Laboratory Diagnosis of Bacterial Infections (I)

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Questions and Answers

Which type of specimen should be collected for diagnosing a urinary tract infection (UTI)?

CSF

Throat swab

Mid-stream urine (correct)

Sputum

What is the purpose of using transport media when specimens are delayed in reaching the laboratory?

To prevent specimen contamination

To preserve specimen volume

To maintain specimen viability (correct)

To enhance bacterial growth

Which microscope is specifically used for the detection of Treponema pallidum?

Ordinary light microscope

Dark field microscope (correct)

Fluorescence microscope

Electron microscope

What is the role of iodine in the Gram stain procedure?

It acts as the mordant Signup and view all the answers

Which type of bacteria appears purple after Gram staining?

Gram positive Signup and view all the answers

What does an unstained (wet mount) preparation primarily demonstrate?

Bacterial motility Signup and view all the answers

Which technique is NOT commonly used for direct detection of bacterial infections?

Serology testing Signup and view all the answers

What is the primary purpose of the Gram stain in microbiology?

To classify bacteria into Gram positive and Gram negative Signup and view all the answers

What is the first step in the Gram staining process?

Fixing the smear Signup and view all the answers

Which of the following is NOT a type of microscope mentioned for bacterial detection?

Phase contrast microscope Signup and view all the answers

What is the primary stain used in the Ziehl-Neelsen staining procedure?

Carbol fuchsin Signup and view all the answers

What color do Gram-positive bacteria appear after Gram staining?

Purple Signup and view all the answers

Which component of the cell wall primarily differentiates Gram-positive from Gram-negative bacteria?

Peptidoglycan layer Signup and view all the answers

What type of media is designed to maintain the viability of organisms during transport?

Transport media Signup and view all the answers

What occurs during the decolorization step of the Ziehl-Neelsen stain?

Use of sulfuric acid with alcohol Signup and view all the answers

Which bacteria are identified using the Ziehl-Neelsen stain?

Mycobacterium spp. Signup and view all the answers

What does blood agar differentiate based on the type of hemolysis produced?

Type of hemolysis Signup and view all the answers

How do acid-fast bacteria appear after the Ziehl-Neelsen stain?

Red Signup and view all the answers

Which of the following is not a characteristic of enriched media?

Contains basic nutritive requirements Signup and view all the answers

What is the purpose of fixing a smear prior to staining?

To prevent the specimen from washing away Signup and view all the answers

Study Notes

Laboratory Diagnosis of Bacterial Infections (I)

This lecture covers the laboratory diagnosis of bacterial infections, focusing on the initial steps.
The process involves three main stages:
- Specimen collection and transport
- Bacterial identification
- Result interpretation and release
Specimen collection and transport:
- Samples should be collected before administering antimicrobial therapy.
- Samples must accurately represent the infectious process. Examples include:
  - Cerebrospinal fluid (CSF) for meningitis
  - Throat/nasopharyngeal swab, sputum for respiratory tract infections (RTIs)
  - Vomit, stool for intestinal infections
  - Mid-stream urine for urinary tract infections (UTIs)
- Samples must be of sufficient volume and collected aseptically in appropriate sterile containers.
- A complete request form is mandatory (including date, sample type, and department).
- Rapid transport to the lab is crucial, using transport media if delays are anticipated.
Bacterial identification:
- Direct detection methods include:
  - Microscopic examination (using various types of microscopes)
  - Culturing bacteria and antibiotics sensitivity testing
  - Antigen detection
  - Molecular tests like PCR (detecting genetic material)
- Indirect detection methods include:
  - Serology, which measures specific antibodies.
Microscopic examination:
- Different types of microscopes are employed, including:
  - Ordinary light microscopes (standard light source).
  - Dark field microscopes are helpful in detecting Treponema pallidum.
  - Fluorescent microscopes use UV light to identify intracellular organisms and species such as Mycobacteria and Chlamydia.
  - Electron microscopes are used mainly for studying viruses.
Unstained wet mount preparation:
- Useful in determining bacterial motility. Examples of bacteria that can be studied include Vibrio cholerae, Salmonella, E. coli, and Proteus.
Stained microscopic techniques:
- These identify organisms based on their staining reactions, shape, and arrangement
  - Gram stain method classifies bacteria into Gram-positive (purple) and Gram-negative (red) types, based on cell wall structure.
    - Gram-positive bacteria have a thick peptidoglycan layer.
    - Gram-negative bacteria have a thin peptidoglycan layer.
    - Gram staining procedure involves multiple steps (primary stain, mordant, decolorization, counterstain, and microscopic examination)
  - Ziehl-Neelsen stain (acid-fast stain) is specialized to identify Mycobacterium spp. which are high in lipid and wax in their cell walls
    - Mycobacterium tuberculosis appear red when using this stain.
Bacterial culture: Clinical samples are cultured on various media, at suitable temperatures, and atmospheric conditions.
- Media types include:
  - Liquid (broth)
  - Solid or semi-solid (agar plates)
- Media can be classified by ingredient:
  - Transport media which supports viability for the duration of the transport to the laboratory . Examples include Stuart, Amies, and Carry & Blair media
  - Culture media which help in bacterial growth and classification. These include ordinary/simple media and enriched media
Enriched media:
- Enriched media support the growth of fastidious bacteria. For example,
  - Löffler's serum agar
  - Blood agar
  - Chocolate agar
Blood agar can be used as:
- Indicator, or differential media, based on hemolysis produced by bacteria:
  - β (complete hemolysis)
  - α (partial hemolysis)
  - γ (no hemolysis)

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Description

This quiz explores the laboratory diagnosis of bacterial infections, emphasizing initial steps. Key topics include specimen collection, bacterial identification, and interpretation of results to ensure accurate diagnosis. Test your understanding of these critical laboratory procedures.