Department of Microbiology Bacteraemia Guideline

Staphylococcus aureus bacteraemia

• Aim
• Background
• About Staphylococcus aureus
• Antimicrobial susceptibilities
• Clinical diferrential diagnosis
• Antimicrobial treatment
• Supplementary Investigations

Quick reference guide to the management of Staphylococcus aureus bacteraemia

This document provides guidelines for doctors on the management of patients with confirmed bacteraemias (blood cultures). This document is supplementary to, and should be used in conjunction with, the antimicrobial guidelines.

Species: Staphylococcus aureus

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Aim

The aim of this guideline is to:

• Provide education to junior microbiology registrars
• Support communication of blood culture results from microbiologists to ward doctors
• Support ward doctors in treating and investigating bacteraemic patients

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Background

The blood culture process: Timings of culture, identification, susceptibility tests and clinical liaison.
How to use this guideline: This guideline should be used to help in the management of patients with a confirmed bacteraemia. The guideline should be used to support interaction with specialist advice e.g. Microbiology.

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About Staphylococcus aureus

S. aureus is a gram positive cocci arranged on a Gram stain as single cells, pairs, tetrads and short chains, but appear predominantly in grape like clusters. S aureus is present in the nose of 30% of healthy people and may be found on the skin. It causes infection most commonly at sites of lowered host resistance, such as damaged skin or mucous membranes.

S. aureus is an important human pathogen and can cause severe infections. Most strains contain a number of virulence factors. Some strains also produce toxins such as Toxic Shock Syndrome Toxin (TTST-1 – a superantigen that is responsible for toxic shock syndrome) and Panton Valentine Leucocidin (PVL – associated with necrotizing pneumonia and some complicated skin and soft tissue infections).

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Antimicrobial susceptibilities

Most strains of S. aureus are resistant to benzylpenicillin due to production of the enzyme penicillinase (a beta-lactamase). Flucloxacillin is stable to this enzyme as are cephalosporins and beta-lactamase inhibitors (such as clavulanic acid and tazobactam).

MRSA (methicillin-resistant S aureus) are resistant to all beta-lactam agents determined by the presence of penicillin binding protein 2a. Glycopeptides (e.g. vancomycin and teicoplanin) and newer agents such as daptomycin and linezolid have activity against MRSA.

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Clinical differential diagnosis

S. aureus is rarely a contaminant of blood cultures and a positive blood culture should always be defined as clinically significant.

The finding of S. aureus in a blood culture should prompt immediate and careful clinical assessment to identify any site of invasion and deep-seated metastatic focus of infection.

The differential diagnosis includes:

• Skin and soft tissue infection e.g. cellulitis/necrotising fasciitis
• Septic arthritis
• Osteomyelitis
• Mastitis
• Pneumonia
• Endocarditis
• Intravenous catheter-related infections
• Brain abscess/subdural empyema
• Prostatitis
• Abscess e.g. renal, abdominal
• Parotitis
• Infected prosthetic material
• Deep spinal infection
• Pyomyositis e.g. psoas abscess

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Antimicrobial treatment

The table below outlines some of the common infections associated with each of the clinical syndromes.

Patients should receive a minimum of 14 days of antimicrobial therapy but duration will depend on source identified and patients with deep seated infections will require a long course if antibiotics.

Oral switch is dependent on source of bacteraemia. Please discuss with Microbiology.

There is strong evidence to suggest that prompt removal or drainage of infected foci improves outcome.

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Supplementary Investigations

It is recommended that all patients with S aureus bacteraemia should be considered for a transthoracic echocardiogram. Transoesophageal echocardiogram may be required in those at high risk of endocarditis (i.e. with abnormal native heart valves or a prosthetic valve), signs of embolic phenomena, or if S. aureus bacteraemia persists with no identified focus of infection.

Consider further investigations as appropriate to source of infection, please see relevant guidelines.

Further Action:

Patients who are confirmed or previously known MRSA positive should be managed in source isolation as per LTHT isolation policy.