Remember the Fite: A Case of Cutaneous Mycobacterium fortuitum Infection.

Abstract

To the Editor: Nontuberculous mycobacteria (NTM) constitute mycobacterial species other than those belonging to the Mycobacterium tuberculosis complex and those that do not cause leprosy, and are divided into 2 major subgroups defined by their ability to grow on solid culture media: (1) fast-growing mycobacteria and (2) slow-growing mycobacteria.1–4Mycobacterium fortuitum (M. fortuitum) is a fast-growing NTM, commonly found in natural and processed water, sewage, and dirt.1–4 When cultured, growth is typically evident after 3-to-5 days.1–4 Skin and soft-tissue infections may be acquired by direct inoculation post-trauma, medical injection, surgery, or tattoo placement.1–4 Cosmetic surgeries performed for implantation and/or liposuction are commonly implicated.5 Presentation of skin lesions is highly variable but may include inflammatory nodules, ulcers, abscesses, cellulitis, or draining sinus tracts, often taking 4 to 6 weeks to develop after inoculation.1–4 Owing to the increasing antibiotic resistance, treatment requires multidrug therapy targeted toward culture sensitivities.1–4 A 45-year-old woman with a recent history of abdominoplasty with fat transfer to her hips in the Dominican Republic, postoperative week 4, presented for a nonhealing wound on the right hip (Fig. 1). The wound started 3 weeks before presentation and progressed despite outpatient (doxycycline/amoxicillin) and inpatient (vancomycin/cefepime) antibiotic therapy. Skin punch biopsies were performed for tissue culture and histopathologic evaluation with special stains (including appropriate positive and negative controls) for fungi and bacteria, including acid-fast bacilli. Hematoxylin & eosin–stained sections demonstrated a robust superficial-to-deep dermatitis with panniculitis, comprised of a polymorphous inflammatory infiltrate including diffuse, poorly formed, noncaseating granulomas (Figs. 2A, B). Fite stain highlighted numerous acid-fast bacilli (Fig. 3A), which also demonstrated Gram positivity with weak Periodic acid–Schiff reactivity (Figs. 3B, C). Ziehl–Neelsen (ZN) stain (repeated twice) was negative (Fig. 3D). Treatment with trimethoprim–sulfamethoxazole and levofloxacin was initiated. Tissue cultures later speciated the organism as M. fortuitum.FIGURE 1.: Nonhealing wound on the right hip after abdominoplasty.FIGURE 2.: A and B, Histopathologic sections demonstrate a robust superficial-to-deep dermatitis with panniculitis, comprised of a polymorphous inflammatory infiltrate including diffuse, poorly formed, noncaseating granulomas (hematoxylin–eosin, original magnification ×20 and ×100).FIGURE 3.: A, Fite stain (original magnification ×200). B, Gram stain (×200). C, Periodic acid–Schiff stain (×200). D, ZN stain (×200).NTM histopathology is not species specific and is typically characterized by a suppurative, granulomatous reaction with tissue necrosis.6 Although not considered gold standard, acid-fast staining is a common complementary method used for diagnosis of NTM, as tissue culture can take weeks to show growth, and molecular techniques are not always available and have variable sensitivity.6,7 As reported for M. fortuitum infections, bacilli can be infrequent in tissue sections, and staining affinity can vary from isolate to isolate, with as few as 10% of bacilli being acid-fast.4M. fortuitum characteristically has short, blunt branches, which more often extend at right angles from their point of origin.4 “Acid-fast” refers to microorganisms whose cell wall has a high lipid content of mycolic acids and long-chain fatty acids, which traditionally is considered to cause them to bind and retain the complex basic dye carbol-fuchsin, even after strong decolorization with acid-alcohol.6,8,9 Many mycobacterial species are reported to be much less acid and alcohol fast as compared with M. tuberculosis and are easily decolorized by standard ZN staining.6,8 One modification is the Fite technique, in which peanut oil is used with the deparaffinizing solvent (xylene), minimizing the exposure of the bacterial cell wall to organic solvents and preserving their acid-fastness.6,10 Scientific evidence regarding the efficacy of acid-fast stains (including ZN and Fite) in clinical practice is lacking, specifically concerning their use for NTM detection.6 Fite staining has demonstrated slightly superior sensitivity (74.6%–43.2%) compared with ZN staining (72.7%–21%) in the detection of mycobacteria, including NTM species.4,6–10 Although numbers in the literature are small, the sensitivity of Fite (50%; 1 of 2 cases) has been found to be superior to ZN (33%; 2 of 6 cases) for identification of M. fortuitum specifically.4,7 Of note, Crothers et al10 reported that ZN staining had a sensitivity of 0% for detecting fast-growing mycobacteria versus 31% for slow-growing mycobacteria (P = 0.04). Nogueira et al1 suggest that fast-growing NTM, including M. fortuitum, may be more sensitive to the discoloration process, and detection may be enhanced by staining modification (ie, Fite). To increase sensitivity for the detection of NTM, pathologists could combine different acid-fast bacilli staining methods, including standard ZN stain, Fite stain, Kinyoun stain, auramine–rhodamine stain, and/or immunohistochemical stain (with polyclonal antimycobacterial antibody).10,11 However, a recent survey of 363 US pathologists reported that 68.2% use no more than 1 acid-fast stain in indeterminate cases.11 In this article, we report a case of cutaneous M. fortuitum infection with Fite positivity and ZN negativity, supporting the use of Fite as a first-line acid-fast stain for diagnosis of suspected fast-growing NTM in the skin.