Download Staphylococcal and streptococcal infections

Staphylococcal and streptococcal infections
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.