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Oxford University Hospitals NHS Foundation Trust
Microbiology

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Antimicrobial susceptibility testing and microbial identification

The Microbiology Department is divided into a number of sections.

Level 7 of the John Radcliffe is home to:

  • autoclaving
  • microbiology specimen reception
  • antibiotic susceptibility testing / bacterial identification (AST/ID)
  • specialised bacteriology
  • general bacteriology
  • the containment level 3 laboratory.

Viral serology and the molecular laboratory are situated on Level 6, although the immunoanalysers used by viral serology are part of the Core Automated Laboratory on Level 4.

This section of the laboratory performs antimicrobial susceptibility testing (AST) and microbial identification (ID) for the whole laboratory.

Susceptibility testing assists the clinician in the choice of antimicrobial agents for therapeutic or prophylactic use in the treatment or prevention of microbial infection. Often, treatment is started empirically before results are available and the susceptibility test result may help to explain a treatment failure and indicate appropriate alternatives. The choice of agents used for empirical therapy is influenced by the antimicrobial susceptibility of local isolates that have recently been tested. In addition, testing is of value for national surveillance purposes.

OUH Microbiology has three BD Phoenix™ analysers that process the bulk of the AST work and a Bruker Microflex™ MALDI-TOF that performs the majority of the microbial ID. Organisms not able to go on the BD Phoenix have a manual susceptibility test following the EUCAST methodology and guidelines:

It is important that identification and susceptibility are reviewed together as this may affect the final interpretation.

Historically microbial ID has been labour-intensive and time consuming. However, in Oxford we use the modern technology of the MALDI-TOF mass-spectrometer. Bacterial ID can now be performed on a single colony and a result obtained, in most cases, within minutes. The quick and precise ID then allows us to tailor the AST accordingly.

The BD Phoenix automates susceptibility testing of commonly isolated bacteria in the laboratory. It takes an average of 16 hours for a result to be produced. It is important that results are available as rapidly as possible reducing unnecessary use of broader spectrum agents and permiting more specific, targeted treatment. This helps to discourage the emergence of resistant bacterial strains and is beneficial to the patient.

There are a number of antibiotic panels used on the machines depending on the type of bacteria isolated, enabling the laboratory to test organisms against the antibiotics most likely to be used when treating infection.