LAUSR

We are grateful for the comments made by Dr Keikha on our clinical case published in Acta Neurologica Belgica. Dr Keikha requests additional information about two aspects that are not clearly explained in our article: the isolation method used for the diagnosis of Nocardia sp. infection, and the techniques employed to identify the species. This information is provided below. The genus Nocardia comprises a heterogeneous group of aerobic microorganisms which are yet to be fully characterised. The first strain was isolated by the French veterinarian Edmond Nocard in 1888 during a bovine farcy epidemic affecting cattle on the island of Guadeloupe [1]. Eppinger subsequently reported the first case of nocardiosis in humans in 1890 [1]. A microbiological analysis is essential to confirm suspicion of Nocardia infection; serology and cutaneous hypersensitivity testing are of no use in these cases [2]. This microorganism grows in most non-selective bacterial culture media [2]. In semi-selective culture media, however, overgrowth of commensal flora may cause false-negative results. The use of selective media, such as Thayer-Martin agar, improves diagnostic performance [1, 2]. Nocardia species may take between 2 days and several weeks to form colonies; cultures should therefore be incubated for at least 3 weeks before a negative result may be reported [1]. Colonies normally form after 3–5 days, displaying the typical characteristics; they normally have a chalky white or cotton ball appearance, and are dry, adhere to the agar, and have a characteristic mould-like smell [2]. Biochemical studies have traditionally been used to identify the different species of Nocardia [1]. At present, the diagnostic techniques of choice for Nocardia infection are molecular biology techniques including restriction fragment length polymorphism (RFLP), and especially sequencing of the rrs gene, which codes for 16S ribosomal ribonucleic acid (rRNA) [1, 3]. Other genes, including segments of the hsp65, secA1, gyrB, rpoB, or sodA genes (the latter, which codes for the enzyme superoxide dismutase, is the most recently identified genetic target), have been used as new genetic targets in the characterisation of Nocardia using the polymerase chain reaction (PCR) [1, 3]. In our case, the microbiological diagnosis was based on a biopsy of surgically resected brain tissue. The specimen was inoculated onto blood, chocolate, and Thayer-Martin agar plates, which were incubated at 37 °C for 10 days under aerobic conditions. Culture yielded a presumptive Nocardia species based on colony morphology and Gram stain results (Gram-positive, filamentous, branching structures). The bacterium was finally identified as Nocardia farcinica by biochemical methods. Susceptibility testing was performed by disc diffusion using Mueller-Hinton agar. Our case occurred 15 years ago; at that time, a combination of biochemical tests was the most widely used method for identifying Nocardia species. Today, molecular biology techniques and mass spectrometry (MALDI-TOF MS) have dramatically changed the way we identify Nocardia species; these techniques are available in reference laboratories [4]. Co-trimoxazole is the most frequently used antibiotic in the treatment of nocardiosis, although some strains, particularly N. farcinica, are resistant to this drug. Antimicrobial susceptibility testing, therefore, continues to be a highly * Carrasco García de León Sira [email protected]