Microbiology Lab



Exercise1: Hanging-Drop and Wet-Mount Preparation

  1. How does true motility differ from Brownian movement?

Intrue motility, bacteria moves freely while in Brownian movement,bacteria move due to collision of water molecule.

  1. What morphological structure is responsible for bacterial motility?

Flagellais the morphological structure responsible for bacterial motility

  1. Why is a wet preparation discarded in disinfectant solution or biohazard container?

Becauseit contains microorganisms that are harmful to human beings andenvironment.

  1. What is the value of a hanging-drop preparation?

Thehanging-drop preparation is valuable for allowing clear observationof the living organisms.

  1. What is the value of a wetmount preparation?

Thewet-mount preparation is valuable for representing motility inmicroorganisms.


  1. Define acidic and basic dyes. What is the purpose of each?

Acidicdyes are negative charged dye while basic dyes are positive chargeddye. Acidic dyes are used on gram-negative bacteria while basic dyesare used on gram-positive bacteria.

  1. What is the purpose of fixing a slide that is to be stained?

Tokill cells by denaturing the enzymes, and stick the cells to theslide hence, prevent wash-off

  1. Why are the specimens to be stained suspended in sterile saline or distilled water?

Toavoid incorrect results by microorganism in water, as well as avoidimpurities.

  1. How does a stained preparation compare with a hanging drop for studying the morphology and motility of bacteria?

Itis easier to perform hanging drop and it provides sufficientinformation unlike the stained preparation.

  1. List at least three types of bacteria whose names reflect their shapes and arrangements, and state the meaning of each name.

Streptococcusmeaning spherical, spirillum pulli meaning spiral shape, andlactobacillus meaning rod shape.


  1. What is the function of the iodine solution in the Gram stain? If it were omitted, how would staining results be affected?

Theiodine solution in gram stain mixes the dye to form insolublesubstance, and if omitted, the bacteria would come off.

  1. What is the purpose of the alcohol solution in the Gram stain?

Thealcohol solution helps in fixing the colours to differentiate gram.

  1. What counterstain is used? Why is it necessary? Could colors other than red be used?

Thecounterstain used is the safranin since it is the most suitable toshow gram-bacterias. No other color can be used apart from red.

  1. What is the advantage of the Gram stain over a simple stain such as methylene blue?

Thegram stain can determine whether a bacterium has a thin or a thickcell wall, and can differentiate between gram-positive andgram-negative bacteria unlike the simple stain.

  1. In what kind of clinical situation would a direct smear report from the laboratory be of urgent importance?

Incase of an infection or a cancer


  1. When an agar plate is inoculated, why is the loop sterilized after the initial inoculum is put on?

Theloop is sterilized after inoculums to minimize the risk ofcontamination.

  1. Distinguish between a pure culture and a mixed culture.

Pureculture consists of single strains or species while mixed culturecontains multiple species or strains.

  1. Define a bacterial colony. List four characteristics by which bacterial colonies may be distinguished.

Bacterialcolony is a group of bacteria from one specimen of bacteria. The fourcharacteristics to distinguish bacterial colony include size, colour,shape, and the constituency.

  1. Why should a Petri dish not be left open for any extended period?

Thepetri dish should not be left open for an extended period to minimizethe possibility of microbe culture growing in the dish andcontamination from microbes.

  1. Why does the streaking method you used to inoculate your plates result in isolated colonies?

Thestreaking method used to inoculate plates results to isolatedcolonies because the bacterial cells spread thinly over the agarsurface.


  1. Discuss the relative convenience of pour and streakplate techniques in culturing clinical specimens.

Thepour-plate and streak-plate technique forces out the mixture ofbacteria into the growth medium and creates enough room forindividual organisms to form distinct colonies.

  1. How do you decide which colonies should be picked from a plate culture of a mixed flora?

Todecide which colonies to pick from plate culture of mixed flora, oneshould first determine the type of desired culture mixed eitherculture with different bacteria and microbes or pure culture with onemicrobe or one strain of bacteria.

  1. Why is it necessary to make pure subcultures of organisms grown from clinical specimens?

Itis necessary to make pure subculture of organisms because it makes iteasy for a medical practitioner to prescribe the most effectiveantibiotic to a patient based on the present bacteria.

  1. What kinds of clinical specimens may yield a mixed flora in bacterial cultures?

Thekinds of clinical specimens that can yield to flora in bacterialculture include oral, GI, and skin specimens.

  1. When more than one colony type appears in pure culture, what are the most likely sources of extraneous contamination?

Whenmore than one colony appears in pure culture, the source ofcontamination may be from transformed or mutant individuals.



  1. How are microorganisms destroyed by moist heat? By dry heat?

Themicroorganisms are destroyed by moist heat or dry heat by exposingthem to a temperature of fifty to seventy degree Celsius.

  1. Are some microorganisms more resistant to heat than others? Why?

Somemicroorganisms are more resistant to heat because they have tougherspore coat and low moisture content. In addition, some havedipicolinic acid and calcium that protect them from heat.

  1. Is moist heat more effective than dry heat? Why?

Moistheat is more effective compared to dry heat due to its capability topenetrate microbial cells.

  1. Why does dry heat require higher temperatures for longer time periods to sterilize than does moist heat?

Dryheat requires high temperature to sterilize than moist heat becauseair is a poor conductor of heat is a good conductor of heat. Further,water has high heat capacity compared to air (Schlegeland Zaborosch, 1995).

  1. What is the relationship of time to temperature in heat sterilization? Explain.

Thelower the temperature the more the sterilization time and vice versa.They both have an inverse proportional relationship.


  1. Define the principles of sterilization with an autoclave and with a dry heat oven.

Autoclaveuses pressurized steam heat to sterilize. The bacteria are subjectedto high temperature and pressurized environment. In other words, thebacteria are killed under high temperature and pressure.

  1. What pressure, temperature, and time are used in routine autoclaving?

Asteam pressure of fifteen to twenty Lb, a steam temperature of 120°cto 125°c and duration of fifteen to forty-five minutes.

  1. What factors determine the time period necessary for steampressure sterilization? Dryheat oven sterilization?

Temperatureand time are the factors that determines the time for steam-pressuresterilization. On the other hand, the temperature of the hot airdetermines the time for dry-heat oven sterilization.

  1. Why is it necessary to use bacteriologic controls to monitor heat- sterilization techniques?

Becauseit is important to ensure sterilization is successful with eachsteam-pressure sterilizer.

  1. When running an endospore control of autoclaving technique, why is one endospore preparation incubated without heating?

Whilerunning an endospore control, one endospore preparation is incubatedwithout heating to determine whether the spore is effectivelydestroyed. In case the spore is not destroyed then, the machine hasa problem.


  1. Define a differential medium and discuss its purpose.

Adifferential medium is an example of culture medium that is used todistinguish microorganism depending on colony appearance. It mainpurpose is to identify microbes.

  1. Define a selective medium and describe its uses.

Selectivemedium is a culture medium that allows the growth of microorganismsdepending on the medium content (Stratton,2012).The main purpose of selective medium is to hold back all the unwantedmicrobes, as well as enhance the microbe growth.

  1. Why is MacConkey agar selective as well as differential?

MacConkeyagar is selective media because it consists of bile salts thatsupport gram-negative microbes growth and crystal violent thatinhibit the gram-positive microbes growth. It is also differentialdue to its neutral red indicator.

  1. Why is blood agar useful as a primary isolation medium?

Bloodagar useful as a primary isolation medium because of the pathogensthat grow effectively in blood.

  1. What is the major difference between Modified ThayerMartin (MTM) and chocolate agar? When would you use MTM rather than chocolate agar?

ModifiedThayer-Martin and chocolate agar are selective medium andnon-selective medium respectively. It is essential to use MTM ratherthan chocolate agar when genital culture is required.


  1. What is the color of phenol red at an acid pH?


  1. What is the function of a Durham tube?

Durhamtube taps the gas that is produced during fermentation.

  1. Why is iodine used to detect starch hydrolysis?

Iodinedissolves in potassium iodide and reacts with starch to produce deeppurple colour to show presence of starch

  1. How is indole produced in SIM medium? How is it detected?

Indoleis produced from the reaction between ferrous ammonium sulphates withferrous sulphide (Walker,2008).Indole is detected by adding chemical reagents after incubationperiod.

  1. How is hydrogen sulfide demonstrated in this medium?

Ahydrogen sulphide is demonstrated in a medium by combining it withiron to form a black precipitate known as ferric sulphide.

  1. What is the advantage of viewing mold structures in a transparent tape preparation?

Becauseit enhances minimum disruption while viewing fungal structure ofcharacteristics morphology.

  1. What fungus can be identified reliably by using the germ tube test?

Candidaalbicans is a fungus that can be identified by use of germ tube test.

  1. Name three stains or reagents that may be used to facilitate the microscopic detection of fungi in clinical samples.

Indiaink, potassium hydroxide, and calcoflour white

  1. What is the main advantage of using the slide culture technique for identifying molds?

Itallows microscopically view the sporulating structure at differentgrowth stage without disrupting their characteristics arrangement.

  1. What is an opportunistic pathogen? Name three fungal specimens.

Opportunisticpathogens are microorganisms that led to infection inimmune-compromised individuals. Aspergilus, Candida, and Cryptococcusare examples of these fungal specimens.

Exercise6: Protozoa and Animal Parasites

  1. Describe the basic structures of protozoa. Can these same structures be seen in bacteria using a light microscope?

Thebasic structures of protozoa are plasma membrane, cytoplasm, nuclear,and locomotive organelle, and they are visible by use of lightmicroscope.

  1. Are any parasitic diseases directly communicable from person to person? If so, how are they transmitted? What kinds of precautions should be taken in caring for persons with directly transmissible parasitic infections?

Someparasitic diseases are communicable from one person to another bydirect contact with an infected person, via insect bite, or byingestion of contaminated food and water. The common precautionsagainst these diseases include vaccination and frequent and properhand washing.

  1. What parasitic forms can be seen in the faeces of a patient with hookworm? Cryptosporidiosis? Tapeworm? Trichinosis?

Thefaeces of a hookworm patient do not have parasitic but hookworm eggs.

  1. What parasitic forms can be seen in the blood of a patient with African sleeping sickness? Filariasis? Amebiasis?

Theparasitic found in the blood of a patient with African sleepingsickness is the trypanosome brucei.

  1. What is meant by the life cycle” of a parasite? What importance does it have to those who take care of patients with parasitic diseases?

Lifecycle of a parasite is the life pattern that a parasite follows itsentire life. It helps to treat patients, as well as detect the stageof the parasite.


Schlegel,H. G., &amp Zaborosch, C. (1995).&nbspGeneralmicrobiology.Cambridge [u.a.: Cambridge Univ. Press.

Stratton,C. W. (2012).&nbspClinicalmicrobiology: Quality in laboratory diagnosis.New York: Demos Medical Pub.

Walker,T. S. (2008).&nbspMicrobiology.Philadelphia, Pa. [u.a.: Saunders.