MLT 101: Microbiology Lecture Notes

Microbiology Study Notes

Introduction to Microorganisms

  • Normal Flora (Normal Biota)

    • These are microorganisms that normally inhabit the body but do not cause disease in healthy individuals.

  • Pathogenic Microorganisms

    • Opportunistic Pathogens: Normally non-pathogenic but can cause disease under certain conditions.

    • Hospital-associated Infections (Nosocomial): Symptoms can appear 48 hours after admission to a healthcare facility.

    • Community-associated Infections: Infections acquired outside of a healthcare setting.

  • Classification and Nomenclature

    • Organisms are classified using the naming system of Genus and Species.

    • Examples:

      • Staphylococcus aureus

      • Streptococcus pyogenes

      • Pseudomonas aeruginosa

Biological Safety

  • Classification of Biological Agents

    • Biological agents are classified based on their hazard to personnel.

    • Biosafety Level 1: Agents that do not usually cause disease (e.g., non-pathogenic strains).

    • Biosafety Level 2: Agents that pose moderate hazards and used in routine microbiology labs.

    • Biosafety Level 3: Agents that are often uncommon and must be handled in a biological safety cabinet.

    • Biosafety Level 4: Highly pathogenic organisms that require the highest level of containment.

  • General Safety Practices in Laboratories

    • Implement proper waste disposal and disinfection processes.

    • Techniques covered include:

    • Disinfection and Sterilization Techniques:

      • Use of chemical disinfectants like bleach (1:10 dilution).

      • Heat or Burning: Effective sterilization methods.

      • Sterilization by Dry Heat: 171°C for 1 hour.

      • Sterilization by Moist Heat: Autoclave at 121°C steam for 15 minutes.

Specimens for Microbiological Examination

  • Specimen Collection Requirements for Culture

    • Containers must be sterile.

    • Transport specimens using transport media if there is a delay in transport.

    • Storage of Specimens: Store at 4-6°C if delayed culture for most specimens.

  • Types of Microbiology Specimens Collected

    • Blood

    • Cerebrospinal fluid

    • Stool

    • Sputum

    • Swabs of various fluids

    • Urine

    • Tissue

    • Body fluids

Basic Equipment and Techniques Used in Microbiology

  • Inoculating Needle or Loop

    • Available in both disposable and reusable forms.

  • Incinerators

    • Used for sterilization of reusable needles and loops.

  • Media

    • Solid Media: Agar with added nutrients.

    • Liquid Media: Broth with added nutrients.

  • Culturing Techniques

    • Dilution Streak Technique: A method to isolate specific organisms.

  • Incubators

    • Set routine bacteria incubations at 35°C (+/- 2°C).

    • Fungal incubations at 30°C.

    • Campylobacter incubations at 42°C.

Media Inoculation/Streaking

Streak for Isolation

  • Urine Isolation Technique

    • Touch loop to the center of the plate and spread the inoculum in a line across the diameter of the plate.

    • Without flaming or re-entering the urine, draw the loop across the plate, crossing the first inoculum streak multiple times to produce isolated colonies.

    • Reference: From Forbes BA, Sahm DF, Weissfeld AS: Bailey and Scott's diagnostic microbiology, ed 12, St Louis, 2007, Mosby.

Identification of Bacteria

  • Smear Preparation and Stains Used in Microbiology

    • Smear Preparation: A crucial step for microscopic examination.

    • Types of Stains:

    • Simple Stains: E.g., Methylene blue, only allows for gross morphology observation.

    • Differential Stains: Used for distinguishing between different types of bacteria (e.g., Gram stain).

      • Gram Staining: Differentiates bacteria into Gram-positive (retains crystal violet) and Gram-negative (does not retain).

      • Acid-fast Stain: Primarily used for detecting Mycobacterium tuberculosis.

  • Morphology of Bacteria

    • Cocci: Can form clusters, chains, pairs, or tetrads.

    • Bacilli: Includes variants like coccobacilli and fusiform bacilli.

  • Microscopy Techniques Used

    • Brightfield Microscopy: Organisms appear dark against a bright background.

Bacterial Cultivation

  • Primary Culture: Initial growth of bacteria from a clinical specimen.

  • Subculture: Transferring a small volume from the primary culture to a new medium.

  • Pure Culture: Growth of a single type of microorganism isolated from mixed cultures.

  • Incubation Time: Cultures should be incubated for 24-48 hours.

  • Classification of Media

    • Enrichment Media: Designed to allow the growth of a specific bacterial pathogen (e.g., charcoal yeast extract for Legionella pneumophila).

    • Supportive Media: Allow most organisms to grow at their normal rates without favoring any specific organism (e.g., Blood agar).

    • Selective Media: Contains dyes, antibiotics, or chemicals that inhibit certain bacteria while promoting the growth of others (e.g., MacConkey agar targets Gram-negative bacteria).

    • Differential Media: Helps in recognizing certain organisms due to distinct features (e.g., Hektoen enteric agar indicates hydrogen sulfide production which forms a black precipitate).

  • Biochemical or Enzymatic Tests

    • Used for precise identification, as microscopic and cultural characteristics alone are insufficient.

Gram Staining Procedure

  1. Crystal Violet Application: The initial stain.

  2. Gram's Iodine Application: Acts as a mordant, enhancing the crystal violet's staining.

  3. Decolorizer: Alcohol or acetone is applied to remove the stain from Gram-negative bacteria.

  4. Safranin Counterstain: Provides color to Gram-negative bacteria after decolorization.

  • Gram-Positive Examples: Staphylococcus aureus

  • Gram-Negative Examples: Escherichia coli

Throat Cultures

  • Collecting the Specimen: A crucial process for accurate detection.

  • Methods for Detection of Group A Beta-Hemolytic Streptococci:

    • Rapid Detection Methods: Include non-culture techniques.

    • Culture on Sheep Blood Agar: Assists in determining hemolytic properties.

    • Types of Hemolysis:

      • Alpha Hemolysis: Partial hemolysis, causing a greenish discoloration.

      • Beta Hemolysis: Complete lysis of red blood cells, clear zones around colonies.

      • Gamma Hemolysis: No hemolysis observed.

Blood Cultures

  • Common Organisms Isolated from Blood:

    • Staphylococcus aureus, Streptococcus pneumoniae, E. coli, Klebsiella pneumoniae.

  • Specimen Collection: Requires careful cleansing of the collection site.

    • Culture Media for Blood: Typically uses enrichment media.

  • Methods for Examination of Blood Cultures:

    • Traditional Method: Incubating the blood cultures and performing blind subcultures after 6-18 hours.

    • Automated Blood Culture Systems: Enable rapid detection and monitoring without the need for visual checks.

Fungi

  • Fungi as a Source of Infection: Generally, from accidental exposure through trauma or inhalation.

  • Collection of Specimens for Fungal Studies: Can include hair, skin, nails, urine, blood.

  • Methods for Detecting Fungi:

    • Direct Microscopic Examination: To visualize fungal elements.

    • Gram Stain: Unable to differentiate fungi from bacteria.

    • Potassium Hydroxide Preparation: Helps in examining fungal elements.

    • India Ink Preparation: Used particularly for encapsulated fungi such as Cryptococcus.

    • Acid-Fast Stain: Employed for certain fungal species.

  • Culture of Fungi:

    • Culture Media: Such as Sabouraud Dextrose Agar.

    • Incubation: At 30°C for 4-6 weeks, checking weekly for growth.

    • Biochemical Screening Methods: For certain fungi are available with results within 72 hours.

Parasites

  • Parasites as a Source of Infection: Often resultant from travel or exposure in refugee populations.

  • Collection of Specimens for Parasite Identification: Commonly involves stool and blood specimens.

  • Methods for Detection of Parasites: Include techniques like wet mount and direct smear, along with antigen detection (e.g., for Giardia lamblia).