Thermostable nuclease: a study of clinical impact

Abstract

Sir, Bacteraemia due to Staphylococcus aureus is associated with a high morbidity and mortality. Early detection and treatment is critical for successful management. Unfortunately, coagulase-negative staphylococci (CoNS) are the most frequent isolates grown from blood cultures, many of which are considered skin contaminants. A test that rapidly and accurately discriminates the two would potentially allow earlier treatment of S. aureus, while reducing unnecessary antimicrobial use. Thermostable nuclease (TSN) is a rapid test (requiring 2– 4 h) that differentiates between S. aureus and CoNS, as S. aureus produces a nuclease that is uniquely and consistently thermostable. 1 TSN testing involves removing 2 –3 mL of blood broth from a blood culture and heating the blood broth in a boiling water bath for 15 min. Once cooled, two to three drops are placed in a 6 mm well cut in the media (Southern Group Laboratory, Corby, UK) and the plate is incubated for 2– 4 h at 378C. A positive reaction, indicating thermonuclease activity, shows an area of clearing at the edge of the well. 1 Addenbrooke’s Hospital is a tertiary referral hospital with 1100 beds. The impact of the TSN test on the immediate management of positive blood cultures was assessed for the calendar month of August 2007. Blood cultures were processed using