Unit 4: Bacterial Culture, Sensitivity and Specimen Collection

1. Reasons for Performing a Bacterial Culture

  • To determine the type(s) of bacteria that are present.
  • To determine which bacteria may be pathogenic.
  • To determine the appropriate antimicrobial treatment.
  • To determine if the patient is a carrier.

2. Purpose of a Sensitivity Test

  • Sensitivity testing allows the determination of which antibiotics the bacteria are sensitive and resistant to.
  • This process is regarded as the Gold Standard for determining antibiotic treatment choice to ensure the resolution of infections.

3. Minimum Inhibitory Concentration (MIC)

  • The Minimum Inhibitory Concentration (MIC) refers to the lowest concentration of an antibiotic that would inhibit bacterial growth.

4. Key Definitions

  • Bactericidal Antibiotic: Antibiotics that kill bacteria.
  • Bacteriostatic Antibiotic: Antibiotics that inhibit the growth or reproduction of bacteria.
  • Endogenous Bacteria: The patient’s own microorganisms that are already living inside the body.
  • Exogenous Source: Bacteria that comes from outside of the body.

5. Acceptability of Collecting Samples

  • True or False: It is acceptable to take a sterile swab of necrotic tissue since the area will still contain pathogenic bacteria.
    • The statement is False.

6. Normal Flora in the External Ear of Small Animals

  • Normal flora microbes of the external ear in small animals (in low numbers) include:
    1. Staphylococcus spp.
    2. Streptococcus spp.
    3. Malassezia (a yeast).

7. Prevalence of Gram-Negative Organisms in Canine Ears

  • True or False: Gram-negative organisms aren’t usually found in canine’s ears.
    • The statement is True.

8. Common Microorganism in Canine Ear Infections

  • A dog that has been swimming and presents with an ear infection (otitis externa) may often have the gram-negative rod Pseudomonas aeruginosa as the culprit.

9. Factors Causing Proliferation of Normal Bacterial Flora in Dog Ears

  • Four factors that could cause abnormal proliferation of the normal bacterial flora in a dog's ear include:
    1. Poor ventilation of the ear canal leading to increased moisture content.
    2. External ear conformation (e.g., folded or floppy ears).
    3. Presence of foreign material.
    4. Hair.

10. Role of Gram Staining

  • True or False: Using a gram stain helps determine whether the bacteria is gram-positive or gram-negative.
    • The statement is True.

11. Gold Standard for Urine Collection for Bacterial Culture

  • The gold standard for urine collection for a bacterial culture is cystocentesis.

12. Incubation Time for Growth Plates

  • After a plate has been streaked for confluent growth, it should be incubated for 24 hours.

13. Full Name for “Milk Culture” Test

  • The full name for a “Milk culture” test that is often performed when confirming mastitis is the California Mastitis Test (C.M.T.).

14. C.M.T. Test Results

  • In the C.M.T., a positive test result indicates the formation of a thick gel when the milk is mixed with a reagent, while a negative result shows that the milk remains liquid.

15. Acceptability of Culturing Bacteria from Blood Samples

  • True or False: It is okay if a small amount of bacteria is cultured from a blood culture since it may have been introduced while obtaining the sample.
    • The statement is False.
    • Explanation: Blood is sterile, and any bacteria found in a blood culture would be viewed as pathogens.

16. Normal Flora in the Respiratory Tract

  • The respiratory tract has a high population of normal flora, whereas the lower respiratory tract should be sterile.

17. Bronchoalveolar Lavage (BAL) Procedure

  • B.A.L. stands for Bronchoalveolar Lavage.
  • Procedure: In a situation where a bacterial (or fungal) infection is suspected in the lower respiratory tract, a BAL can be performed to obtain a sterile sample. A sterile catheter or bronchoscope is passed into the lower airways of the anesthetized patient. A small amount of sterile saline is instilled, and the fluid is immediately suctioned back out.

18. Staining of Acid-Fast Bacteria

  • Acid-fast bacteria do not stain well with Gram stain, indicating their unique polymeric structure that includes mycolic acids in their cell walls.

19. Microscopical Distinctions between Cocci and Rods

  • Microscopically, cocci are round or spherical in shape, whereas rods are elongated, cylindrical, or rod-shaped.

20. Gram Positive Cocci Normal Flora

  • Two Gram-positive cocci that are considered normal flora:
    1. Staphylococcus aureus: Normal inhabitant of the skin in cats and large animals.
    2. Staphylococcus pseudintermedius: Normal flora of the skin of dogs.

21. Laboratory Procedure and Results

  • Referral Diagnostic Laboratory: Features of laboratory procedures include cost, time for results, overgrowth of bacteria, accuracy of results, false negative results, identification of bacteria types, and antibiotic sensitivity.
    • Typical costs and result times:
    • Diagnostic testing typically less expensive, with quicker results due to immediate plating.
    • Higher costs and longer result times often due to shipping delays for referral labs.

22. Bacteria Associated with Cat Bites or Abscesses

  • The gram-negative bacteria often associated with cat bites or abscesses is Pasteurella multocida.

23. Features to Describe Isolated Colonies

  • Six different features used to describe an isolated colony grown on a culture medium:
    1. Size
    2. Color
    3. Shape
    4. Texture
    5. Optical Properties
    6. Elevation

24. Differentiating Staphylococcus spp. and Streptococcus spp.

  • The catalase test differentiates between Staphylococcus spp. and Streptococcus spp., as follows:
    • A sample of the colony is placed onto a glass slide with a sterile loop.
    • A drop of 3% hydrogen peroxide is added; if bubbles appear, it indicates Staphylococcus spp. If no bubbles appear, it indicates Streptococcus spp..