Pathogenic Gram-Positive Cocci: Staphylococcus, Streptococcus, and Enterococcus

Pathogenic Gram-Positive Bacteria

Introduction

This section covers the structure, physiology, pathogenicity, epidemiology, diseases, diagnosis, treatment, and prevention of major Gram-positive cocci, focusing on Staphylococcus, Streptococcus, and Enterococcus. These bacteria are significant human pathogens responsible for a wide range of diseases, from mild skin infections to life-threatening systemic illnesses.

Staphylococcus: Structure, Physiology, and Pathogenicity

Structure and Physiology

• Cell Arrangement: Cells occur in grape-like clusters.

• Motility: Non-motile.

• Oxygen Requirements: Facultative anaerobes.

• Catalase: Catalase-positive (distinguishes from Streptococcus).

• Antibiotic Resistance: Most are resistant to Penicillin G.

• Salt Tolerance: Can tolerate high salt concentrations, allowing survival on human skin.

• Environmental Resistance: Tolerant of desiccation, radiation, and heat; survive on environmental surfaces.

Medically Important Species

• Staphylococcus aureus: More virulent; causes a variety of diseases depending on infection site; coagulase positive.

• Staphylococcus epidermidis: Part of normal skin microbiota; causes opportunistic infections, especially in immunocompromised hosts or with indwelling devices.

Pathogenicity Mechanisms

Staphylococcal infections occur when bacteria breach the body's physical barriers. Pathogenicity is due to:

• Structural defenses against phagocytosis:

  • Slime layers (capsules): Inhibit leukocyte chemotaxis and phagocytosis; facilitate attachment to surfaces.

  • Protein A (S. aureus): Binds IgG, inhibiting opsonization and complement activation.

  • Bound coagulase (S. aureus): Converts fibrinogen to fibrin, forming clots that hide bacteria from immune cells.

• Enzymes:

  • Cell-free coagulase (S. aureus): Triggers blood clotting.

  • Hyaluronidase (S. aureus): Breaks down hyaluronic acid, aiding tissue invasion.

  • Staphylokinase (S. aureus): Dissolves fibrin clots, allowing bacterial spread.

  • Lipases (S. aureus & S. epidermidis): Digest lipids, enabling growth on skin and in oil glands.

  • Beta-lactamase (S. aureus): Breaks down penicillin, conferring antibiotic resistance.

• Toxins (mainly S. aureus):

  • Cytolytic toxins: Disrupt cell membranes; leukocidin lyses leukocytes.

  • Exfoliative toxins: Cause skin cells to separate and slough off.

  • Toxic-shock syndrome toxin (TSST-1): Causes systemic effects.

  • Enterotoxins: Cause food poisoning symptoms.

Virulence Factor Comparison Table

Virulence Factor	S. aureus	S. epidermidis
Protein A	+	−
Coagulase	+	−
Slime layer	+	+
Catalase	+	+
Hyaluronidase	+	−
Staphylokinase	+	−
Lipase	+	+
β-lactamase	+	−
Toxins	+	−

Epidemiology

• S. epidermidis: Ubiquitous on human skin.

• S. aureus: Primarily found on moist skin folds.

• Both species can colonize the upper respiratory, gastrointestinal, and urogenital tracts.

• Transmission occurs via direct contact and fomites.

• Handwashing and aseptic techniques are critical for prevention.

Staphylococcal Diseases

• Exotoxin-mediated diseases (S. aureus):

  • Gastroenteritis (food poisoning): Non-invasive; caused by ingestion of enterotoxin-contaminated food.

  • Scalded Skin Syndrome: Cutaneous disease caused by exfoliative toxin.

  • Toxic Shock Syndrome (TSS): Systemic disease caused by TSST-1.

• Direct organ invasion:

  • Bacteremia: Bacteria in the blood.

  • Endocarditis: Infection of the heart lining.

  • Pneumonia and empyema: Lung infections.

  • Osteomyelitis: Infection of bone and bone marrow.

  • Cutaneous infections: Impetigo, folliculitis.

Diagnosis, Treatment, and Prevention

• Diagnosis: Gram-positive cocci in clusters; coagulase test for S. aureus.

• Treatment: Methicillin; vancomycin for MRSA (methicillin-resistant S. aureus).

• Prevention: Hand antisepsis is crucial, especially in healthcare settings.

Streptococcus: Structure, Physiology, and Pathogenicity

Structure and Physiology

• Cell Arrangement: Cocci in pairs or chains.

• Oxygen Requirements: Facultative anaerobes.

• Catalase: Catalase-negative (distinguishes from Staphylococcus).

• Classification: Based on hemolytic activity (alpha, beta, gamma) and Lancefield grouping (A, B, etc.).

• Significant Human Pathogens: Groups A and B.

Group A Streptococcus (Streptococcus pyogenes)

• Structural defenses:

  • M protein: Destabilizes complement, inhibits phagocytosis.

  • Hyaluronic acid capsule: Camouflages bacteria from immune cells.

• Enzymes:

  • Streptokinases: Break down blood clots, aiding spread.

  • Deoxyribonucleases: Reduce pus viscosity, facilitating spread.

  • C5a peptidase: Degrades complement protein C5a, reducing leukocyte recruitment.

  • Hyaluronidase: Facilitates tissue invasion.

• Toxins:

  • Pyrogenic toxins: Cause fever, rash, and shock.

  • Streptolysins: Lyse red and white blood cells, and platelets.

Epidemiology

• Infects pharynx or skin; disease often follows depletion of normal microbiota.

• Spread via respiratory droplets.

Diseases Caused by S. pyogenes

• Local invasion/exotoxin release:

  • Pharyngitis (strep throat): Inflammation of the pharynx.

  • Scarlet fever: Rash following pharyngitis, caused by pyrogenic toxin.

  • Streptococcal TSS: Severe multisystem infection, can cause organ failure and death.

  • Pyoderma: Pus-producing skin lesion.

  • Erysipelas: Infection and inflammation of lymph nodes.

  • Cellulitis: Inflammation of deeper skin layers.

  • Necrotizing fasciitis: Rapid tissue destruction, high mortality.

• Delayed antibody-mediated diseases:

  • Rheumatic fever: Autoimmune response damaging heart valves and muscle.

  • Glomerulonephritis: Immune complexes in kidneys cause inflammation, hypertension, and possible irreversible damage.

Diagnosis, Treatment, and Prevention

• Diagnosis: Gram-positive cocci in chains/pairs; rapid strep test for respiratory infections.

• Treatment: Penicillin is effective.

• Prevention: Antibodies against M protein provide protection.

Group B Streptococcus (Streptococcus agalactiae)

• Structure: Cocci in chains; distinguished from group A by group-specific antigens and smaller beta-hemolysis zone.

• Pathogenicity: Infects newborns lacking specific antibodies; produces enzymes of unclear function.

• Epidemiology: Colonizes GI, genital, and urinary tracts; newborns often infected at birth.

Diseases, Diagnosis, and Prevention

• Diseases: Neonatal bacteremia, meningitis, pneumonia; also affects immunocompromised adults.

• Diagnosis: ELISA test for group B antigen.

• Treatment: Penicillin or ampicillin.

• Prevention: Prophylactic penicillin during childbirth; immunization of women can protect future children.

Other Beta-Hemolytic Streptococci

• Streptococcus equisimilis: Causes pharyngitis.

• Streptococcus anginosus: Produces pus-filled abscesses.

• Penicillin is effective against both.

Alpha-Hemolytic Streptococci: The Viridans Group

• Produce green pigment on blood agar (alpha-hemolysis).

• Lack group-specific carbohydrates; not classified by Lancefield system.

• Inhabit mouth, pharynx, GI, genital, and urinary tracts.

• Opportunistic pathogens: cause abscesses, dental caries, endocarditis, and meningitis if entering the blood.

Streptococcus pneumoniae

• Structure: Diplococci, form unpigmented alpha-hemolytic colonies on blood agar; lack Lancefield antigens.

• Pathogenesis and Epidemiology: Present in mouths/pharynx; disease occurs when bacteria reach lungs. Most common in children and elderly.

• Diseases: Pneumococcal pneumonia, sinusitis, otitis media, bacteremia, endocarditis, meningitis (high mortality).

Diagnosis, Treatment, and Prevention

• Diagnosis: Gram stain of sputum; Quellung reaction confirms capsule.

• Treatment: Penicillin (resistant strains exist).

• Prevention: Vaccine from purified capsule material.

Enterococcus: Structure, Physiology, and Pathogenicity

Structure and Physiology

• Previously classified with group D streptococci; now a separate genus.

• Form short chains and pairs; lack a capsule.

• Two main species: Enterococcus faecalis and E. faecium.

• All enterococci inhabit the intestinal tracts of animals.

Pathogenesis, Epidemiology, and Disease

• Normally found in the human colon; rarely pathogenic there.

• Can cause disease if introduced to other body sites (e.g., urinary tract, bloodstream).

• Important cause of healthcare-associated infections, especially in immunocompromised patients.

Diagnosis, Treatment, and Prevention

• Diagnosis: Distinguished from S. pneumoniae by bile sensitivity.

• Treatment: Often resistant to antimicrobials; infections are difficult to treat.

• Prevention: Good hygiene and aseptic techniques are essential; prevention is challenging in healthcare settings.

Summary Table: Characteristics of Pathogenic Streptococci

Lancefield Group	Scientific Name	Hemolytic Pattern	Significant Characteristics	Characteristic Diseases
A	S. pyogenes	Large zone of beta-hemolysis	1-2 mm white colonies; bacitracin sensitive	Pharyngitis, rheumatic fever, scarlet fever, pyoderma, erysipelas, cellulitis, TSS, necrotizing fasciitis, glomerulonephritis
B	S. agalactiae	Small zone of beta-hemolysis	2-3 mm buttery colonies; bacitracin resistant	Puerperal fever, neonatal bacteremia, meningitis, pneumonia
C	S. equisimilis	Large zone of beta-hemolysis	1-2 mm white colonies	Pharyngitis, glomerulonephritis
C, F, or G	S. anginosus	Small zone of beta-hemolysis	1-2 mm white colonies	Pharyngitis
—	S. mutans (Viridans group)	Alpha-hemolysis	Green pigment on blood agar	Dental caries, rarely bacteremia, meningitis, endocarditis
—	S. pneumoniae	Alpha (aerobic), beta (anaerobic)	Diplococci; capsule required for pathogenicity; bile sensitive	Pneumonia, sinusitis, otitis media, bacteremia, endocarditis, meningitis
D	Enterococcus faecalis, E. faecium	None (gamma-hemolysis)	Diplococci or short chains; no capsule; bile insensitive	UTIs, bacteremia, endocarditis, wound infections

Key Concepts and Review Questions

• Most staphylococci are resistant to penicillin because they express the enzyme: Beta-lactamase.

• Streptococcus pyogenes produces ________ which can cause fever, rashes, and shock: Pyrogenic toxins.

• Enterococcus can be differentiated from Streptococcus pneumoniae by: Growth in bile.