1. General principles of microbiological diagnosis.Staphylococcus spp.(1)

General Principles of Microbiological Diagnosis

  • Laboratory diagnosis of infectious diseases, especially those caused by Staphylococcus spp.

  • Presented by Dr. Denitsa Tsaneva-Damyanova.

Clinical Microbiology

  • Clinical microbiology identifies etiologic agents of disease.

  • Laboratories test specimens from patients to detect microorganisms that may cause illness.

  • Provide information about antimicrobial drug activity against identified microorganisms.

Detection of Bacteria in Clinical Specimens

  • Key procedures for bacterial detection:

    1. Appropriate Specimen Collection

    2. Microscopy

    3. Cultivation in artificial media for isolation and biochemical activity.

    4. Isolation of Pure Culture

    5. Identification of pure cultures using biochemical, serological methods, phage typing.

    6. Antimicrobial Susceptibility Testing

    7. Serology

    8. Result Interpretation

Signs and Symptoms

  • Characteristic signs and symptoms guide presumptive clinical diagnosis.

  • Symptomatic assessments can be localized or systemic.

  • **Specimen Types:

    • Sputum

    • Blood

    • Feces

    • Serum

    • Tissue biopsy**

  • Tests Selection:

    • Selective and non-selective media

    • Serologic tests, biochemical tests (e.g., complement fixation, agglutination, ELISA)

    • Definitive diagnosis

    • Antimicrobial susceptibility testing supports treatment decisions.

Specimen Collection Requirements

  • Specimens must reflect the disease process;

  • Should be collected quickly and processed to maximize pathogen detection.

  • Specimens can be obtained from various sources:

    • Blood, cerebrospinal fluid, abdominal fluids, respiratory tract, urine, etc.

Microscopy in Microbiology

  • Used for initial detection and preliminary identification of microorganisms.

  • Sensitivity depends on microorganism count; specificity on unique morphological characteristics.

  • Different microscopy types include:

    1. Brightfield Microscopy

    2. Darkfield Microscopy

    3. Phase-Contrast Microscopy

    4. Fluorescent Microscopy

    5. Electron Microscopy

Culturing Microorganisms

  • Infection confirmed by isolating and culturing microorganisms in media:

    • Cultures can be grown in liquid or on solid media.

    • Liquid media: More sensitive for isolating small numbers but requires further subculturing for mixed cultures.

    • Solid media: Allows for isolation and quantification of colonies.

    • Chlamydia and viruses are cultured in cell cultures, while some may require inoculation into animals.

Types of Media for Cultivation

  • Minimal Essential Media:

    • Contains primary growth precursors.

  • Enriched Media:

    • Contains additives like blood, milk to enhance bacterial growth.

  • Selective Media:

    • Suppress growth of unwanted organisms to enhance specific ones.

  • Differential Media:

    • Distinguishes between different microorganisms based on biochemical characteristics.

Typical Media Examples

  • Enriched Media:

    • Blood Agar, Chocolate Agar, Mueller-Hinton Agar, Sabouraud Dextrose Agar.

  • Selective Media:

    • MacConkey Agar for gram-negative bacteria, and various others for specific pathogens (e.g., Salmonella).

Staphylococcus Genus Overview

  • Characteristics:

    • Includes at least 45 species, divided into catalase-positive (e.g., S. aureus) and catalase-negative (e.g., S. epidermidis).

  • Generally, they reside harmlessly on skin and mucous membranes or can be pathogenic.

  • Microscopically appear as grape-like clusters (0.5-1.5 μm), non-motile, and some form capsules.

Staphylococcus aureus Properties

  • Forms smooth, convex colonies on agar (2-4 mm) with varying pigmentations.

  • Highly resilient, living in environments with salt concentrations up to 20%.

  • Exhibits psychrophilic properties, growing at low temperatures (0°C).

Biochemical Features of Staphylococcus

  • Includes various antigens, such as Peptidoglycan, teichoic acids, and Protein A.

  • Exotoxins have various pathogenic effects, including cytolytic effects, exfoliative responses, and toxic shock.

Exotoxins and Their Roles

  • Cytotoxins: Affect red blood cell membranes, causing hemolysis.

  • Exfoliative Toxins: Lead to conditions like Staphylococcal Scalded Skin Syndrome (SSSS).

  • Enterotoxins: Cause food poisoning, resulting in severe gastrointestinal symptoms.

  • Toxic Shock Syndrome Toxin (TSST-1): Functions as a super antigen, impacting immune responses.

Pathogenic Mechanisms

  • Disease manifestations arise from bacterial toxin production or direct tissue invasion.

  • Localized infections: Include cases like impetigo and endocarditis.

Antibiotic Resistance Trends

  • Rapid development of resistance since penicillins' introduction.

  • Methicillin-resistant strains (MRSA) remain a major concern, showing resistance to various antibiotics.

  • Current antibiotics include macrolides, tetracyclines, and Vancomycin, with increasing resistance observed.

Vaccines and Treatment Approaches

  • Development of staphylococcal vaccines and serums, including autovaccines and γ-globulins.