20: SKIN AND UPPER RESPIRATORY BACTERIA
- Page ID
- 157089
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Differentiate between alpha, beta, and gamma hemolysis on blood agar plates.
- Use hemolytic patterns to identify potential pathogenic organisms, such as Streptococcus pyogenes.
BACKGROUND
The skin and upper respiratory tract are ideal environments for microbial growth. Under normal conditions, the microorganisms that naturally reside in these areas—known as the normal flora—do not cause harm. In fact, many of these microbes are beneficial and contribute to the overall health of the tissues they inhabit.
The throat is a particularly accessible part of the upper respiratory tract for sampling and analysis. It also serves as a common entry point for infectious agents. The normal flora of the throat includes species from several genera, such as Streptococcus, Staphylococcus, Branhamella, Corynebacterium, Haemophilus, Actinomyces, and Bacteroides. Many of these organisms can be preliminarily identified using selective media and a Gram stain.
Interestingly, pathogenic organisms can belong to the same genera as the normal flora. Examples include Streptococcus pyogenes, a major cause of strep throat, and Corynebacterium diphtheriae, which causes diphtheria. Other pathogens in the throat may include yeast and viruses.
Of particular importance is Streptococcus pyogenes, a frequent upper respiratory tract pathogen. This organism causes strep throat and produces enzymes capable of lysing red blood cells and degrading hemoglobin, contributing to tissue damage during infection. A classic diagnostic method involves growing throat swabs on blood agar, a medium that contains intact red blood cells. Colonies of S. pyogenes often produce a clear zone around them, where the red blood cells have been completely lysed. This phenomenon is called beta hemolysis and serves as a strong indicator of a pathogenic streptococcal infection.
In contrast, many nonpathogenic streptococci produce enzymes that only partially lyse red blood cells. This results in greenish discoloration and partial clearing around colonies, known as alpha hemolysis. Some organisms do not lyse red blood cells at all; these are referred to as gamma hemolytic or non-hemolytic. Blood agar is a useful tool for identifying the hemolytic characteristics of streptococci, and a follow-up Gram stain can help differentiate other organisms that may appear on the plate.
The skin is another body site rich in normal flora. One of their primary benefits is their ability to outcompete potential pathogens for space and resources. Many skin bacteria also produce enzymes or metabolic byproducts that inhibit the growth of invading microbes, including harmful bacteria, fungi, and viruses.
However, when the protective barrier of the skin is compromised—through irritation, cuts, or invasive medical procedures, the same organisms can become opportunistic pathogens. This risk is a major concern in surgical settings, where aseptic technique is essential to prevent the introduction of skin flora into sterile body sites. Even common skin organisms can become dangerous if they gain access to the bloodstream or internal tissues.
Preliminary identification of skin flora can be achieved using a Gram stain in combination with colony characteristics, such as color and shape. Understanding the typical microorganisms found in the throat and on the skin is essential for distinguishing between normal and pathogenic microbes during clinical evaluations.
TYPICAL THROAT FLORA AND PATHOGENS
Organism |
Morphology |
Staphylococcus |
Gram-positive cocci in clusters |
Streptococcus |
Gram-positive cocci in chains |
Branhamella |
Gram-positive cocci in pairs |
Corynebacterium |
Gram-positive pleomorphic rods |
Haemophilus |
Gram-positive pleomorphic rods |
Actinomyces |
Gram-positive rods in filaments |
Bacteroides |
Gram-negative pleomorphic rods |
TYPICAL SKIN FLORA
|
Organism |
Morphology and Colony Appearance |
Staphylococcus epidermidis |
Gram-positive cocci in clusters, small white colonies |
Staphylococcus aureus |
Gram-positive cocci in clusters, yellowish colonies |
Propionibacterium acnes |
Gram-positive pleomorphic rods |
Corynebacterium spp. |
Gram-positive pleomorphic rods, waxy colonies |
Pityrosporum spp. |
Small budding yeasts |
Streptococcus pyogenes |
Gram-positive cocci in chains |
MATERIALS (Per Student)
1 Nutrient agar plate
1 DI water tube
1 Sterile swab
METHODS/PROCEDURES
Choose a sampling area
Select an area of your skin approximately 2 inches by 2 inches. If the area you wish to test is not easily accessible
(such as your back or upper thigh), you may take sterile swabs to the restroom for privacy.
2. Moisten the swab
Dip a sterile swab into the tube of sterile water to dampen it. Gently roll the swab against the inner wall
of the tube to remove excess moisture. The swab should be damp but not dripping.
3. Swab the skin
Thoroughly swab the entire 2 x 2 inch skin area using firm but gentle pressure. Roll the swab between
your fingers as you go to ensure all surfaces of the cotton tip make contact with the skin.
4. Inoculate the agar plate
Immediately after swabbing your skin, gently rub the swab across the entire surface of the nutrient
agar plate. Roll the swab while streaking to transfer microorganisms from all sides of the swab to the
agar surface.
5 Label and seal the plate
Write your last name, the date, and the area sampled on the bottom of the Petri dish (the smaller,
deeper half). Use a small piece of tape to securely seal the lid to the base of the dish.
6. Incubate the plate
Place the labeled and sealed dish in the incubator as directed by your instructor.
UPPER RESPIRATORY FLORA
Important Safety Note: Only the instructor may perform throat swab collections.
Do not attempt to swab your own throat or another student’s.
If you are interested in participating in a throat culture test, sign up on the board. This activity is optional. and not required for credit.
For those who choose to participate, a sterile swab will be used by the instructor to collect microorganisms from the back of your throat. The swab will then be streaked onto a blood agar plate. This medium is useful for detecting hemolysis—the breakdown of red blood cells—which can help differentiate between normal flora and potential pathogens.
Print a hard copy to bring to lab (PDF).
👉 If you are filling this out on a digital iPad or tablet please note put your name here and take a screen shot.
You are also welcome to print the PDF and turn in a physical copy of the following.
Exercise #26 Skin and Respiratory Flora
NAME ______________________
EXPECTATIONS
How many total colonies do you think will grow on your skin culture plate?______________________
How many different types of colonies (based on appearance) do you expect to see? ____________
Which do you expect to show a greater variety of microorganisms—the skin cultures or the throat cultures? Briefly explain your reasoning.
RESULTS
Draw your observations of the skin test plate after incubation.
Draw your Gram stain and controls below. Use accurate colors that reflect the colors seen on the slide below.
Alpha and Beta Hemolysis blood agar plates are on display at the instructor bench. Observe the growth on the plates then draw what you see below. Identify any hemolysis you see.
CONCLUSIONS
1. How accurate was your prediction about the number and types of colonies that grew on your skin culture plate? Explain how your results are compared to your initial guess.
2. How does alpha hemolysis differ from beta hemolysis in appearance and cause?
3. What does the term opportunistic pathogen mean in the context of normal flora?
4. Why is the ability to identify alpha, beta, and gamma hemolysis important in clinical microbiology?