers may use an autoclave to isolate them inculture.
THE POPULATION GROWTH CURVEIn reality, a population of bacteria does not maintain its potentialgrowth rate and doesnot double endlessly, because in most systemsnumerous factors prevent the cells fromcontinuously dividing attheir maximum rate. Quantitative laboratory studies indicatethat apopulation typically displays a predictable pattern, or growthcurve, over time.The method traditionally used to observe the populationgrowth pattern is a viablecount technique, in which the totalnumber of live cells is counted over a given timeperiod. In brief,this method entails (1) placing a tiny number of cells into asterileliquid medium; (2) incubating this culture over a period of severalhours; (3)sampling the broth at regular intervals during incubation;(4) plating each sample ontosolid media; and (5) counting the numberof colonies present after incubation.Microbits 7.6 gives the detailsof this process.STAGES IN THE NORMAL GROWTH CURVEThe system of batch culturing is closed,meaning that nutrients and space are finite andthere is no mechanismfor the removal of waste products. Data from an entiregrowthperiod of 3 to 4 days typically produce a curve with a seriesof phases termed the lagphase, the exponential growth (log) phase, the stationary phase, and the deathphase.I-The lag phase is a relatively ―flat‖ period on the graph whenthe population appearsnot to be growing or is growing at less thanthe exponential rate. Growth lagsprimarily 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.Thecells reach the maximum rate of cell division during theexponential growth (log)phase, a period during which the curveincreases geometrically. This phase willcontinue as long as cellshave adequate nutrients and the environment is favorable.III-At the stationary growth phase, the population enters a survivalmode in whichcells stop growing or grow slowly. The curvelevels off because the rate of cellinhibition 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 inexponentialnumbers (literally perishing in their own wastes), and they areunable tomultiply. The curve now dips downward as the deathphase begins.The speed with which death occurs depends on the relativeresistance of the speciesand how toxic the conditions are, but itis usually slower than the exponential growthphase. 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 sothat cultures will remain viable as long as possible.
(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 forma sugar-phosphate linkage that becomes the backbone of each strand. Each sugarattaches in a repetitive pattern to two phosphates. One of the bonds is to the number5_ (read ―five prime‖) carbon on deoxyribose, and the other is to the 3_ carbon, whichconfers a certain order and direction on each strand The nitrogenous bases, purinesand pyrimidines, attach by covalent bonds at the 1_ position of the sugar. They spanthe center of the molecule and pair with appropriate complementary base from theother side of the helix. The paired bases are so aligned as to be joined by hydrogenbonds. 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 ofhydrogen bonds between certain bases. Thus, in DNA, the purine adenine (A) pairswith the pyrimidine thymine (T), and the purine guanine (G) pairs with thepyrimidine cytosine (C).
(Transcription& Translation /Lecture 11)TranscriptionThe first step in gene expression is the production of anRNA copy of the DNAsequence encoding the gene, aprocess called transcription. To understand themechanismbehind the transcription process, it is useful to focusfirst on RNApolymerase, the remarkable enzyme responsiblefor carrying it outPromoterTranscription starts at RNA polymerase binding sitescalled promoters on the DNAtemplate strand. A promoteris a short sequence that is not itself transcribed bythepolymerase that binds to it
.Lec 13/Antimicrobial chemotherapy.………….……..DRUG, MICROBE, HOST—SOME BASICINTERACTIONS(1) The drug is administeredto the host via a designated route. Delivery isprimarilyby oral, circulatory, muscular, and cutaneous routes.(2) The drug isdissolved in body fluids.(3) The drug is delivered to the infectedarea (extracellular orintracellular).(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.
MECHANISMS OF DRUG ACTIONAntimicrobial 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.
OVERVIEW OF LABORATORY TECHNIQUESThe routes taken in specimen analysis are the following:(1) directtests using microscopic, immunologic, or other specific methods that provideimmediate clues as to the identity of the microbe ormicrobes in the sample and(2) cultivation, isolation, and identification of pathogens using a wide variety ofgeneral and specific tests . Most test results fall into two categories:presumptivedata, which place the isolated microbe (isolate) in a preliminarycategorysuch as a genus, and more specific, confirmatorydata, which provide more definitiveevidence of a speciesBiochemical Testing The physiological reactions of bacteria tonutrients and othersubstrates provide excellent indirect evidence ofthe types of enzyme systems presentin a particular speciesthepolymerase chain reaction(PCR). This method can amplify DNA present insamples even intiny amounts, which greatly improves the sensitivity of the test PCRtests are being used or developed for a wide varietyof bacteria, viruses, protozoa, andfungi.
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 substancessuchas antibodies. Active immunity is marked by several characteristics:(1) It is an essential attribute of an immunocompetent individual; (2) it creates amemory that renders the person ready for quick action upon reexposure to that sameantigen; (3) it requires several days to develop; and (4) it lasts for a relatively longtime, sometimes for life. Active immunity can be stimulated by natural or artificialmeans.Passive immunity occurs when an individual receives immune substances(antibodies) that were produced actively in the body of another human or animaldonor. The recipient is protected for a time even though he or she has not had priorexposure to the antigen. It is characterized by: (1) lack of memory for the originalantigen, (2) lack of production of new antibodies against that disease, (3) immediateonset of protection, and (4) short-term effectiveness, because antibodies have alimited 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 PreparationA vaccine must be considered from the standpoints of antigen selection,effectiveness, ease in administration, safety, and cost.In natural immunity, aninfectious agent stimulates appropriate B and T lymphocytes and createsmemory clones. In artificial active immunity, the objective is to obtain this sameresponse with a modified version of the microbe or its components.A safe and effective vaccine should mimic the natural protective response, notcause 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 followingantigenic stimulants (1) killed whole cells or inactivated viruses, (2) live, attenuatedcells or viruses, (3) antigenic components of cells or viruses, or (4) geneticallyengineered microbes or microbial antigens. Large, complex antigens such as wholecells or viruses are very effective immunogens. Depending on the vaccine, these areeither killed or attenuated.