Answer to July 2018 Photo Quiz

Answer: Brain abscess caused by Nocardia farcinica. The brain abscess grew Nocardia farcinica. The Gram stain of the organism showed long, thin, beaded, branching Gram-positive bacilli. The bacterium was partially acid fast with modified Kinyoun staining. The colonies appeared smooth and moist on sheep blood agar, in contrast to the typical dry, chalky, wrinkled colony morphology of other Nocardia species. The organism was identified as N. farcinica by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS), with a log score of 2.173 (Biotyper RTC version 4.0.11) (Bruker Daltonics, Billerica, MA), in our laboratory, and this identification was later confirmed with secA1 gene sequencing in a reference laboratory, with a 100% match to the type strain.

N. farcinica is a common cause of nocardial brain abscesses, 45% of which present as primary abscesses (1). This organism is associated with high mortality rates, as high as 55% in immunocompromised patients despite treatment. Long-term therapy with high-dose prednisone has been shown as a risk factor for Nocardia infection (2, 3). Definitive identification of Nocardia is clinically important because antibiotic susceptibilities differ greatly within the genus, with N. farcinica being resistant to multiple antibiotics, including third- and fourth-generation cephalosporins, doxycycline, tobramycin, and clarithromycin (4).

In most clinical laboratories, Nocardia isolates are sent to a referral laboratory with molecular capabilities for 16S rRNA or secA1 gene sequencing or, more recently, multilocus sequencing analysis (MLSA) (5, 6). Although these techniques provide accurate species identification, the long turnaround time can delay treatment, resulting in poor patient outcomes (7). Recent studies have evaluated the use of MALDI-TOF MS to identify Nocardia. Currently, Nocardia species are absent in the FDA-cleared databases of Bruker and bioMérieux. It has been shown that database enhancement through library construction can overcome this limitation, especially for the low-prevalence species (6, 8, 9). Modified extraction procedures using heat or silica beads improve sensitivity (8). The use of young colonies after 18 to 48 h of growth yields better results than the use of colonies from older culture (9).

Based on the identification of N. farcinica by MALDI-TOF MS, the antibiotic regimen of our patient was changed to linezolid and trimethoprim-sulfamethoxazole. The patient remained stable until an episode of heart failure exacerbation a month later, and he ultimately passed away. The N. farcinica isolate was confirmed to be susceptible to amikacin, amoxicillin-clavulanic acid, moxifloxacin, linezolid, and trimethoprim-sulfamethoxazole, intermediate to ceftriaxone, imipenem, doxycycline, and minocycline, and resistant to ciprofloxacin, tobramycin, and clarithromycin by broth microdilution (10).

See https://doi.org/10.1128/JCM.01207-16 in this issue for photo quiz case presentation.