Streptococci in the Normal Flora



Diagnostic Microbiology In Action
  Microbiology of the Respiratory Tract
      Isolation and Identification of Staphylococci
      Staphylococci in the Normal Flora
    Streptococci, Pneumococci, and Enterococci
      Isolation and Identification of Streptococci
      The CAMP Test for Group B Streptococci
      Identification of Pneumococci
      Identification of Enterococci
      Streptococci in the Normal Flora
    Haemophilus, Corynebacteri and Bordetella
    Clinical Specimens from the Respiratory Tract
      Laboratory Diagnosis of a Sore Throat
      Laboratory Diagnosis of Bacterial Pneumonia
      Antimicrobial Susceptibility Test of an Isolate from a Clinical Specimen

Purpose To study the normal flora of the throat
Materials Sheep blood agar plate
Sterile swab
Sterile tongue depressor
  • Simulated swab from suspected “strep” throat patient
  • Kit for detection and confirmation of group A streptococcal antigen from a throat swab

  1. Figure 21.4 diagrams the correct method for collecting a throat culture. Note that the tongue is held down out of the way and the throat swab is lightly touching the posterior wall of the pharynx.
  2. The instructor will demonstrate the method. Observe carefully.
  3. Now take a throat culture from your laboratory partner. The “patient” should be positioned in good light so that you can see the back of the throat and the position of the swab as you insert it. Gently and quickly swab the posterior membranes of the throat, being careful not to touch the swab to any other tissues as you insert or remove it.
  4. Inoculate a blood agar plate by rolling the swab over a small area near one edge. Streak the plate with the inoculating loop in a manner to obtain isolated colonies (review fig. 9.1).
  5. Discard the swab and tongue depressor in a container of disinfectant.
  6. With your sterilized loop, make a few stabs in the agar at the area where the swab was rolled. Do not stab through to the bottom of the agar layer. Incubate the plate at 35°C for 24 hours.
  7. If the group A streptococcal antigen test kit is available, take a second swab from your “patient” (step 3). Test both your “patient” swab and the simulated swab from the “strep” throat patient following the manufacturer’s instructions carefully.
Taking a throat culture. The swab should touch only the pharyngeal membranes
Figure 21.4 Taking a throat culture. The swab should touch only the pharyngeal membranes.

  1. If the streptococcal antigen detection test was performed, record the results.
    Your “patient” (+ or −)_______
    “Strep” throat patient (+ or −)_______
    Complete the following if the test you used was an EIA test.
    Color of the positive test:_______
    Color of the negative test:_______
  2. Examine the incubated culture plate carefully. How many colonies of different types can you distinguish? Describe each colony type in the following table.
  3. Hold the plate against a good light. Do you see any hemolytic colonies? Indicate type of hemolysis shown by each colony recorded in the table.
  4. Make a Gram stain of one colony of each type and record results in the table.
  5. Enter your tentative identification of each colony in the table and the additional tests needed to complete the identification.
  6. Did the culture of your “patient’s” throat confirm the results of the swab antigen detection test?