Staphylococcus (Bacteriology) (Medical Microbiology and Infection)

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Staphylococci are part of the normal flora and important human pathogens. There are more than 26 species but only a few are pathogenic. Staphylococcus aureus is the most invasive species, which can be differentiated from other species by its possession of the enzyme coagulase.

Staphylococcus aureus

Asymptomatic carriage of S. aureus is found in up to 40% of healthy people, in the nose, skin, axilla or perineum. This is important in healthcare workers especially if they carry an invasive or resistant strain (e.g. MRSA).

Pathogenesis

Staphylococcus aureus has many potential pathogenicity determinants such as coagulase, which catalyses the conversion of fibrinogen to fibrin thereby providing protection.

Determinant

Activity

Effect

Coagulase

Converts fibrinogen to fibrin

May form protective barrier

Adhesion molecules


Bind fibronectin and collagen

Assist adherence

Lytic enzymes

Lipase

Breaks down host tissue

Protein toxins

Panton-Valentine leucocidin (PVL) toxin

Tissue damage

Toxic shock syndrome toxin (TSST)

Shock and toxicity

Enterotoxins

Diarrhoea

Biofflm formation

Slower growth in an extracellular matrix

Difficult to treat with antibiotics, adhere to plastics

Clinical importance

Staphylococcus aureus causes a wide range of infectious syndromes.

• Primary skin infection - impetigo, which is transmitted from person to person.

• Secondary skin infections – associated with eczema, surgical wounds, intravenous devices, burns.

• Pneumonia - rare, but may follow influenza and progress rapidly with cavity .

• Endocarditis – can be rapid and destructive; associated with intravenous drug misuse or colonization of intravenous devices.

• Osteomyelitis.

• Septic arthritis.

Laboratory diagnosis

Staphylococcus aureus grows readily on most laboratory media. Selective medium contains high salt, to which S. aureus is relatively tolerant. Phenotypic identification depends on demonstrating coagulase, catalase enzymes and typical ‘cluster of grapes’ morphology on Gram stain. Typing by molecular means can support interventions to control outbreaks.

Antibiotic susceptibility

The history of the susceptibility of S. aureus is a lesson in the history of antimicrobial chemotherapy.

1 It was initially susceptible to penicillin, but strains that produced β-lactamase soon predominated, so methicillin and related agents (e.g. flucloxacillin) were introduced and replaced penicillin.

2 Methicillin-resistant S. aureus (MRSA) emerged. Resistance is caused by possession of the mecA gene, which codes for a penicillin-binding protein that binds the drug less well. Glycopeptides, such as vancomycin or teicoplanin, started to be required for these strains.

3 Intermediate or heteroresistance to glycopeptides emerged as an increasing issue and fully glycopeptide-resistant strains (GRSA) have now been described, resistance being mediated by the vanA vanB genes acquired from enterococci.

Other antibiotics that remain effective include linezolid, aminoglycosides, erythromycin, clindamycin, fusidic acid, chlo-ramphenicol and tetracycline.

In methicillin-sensitive strains, first- and second-generation cephalosporins are effective. Fusidic acid may be given with another agent; it is often given in bone and joint infections because of its tissue penetration.

Prevention and control

Staphylococcus aureus spreads by airborne transmission and via the hands of healthcare workers. Patients colonized or infected with MRSA or GRSA should be isolated in a side room with wound and enteric precautions. Staff may become carriers and disseminate the organism widely in the hospital environment. Carriage may be eradicated by using topical mupirocin and chlorhexidine.

Staphylococcus epidermidis

Staphylococcus epidermidis is the most important of the coagulase-negative staphylococci (CoNS). Once dismissed as contaminants, these organisms are now recognized as pathogens if conditions favour their multiplication.

Clinical importance

Staphylococcus epidermidis causes infection of intravenous can-nulae, long-standing intravascular prosthetic devices, ventricu-loperitoneal shunts and prosthetic joints, which may lead to bacteraemia or endocarditis and require the removal of the prosthesis. Biofilm production contributes to their pathogenicity.

Laboratory diagnosis

Staphylococcus epidermidis grows readily on laboratory media; coagulase is not produced. Speciation is by biochemical testing. DNA restriction patterns or other molecular techniques may be needed to determine whether strains are identical. S. epidermidis and other CoNS are common contaminants in blood cultures, requiring careful evaluation of their clinical significance.

Antibiotic susceptibility

This group of organisms is uniformly susceptible to vancomycin and usually to teicoplanin. They may also be susceptible to any of the other agents used for S. aureus infection, but this is unpredictable. Treatment must be guided by in vitro testing.

Staphylococcus haemolyticus

Less common than S. epidermidis, Staphylococcus haemolyticus causes a similar disease pattern. It differs from S. epidermidis in that it causes haemolysis on blood agar. More importantly, it is naturally resistant to teicoplanin; significant infections require vancomycin therapy.

Staphylococcus saprophyticus

These CoNS are a common cause of urinary tract infection in young women. They can be rapidly distinguished from other species by their resistance to novobiocin.

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