Staphylococcal infections
- More than 30 staphylococcal species exist
- All are part of normal skin and mucous membrane flora
- They are either coagulase-positive or negative
- The most important coagulase-positive species is Staph. aureus
Staph aureus
- 30% adults carry Staph. aureus in their anterior nares
- Carriers transfer the organism to skin allowing a portal of entry
- The organism has several putative determinant of pathogenicity including:
- Cell wall constituents
- Cell surface proteins
- Toxins (e.g. haemolysins and leukocidins)
- Enzymes (e.g. coagulase, protease, hyaluronidase)
- The organism is both aerobic and anaerobic on blood agar
- Microscopically it is gram-positive
- Forms clusters on solid media
- There is increasing spread of clones resistant to beta-lactam antibiotics (e.g. MRSA)
- Clinically it produces skin and soft tissue infections including
- Impetigo
- Folliculitis
- Cellulitis
- Deeper infections may occur after trauma or surgery
- Metastatic infection may result in
- Endocarditis
- Pericarditis
- Osteomyelitis
- Lung abscesses
- Treatment is with antistaphylococcal antibiotics (e.g. flucloxacillin)
- In MRSA Vancomycin is the treatment of choice
Coagulase-negative staphylococci
- Staph. epidermidis and Staph. saprophyticus are the commonest human pathogens
- Staph. epidermidis is a common cause of nosocomial bacteraemia
- Often associated with indwelling catheters and prosthetic materials
- Is a common cause of prosthetic valve endocarditis
- Its is often multiply antibiotic resistant
- Treatment may require removal of line or prosthesis
Streptococcal infections
- Streptococci are gram-positive cocci
- More than 30 species have been identified
- On solid media they grow in pairs or chains
- They are catalase negative
- Beta-haemolytic streptococci are classified according to their Lancefield group
- The following are human pathogens
- Strep. pyogenes (group A Streptococcus)
- Group C and G streptococci
- Strep. pneumoniae (pneumococcus)
- Group B Streptococcus
- viridans group streptococci
- Enterococcus
Strep. pyogenes
- Important human pathogen
- Causes various cutaneous and systemic infections including
- Streptococcal pharyngitis
- Scarlet fever
- Rheumatic fever
- Post-streptococcal glomerulonephritis
- The bacteria is sensitive to penicillin
Strep. pneumoniae
- Common bacterial pathogen
- Found in the nasopharynx of 20% of adults
- On a Gram-stain it appears as a diplococcus
- It is alpha-haemolytic on blood agar
- Common cause of localised and systemic infections including
- Otitis media
- Sinusitis
- Meningitis
- Pneumonia
- Endocarditis
- Osteomyelitis
- Infection can be prevented by the pneumococcal vaccine
- Resistance to penicillin is increasing worldwide
Viridans group streptococci
- The viridans group of streptococci are a diverse group of organisms
- They are respiratory, gastrointestinal and oral cavity commensals
- Infection usually occurs in immunocompromised hosts
- Principal virulence trait is to adhere to cardiac valves and cause endocarditis
- Account for 30 - 40% of cases of endocarditis
- Most occur in patients with valvular heart disease
- Other risk factors include:
- Prosthetic heart valves
- Intravenous drug abuse
- Most are viridans streptococcal species are sensitive to penicillin
Enterococcus spp.
- Enterococci are facultative anaerobes
- They are common commensal of the gastrointestinal tract
- They are significant cause of nosocomial infection including
- Urinary tract infections
- Endocarditis
- Intra-abdominal infection
- Risk factors for infection include
- Severe underlying disease
- Previous surgery
- Previous antibiotic therapy
- Renal failure
- The presence of vascular or urinary catheters
- Mortality from enterococcal infection is high
- Intrinsically resistant to beta-lactams and aminoglycosides
- They can also acquire resistance to Vancomycin
- Management of Vancomycin-resistant enterococcus (VRE) is difficult
Bibliography
Herwaldt L A. Staphylococcus aureus nasal carriage and surgical site infections. Surgery
2003; 134 (Suppl 5): S2-S9.
Lowy F D. Staphylococcus aureus infections. N Engl J Med 1998; 339: 520-552. |