Department of Microbiology Bacteraemia Guideline

Streptococcal Bacteraemia

Quick reference guide to the management of streptococcal bacteraemia

• Aim
• Background
• Bacteriological differential diagnosis
• Clinical differential diagnosis
• Technical issues
• Antimicrobial treatment
• Supplementary investigations

This document provides guidelines for [state target audience: users of the guidelines]  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: Streptococcus

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

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.

Bacteriological differential diagnosis

Streptococci are normally seen as Gram positive cocci in chains or pairs on Gram stain. Whilst morphology of chains may give an indication of the streptococcal species, it is unreliable and culture results the next day should be used to confirm the diagnosis.

Common Streptococcal species

 Beta –haemolytic streptococci:

• S. pyogenes (LancefieldGroup A) 
• S. agalactiae (Lancefield group B)
• S. dysgalactiae (Lancefield Group C &G) 
• Enterococci (Lancefield Group D)

 Alpha haemolytic streptococci including

• S. pneumoniae
• Viridans (oral) streptococci a composite term encompassing a number of different streptococcal species including S. mitis/mutans/salivarius and sanguinis.

Uncommon bacteriological diagnoses

• Streptococcus anginosus is a group composed of S. intermedius, S. constellatus, and S .anginosus.  This group wasformally known as S. milleri and is associated with abscess formation e.g. liver abscess.

Certain subspecies of S. gallolyticus/infantarius (formally part of the S. bovis group) may be associated with colonic malignancy/endocarditis

Clinical differential diagnosis

There is a wide differential diagnosis for Streptococcal infections; this reflects the varied site and numerous species that may be cultured. A full systems review is recommended to identify the source of a streptococcal bacteraemia. The differential diagnosis includes:

• Pneumonia
• Peritonitis
• Osteomyelitis
• Urinary tract infection
• Intra-abdominal infection
• Skin and soft tissue infection e.g. cellulitis/necrotising fasciitis
• Septic arthritis
• Meningitis
• Endocarditis 
• Puerperal sepsis
• Contamination:  “viridans streptococci” can occasionally be contaminants.

Repeat negative blood cultures whilst off antibiotics are required to confirm a diagnosis of contamination. This is essential as viridans streptococci are a common cause of endocarditis.

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. Some more unusual gram positive cocci may appear indistinguishable on gram stain. This should be considered in the context of the clinical diagnosis. 
Additional laboratory tests available

Streptococci are catalase negative Gram positive bacteria that are arranged in pairs or chains. The laboratory will perform a number of different tests to further identify streptococcal species. (There are a number of different ways to further identify streptococci including assessment of type of haemolysis, Lancefield grouping, biochemical tests, mass-spectrometry and 16S PCR).

Antimicrobial treatment

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

Supplementary Investigations

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

Further Action:

Notification to the local health protection unit is required for invasive Group A Streptococcal infections and Meningitis with S. pneumoniae.

References

1. BNF
2. EMC