Bacteriology for surgeons

Microbiology testing is increasingly automated with same-day PCR tests for organisms such as MRSA and C. difficile now widely available. Despite these advances, the pus and tissue samples of most interest to surgeons take at least two days to produce full culture and sensitivity results. There is good evidence that appropriate early antibiotics reduces morbidity in septic shock, and 8 hours delay is associated with an approximately 10-fold increase in mortality. Treatment of severe surgical infections therefore requires an empirical choice of antibiotics, based on knowledge of local flora and resistance rates. If the sample is sent immediately for microscopy, the regimen can be amended on day one if necessary. A basic knowledge of bacterial taxonomy will enable the surgical trainee to understand why different antibiotics are required in different situations, and to refine empirical therapy as supplementary results become available.

The Gram stain

First devised in 1844, this staining technique is still fundamental to bacteriology today. It allows prediction of the cause of infection within minutes, and at minimal cost. Laboratories base subsequent biochemical identification and susceptibility testing on the initial Gram stain of the surgical sample, so it is crucial that samples are collected carefully, and transported promptly.