Mycobacterium celatum as a cause of disseminated infection in an AIDS patient. A case report and review of the literature.

Fernando García-Garrote^1, María Jesús Ruiz-Serrano¹ Jaime Cosín¹ Luis Alcalá¹Arturo Ortega² Emilio Bouza¹
Department of Microbiology and Infectious Diseases. Hospital General Universitario."Gregorio Marañón".
² Mycobacteriology Reference Laboratory, Instituto de Salud Carlos III. Madrid. España.

 
    Disseminated infections due to non-tuberculous mycobacteria (NTM) are frequent in HIV infected patients (1). The Mycobacterium. avium complex (MAC) accounts for most non-tuberculous disseminated infections but recently species such as M. genavense, M. simiae and M. haemophilum have emerged as potential pathogens in AIDS patients. Mycobacterium celatum is a recently discovered microorganism causing disseminated infection in immunocompromised patients and could be misidentified (2). We report a case of M. celatum infection in an HIV infected patient.

A 30-year-old male intravenous drug abuser infected with HIV, was admitted to the Hospital with a 7 day history of fever, chest pain, cough and yellowish sputum production and significant weight loss during last month. The CD4 T-cell count was 33 cells/m l. The patient who had not received antiretroviral therapy, had been diagnosed as suffering cerebral toxoplasmosis and bilateral pneumonia two months previously. At the time of the present admission ocular CMV disease was also present. A chest X-ray showed perihiliar bilateral infiltrates.

We processed three sputum specimens, urine and blood cultures in the mycobacteriology laboratory. Sputa and urine were decontaminated by Karlson’s procedure and blood cultures (BACTEC 9240, Becton Dikinson Mycrobiology System, Cockeysville, Md) were processed using lysis-centrifugation treatment (3). AFB were seen in sputum smears. The sputa showed a positive PCR for M. tuberculosis (MTB) (Amplicor, Roche Diagnostics Systems) but the microorganism was not recovered by culture. The urine specimens processed were negative. Blood cultures showed growth of microorganisms which were identified as M. celatum. Small, smooth, dome-shaped colonies, similar to MAC strains, grew only in the blood cultures after 41 days. The isolate was identified using two chemiluminiscent DNA probes (Accuprobe,Gen Probe, San Diego), one specific for MAC and the other for M. tuberculosis, according to the manufacturer’s recommendations. The amount of chemiluminescence emitted from hydrolysis of the DNA-RNA complex was estimated using a PAL luminometer and quantified as relative light units (RLU). The cut-off point was established at 900 RLU. We obtained a negative result with the MAC probe and a weak positive-result with the MTB probe (1010 RLU). To confirm identification, gas-liquid chromatography (GLC) and PRA (PCR and restriction pattern analysis) were kindly performed by at the Mycobacteriology Reference Laboratory of one of the authors. The isolate was identified as M. celatum.

In spite of having received drugs which included rifampin, isoniazid and pyrazinamide, our patient died 15 days after admission. Postmortem examination was not possible.

In 1993, Butler et al described 24 isolates of a new species of mycobacterium and, using 16S rRNA as a basis, proposed the name M.celatum, further differentiated into two genotypes 1 and 2 with a 10-nucleotide difference (4). Disseminated infection in AIDS patients was first reported in Italy and more recently a retrospective study with seven cases has been published by Tortoli et al including the first two isolates (2,5,6). On the basis of 16S rRNA sequencing, all these strains were assigned to the genotype 1. Fourteen clinical isolates causing disseminated infection in six patients have been reported in Great Britain. Analysis of 16S rRNA showed some nucleotide differences from the two genotypes previously described and a new group of M.celatum type 3 was proposed (7). Zurawski et al have reported two cases of pneumonia and bacteremia in AIDS patients in the USA but information about genotypes was not available (8). Only one report of cervical lymphadenitis in an immunocompetent patient has been published (9)

M. celatum is a new species of mycobacterium recently described as a slow growing nonphotochromogenic mycobacterium whose cells are acid-fast, slender and predominantly rod-shaped. It does not form cords or branches. Colonies are predominantly small, smooth, dome- shaped and unpigmented and, like MAC strains, they become pale yellow when cultures are old (8-12 weeks) (4). Identification in the laboratory is problematic because biochemical tests give results similar to those produced by other mycobacteria (M. avium, M. xenopi) (4,5). Besides, DNA probes may also give misleading results because the M. tuberculosis DNA probe sometimes shows cross reactivity with M. celatum. This in turn, leads to false-positive results because the rRNA 16S sequence is quite similar to M. tuberculosis (10). Positive biochemical reactions for M. celatum are 3- and 14-arylsulfatase, heat stable catalase test, tellurite reduction and pyrazinamidase test. M. celatum is negative for niacin production, tween opacity nitrate reduction, hydrolysis of tween 80 and urease activity (4). M. celatum cannot be distinguished from M. avium and M. xenopi by thin-layer cromatography (TLC) but, although the GLC profiles of M. xenopi and M. celatum are very similar, this technique allows them to be differentiated (4) on the basis of the relative amounts of secondary alcohols (2-ecosanol 2-OH-C20:0 and 2-docosanol 2-OH-C22:0).

Presumptive identification should include microscopic "birds nest" colony morphology, inability to grow at 45ºC, a positive 3-day arylsulfatase test and resistance to 2 m g of rifabutin per ml. M. celatum may also be suspected when a nonpigmented mycobacterium behaves like MAC or M. xenopi in biochemical tests and fails in hybridization with a genetic probe for MAC or gives negative or weak positive results with the MTB DNA probe. M.celatum type 2 is the only genotype that has not shown cross reactivity with the commercial DNA-probes for MTB identification (7,10). Another point of interest is the positive result with PCR for MTB. To our knowledge, there is only one previous report in which the Amplicor MTB PCR Detection Kit was used. The fourteen type 3 strains gave negative results (7). Definitive identification must be performed on the basis of chromatographic mycolic patterns and molecular genetic analysis.

Information on the antimicrobial susceptibility of M. celatum is scarce and it has been described as variable in previous reports (4,5). This variability can be attributed to the testing procedures used and the existence of 3 genetic subtypes within the same species (7). It is clear that M. celatum is susceptible to quinolones and clarithromycin and completely resistant to rifampin, capreomycin, isoniazid and pyrazinamide (5). The susceptibility pattern of our strain was determined by the proportions method (11), and it was found to be resistant to isoniazid, rifampin, streptomycin and ethambutol but susceptible to ethionamide.

M. celatum should be added to the NTM causing disseminated infection in AIDS patients. Special alerting for this microorganism is required as it can easily be missed or misinterpreted in the laboratory.

 
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