BackSkin and Throat Flora: Microbiota, Pathogenicity, and Laboratory Identification
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Skin Flora
Introduction to Skin Microbiota
The human skin is colonized by a diverse community of microorganisms, collectively known as the skin microbiota. These microbes inhabit the superficial layers of the epidermis and the upper portions of hair follicles. The majority of these organisms are non-pathogenic and play a protective role by occupying host cell receptors, thereby preventing colonization by pathogenic microbes.
Microbiota: The community of microorganisms living on body surfaces exposed to the environment.
Resident Microbes: Typically harmless and beneficial, but can become opportunistic pathogens under certain conditions (e.g., breaks in the skin).
Transmission: Direct contact with animate or inanimate surfaces (fomites) is a key method of microbial transmission.
Handwashing and Microbial Removal
Handwashing is a critical practice for reducing the transmission of potentially harmful microbes, especially in healthcare and food service settings. Laboratory experiments culturing the tactile surfaces of fingers before and after washing demonstrate the effectiveness and limitations of handwashing.
Experiment: Skin of fingers cultured at four time points: after wetting (Time 0), after 2 minutes (Time 2), 4 minutes (Time 4), and 6 minutes (Time 6) of washing.
Observation: The number of bacterial colonies may increase after washing, but the diversity of colony types decreases.
Explanation: Washing removes transient, detergent-susceptible bacteria, while tightly adhered resident microbiota remain.
Staphylococci
Medical Importance and Identification
Members of the genus Staphylococcus are common skin and mucous membrane microbiota. While many are opportunistic pathogens, Staphylococcus aureus is a significant pathogen capable of causing a range of infections, including skin abscesses, pneumonia, and toxic shock syndrome. Differentiation between S. aureus and other staphylococci is essential in clinical microbiology.
Blood Agar: Used to assess hemolysis; β-hemolysis is associated with more pathogenic strains.
Mannitol Salt Agar (MSA): Selective-differential medium for isolating and identifying Staphylococcus species.
Mannitol Salt Agar (MSA)
MSA contains 7.5% NaCl, which inhibits most bacteria except Staphylococcus spp. It also contains mannitol and phenol red indicator. S. aureus ferments mannitol, producing acid and turning the indicator yellow, while most other staphylococci do not ferment mannitol.
Selective Agent: Sodium chloride (NaCl)
Differential Agent: Mannitol (sugar alcohol)
Indicator: Phenol red (yellow in acid, red in neutral/alkaline)
Coagulase Test
The coagulase test distinguishes S. aureus (coagulase-positive) from other staphylococci (coagulase-negative). Coagulase exists in two forms: free and bound (clumping factor). The tube test detects both forms, while the slide test detects only bound coagulase.
Tube Test: Organism added to rabbit plasma; clotting indicates a positive result.
Slide Test: Organism mixed with plasma on a slide; agglutination within 1–2 minutes indicates bound coagulase.
Latex Agglutination Test
The BACTITM STAPH Latex Slide Agglutination Test uses latex beads coated with proteins that react with bound coagulase and protein A, both associated with the cell wall of S. aureus. Agglutination indicates a positive result.
Throat Flora
Introduction to Throat Microbiota
The upper respiratory tract (URT) harbors a diverse microbiota, including organisms also found on the skin. Most are non-pathogenic and beneficial, but under certain conditions, resident microbes can become opportunistic pathogens, causing abscesses or septicemia. Coughing and sneezing facilitate microbial transmission.
Key Pathogens: Streptococcus pyogenes (Group A) and Streptococcus pneumoniae
Streptococci: Identification and Classification
Streptococci are initially cultured on blood agar to assess hemolysis, which is important for identification. They are also classified into serological groups (A, B, C, D, etc.) based on cell wall polysaccharides. Sensitivity to antibiotics (e.g., bacitracin, optochin) further differentiates species.
Group A Streptococcus (S. pyogenes): Sensitive to bacitracin
S. pneumoniae: Sensitive to optochin
Throat Swab and Hemolysis
Throat swabs are commonly cultured on sheep blood agar to evaluate hemolysis:
α-hemolysis: Partial hemolysis, green halo (e.g., viridans streptococci)
β-hemolysis: Complete hemolysis, clear halo (e.g., S. pyogenes)
γ-hemolysis: No hemolysis
Catalase Test
The catalase test differentiates staphylococci (catalase-positive) from streptococci (catalase-negative). It should not be performed directly on blood agar to avoid false positives.
Streptolysins
Streptolysin S is a β-hemolysin produced by S. pyogenes that is stable in oxygen and detected on the surface of blood agar. Streptolysin O is oxygen-labile and detected only within the agar (by stabbing the plate). The zone of hemolysis is sharper in the stab (β-hemolysis) than on the surface (α-hemolysis).
Group A Streptococcus Antigen Test
This rapid immunochromatographic assay detects Group A streptococcal antigen directly from throat swabs. A positive result shows two colored lines (control and test), while a negative result shows only the control line.
Summary Table: Key Laboratory Media and Tests for Skin and Throat Flora
Test/Medium | Purpose | Key Result |
|---|---|---|
Blood Agar | Assess hemolysis (α, β, γ) | β-hemolysis: S. pyogenes; α-hemolysis: viridans streptococci |
Mannitol Salt Agar (MSA) | Selective for Staphylococcus; differential for S. aureus | Yellow color: S. aureus ferments mannitol |
Coagulase Test | Differentiates S. aureus from other staphylococci | Clotting/agglutination: positive for S. aureus |
Catalase Test | Differentiates staphylococci (+) from streptococci (-) | Bubbles: positive (staphylococci) |
Group A Strep Antigen Test | Rapid detection of S. pyogenes | Two lines: positive; one line: negative |
