Staphylococci

Staphylococci are a diverse group of bacteria with considerable clinical significance. Their ability to cause a wide range of infections, combined with various virulence factors and increasing antibiotic resistance, underscores the importance of accurate identification and effective management strategies. A thorough understanding of their characteristics, pathogenic mechanisms, and laboratory diagnosis is essential for healthcare professionals to combat infections caused by these opportunistic pathogens.

General Character

• Genus: Staphylococcus
• Family: Staphylococcaceae
• Gram Staining: Staphylococci are Gram-positive, appearing purple under a microscope due to their thick peptidoglycan layer.
• Shape and Arrangement:
  • Shape: They are spherical (cocci) in shape.
  • Arrangement: Staphylococci are typically found in clusters resembling grapes due to division in multiple planes without separation.
• Oxygen Requirements: Staphylococci are facultative anaerobes. This means they can grow in both the presence and absence of oxygen, allowing them to inhabit diverse environments, including human skin and mucosal surfaces.

Morphology

• Cell Wall Structure:
  • Composed of a thick peptidoglycan layer (approximately 30-80% of the cell wall) provides structural integrity and helps resist osmotic pressure.
  • It contains teichoic acids that play roles in cell wall maintenance and regulation of cell growth.
• Capsule:
  • Some species, particularly Staphylococcus aureus, possess a polysaccharide capsule that helps evade phagocytosis by immune cells.
• Surface Structures:
  • Slime Layer: Contributes to biofilm formation, particularly in coagulase-negative staphylococci (CoNS), enhancing adherence to surfaces like catheters and prosthetic devices.
• Flagella: Staphylococci are non-motile, lacking flagella and using other mechanisms for adherence and colonization.

Cultural Characteristics

• Growth Media:
  • Blood Agar: Supports the growth of various staphylococcal species and allows for hemolysis observation. S. aureus shows β-hemolysis, while S. epidermidis is usually non-hemolytic.
  • Mannitol Salt Agar (MSA): A selective medium for staphylococci. It contains high salt concentration (7.5% NaCl) and mannitol. S. aureus ferments mannitol, resulting in a yellow colour change, while S. epidermidis does not ferment it and remains red.
• Colony Appearance:
  • Colonies of S. aureus are typically golden-yellow due to carotenoid pigments, while S. epidermidis colonies are usually white or off-white.
• Temperature and pH Range:
  • The optimal growth temperature is around 37°C, but they can grow between 15°C and 45°C.
  • They can tolerate a broad pH range, but neutral pH is preferred for optimal growth.

Biochemical Reactions

• Catalase Test: Staphylococci produce the enzyme catalase, which converts hydrogen peroxide into water and oxygen, resulting in bubbling. This distinguishes them from streptococci, which are catalase-negative.
• Coagulase Test:
  • S. aureus: Positive for coagulase, causing plasma to clot.
  • Coagulase-negative Staphylococci (CoNS), like S. epidermidis, are usually coagulase-negative.
• Mannitol Fermentation:
  • S. aureus: Ferments mannitol, producing acid and causing a colour change in MSA.
  • S. epidermidis: Does not ferment mannitol.
• Other Biochemical Tests:
  • Oxidase Test: Negative.
  • Urease Test: Variable; most strains of S. saprophyticus are urease-positive, while others are typically negative.
  • DNase Test: Positive for S. aureus, indicating the ability to degrade DNA. This is a differentiating feature for identifying S. aureus.

Pathogenicity

• Virulence Factors:
  • Toxins:
    • Enterotoxins: Heat-stable toxins responsible for food poisoning, leading to gastrointestinal symptoms.
    • Toxic Shock Syndrome Toxin (TSST-1): A superantigen that can cause toxic shock syndrome characterized by fever, rash, and multi-organ failure.
    • Alpha-toxin: A pore-forming toxin that disrupts cell membranes, causing tissue damage and necrosis.
  • Enzymes:
    • Coagulase: Promotes clot formation, helping to shield the bacteria from immune responses.
    • Hyaluronidase: Breaks down hyaluronic acid in connective tissue, facilitating tissue invasion.
    • Lipase: Allows for colonization of sebaceous glands by breaking down lipids.
  • Adhesins: Surface proteins that promote binding to host tissues, critical for colonization and infection.
• Clinical Infections:
  • Skin and Soft Tissue Infections: Commonly cause boils, abscesses, impetigo, and cellulitis.
  • Invasive Infections: This can lead to more severe conditions like endocarditis (infection of the heart valves), osteomyelitis (bone infection), and septic arthritis.
  • Respiratory Infections: S. aureus can cause pneumonia, especially in immunocompromised individuals or those with chronic lung disease.
  • Food Poisoning: Resulting from ingestion of food contaminated with enterotoxins, leading to nausea, vomiting, and diarrhoea.
  • Opportunistic Pathogen:
    • S. epidermidis: Often associated with infections related to medical devices (e.g., catheters, prosthetic joints) due to biofilm formation.
    • S. saprophyticus: A common cause of urinary tract infections, particularly in young women.

Laboratory Diagnosis

• Specimen Collection: Obtaining samples from infected sites (e.g., skin lesions, abscesses, blood, urine). The choice of specimen depends on the clinical presentation.
• Microscopic Examination:
  • A Gram stain of the specimen reveals Gram-positive cocci in clusters.
  • This initial finding guides further testing and identification.
• Culture Techniques:
  • Inoculation: Samples are inoculated on blood agar and MSA to promote growth and identify species.
  • Incubation: Cultures are typically incubated at 35-37°C for 24-48 hours.
  • Colony Morphology Examination: Observing colony colour and morphology aids in preliminary identification.
• Biochemical Testing:
  • Catalase Test: Distinguishes staphylococci from streptococci.
  • Coagulase Test: Essential for differentiating S. aureus from CoNS.
  • Mannitol Fermentation Test: Further identifies S. aureus based on mannitol fermentation.
  • Additional tests may include the DNase and urease tests, depending on the suspected species.
• Molecular Methods:
  • Polymerase chain reaction (PCR) and other nucleic acid amplification techniques can provide rapid and specific identification of staphylococci and detection of antibiotic resistance genes.
  • These methods are especially useful in outbreaks or cases of severe infections.

Antibiotic Resistance

• Methicillin-resistant Staphylococcus aureus (MRSA): A significant public health concern due to its resistance to beta-lactam antibiotics. MRSA strains are often associated with severe infections and are prevalent in both hospital and community settings.
• Vancomycin-Resistant Staphylococcus aureus (VRSA): The emergence of strains resistant to vancomycin complicates treatment options.
• Coagulase-negative Staphylococci: Increasingly recognized for their resistance to multiple antibiotics, posing challenges in treating infections associated with implanted medical devices.