chapter 20 medical microbiology. elsevier items and derived items © 2008 by saunders, an imprint of...

CHAPTER 20CHAPTER 20MEDICAL MICROBIOLOGY

Elsevier items and derived items © 2008 by Saunders, an imprint of Elsevier Inc. 2

PRETESTPRETEST

True or False1. Microbiology is the scientific study of

microorganisms and their activities.

2. A disease that can be spread from one person to another is known as an infectious disease.

3. Droplet infection is the transfer of pathogens from a fine spray emitted from a person already infected with the disease.

4. Streptococci are round bacteria that grow in pairs.

5. Chickenpox is caused by a virus.


PRETEST, CONT.PRETEST, CONT.

True or False6. The course adjustment on a microscope is used to

obtain precise focusing of an object.

7. The purpose of transport media is to provide nutrients for the multiplication of the specimen.

8. A throat specimen should be collected from the tonsillar area and posterior pharynx.

9. A wet mount is used to examine microorganisms in the living state.

10. A smear is material spread on a slide for microscopic examination.


Content OutlineContent Outline

1. Microbiology: the scientific study of microorganisms and their activities

a. Microorganisms: Tiny living plants or animals that cannot be viewed with the naked eye

2. Anton van Leeuwenhoek (1632-1723)a. Designed a magnifying glass strong

enough for viewing microorganisms• Precursor of modern microscope

b. First to observe and describe protozoa and bacteria

Introduction to Microbiology


Introduction to Microbiology, cont.


3. Microbiology: Deals with unicellular (one-celled) microscopic organisms

4. All life processes of a microorganism are performed by one cell including:

a. Ingestion of food substances and their use for energy

b. Growth

c. Reproduction

d. Excretion




5. Microorganisms are ubiquitous

a. Means they are found everywhere:

• Air

• Food

• Water

• Soil

• Plants

• Animals

• Humans




6. Only a small number of microorganisms are pathogenic (disease-producing)

7. Pathogen infects host: produces symptoms of that disease

a. Assists physician in diagnosing that disease




8. Microbiologic tests:

a. Also help physician identify pathogen causing disease

9. Identification of pathogen: leads to proper treatment

10.Most microbiologic tests: performed at outside laboratory

a. MA often responsible for collecting the specimen


Normal FloraNormal Flora

1. Every individual has normal flora:

2. Normal flora: Harmless microorganisms that normally reside in many parts of the body but do not cause disease

a. Examples of body areas with an abundant normal flora• Surface of skin

• Mucous membranes of gastrointestinal (GI) tract

• Respiratory tract

• Genitourinary tract


The Normal Flora, cont. The Normal Flora, cont.

3. Normal flora may be beneficial

a. Example:• Normal flora in intestinal tract:

– Feed on potentially harmful microorganisms

– Synthesize vitamin K (needed for blood clotting)

From Mahon CR, Manuselis G, Jr: Textbook of diagnostic microbiology, ed 2, Philadelphia, 2000, Saunders.


The Normal Flora, cont. The Normal Flora, cont.

4. Sometimes normal flora can become pathogenic and cause disease

a. May occur during lowered body resistance

Courtesy Cathy Bissonette


Infection Infection

1. Infection: invasion of the body by pathogenic microorganisms

a. If body conditions are favorable• Pathogen grows and multiplies

• Results in an infectious disease

– Infectious disease: a disease caused by a pathogen that produces harmful effects on its host


Infection, cont. Infection, cont.

2. Not all invading pathogens are able to cause disease

3. Body attempts to stop invasiona. Uses second line of defense mechanisms:

• Inflammation: a protective response of the body to the entrance of foreign matter.

– The purpose of inflammation is to destroy invading microorganisms and to repair injured tissue

• Phagocytosis by white blood cells (WBCs)

• Production of antibodies



4. First line of defense: works to prevent entrance of pathogens into body

a. Examples:• Skin

• Mucous membranes

• Mucus and cilia in nose



• Coughing and sneezing

• Tears and sweat

• Acid nature of urine and vaginal secretions

• Hydrochloric acid secreted by stomach



5. If body defenses are successful: pathogens are destroyed

a. Individual does not get infectious disease

6. If body defenses are overcome by pathogen

a. Individual gets infectious disease



7. Many infectious diseases are contagiousa. Contagious: capable of being

transmitted directly or indirectly from one person to another • Droplet infection: inhalation of

pathogens from a fine spray emitted by person infected with disease

• When infected individual exhales (breathing, talking, coughing, sneezing)

– Pathogens are dispersed on minute liquid particles

• Infected individual: should cover mouth when coughing and sneezing


Stages of an Infectious Disease

Stages of an Infectious Disease

1. Infection: invasion and multiplication of pathogenic microorganisms in the body

2. Incubation period: interval of time between invasion by a pathogen and appearance of first symptoms of disease

a. May range from a few days to several months

b. Pathogen is growing and multiplying during this time


Stages of an Infectious Disease, cont.


3. Prodromal period: first symptoms appear

a. Examples:• Headache

• Feeling of illness

From Mahon CR, Manuselis G Jr: Textbook of diagnostic microbiology, ed 2, Philadelphia, 2000, Saunders.




4. Acute period: disease is at its peak and symptoms fully developed

a. Example: fever

5. Decline period: symptoms begin to subside

6. Convalescent period: patient regains strength and returns to good health


Microorganisms and DiseaseMicroorganisms and Disease

1. Groups of microorganisms known to contain species capable of causing human disease:

a. Bacteria

b. Viruses

c. Protozoa

d. Fungi (yeast)

e. Animal parasites

Courtesy Cathy Bissonette.


BacteriaBacteria

1. Microscopic single-celled organisms

2. Of 1700 species known to dwell in humans: only about 100 produce human disease

3. Antibiotics: help in controlling bacterial infections

a. Not effective against viral infections

4. Classified according to shape


Classification of Bacteria by Shape

Classification of Bacteria by Shape


Staphylococci Staphylococci

5. Cocci: round shape

a. Staphylococci• Round bacteria that grow in grapelike clusters



Staphylococci, cont.Staphylococci, cont.

• S. epidermidis: normally present on skin surface and mucous membranes of the mouth, nose, throat, intestines

– Usually nonpathogenic

– Break in the skin can allow invasion

1) Example: cut

2) Results in a mild infection



• S. aureus: associated with pathologic conditions - Boils

- Carbuncles

- Pimples

- Impetigo

- Abscesses

- Staphylococcal food poisoning

- Wound infections



• Staphylococcal infections usually cause pus formation (suppuration)

– Termed: pyogenic infections



Streptococci Streptococci

b. Streptococci• Round bacteria that

grow in chains

From Mahon CR, Manuselis G Jr: Textbook of diagnostic microbiology, ed 2,

Philadelphia, 2000, Saunders.


Streptococci, cont. Streptococci, cont.

• Before antibiotics: major cause of human death

• Diseases caused by streptococci:

- Streptococcal sore throat

- Scarlet fever

- Rheumatic fever

- Pneumonia- Puerperal sepsis

- Erysipelas

- Carbuncles

- Impetigo From Mahon CR, Manuselis G Jr: Textbook of diagnostic microbiology,

ed 2, Philadelphia, 2000, Saunders.


Diplococci Diplococci

c. Diplococci• Round bacteria that grow in pairs

• Diseases caused by diplococci:

– Pneumonia

– Gonorrhea

– Meningitis




Bacilli Bacilli

6. Bacilli: rod shaped bacteria

a. Found in soil and air



Bacilli, cont. Bacilli, cont.

b. Some are able to form spores

• Spore: a hard, thick-walled capsule formed by some bacteria that contains only the essential parts of the protoplasms of the bacterial cell

• Enables them to resist adverse conditions

– Heat

– Disinfectants



• Diseases caused by bacilli:

– Botulism

– Tetanus

– Gas gangrene

– Gastroenteritis caused by Salmonella food poisoning

– Typhoid fever

– Pertussis

– Bacillary dysentery

– Diphtheria

– Tuberculosis Courtesy Cathy Bissonette.



c. Escherichia coli (E. coli): a species of bacillus• Normal flora of the large intestine




• Normally harmless

• May cause a urinary tract infection (UTI) if enters urinary tract due to:

– Lowered resistance

– Poor hygienic practices




Spirilla Spirilla

7. Spirilla: spiral or curved shaped bacteria

a. Treponema pallidum

b. Causative agent of syphilis

c. Cannot be grown in culture media• Diagnosis of syphilis made through serologic

tests


Spirilla, cont.Spirilla, cont.

b. Vibrio cholerae• Causative agent of cholera

– Immunization and proper sanitation: have eliminated cholera in United States


VirusesViruses

1. Smallest living organisms

2. Electron microscope must be used to view them

3. Can infect: plants, animals, humans

4. Use nutrients inside host cells

a. For metabolic and reproductive needs


Viruses, cont. Viruses, cont.

5. Diseases caused by viruses:

a. Influenza

b. Chickenpox

c. Rubeola (measles)

d. Rubella (German measles)

e. Mumps


Viruses, cont. Viruses, cont.

f. Poliomyelitis

g. Smallpox

h. Rabies

i. Herpes simplex

j. Herpes zoster


Viruses, cont.Viruses, cont.

k. Yellow fever

l. Hepatitis

m. Majority of diseases in upper respiratory tract• Example: Common cold


What Would You Do?What Would You Not Do?



MicroscopeMicroscope

1. Compound microscope: used most often in medical office

a. Consists of two-lens system

2. Components:

a. Support system

b. Optical system


Parts of a MicroscopeParts of a Microscope


Support SystemSupport System

1. Frame

a. Supports microscope

b. Consists of:• Base: for support

• Arm:

– For carrying it

– To support magnifying and adjusting systems


Support System, cont. Support System, cont.

2. Stage

a. Flat, horizontal platform

b. Slide is placed on stage



c. Has small round opening in center• Permits light from below to pass through the

object being viewed and up into lenses

d. Standard microscope: has metal clips to hold slide in place• Slide must be moved by hand



e. Mechanical stage: allows movement of slide in vertical or horizontal position using adjustment knobs• Needed for:

– Differential white blood cell count

– Inspection of gram-stained smear



3. Light source

a. Located at the base

b. Consists of: built-in illuminator • Switch for turning on and off

c. Light directed to condenser above• Then through object to be viewed



4. Condenser

a. Compound microscopes have lens system between light source and object• Known as

substage condenser



b. Popular type: Abbe's condenser• Consists of two lenses

• Collects and concentrates light rays and directs them up

– Brings light rays to a focus on the object so it is well-illuminated



5. Diaphragm

a. Controls the amount of light focused on an object

b. Amount of light controlled by iris diaphragm

c. Located beneath or inside condenser

d. Consists of: • Series of horizontally arranged interlocking

plates

• With central opening (aperture)



e. Diaphragm lever: increases or decreases aperture• Increases or

decreases light admitted



f. As magnification increases: more light is needed• Example: Oil immersion requires increased

light intensity

g. Low-power objective• Do not need as much light



h. Degree of illumination: also influenced by density of the object• Stained structures: require more light than

unstained specimens

6. Adjustment knobs

a. Coarse adjustment: to obtain an approximate focus quickly

b. Fine adjustment: to obtain precise focusing



c. Adjustment knobs:

-May be mounted as two separate knobs

-May be placed together


Optical SystemOptical System

1. Compound microscopes have two-lens magnification system

a. Magnification: The ratio of the apparent size of an object viewed through the microscope to the actual size of the object

2. The Eyepiece

a. First lens system

b. Also called ocular lens


Optical System, cont.Optical System, cont.

c. Located at top of body tube

d. Marked 10×(magnifies 10 times)

e. Monocular microscope: has one eyepiece

f. Binocular microscope: has two eyepieces


Optical System, cont. Optical System, cont.

3. Objective Lenses

a. Second lens system

b. Three objective lenses: located on a revolving nosepiece

c. Each objective has a different degree of magnification: engraved on metal shaft• Low-power objective: 10×

• High-power objective: 40×

• Oil-immersion objective: 100×



d. May have color-coded rings to identify objective• Example:

– Low-power objective: yellow ring

– High-power objective: blue ring

– Oil-immersion objective: white ring



e. Metal shafts of objectives: differ in length• Low-power objective: shortest length

• High-power objective: length between low and oil objectives

• Oil-immersion objective: longest length



• Low-power total magnification:

-100× (10×10)

• High-power total magnification:

-400× (10×40)

• Oil-immersion magnification:

-1000× (10×100)

f. Total magnification: multiply the ocular magnification by the objective magnification:



4. Focus

a. Barrel focus: body tube or barrel moves while stage remains stationary

b. Stage focus: stage moves while body tube remains stationary



5. Low and High Power

a. Low-power objective used for:• Initial focusing and light adjustment

• Initial observation and scanning requirements

b. High-power objective used for:• A more thorough study of the slide



c. Working distance: distance between tip of lens and slide• High-power objective: working distance is

short

• Care must be taken when using high-power objective

– To prevent it from striking and breaking slide

– To prevent damaging the lens



d. Most compound microscopes are parafocal• Parafocal: once focused on low power the

nosepiece can be rotated to high power and focused with fine adjustment



6. Oil Immersion

a. Provides highest magnification

b. Used to view very small structures or the detail of structures• Microorganisms

• Blood cells



c. Oil immersion has a very short working distance• Lens nearly rests on

slide



d. Must use immersion oil• Does not dry out

when exposed to air for a long time

e. Drop of oil placed on slide



• Oil resides between objective and slide

• Provides path for light to travel between slide and lens

• Prevents scattering of light rays

– Permits clear viewing of small structures



f. Oil immersion used for:• Differential white blood

counts

• Examination of gram-stained smears


Care of the MicroscopeCare of the Microscope

1. Carry with two hands: one on arm and one under base

a. Place down gently: prevents jarring it• Could damage delicate parts


Care of the Microscope, cont. Care of the Microscope, cont.

2. Do not touch the lenses: prevents fingerprints

a. Avoid wearing mascara: difficult to remove from lens

3. When not in use: keep covered with plastic dust cover

a. Store in case or cupboard

b. Rotate nosepiece to low objective • Position as close as possible to stage



4. Clean by washing enameled surfaces with soap and water

a. Dry thoroughly with a soft cloth

5. Clean stage after each use:

a. Wipe stage with gauze or tissue

b. If immersion oil gets on stage:• Remove with gauze moistened with xylene



6. Ocular, objectives and condenser

a. Consist of hand-ground optical lenses• Must keep spotlessly clean

– Use dry lens paper



• Optical glass: softer than ordinary glass

– Do not use tissue or gauze to clean: scratches the lens

• If lenses are especially dirty

– Clean with commercial lens cleaner or xylene

– Dry and polish with lens paper


Clean with lens paper:Clean with lens paper:



7. Light source: keep free of dust, lint, and dirt

a. Periodically polish with lens paper



8. Microscope that is malfunctioning

a. Have repaired by qualified service person• Do not try to fix yourself

– May result in further damage


Microbiologic Specimen Collection

Microbiologic Specimen Collection

1. Purpose:

a. To identify pathogen causing the disease• Assists in the diagnosis

– Example: Obtaining a urine specimen to assist in diagnosis of a urinary tract infection

2. Specimen: a small sample or part taken from the body to show the nature of the whole


Microbiologic Specimen Collection, cont.


3. MA often responsible for collecting microbiologic specimens

a. Examples: throat, nose, and wound specimens

4. MA may assists physician in collecting specimens

a. Examples: vagina, cervix, urethra, rectum




5. Sterile swab used to collect specimen

a. Swab: small piece of cotton wrapped around the end of a slender wooden or plastic stick• Passed across body surface or opening

– To obtain a specimen for microbiologic analysis


Sterile SwabSterile Swab




6. To prevent inaccurate test results:

a. Use good techniques of medical and surgical asepsis

b. Do not contaminate with extraneous microorganisms

• Extraneous microorganisms: undesirable microorganisms that can enter the specimen in various ways.

– They grow and multiply

– Can obscure and prevent identification of pathogens that might be present




c. To prevent contamination of a specimen with extraneous microorganisms:• Supplies used to collect specimen must be

sterile

– Examples: Swabs and specimen containers




• Do not allow swab to touch areas surrounding collection site

– Example: Throat specimen: Do not allow swab to touch inside of mouth




7. Follow OSHA Bloodborne Pathogens Standard when performing microbiologic procedures:

a. Wear gloves

b. No eating, drinking, smoking, or applying makeup• Pathogens can be transmitted through hand-

to-mouth contact

c. Do not lick labels for specimen containers

d. Cover breaks in skin with a bandage




e. If accidentally touch specimen: • Wash area immediately with soap and water

f. If specimen comes in contact with worktable: • Immediately clean with soap and water

followed by disinfectant (e.g., phenol)

g. At end of day:• Clean worktable with disinfectant




8. After collection: place specimen in proper container

a. Securely fasten lid

b. Label with:• Patient's name

• Date

• Source of specimen

• Initials


Handling and Transporting Microbiologic Specimens

Handling and Transporting Microbiologic Specimens

1. Delay in processing specimen:

a. May cause death of pathogens

b. Overgrowth of specimen by normal flora (usually collected along with pathogen)

2. If specimen analyzed in office:

a. Examine under microscope or culture immediately


Handling and Transporting Microbiologic Specimens, cont.


3. If specimen transported to outside laboratory by courier

a. Place in transport medium

(e.g., Starswab II, Culturette)• Prevents drying of specimen

• Preserves specimen in its original state

b. Outside laboratory: provides instructions on care and handling of specimen




c. Specimen must be accompanied by laboratory request which includes:• Physician's name and address

• Patient's name, age, and gender

• Date and time of collection

• Type of microbiologic examination requested




• Source of specimen

– Examples: throat, wound, urine

• Physician's clinical diagnosis

– Clinical diagnosis: a tentative diagnosis of a patient's condition obtained through the evaluation of the health history and the physical examination without the benefit of laboratory or diagnostic tests




• If patient is receiving antibiotic treatment

– Antibiotics suppress growth of bacteria

– Could cause false-negative results




4. Wound Specimens

a. Must be collected from within wound (rather than from surface of wound)• To obtain accurate and reliable test results

b. Usually two swabs used




c. Swab inserted into area of wound that contains the most drainage

d. Swab gently rotated from side to side• To completely absorb microorganisms present

e. Swab placed in specimen container

f. Process repeated with second swab


Collection and Transport System

Collection and Transport System

1. Types available

a. Culturette

b. Starswab II


Collection and Transport System, cont.


2. Consist of sterile swab and plastic tube that contains a transport medium

a. Comes packaged in peel-apart envelope

b. Should be stored at room temperature




3. Procedure for use

a. Complete laboratory request form

b. Sanitize hands

c. Apply gloves




d. Check expiration date

e. Peel open envelope

f. Remove cap from the collection tube

g. Remove cap/swab unit from envelope

h. Collect specimen using aseptic technique• Do not allow swab to touch any other area




i. Insert swab into collection tube

j. Push cap/swab as far as it will go to completely immerse in transport medium

k. Remove gloves and sanitize hands




l. Label tube

m. Place in biohazard specimen transport bag • Place laboratory request in outside pocket

n. Chart procedure

o. Transport to laboratory to laboratory within 24 hours


CulturesCultures

1. Once a microbiologic specimen is collected

a. Examined to determine type of microorganisms present

2. Most specimens: only contain a small number of pathogens

a. Must induce pathogens to grow and multiply


Cultures, cont. Cultures, cont.

3. Culture medium: a mixture of nutrients on which microorganisms are grown in the laboratory

a. Must contain growth requirements:• Presence or absence of oxygen

• Proper nutrition

• Temperature

• pH

• Moisture



b. Solid culture medium • Blood agar most commonly used

– Sheep's blood added to agar

1) Contains nutrients to grow bacteria




c. Liquid culture medium• Referred to as broth

• Usually contained in a tube

• Example: nutrient broth



4. Culture media: must be stored in refrigerator

a. Warm to room temperature before use

b. Do not use cold culture medium• Results in death of microorganisms



5. Petri plate: frequently used to hold solid culture medium

a. Consists of shallow circular dish with a cover (made of glass or clear plastic)

b. Microorganisms cultured on surface of medium in plate

c. Allows examination of culture• Prevents microorganisms from entering or

escaping



d. Culture: a mass of microorganisms growing in a laboratory culture medium




6. Most offices use commercially prepared culture media in disposable plastic Petri plates

a. Plates are packaged in plastic bag• Prevents medium from drying out

b. Stored in refrigerator with medium side facing up• Prevents condensation on surface of medium


Cultures, cont.Cultures, cont.

c. Expiration date: check before using• Do not use plates that are:

– Expired

– Dried out

– Contaminated



7. Inoculation of solid culture medium in Petri plate

a. Specimen swab is lightly rolled across surface of medium (streaking)• Only remove Petri plate cover during streaking

of specimen

– Prevents contamination with extraneous microorganisms

b. Culture is incubated for 24 to 48 hours



8. Most specimens for analysis contain a mixture of microorganisms

a. Mixed culture: contains two or more types of microorganisms• Suspected pathogen must be separated to

analyze

b. Pure culture: contains only one type of microorganism• Tests to identify pathogen are performed on

pure culture


Cultures, cont.Cultures, cont.

9. Viruses cannot be grown by the above method

a. Are identified by:• Culturing on living tissue

• Serologic tests


Streptococcus TestingStreptococcus Testing

1. Streptococcal sore throat (streptococcal pharyngitis)

a. Primarily affects children and young adults

b. Causative agent: group A beta-hemolytic streptococcus


Streptococcus Testing, cont. Streptococcus Testing, cont.

2. Potentially serious condition

a. Small percentage of patients develop a poststreptococcal sequela

• Sequela: a morbid secondary condition that occurs as a result of a less serious primary infection

– Example: rheumatic fever

b. Early diagnosis and treatment of “strep throat” with antibiotics is important


Rapid Streptococcus TestsRapid Streptococcus Tests

1. Detects group A streptrococci from a throat swab


Rapid Streptococcus Tests, cont.


a. Requires only 4 to 10 minutes to process• Diagnosis can be made while patient is at

office

– If streptococci present: antibiotics prescribed




2. Direct antigen identification test: most frequently used

a. Particles sensitized to streptococcal antibody are combined with specimen (antigen)

b. Produces a color change if streptococcal is present (through antigen-antibody reaction)

c. Test includes a control• To ensure results are accurate and reliable




3. Includes a control to verify the results are accurate and reliable

4. Advantage

a. Provides physician with immediate results




5. Examples of tests

a. Q Test Strep

b. Clearview Strep A

c. Quick Vue In-Line Strep A


Hemolytic Reaction and Bacitracin SusceptibilityHemolytic Reaction and Bacitracin Susceptibility

1. Streptococci classified into three types based on their hemolytic properties:

a. Alpha

b. Beta: produces streptolysin (exotoxin) that hemolyzes red blood cells (RBCs)

c. Gamma


Hemolytic Reaction and Bacitracin Susceptibility, cont.


2. Further divided according to their antigenic properties

a. Into 15 subgroups designated by letters A through O• Group A streptococci: susceptible to the

antibiotic bacitracin

– Are destroyed by it




3. Test uses these hemolytic and antigenic properties to interpret test results

a. Causative agent of strep throat:

• Group A beta-hemolytic streptococcus




4. Testing procedure:

a. Throat specimen inoculated on blood agar medium

b. Bacitracin disc placed on surface of medium

c. Medium incubated for 18 to 24 hours




c. Plate examined for hemolytic and antigenic reactions

d. Positive reaction for Group A beta-hemolytic streptococcus

• Hemolytic reaction: clear wide colorless zone around bacterial colonies

– Streptolysin from beta streptococci has hemolyzed RBCs

• Antigenic reaction: clear zone of inhibition around the bacitracin disk

– Group A streptococci are susceptible to and destroyed by bacitracin


Bacitracin Susceptibility TestBacitracin Susceptibility Test



Sensitivity TestingSensitivity Testing

1. Determines susceptibility of pathogenic bacteria to antibiotics

a. Allows physician to determine best antibiotic to use to treat patient's condition

2. Must be performed on a pure culture


Sensitivity Testing, cont. Sensitivity Testing, cont.

3. Disk-diffusion method

a. Most common method

b. Use disks impregnated with various antibiotics• Dropped on surface of solid culture medium in

Petri plate inoculated with pathogen



c. Culture is incubated• Allows antibiotics to diffuse into culture

medium

d. If pathogen is susceptible to an antibiotic:

e. Clear zone without bacterial growth around disk• Indicates antibiotic is effective against

pathogen



e. If pathogen is resistant to an antibiotic• No clear zone around disk

– Indicates antibiotic was unable to kill pathogen


Sensitivity TestingSensitivity Testing



Microscopic Examination of Microorganisms

Microscopic Examination of Microorganisms

1. Can be examined in fixed or in living state

2. Examining microorganisms in a fixed state

a. Most microorganisms are examined this way

b. Preparation of a smear through heat fixation: followed by staining• Example: Gram staining

• Easier to examine when stained


Microscopic Examination of Microorganisms, cont.


3. Examining microorganisms in a living state

a. Allows visualization of movement

b. Helpful in identification of certain motile microorganisms• Example: Trichomonas vaginalis




c. Microorganisms suspended in liquid medium• Microorganisms are free to move about

d. Wet mount: most common method used


Wet Mount MethodWet Mount Method

1. MA prepare slide

2. Drop of fluid containing organism: placed on glass slide

a. Covered with coverslip

b. Coverslip ringed with petroleum jelly• Provides seal

– Reduces rate of evaporation

– Evaporation could lead to drying out of slide and death of specimen


Wet Mount Method, cont. Wet Mount Method, cont.

1. Slide placed under microscope

2. Examined under high power by physician

3. Slide discarded in biohazard sharps container


Wet Mount MethodWet Mount Method


SmearsSmears

1. Smear: material spread on a slide for microscopic examination


Smears, cont. Smears, cont.

2. Prepared:

a. Directly from specimen swab

b. Specimen first grown on culture medium

3. Most smears must be stained before viewing under microscope


Smears, cont. Smears, cont.

4. Smears can be prepared and examined immediately

a. Helpful when time is a factor

5. Provides a preliminary clue

a. While more time-consuming tests are being performed


Gram StainGram Stain

1. Used in combination with other tests: to diagnose and treat infectious diseases

2. Bacteria in a smear are colorless

a. Makes them difficult to identify

b. Staining allows for direct viewing of bacteria• Size

• Shape

• Growth patterns


Gram Stain, cont. Gram Stain, cont.

3. Discovered by Christian Gram in 1883

a. Differentiates bacteria on basis of their color reactions to various stains



4. Gram-positive bacteria: exhibit a purple color

a. When treated with purple crystal violet dye: gram-positive bacteria permanently retain this dye




5. Gram-negative bacteria: exhibit pink or red color

a. Gram-negative bacteria are unable to retain purple dye: are counterstained with a red dye

6. Staining characteristics of bacteria:

a. Caused by differences in the chemical composition of bacterial cell walls





7. Infectious diseases caused by gram-positive bacteria:

a. Streptococcal sore throat

b. Scarlet fever

c. Rheumatic fever

d. Diphtheria


Gram Stain, cont.Gram Stain, cont.

e. Lobar pneumonia

f. Tetanus

g. Botulism




8. Infectious diseases caused by gram-negative bacteria:

a. Whooping cough

b. Gonorrhea

c. Meningitis





d. Bacillary dysentery

e. Cholera

f. Typhoid fever

g. Plague





9. Categories of gram stained bacteria:

a. Gram-positive rods

b. Gram-negative rods

c. Gram-positive cocci

d. Gram-negative cocci

• Example: gonorrhea caused by a gram-negative diplococcus


Prevention and Control of Infectious Diseases

Prevention and Control of Infectious Diseases

1. Practice good techniques of medical asepsis.

2. Obtain proper nutrition and rest.

3. Use good hygienic measures.


Prevention and Control of Infectious Diseases, cont. Prevention and Control of Infectious Diseases, cont.

4. Infected individuals should contact their physicians to ensure early diagnosis and treatment.

5. MA responsible for educating community members

a. Practices that reduce transmission of pathogens

b. Practices that control and prevent infectious diseases


POSTTESTPOSTTEST

True or False1. Microorganisms that reside in the body but do not

cause disease are known as transient flora.

2. The invasion of the body by a pathogenic microorganism is known as infection.

3. The interval of time between the invasion by a pathogen and the first symptoms of disease is known as the prodromal period.

4. Staphylococcal infections usually result in pus formation.

5. E. coli normally reside in the urinary tract.


POSTTEST, CONT.POSTTEST, CONT.

True or False6. The high-power objective has a magnification of

40x.

7. Examination of urine sediment requires the use of the oil immersion objective.

8. A mixed culture contains two or more types of microorganisms.

9. The purpose of sensitivity testing is to identify the type of microorganism present.

10. When viewed under a microscope gram-positive bacteria appear pink or red.

chapter 20 medical microbiology. elsevier items and derived items © 2008 by saunders, an imprint of...

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