Lecture Notes on Staphylococcus and Related Organisms

Overview of Staphylococcus, Micrococcus, and Related Organisms

  • Discusses catalase positive gram positive cocci,

  • Focuses primarily on Staphylococcus species.

Staphylococcus General Characteristics

  • Non-motile and non-spore forming organisms.

  • Associated with human infections as they are normal microbiota of skin and mucosal surfaces.

Gram Stain Characteristics

  • Staphylococcus species appear as gram positive cocci on gram stain:

    • Possess a thick layer of peptidoglycan in their cell wall.

    • Retain crystal violet stain effectively.

    • Arranged in clusters resembling grape-like formations: identified as "gram positive cocci in clusters."

    • Typically large cocci, easily recognizable on smear.

Clinically Significant Staphylococcus Species

Major Species

  1. Staphylococcus aureus

    • Major focus of clinical significance.

    • Associated with skin and soft tissue infections.

    • Not limited to skin infections; also causes:

      • Toxic shock syndrome

      • Food poisoning

      • Scalded skin syndrome

  2. Staphylococcus saprophyticus

    • Primarily linked to urinary tract infections in sexually active young women.

    • Not part of normal flora; considered pathogenic when found.

  3. Staphylococcus epidermidis

    • Normal skin flora but a common cause of nosocomial (healthcare-acquired) infections.

    • Often a contaminant in wound infections and blood cultures if the collection site wasn't cleaned adequately.

  4. Staphylococcus lugdunensis

    • Associated with nosocomial infections.

    • Similar virulence to S. aureus, potentially severe.

Terminology and Reporting

  • Informally referred to as "staph."

  • Use abbreviated terms in reports, avoiding slang to maintain clarity and professionalism.

Growth Media for Staphylococci

  • Staphylococci thrive on:

    • Chocolate agar

    • Sheep blood agar

    • Thioglycolate broth

Selective Media

  • Mannitol salt agar

    • Inhibits most bacteria; S. aureus ferments mannitol, producing a yellow halo.

  • PEA agar (Phenylethyl alcohol agar)

    • Inhibits gram negative organisms; selects for gram positives.

  • CNA agar (Colistin-Nalidixic Acid agar)

    • Also selects for gram positive cocci.

  • Chrome agar

    • Selective for certain clinically important species, including MRSA (methicillin-resistant Staphylococcus aureus). Produces specific color changes indicating growth.

Identification Tests for Staphylococcus

Biochemical Tests

  1. Catalase Test

    • Differentiates Staphylococcus (catalase positive) from Streptococcus (catalase negative).

    • Positive test produces bubbling with hydrogen peroxide due to catalase action.

    • Reaction: H<em>2O</em>2<br>ightarrowH<em>2O+O</em>2ext(bubblesindicatepositiveresult)H<em>2O</em>2 <br>ightarrow H<em>2O + O</em>2 ext{ (bubbles indicate positive result)}

  2. Coagulase Test

    • Tests for enzyme coagulase that clots plasma:

      • Free coagulase: detected in tube test; positive if clot forms after 4-24 hours in rabbit plasma.

      • Bound coagulase: detected in slide test by simultaneous mixing with plasma; positive if agglutination occurs.

Detailed Examination of Staphylococcus aureus

Characteristics

  • Non-motile, non-encapsulated, aerobic (or facultative anaerobe).

  • Most virulent among Staphylococcus species.

  • Distinct morphology on blood agar:

    • Appears buttery, creamy yellow with beta hemolysis (complete lysis of red blood cells).

Reservoirs and Transmission

  • Primary reservoirs include:

    • Nostrils (nares)

    • Other mucosal surfaces: vagina, axillae, pharynx.

Carriers

  • Persistent carriers: harbor a single strain.

  • Intermittent carriers: harbor different strains over time.

  • Non-carries: do not harbor organisms.

Infection Acquisition

  • Generally occurs due to trauma to sterile sites:

    • Types of trauma include burns, rashes, minor cuts, needle sticks.

  • Predisposing conditions:

    • Chronic infections, indwelling devices, skin injuries, immune defects.

Infections Caused by S. aureus

Skin Lesions

  1. Furuncle (Boil)

    • Painful infections surrounding hair follicles filled with pus.

  2. Carbuncle

    • Cluster of boils, more severe, deeper, usually leaves scars.

  3. Folliculitis

    • Mild inflammation around hair follicles.

  4. Bullous Impetigo

    • Highly contagious, seen in children.

  5. Scalded Skin Syndrome (Ritter's disease)

    • Visible skin shedding due to exfoliative toxin; severe in newborns, associated with hypersensitivity.

  6. Toxic Shock Syndrome (TSS)

    • Caused by super absorbent tampons; symptoms include high fever, rash, diarrhea, can lead to DIC.

    • Potential fatality rate of 2-5% due to multi-organ failure.

  7. Food Poisoning

    • Caused by enterotoxins (A, B, C, D) found in contaminated food.

    • Symptoms appear 2-8 hours after ingestion; nausea, vomiting; resolved within 24-48 hours.

Other Infections

  • Pneumonia: Often secondary to influenza infections.

  • Bacteremia: Common in IV drug users, often presents with fever.

  • Osteomyelitis: Infects bones, often follows bacteremia.

  • Arthritis: Occurs with exposure to joints.

Virulence Factors of S. aureus

  1. Capsule

    • Aids in inhibiting phagocytosis; acts as a biofilm to adhere to surfaces.

  2. Cell Wall Characteristics

    • High peptidoglycan content activates immune responses; leads to tissue damage.

  3. Protein A

    • Binds immunoglobulins, evading immune response by binding complement.

  4. Exotoxins (Hemolysins)

    • Alpha, beta, delta, and gamma toxins; cytotoxic effects on red blood cells and tissues, promoting necrosis and inflammation.

  5. Superantigens: Enterotoxins activate non-specific T cells leading to cytokine storms and extreme immune reactions.

  6. Teichoic Acids: Contribute to pathogenicity by mediating attachment to host tissues.

  7. Enzymes:

    • Coagulase, hyaluronidase (spreading), lipase (break down skin lipids), beta-lactamase (confers resistance to beta-lactam antibiotics).

  8. PVL (Pantone Valentine leukocidin):

    • Associated with necrotizing skin infections, notably affecting white blood cells.

Conclusion

  • Understanding the pathogenic mechanisms and characteristics of Staphylococcus species is essential for identification and treatment in clinical microbiology settings.