05: Staphylococci

Question 1

Give a general description of the Staphylococci spp.

Answer 1

• Members of the Micrococcaceae family: S. aureus, epidermis & saprophyticus.
• Extracellular, pyogenic pathogens (produce pus, abcesses).
• Grouped as coagulase positive (S. aureus) or negative (all others).
• Nonsporulating and nonmotile, gram positive cocci in grape-like clusters.
• Extremely hardy; survive for prolonged periods of time on environmental surfaces.

Question 2

Describe the laboratory identification of Staphylococcus.

Answer 2

• Gram-positive cocci in grape-like clusters
  
  • Coccus = spherical shape
• Form soft, round convex colonies on agar
  
  • S. aureus colonies are beta-hemolytic: complete lysis of RBCs; area appears lightened (golden) and transparent
• All staphylococci are catalase positive (test with H₂O₂).
  
  • Coagulase and mannitol (fermentation) tests distinguish between S. aureus and S. epidermis

Question 3

Describe the structural components of S. aureus.

Answer 3

• Cell envelope composed of microcapsule and cell wall consisting of alternating units of N-acetylglucosamine and N-acetyl muramic acid.
• Additional components of cell wall: lipoteichoic acid and family of structurally related surface proteins that facilitate bacterial adherence to host cell surfaces (MSCRAMMs: microbial surface components recognizing adhesive matrix molecules)
  
  • Fibronectin, fibrinogen & collagen binding proteins ==> tropism for organism to infect particular tissues (e.g., collagen binding protein –> bones & joints)
• Peptidoglycan + lipoteichoic acid = bacterial components responsible for inducing sepsis syndrome.

Question 4

Describe the enzymes produced and secreted by Staphylococci.

Answer 4

• Catalase: Converts hydrogen peroxide to water and oxygen
• Coagulase: Converts fibrinogen to fibrin; primary test to distinguish S. aureus (+) from other Staph species (-).
• Hyaluronidases: Hydrolyze hyaluronic acids and contribute to tissue breakdown and spread of staph across tissue barriers.
• Beta-lactamases: Hydrolyze the beta-lactam ring of penicillins and cephalosporins (beta lactam antibiotics), thus providing bacterial resistance

Question 5

Describe the toxins produced and secreted by Staphylococci.

Answer 5

• Superantigen family, including toxic shock syndrome toxin-1 (TSST-1), enterotoxins and exfoliative (epidermolytic) toxins (serine proteases that split human skin at upper surface ==> scalded skin syndrome; image below).
  
  • Cause non-specific activation of T-cells –> polyclonal T-cell activation –> cytokine storm –> food poisoning, TSS
• Membrane damaging toxins:
  
  • Leukocidin: associated with soft tissues and necrotizing pulmonary infections
  • Alpha toxin: cytotoxic to host cell membranes; may play role in producing sepsis syndrome

Question 6

Describe the epidemiology of Staphylococcus.

Answer 6

• Humans are the natural reservoir.
• Coagulase negative staphylococci (S. epidermidis & hemolyticus) part of normal skin flora & anterior nares (S. aureus present in nares of 20-40% of normal population; some may also be present in oropharynx).
  
  • Carriage increased in dialysis patients, injection drug users, diabetics & HIV-infected subjects.
• Infection via autoinoculation (most common) or transmission from carrier to patient.
• Staphylococci among most common causes of community and hospital-based infections.
• Increasing number of community-based infections caused by methicillin-resistant staphylococci (e.g., MRSA).

Question 7

Name common diseases caused by Staphylococcus aureus.

Answer 7

• Skin/soft tissue infections
• Bacteremia (bacteria in blood) - sepsis, metastatic seeding
• Endocarditis
• Musculoskeletal infections
• Respiratory tract infections
• Toxin-related diseases
  
  • TSST-1, scalded skin syndrome
  • Food poisoning

Question 8

What is collateral damage? Give an example as it pertains to S. aureus.

Answer 8

Collateral damage refers to the notion that antibiotics have an effect on commensal flora (including Staph), which may result in strains gaining resistance to these antibiotics.

Example: HIV individuals with low CD4 counts are given prophylactic antibiotics, however such antibiotics may cause selection for antimicrobial resistant S. aureus..

Question 9

Describe how infection of Staphylococcus occurs.

Answer 9

• Disease as a result of mechanical breach of skin or mucosal barriers, or elaboration of toxins.
• S. aureus unlikely to cause local/systemic disease in the absence of trauma (albiet minor); rather, they persist as commensals.
• Coagulase positive & negative bacteria frequently cause prosthetic device (e.g., intravascular catheter) related infections (reduced inoculum in presence of foreign material).
• Establishment of infection: adherence –> colonization –> invasion –> spread –> host response.
• Staphylococci elaborate enzymes providing nutrients for the bacteria and facilitate spread to adjacent tissue and other organs.
• Staphylococci are noted for their ability to spread to other tissues, establishing metastatic foci of infection.
• Primary host response: polymorphonuclear leukocyte (PMN) infiltration –> vascular thrombosis & tisse necrosis –> abscess formation.

Question 10

Describe toxic shock syndrome toxin 1 (TSST-1).

Answer 10

• TSST-1 is a 22kDa exotoxin protein whose expression is subject to regulatory control.
• TSST-1 gene (tst) is on the chromosome and appears to be part of a mobile element.
• S. aureus isolates from menstruation-associated TSS express toxin (>95%).
• In nonmenstrual isolates, ~50% express TSST-1. Other structurally-related enterotoxins also cause TSS in this setting.
• Sx: sunburnt rash, strawberry tongue

Question 11

Describe the mechanism of superantigen-mediated diseases (e.g., TSS).

Answer 11

• Superantigens are T cell mitogens (encourage cells to divide).
• Disease is due to ability of these toxins to bind antigen presenting cells’ MHC II molecules outside the peptide groove.
• Superantigens then bind T cells via the variable region, resulting in massive T cell activation and the release of large quantities of cytokines, including:
  
  • IL-1
  • IL-2
  • TNF (tumor necrosis factor)
  • Interferon gamma
• Cytokines migrate to vascular endothelium, causing capillary leak and hypotension.
• Simultaneously, liver damage results in decreased endotoxin clearance, which magnifies cytokine release.
• Results in multiorgan disease similar in clinical presentation to septic shock (significant morbidity & mortality).

Question 12

Describe the mechanism of staphylococcal food poisoning.

Answer 12

• Results from ingestion of heat-stable enterotoxin (does not require presence of viable staphylococci).
• Enterotoxins stimulate vagus nerve and CNS vomiting system; also increase peristalsis; intestinal inflammation –> diarrhea.
• Active site is distinct from site inducing TSS.
• Rapid onset of sx after ingestion (~6hr), but self-limited (~24hr).

Question 13

Desribe the coagulase negative staphylococci.

Answer 13

• Relatively avirulent bacteria.
• Part of normal skin flora.
• S. epidermidis is most common pathogen among coagulase negative species.
• Unique niche for infections: prosthetic devices (e.g., IV catheters, prosthetic heart valves, prosthetic hips).
  
  • In part due to elaboration of extracellular polysaccharide known as slime or biofilm.
• Frequent contaminant in cultures.

Question 14

Describe the treatment and prevention of Staphylococcus aureus infections.

Answer 14

• Surgically drain abscesses, remove infected prosthetic material
• Treat with appropriate antibiotics:
  
  • Beta-lactam antibiotics (e.g., penicillins) ==> resistance called Methicillin-Resistant SA (MRSA)
  • Vancomycin ==> resistance called Vancomycin-Resistant SA (VRSA); treat with daptomycin or linezolid
• Prevent by eliminating colonization in high risk individuals (topical application of antibiotics to nares); potential staphylococcal vaccines and antibodies currently under investigation.