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Other than “Rhodococcus

      To the Editor:
      Franco-Paredes and Ray report a case of persistent acid-fast positive smear due to Rhodococcus equi.
      • Franco-Paredes C.
      • Ray S.
      Cause of persistent acid-fast positive smears in pulmonary tuberculosis.
      The reported case reminds us of the possibility of non-Mycobacterium species microorganism with positive acid-fast stain. However, we are concerned about 2 important issues that need to be clarified.
      First, the accurate identification of Rhodococcus spp based on conventional biochemical laboratory methods carries the risk of misidentification. Particularly, other members of Actinomycetes, such as Gordonia, and Tsukamurella spp, are difficult to be differentiated from Rhodococcus spp.
      • Savini V.
      • Fazii P.
      • Farvaro M.
      • et al.
      Tuberculosis-like pneumonias by the aerobic actinomycetes Rhodococcus, Tsukamurella and Gordonia.
      In the previous study of 8 patients with Tsukamurella infection and 2 patients with Tsukamurella colonization, all of the 10 clinical isolates were misidentified as Rhodococcus spp and accurately identified as Tsukamurella to the species level by molecular methods.
      • Liu C.Y.
      • Lai C.C.
      • Lee M.R.
      • et al.
      Clinical characteristics of infections caused by Tsukamurella spp. and antimicrobial susceptibilities of the isolates.
      Another study of Gordonia species had similar findings: 15 of 66 clinical isolates initially identified as Rhodococcus spp by conventional methods were further confirmed as Gordonia spp by 2 molecular methods.
      • Lai C.C.
      • Wang C.Y.
      • Liu C.Y.
      • et al.
      Infections caused by Gordonia species at a medical centre in Taiwan, 1997 to 2008.
      As these 3 pathogens can cause similar clinical manifestations of pulmonary infections—like tuberculosis,
      • Savini V.
      • Fazii P.
      • Farvaro M.
      • et al.
      Tuberculosis-like pneumonias by the aerobic actinomycetes Rhodococcus, Tsukamurella and Gordonia.
      it suggests that advanced molecular methods should be required for correct diagnosis to avoid misidentification.
      Second, although Rhodococcus spp can show positive acid-fast smear, it has branching filamentous morphology, and often has a bead appearance. These characteristics under microscope would be helpful for differentiating from Mycobacterium spp. Thus, in this demonstrated case, a more detailed microscopic examination of morphology by an experienced microbiologist may lead to a prompt and correct diagnosis.
      In conclusion, we would like to remind physicians about 2 uncommon pathogens—Gordonia and Tsukamurella—which are hard to differentiate from Rhodococcus by conventional laboratory methods. In addition, the detailed morphologic examinations would be helpful for diagnosis.

      References

        • Franco-Paredes C.
        • Ray S.
        Cause of persistent acid-fast positive smears in pulmonary tuberculosis.
        Am J Med. 2012; 125: e3-e4
        • Savini V.
        • Fazii P.
        • Farvaro M.
        • et al.
        Tuberculosis-like pneumonias by the aerobic actinomycetes Rhodococcus, Tsukamurella and Gordonia.
        Microbes Infect. 2012; 14: 401-410
        • Liu C.Y.
        • Lai C.C.
        • Lee M.R.
        • et al.
        Clinical characteristics of infections caused by Tsukamurella spp. and antimicrobial susceptibilities of the isolates.
        Int J Antimicrob Agents. 2011; 38: 534-537
        • Lai C.C.
        • Wang C.Y.
        • Liu C.Y.
        • et al.
        Infections caused by Gordonia species at a medical centre in Taiwan, 1997 to 2008.
        Clin Microbiol Infect. 2010; 16: 1448-1453

      Linked Article

      • Causes of Persistent Acid-Fast Positive Smears in Pulmonary Tuberculosis
        The American Journal of MedicineVol. 125Issue 12
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          Three months after initiating chemotherapy for pulmonary tuberculosis, the number of acid-fast bacilli in sputum smears are negative in more than 95% of cases.1,2 The main reasons for persistently positive acid-fast smears include nonadherence to treatment and drug resistance.3 In a small number of patients with pulmonary tuberculosis who are receiving effective chemotherapy, the presence of acid-fast staining organisms in smears are due to nonviable bacilli or coinfection4 with other acid-fast organisms, and it may not represent treatment failure.
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      • The Reply
        The American Journal of MedicineVol. 126Issue 7
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          We appreciate the important observations made by Su et al1 regarding the tuberculosis-like pneumonias caused by bacterial organisms of the Actinomycetales order. The Actinomycetales group includes phylogenetically diverse but morphologically similar aerobic and anaerobic actinomycetes such as Actinomyces, Rothia, Williamsia, Gordonia, Tsukamurella, and Rhodococcus.2 As a group, these organisms exhibit filamentous branching structures, which may fragment into rods, or coccoid forms. This morphological variability is one of the reasons why acid-fast bacilli organisms such as Mycobacterium, Tsukamurella, and Rhodococcus (aerobic actinomycetes) may be confusing even for experienced microbiologists.
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