Clinical Microbiology: Staphylococci, Streptococci, Haemophilus, and Moraxella

The Staphylococci

Taxonomy and Classification

The genus Staphylococcus belongs to the family Micrococcaceae and includes both coagulase-positive and coagulase-negative species. Coagulase-positive species, such as S. aureus, are primary pathogens, while coagulase-negative species, including S. epidermidis, S. saprophyticus, and S. haemolyticus, are often part of the normal flora or opportunistic pathogens.

• Coagulase Positive: S. aureus

• Coagulase Negative: S. epidermidis, S. saprophyticus, S. haemolyticus

• Micrococcus: Often a contaminant

Gram Reaction and Morphology

Staphylococci are Gram-positive, spherical cells (0.5–1.5 μm) that appear in singles, pairs, tetrads, or clusters resembling "bunches of grapes."

• Nonmotile, non-spore-forming, nonencapsulated

• Catalase positive, oxidase negative

• Glucose fermenters, primarily aerobic, some facultatively anaerobic

• Bacitracin resistant

• Colony morphology: Buttery, cream, or white colored

Staphylococcus aureus

S. aureus is the primary pathogen of the genus, responsible for a range of superficial to systemic infections. It colonizes the anterior nares, axilla, vagina, and pharynx, and is carried by 20–30% of the population.

• Infections: Skin, sepsis, hospital-acquired infections, MRSA

• Transmission: Traumatic introduction, direct contact, inanimate objects

• Predisposing conditions: Chronic infections, indwelling devices, skin injuries, immune defects

• Pus formation: Accumulation of neutrophils, bacterial cells, and extravascular fluid

Virulence Factors of S. aureus

Virulence factors contribute to pathogenicity and include toxins, enzymes, and structural proteins.

• Enterotoxins: Heat-stable exotoxins causing diarrhea and vomiting; implicated in food poisoning and toxic shock syndrome

• Exfoliatin: Epidermolytic toxin

• TSST-1: Toxin Shock Syndrome Toxin-1, stimulates T cell production and cytokines

• Cytolytic toxins: Hemolytic toxins (alpha, beta, gamma, delta), Panton-Valentine leukocidin

• Extracellular enzymes: Hyaluronidase, staphylokinase, coagulase, lipase, penicillinase, DNase, beta-lactamase

• Protein A: Binds Fc region of IgG, blocks phagocytosis

Clinical Infections

• Skin and wound infections: Impetigo, furuncles, carbuncles, surgical wounds, scalded skin syndrome, toxic shock syndrome

• Food poisoning: Enterotoxin A is most common; foods include meat, dairy, bakery goods, salads

Coagulase-Negative Staphylococci

These species are part of the indigenous flora but can cause infections, especially in immunocompromised patients or those with prosthetic devices.

• S. epidermidis: Hospital-acquired infections, slime layer aids adherence and avoids phagocytosis, common in UTIs

• S. saprophyticus: UTIs in young sexually active women, significant even in low numbers in urine cultures

• S. haemolyticus: Rarely implicated, associated with opportunistic infections

The Streptococci

Characteristics

Streptococci are Gram-positive cocci, usually capsulated, nonmotile, non-spore-forming, and facultative anaerobes. They are catalase negative and fastidious.

• Arrangement: Chains or pairs

• Catalase negative: Differentiates from staphylococci

Classification

Streptococci are classified by oxygen requirements, serology (Lancefield classification), and hemolysis on blood agar.

• Hemolysis types:

  • Alpha (α): Partial hemolysis, green discoloration (e.g., S. pneumoniae, S. viridans)

  • Beta (β): Complete hemolysis, clear zone (e.g., S. pyogenes, S. agalactiae)

  • Gamma (γ): No hemolysis (e.g., group D, Enterococcus spp.)

• Lancefield classification: Based on C-carbohydrate antigen; groups A, B, D are most frequent

Group A Streptococci (S. pyogenes)

Group A includes only S. pyogenes, responsible for 90% of pharyngitis cases and a variety of diseases.

• Diseases: Strep throat, impetigo, erysipelas, cellulitis, necrotizing fasciitis, toxic shock-like syndrome

• Erysipelas: Acute dermal infection, treated with penicillin or erythromycin

• Strep throat: Spread by saliva/nasal secretions, incubation 2–4 days, treat promptly to avoid complications

Diagnosis and Treatment of Strep Throat

• Symptoms: Sore throat, slight fever, pus in throat

• Diagnosis: Molecular tests, throat swab, blood agar (β-hemolysis)

• Scarlet fever: Lysogenic phage expresses erythrogenic toxin, causes rash and strawberry tongue

• Treatment: Penicillin G or erythromycin

Poststreptococcal Diseases

• Rheumatic fever: Autoimmune disease affecting heart, joints, nervous system

• Acute glomerulonephritis: Inflammatory disease of renal glomeruli, due to Ag/Ab complex deposition

Pneumonia Caused by Streptococcus pneumoniae

S. pneumoniae is a Gram-positive, alpha-hemolytic organism, often part of normal respiratory flora. Infection occurs when host resistance is lowered.

• Virulence factor: Capsular polysaccharide protects against phagocytosis

• Pathogenesis: Rapid bacterial growth in alveolar spaces

• Treatment: Penicillin G, cefotaxime, ofloxacin, erythromycin, tetracycline

• Vaccine: Pneumovax 23 for elderly and immunocompromised

Pathogenesis and Virulence Factors

• M protein: Interferes with opsonization and lysis

• Lipoteichoic acid and F protein: Adhesion

• Hyaluronic acid capsule: Camouflages bacteria

• Enzymes: Streptokinases, deoxynucleases, C5a peptidase

• Pyrogenic toxins: Stimulate cytokine release

• Streptolysins: O and S lyse RBCs, WBCs, platelets

Disease Classification

• Suppurative (pus-producing): Pharyngitis, impetigo, scarlet fever, pyoderma, necrotizing fasciitis

• Non-suppurative: Rheumatic fever, glomerulonephritis

Differentiation of Streptococci

Laboratory tests are used to differentiate between streptococcal species.

• Lancefield classification

• Bacitracin susceptibility: S. pyogenes (Group A) is susceptible

• CAMP test: S. agalactiae (Group B) is positive

• Optochin test: S. pneumoniae is sensitive

• Bile solubility test: S. pneumoniae is soluble

Key Differentiation Table

	CAMP Test	Bacitracin Sensitivity	Hemolysis
S. pyogenes	Negative	Susceptible	Beta (β)
S. agalactiae	Positive	Resistant	Beta (β)

	Insulin Fermentation	Bile Solubility	Hemolysis
S. pneumoniae	Soluble	Sensitive	Alpha (α)
Viridans	Insoluble	Resistant	Alpha (α)

Haemophilus and Moraxella

Haemophilus

Haemophilus species are tiny Gram-negative coccobacilli, often fastidious and requiring specific growth factors. Most are nonpathogenic, but H. influenzae and H. ducreyi are notable pathogens.

• Characteristics: Facultative anaerobes, oxidase positive, optimal growth in 5–10% CO2

• Growth requirements: Hematin (X factor), NAD (V factor)

• Isolation: Chocolate agar, horse blood agar with bacitracin

• Identification: X and V factor requirements, ALA porphyrin test, quad plates, hemolysis, biochemical tests

Haemophilus influenzae

• Typeable strains: Encapsulated, six serotypes (a–f), serotype b most frequent

• Infections: Meningitis, epiglottitis, cellulitis, septic arthritis, pneumonia

• Non-typeable strains: No capsule, normal flora of upper respiratory tract

• Localized infections: Otitis media, sinusitis, conjunctivitis

• Immunization: Unvaccinated infants and children at risk

Other Haemophilus Species

• H. influenzae biogroup aegyptius: Purulent conjunctivitis (pink eye)

• H. ducreyi: Chancroid, sexually transmitted, not normal flora

• H. hemolyticus: Occasional normal flora, beta hemolytic on sheep blood agar

• H. parainfluenzae: Normal flora, rare infections outside respiratory tract

Moraxella catarrhalis

Moraxella catarrhalis is a Gram-negative diplobacillus, normal flora of the upper respiratory tract but increasingly recognized as an opportunistic pathogen.

• Diseases: Otitis media, sinusitis, pneumonia, endocarditis, bacteremia, wound infections

• Isolation: Nutrient, blood, or chocolate agar

• Biochemical properties: Oxidase positive, DNAse positive, may produce beta-lactamase

• Fails to produce acid from carbohydrates

• Other species: M. nonliquefaciens, M. osloensis are key eye pathogens

Summary Table: Key Laboratory Tests for Differentiation

Test	Purpose	Positive Result	Negative Result
Bacitracin Sensitivity	Identify Group A Strep (S. pyogenes)	Zone of inhibition	No inhibition
CAMP Test	Identify Group B Strep (S. agalactiae)	Arrowhead hemolysis	No arrowhead
Optochin Test	Identify S. pneumoniae	Zone >14 mm	Zone <13 mm
Bile Solubility	Identify S. pneumoniae	Clearing	Turbid

Additional info: The notes cover clinical microbiology topics relevant to bacterial taxonomy, morphology, virulence, pathogenesis, and laboratory identification, focusing on staphylococci, streptococci, Haemophilus, and Moraxella. These are central to chapters on Diversity of Bacteria, Pathogenesis, Skin and Eye Diseases, Respiratory Diseases, and Diagnosing Diseases.