Micrococcaceae and Streptococcaceae: Structure, Pathogenicity, and Clinical Relevance

Micrococcaceae

General Characteristics

The family Micrococcaceae includes the genera Micrococcus and Staphylococcus. These bacteria are notable for their Gram-positive cell walls, catalase positivity, coccoid shape, and pigment production.

• Gram-positive: Retain crystal violet stain due to thick peptidoglycan layer.

• Catalase positive: Produce the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen.

• Coccoid shape: Spherical cells, often arranged in characteristic patterns.

• Pigment producers: Many species produce colored colonies.

Genus Micrococcus

• Gram-positive, catalase positive, coagulase negative

• Susceptible to Bacitracin

• Growth in tetrads: Cells often appear in groups of four.

• Non-pathogenic: Generally considered harmless.

• Example: Micrococcus luteus

  • Normal skin flora, especially on extremities

  • Also found in soil

Genus Staphylococcus

• Gram-positive, catalase positive

• Growth in grape-like clusters

• Can be normal flora or opportunistic pathogens

Staphylococcus saprophyticus

• Normal flora of skin and mucosa of genitourinary tract

• Causes community-acquired urinary tract infections (UTIs) in young, sexually active females

• Infections outside urinary tract are rare

Staphylococcus epidermidis

• Normal flora of skin and mucous membranes (respiratory system)

• Non-pathogenic but can be found in wounds

• DNase negative, coagulase negative

• Unable to ferment mannitol

• Grows in 7.5% NaCl

• Gamma hemolytic (no hemolysis)

Staphylococcus aureus

• Normal flora of skin and upper respiratory tract (especially nose and throat)

• Pathogenic under certain conditions (e.g., immunosuppression, wounds)

• Most common cause of skin infections

• Associated with boils, pimples, impetigo, pneumonia, carditis, meningitis, arthritis

• Leading cause of nosocomial (hospital-acquired) infections

• MRSA and VRSA: Resistant to most antibiotics; infections can be fatal

Diseases and Syndromes Caused by S. aureus

• Scalded Skin Syndrome

• Toxic Shock Syndrome (TSS)

  • High fever, rash, vomiting, diarrhea, sometimes death

  • First recognized in menstruating women using tampons

  • Also occurs post-surgery

  • Caused by toxic shock toxin (a superantigen) that triggers massive T cell activation and inflammatory response

• Food Poisoning

  • Caused by enterotoxin A (a superantigen)

  • Symptoms: Severe but short-lived diarrhea and vomiting

Virulence Factors of S. aureus

• Coagulase positive: Enzyme induces fibrin clot formation, protecting bacteria from phagocytosis

• Leukocidin production: Destroys white blood cells, contributing to pus formation

• Proteolytic enzymes:

  • Hyaluronidase

  • Fibrinolysin

  • Lipase

  • Ribonuclease

  • Deoxyribonuclease (DNase positive)

• Growth in 7.5% NaCl

• Ferments mannitol

• Beta hemolytic (complete hemolysis on blood agar)

Streptococcaceae

General Characteristics

The family Streptococcaceae includes Streptococcus, Lactococcus, and Enterococcus species. These bacteria are Gram-positive cocci, catalase negative, and are classified into Lancefield groups based on cell wall antigens.

• Catalase negative

• Lancefield grouping: Groups A through O (based on carbohydrate antigens)

  • Group A: Beta hemolytic strains in humans

  • Group B: Associated with animals (e.g., mastitis in cows)

  • Group D: Enterococci

  • Group N: Lactococci

Streptococcus pyogenes (Group A Streptococcus)

• Frequently isolated from upper respiratory tract

• Causative agent of streptococcal pharyngitis (strep throat): Severe sore throat, enlarged tonsils, exudates, tender lymph nodes, mild fever

• Can cause otitis media, mastitis, impetigo

• Rheumatic fever: Immune-mediated damage to heart, joints, kidneys due to cross-reactive antibodies

• Scarlet fever: Caused by erythrogenic toxin (exotoxin) encoded by lysogenic bacteriophage; symptoms include pink-red rash, fever, blood vessel damage

• Necrotizing fasciitis (rare): Rapid, severe infection of subcutaneous tissue; "flesh-eating bacteria"; high mortality

• Streptococcal Toxic Shock-like Syndrome: Similar to S. aureus TSS; high mortality; associated with soft-tissue infections

• Beta hemolytic, esculin hydrolysis negative, bacitracin sensitive, optochin resistant, DNase positive

Streptococcus pneumoniae

• Frequently isolated from upper respiratory tract

• Causative agent of pneumonia (often secondary to other respiratory infections)

• Cells typically in pairs (diplococci), often encapsulated

• Invasive strains cause strong inflammatory response in alveoli, leading to fluid accumulation and reduced lung function

• Pneumococcal pneumonia: Untreated mortality ~30%; treated 5-10%

• Alpha hemolytic, esculin hydrolysis negative, optochin sensitive, bacitracin resistant

Streptococcus agalactiae (Group B Streptococcus)

• Normal flora of female genital and lower gastrointestinal tract

• Can be transmitted to neonates, especially after premature rupture of membranes

• Can cause septic arthritis, postpartum infections (endometritis, pelvic abscesses, septic shock)

• Neonatal infections: Sepsis, fever, meningitis, respiratory distress, lethargy, hypotension

• Early onset (first 5 days of life) or late onset (7 days to 3 months after birth)

Streptococcus bovis

• Esculin hydrolysis positive

• Alpha hemolytic (weak) or gamma hemolytic

• Unable to grow in 6.5% NaCl

Streptococcus salivarius

• Associated with dental caries (tooth decay)

• Ferments sugars via lactic acid fermentation; lactic acid demineralizes tooth enamel, predisposing to infection

Enterococcus faecalis

• Isolated from fecal samples (enteric origin)

• Esculin hydrolysis positive

• Gamma hemolytic or weakly alpha hemolytic

• Resistant to bacitracin and optochin

• Able to grow in 6.5% NaCl

Comparative Table: Key Biochemical and Clinical Features

Organism	Hemolysis	Catalase	Coagulase	Salt Tolerance (7.5% NaCl)	Mannitol Fermentation	DNase	Esculin Hydrolysis	Bacitracin Sensitivity	Optochin Sensitivity	Clinical Significance
Micrococcus luteus	Gamma	+	-	Variable	-	-	-	+	-	Normal skin flora
Staphylococcus aureus	Beta	+	+	+	+	+	-	-	-	Skin infections, TSS, food poisoning
Staphylococcus epidermidis	Gamma	+	-	+	-	-	-	-	-	Normal flora, opportunistic infections
Streptococcus pyogenes	Beta	-	-	-	-	+	-	+	-	Strep throat, scarlet fever, rheumatic fever
Streptococcus pneumoniae	Alpha	-	-	-	-	-	-	-	+	Pneumonia, meningitis
Streptococcus agalactiae	Beta	-	-	-	-	-	-	-	-	Neonatal sepsis, postpartum infections
Streptococcus bovis	Alpha (weak)/Gamma	-	-	-	-	-	+	-	-	Endocarditis, colon cancer association
Enterococcus faecalis	Gamma/Alpha (weak)	-	-	+	-	-	+	-	-	UTIs, endocarditis

Key:

• + : Positive

• - : Negative

• Hemolysis: Beta (complete), Alpha (partial/green), Gamma (none)

Additional info:

• Superantigens are a class of antigens that result in excessive activation of the immune system, particularly T cells, leading to massive cytokine release and systemic inflammation.

• Esculin hydrolysis is a biochemical test used to differentiate bacterial species based on their ability to hydrolyze esculin to esculetin and glucose.

• Salt tolerance (growth in 6.5% or 7.5% NaCl) is used to distinguish between genera and species, especially in clinical microbiology.