Department of Microbiology Bacteraemia Guideline

Staphylococcal bacteraemia

• Aim
• Background
• Bacteriological differential diagnosis (Gram guideline only)
• Clinical differential diagnosis
• Technical issues
• Antimicrobial treatment
• Supplementary investigations

Quick reference guide to the management of Staphylococcal 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.

Gram stain: Staphylococcus

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Aim

The aim of this guideline is to:

• Provide education to junior microbiology registrars
• Support communication of Gram stain 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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Bacteriological differential diagnosis (Gram guideline only)

Staphylococcus species are Gram positive cocci arranged on a Gram stain as single cells, pairs, tetrads and short chains, but appearing predominantly in grape-like clusters. They commonly colonise the skin and mucous membranes of humans and animals. They can cause infections or colonise without any associated infection.

Common Staphylococcal species

• Staphylococcus aureus including:
  • Meticillin- sensitive S.aureus (MSSA)
  • Meticillin-resistance S.aureus (MRSA)

S. aureus isan important human pathogen and has been associated with severe infections. It produces virulence factors such as protein A, capsular polysaccharides and α toxin. Some strain of S. aureus produce Toxic Shock Syndrome Toxin (TTST-1), Panton Valentine Leucocidin (PVL) and other toxins. Meticillin-resistance in S. aureus (MRSA) is determined by the presence of penicillin-binding protein, PBP 2a, which confer resistance to all β lactam agents.

• Coagulase negative staphylococci

Coagulase negative staphylococci (CNS) are frequently encountered as culture contaminants, being part of the normal skin colonising bacteria e.g. Staphylococcus epidermidis. However, they also account for approximately 30% of health care-associated bloodstream infections. Most of these infections are associated with intravascular catheters or prosthetic medical devices. Repeat negative blood cultures whilst off antibiotics may be required to confirm a diagnosis of contamination.

Uncommon bacteriological diagnoses

• Micrococcus species. These organisms are found in the environment and as transient flora of the skin of human and other mammals. They produce yellow or pink colonies on agar media and are associated with opportunistic infections e.g. intravascular catheter-related infections
• Rothia species. They are found as coloniser of the human oral cavity. Rare cause of sepsis with cases of opportunistic infections having been reported.

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Clinical differential diagnosis

There is a wide differential diagnosis for Staphylococcal infections. A full systems review is recommended to identify the source of a Staphylococcal bacteraemia. 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
• Pyomyositis e.g. psoas abscess

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Technical issues

Common bacteriological misdiagnoses
Gram positive cocci include Staphylococcus and Streptococcus. It is possible for these genera of bacteria to be mis-identified by Gram stain i.e. what looks like a Staphylococcus may be a Streptococcus.

Additional laboratory tests available

Staphylococci are catalase positive Gram positive bacteria that are arranged in clusters. The laboratory will confirm the identification of Staphylococcus using biochemical tests ( such as detection of coagulase enzyme) and MALDI-TOF analysis. On occasion, molecular diagnosis using 16S PCR has been used for identification.

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Antimicrobial treatment

• Antimicrobial therapy can often be improved with knowledge of the Gram stain result. An antibiotic may be started, stopped, or have its dose changed.
• Antibiotic therapy should always be reviewed the day after the Gram result as bacterial growth will have allowed further speciation of the bacteria, and sensitivity tests to be completed.

History of the following are important to determine appropriate antimicrobial therapy

•  
  • previous staphylococcal infections
  • risk of MRSA including previous colonisation
  • source of infection
  • Allergy status and nature of allergy

The table below outlines some of the common organisms associated with each of the clinical syndromes. Please be aware that streptococci can present in unusual ways, and that this list is by no means exhaustive

CNS=Coagulase negative staphylococcus

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Supplementary Investigations

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

Further Action:

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