Letter to the editor - Nocardia identification.

To The Editor,

This letter is with regards to “Disseminated Nocardia cyriacigeorgia causing pancreatitis in [1]. The author(s) of that comment had questions about the isolation of the specific Nocardia organism as well as the methods of species identification.

The microbiology lab initially detected a branching Gram-positive bacillus that was modified acid-fast positive, concerning for Nocardia. The specimen was sent out to the University of Texas Health Northeast Testing Lab in Tyler, Texas (USA) for further identification. Partial 16S rRNA gene sequencing revealed that the isolate was a 100% match to Nocardia cyriacigeorgica type strain.

The 16S rRNA gene sequence is composed of 1550 base pairs, and is found universally in all bacteria. The sequence has both conserved and variable regions, allowing for accurate identification and differentiation between bacterial species. Studies have shown that the initial 500 base pair sequence can provide adequate information for identification while improving testing efficiency [2]. For Nocardia isolates, sequencing identification utilizing the first 500 base pairs in reference samples demonstrated 100% accuracy when compared to phenotypic analysis in combination with more extensive DNA sequencing of multiple bacterial proteins. In that study, unknown isolates were able to be identified with 99% accuracy using the first 500 base pairs when compared with reference sample sequences [3]. The reference laboratory (UT Health Northeast) utilizes not only partial 16S rRNA sequencing for Nocardia species identification, but also sequencing of other proteins such as secA or full 16S rRNA sequencing if needed [4].

There is considerable difficulty in identifying atypical pathogens such as Nocardia and Mycobacteria, though high clinical suspicion in an immunocompromised patient can prompt investigators to pursue additional specialized testing. Sequencing techniques utilizing the 16S rRNA gene can provide relatively rapid and accurate identification of these organisms, leading to timely and appropriate antibiotic therapy.