BSc Nursing, third semester, pathogenic microorganism, nodes. Could-- You could start with the Streptococcus. Staphylococcus.
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
I now have comprehensive content from Medical Microbiology 9e. Let me compile a thorough, well-structured answer for a BSc Nursing 3rd semester student.
Pathogenic Microorganisms: Staphylococcus & Streptococcus
BSc Nursing - 3rd Semester | Microbiology
STAPHYLOCOCCUS
Classification & Basic Characteristics
Staphylococci are Gram-positive cocci arranged in irregular grape-like clusters ("staphyle" = bunch of grapes in Greek). They are:
- Catalase-positive (distinguishes them from Streptococcus)
- Non-motile, non-spore-forming
- Facultative anaerobes
- Can survive on dry surfaces for long periods due to a thick peptidoglycan cell wall and no outer membrane
Key species of clinical importance:
| Species | Coagulase | Clinical Significance |
|---|---|---|
| S. aureus | Positive | Most virulent; major pathogen |
| S. epidermidis | Negative | Opportunistic; catheter/implant infections |
| S. saprophyticus | Negative | UTIs in young women |
Staphylococcus aureus
Virulence Factors
Structural components:
- Protein A - binds IgG antibodies backwards (Fc region), blocking phagocytosis
- Capsule - resists phagocytosis
- Peptidoglycan - resists drying; triggers inflammation
- Teichoic acid - helps adherence to mucosal surfaces
Toxins:
| Toxin | Action | Disease |
|---|---|---|
| Exfoliative toxins (A & B) | Cleave desmoglein-1 in skin | Staphylococcal Scalded Skin Syndrome (SSSS) |
| Enterotoxins (A-E, etc.) | Heat-stable superantigens | Food poisoning |
| Toxic Shock Syndrome Toxin-1 (TSST-1) | Superantigen; massive cytokine release | Toxic Shock Syndrome |
| Panton-Valentine Leukocidin (PVL) | Destroys leukocytes | Necrotizing pneumonia, severe furunculosis |
| Alpha-toxin (α-hemolysin) | Pore-forming; lyses RBCs & other cells | Tissue destruction |
Enzymes:
| Enzyme | Action |
|---|---|
| Coagulase | Converts fibrinogen to fibrin; "walls off" infection (key ID test for S. aureus) |
| Hyaluronidase | Breaks down connective tissue; spreads infection |
| Staphylokinase | Dissolves fibrin clots |
| DNase | Breaks down DNA |
| Lipase | Helps colonize sebaceous skin areas |
Epidemiology
- Normal flora on human skin and nasal mucosa (~30% of population are nasal carriers)
- Spread by direct contact or contaminated fomites (bed linens, clothing)
- MRSA (Methicillin-Resistant S. aureus) is now the most common cause of community-acquired skin and soft-tissue infections - a major public health concern worldwide
Risk groups:
- Infants (SSSS)
- Surgical patients, patients with catheters/prostheses
- IV drug users
- Immunocompromised patients
Diseases Caused by S. aureus
1. Toxin-mediated diseases:
- Scalded Skin Syndrome (SSSS) - mainly in neonates and infants; diffuse bullous desquamation; no organisms in the blisters
- Food Poisoning - rapid onset (1-6 hrs) vomiting, diarrhea, abdominal cramps after ingestion of pre-formed heat-stable enterotoxin; resolves within 24 hrs
- Toxic Shock Syndrome (TSS) - fever, hypotension, diffuse erythematous rash, multi-organ failure; associated with tampon use or wound infections
2. Suppurative (pus-forming) infections:
- Impetigo - pus-filled vesicles on erythematous base
- Folliculitis - infection of hair follicles
- Furuncles (boils) - large, painful, pus-filled nodules
- Carbuncles - coalescence of furuncles extending into subcutaneous tissue; associated with systemic signs (fever, bacteremia)
3. Systemic infections:
- Bacteremia and Endocarditis - bacterial seeding of heart valves
- Pneumonia - especially post-influenza; severe necrotizing form possible
- Osteomyelitis - destruction of bone (especially metaphysis of long bones in children)
- Septic arthritis - purulent joint effusion
- Meningitis - especially in patients with shunts
Diagnosis
- Gram stain - Gram-positive cocci in clusters
- Culture - grows rapidly on non-selective media; selective media: mannitol-salt agar (ferments mannitol; colonies turn yellow), chromogenic agar
- Coagulase test - positive = S. aureus (tube/slide test)
- NAAT (nucleic acid amplification) - screening for MRSA carriage
- Catalase test - positive (bubbles with H₂O₂); distinguishes from Streptococcus
Treatment
| Infection Type | Treatment |
|---|---|
| Localized (abscess) | Incision and drainage (I&D) |
| Non-MRSA systemic | Beta-lactams (oxacillin, nafcillin) |
| MRSA | Vancomycin (IV) - drug of choice; alternatives: daptomycin, linezolid |
| Oral MRSA | Trimethoprim-sulfamethoxazole (TMP-SMX), doxycycline, clindamycin, linezolid |
| Food poisoning | Symptomatic only |
STREPTOCOCCUS
Classification
Lancefield grouping (based on cell wall carbohydrate antigen):
- Group A = S. pyogenes
- Group B = S. agalactiae
- Group D = Enterococcus (now reclassified)
Hemolysis pattern on blood agar:
| Hemolysis | Pattern | Example |
|---|---|---|
| Beta (β) | Complete - clear zone | S. pyogenes (Group A), S. agalactiae (Group B) |
| Alpha (α) | Partial - green zone (incomplete lysis) | S. pneumoniae, viridans streptococci |
| Gamma (γ) | No hemolysis | Enterococcus |
Streptococcus pyogenes (Group A Streptococcus - GAS)
Basic Features
- Spherical cocci, 1-2 µm diameter
- Arranged in short chains in clinical specimens; longer chains in liquid media
- Beta-hemolytic on blood agar
- Catalase-negative (distinguishes from Staphylococcus)
- Bacitracin sensitive (key lab test - inhibited by bacitracin)
Virulence Factors
Structural:
- M protein - major virulence factor; anti-phagocytic; over 200 serotypes based on emm gene; class I M proteins are associated with rheumatic fever
- Hyaluronic acid capsule - antigenically similar to human connective tissue, so not recognized as foreign; anti-phagocytic
- Lipoteichoic acid + F protein - adhere to fibronectin on host cells
- Group A carbohydrate (Lancefield antigen) - N-acetylglucosamine + rhamnose dimer; used for classification
Toxins:
- Erythrogenic (pyrogenic) toxins (SPE A, B, C) - superantigens; cause the scarlet fever rash; SPE A is associated with toxic shock-like syndrome
- Streptolysin O (SLO) - oxygen-labile; lyses RBCs and leukocytes; immunogenic - antibodies (ASO titer) used diagnostically
- Streptolysin S (SLS) - oxygen-stable; responsible for the beta-hemolysis seen on blood agar plates
Enzymes:
- Streptokinase (fibrinolysin) - dissolves fibrin clots; aids spread
- Hyaluronidase - "spreading factor"; breaks down connective tissue
- DNase (streptodornase) - liquefies pus; aids spread
- C5a peptidase - destroys complement component C5a, impairing neutrophil recruitment
Diseases Caused by S. pyogenes
Suppurative (pus-forming) diseases:
- Pharyngitis ("strep throat") - most common bacterial pharyngitis; tonsillar exudates, fever, no cough
- Impetigo - honey-crusted skin lesion; more common in children
- Cellulitis - spreading skin/soft tissue infection
- Erysipelas - superficial skin infection with sharp raised borders; typically on face
- Necrotizing fasciitis - "flesh-eating bacteria"; rapid destruction of fascia and muscle; high mortality
- Scarlet fever - pharyngitis + strawberry tongue + diffuse sandpaper rash (from pyrogenic toxin)
Nonsuppurative (post-infectious/immunological) complications:
- Rheumatic Fever - occurs 2-4 weeks after pharyngitis; involves heart (carditis), joints (polyarthritis), skin (erythema marginatum, subcutaneous nodules), and CNS (Sydenham's chorea); caused by molecular mimicry between M protein and cardiac tissue
- Post-streptococcal glomerulonephritis (PSGN) - occurs 1-3 weeks after pharyngitis or skin infection; immune complex deposition in glomeruli; hematuria, proteinuria, hypertension
Diagnosis
- Gram stain - Gram-positive cocci in chains
- Culture on blood agar - beta-hemolytic colonies
- Rapid antigen detection test (RADT) - detects Group A carbohydrate antigen; quick but less sensitive
- Throat culture - gold standard for pharyngitis
- ASO titer (Anti-Streptolysin O) - elevated in post-streptococcal disease (rheumatic fever, PSGN)
Treatment
- Penicillin G (or V) - drug of choice; S. pyogenes has never developed resistance to penicillin
- Amoxicillin - commonly used orally
- Azithromycin or clindamycin - for penicillin-allergic patients
- Importance of completing the course - to prevent rheumatic fever
Streptococcus pneumoniae (Pneumococcus)
Basic Features
- Elongated diplococci (pairs) arranged in short chains; lancet-shaped
- Alpha-hemolytic on blood agar (green zone)
- Optochin sensitive (key lab test) and bile soluble
- Identified by: catalase negative, optochin susceptibility, bile solubility
Virulence Factors
- Polysaccharide capsule - most important; >90 serotypes; anti-phagocytic
- Pneumolysin - lyses host cells and inhibits complement
- IgA protease - cleaves secretory IgA; helps colonize respiratory mucosa
- Teichoic acid (C-polysaccharide) - activates complement; triggers inflammation
- Surface adhesins - help colonize nasopharynx
Diseases
- Lobar pneumonia (most common cause in adults)
- Meningitis (most common cause in adults; also in children after Hib vaccine)
- Otitis media (most common cause in children)
- Sinusitis
- Bacteremia/Sepsis - fulminant in asplenic patients
Diagnosis
- Gram stain: Gram-positive diplococci (lancet-shaped)
- Culture on sheep blood agar (enriched media)
- Optochin sensitivity test
- Quellung reaction - capsule swelling with specific antibody (gold standard for serotyping)
- Antigen tests for C polysaccharide (CSF - highly sensitive)
- NAAT for meningitis
Treatment & Prevention
- Penicillin (if susceptible); high-dose amoxicillin
- Cephalosporins for resistant strains; vancomycin for severe/resistant meningitis
- Vaccines: PCV13 (13-valent conjugate, for children) and PPSV23 (23-valent polysaccharide, for adults >65 and high-risk groups)
Streptococcus agalactiae (Group B Streptococcus - GBS)
- Beta-hemolytic, Group B Lancefield antigen
- CAMP test positive (key lab test) - enhances RBC lysis in presence of S. aureus
- Hippurate hydrolysis positive
- Major pathogen in neonates and pregnant women
Diseases:
- Neonatal sepsis and meningitis - early-onset (within 7 days) or late-onset
- Postpartum endometritis in mothers
- UTI in pregnant women
Treatment: Penicillin; for high-risk pregnancies, intrapartum penicillin is given at least 4 hours before delivery
QUICK COMPARISON TABLE
| Feature | Staphylococcus aureus | Streptococcus pyogenes | S. pneumoniae |
|---|---|---|---|
| Shape/Arrangement | Clusters | Chains | Pairs (diplococci) |
| Gram stain | +ve | +ve | +ve |
| Catalase | Positive | Negative | Negative |
| Hemolysis | Beta | Beta | Alpha |
| Key enzyme | Coagulase (+) | Coagulase (-) | - |
| Key ID test | Coagulase test | Bacitracin sensitive | Optochin sensitive |
| Key toxin | TSST-1, exfoliatin | Streptolysin O/S | Pneumolysin |
| Drug of choice | Vancomycin (MRSA) | Penicillin G | Penicillin |
NURSING RELEVANCE - KEY POINTS
- Infection control - MRSA is spread by contact; hand hygiene and standard precautions are the most effective prevention
- ASO titer monitoring - nurses should recognize elevated ASO titer as a marker for post-streptococcal complications
- Completing antibiotic courses - especially for Strep throat, to prevent rheumatic fever
- Neonatal care - GBS screening of pregnant women (vaginal/rectal swab at 35-37 weeks) and prophylactic penicillin in labor
- SSSS in infants - handle with care; skin barrier is compromised; prevent secondary infections
- Vaccine counseling - educate patients on pneumococcal vaccines (especially elderly, asplenic, immunocompromised)
Sources: Medical Microbiology 9e (Murray et al.), p. 1909-2700; Goldman-Cecil Medicine.
This is a shared conversation. Sign in to Orris to start your own chat.