Related strains of Mycobacterium avium cause disease in children with AIDS and in children with lymphadenitis

Dose response models and a quantitative microbial risk assessment framework for the Mycobacterium avium complex that account for recent developments in molecular biology, taxonomy, and epidemiology

Mycobacterium avium complex (MAC) is a group of environmentally-transmitted pathogens of great public health importance. This group is known to be harbored, amplified, and selected for more human-virulent characteristics by amoeba species in aquatic biofilms. However, a quantitative microbial risk assessment (QMRA) has not been performed due to the lack of dose response models resulting from significant heterogeneity within even a single species or subspecies of MAC, as well as the range of human susceptibilities to mycobacterial disease. The primary human-relevant species and subspecies responsible for the majority of the human disease burden and present in drinking water, biofilms, and soil are M. avium subsp. hominissuis, M. intracellulare, and M. chimaera. A critical review of the published literature identified important health endpoints, exposure routes, and susceptible populations for MAC risk assessment. In addition, data sets for quantitative dose-response functions were extracted from published in vivo animal dosing experiments. As a result, seven new exponential dose response models for human-relevant species of MAC with endpoints of lung lesions, death, disseminated infection, liver infection, and lymph node lesions are proposed. Although current physical and biochemical tests used in clinical settings do not differentiate between M. avium and M. intracellulare, differentiating between environmental species and subspecies of the MAC can aid in the assessment of health risks and control of MAC sources. A framework is proposed for incorporating the proposed dose response models into susceptible population- and exposure route-specific QMRA models.

Association between 16S-23S internal transcribed spacer sequence groups of Mycobacterium avium complex and pulmonary disease

Organisms within the Mycobacterium avium complex (MAC) may have differential virulence. We compared 33 subjects with MAC pulmonary disease to 75 subjects with a single positive culture without disease. M. avium isolates were significantly more likely to be associated with MAC pulmonary disease (odds ratio = 5.14, 95% confidence interval = 1.25 to 22.73) than M. intracellulare.

Genetic diversity of Mycobacterium avium isolates recovered from clinical samples and from the environment: molecular characterization for diagnostic purposes

Isolation of Mycobacterium avium complex (MAC) organisms from clinical samples may occur in patients without clinical disease, making the interpretation of results difficult. The clinical relevance of MAC isolates from different types of clinical samples (n = 47) from 39 patients in different sections of a hospital was assessed by comparison with environmental isolates (n = 17) from the hospital. Various methods for identification and typing (commercial probes, phenotypic characteristics, PCR for detection of IS1245 and IS901, sequencing of the hsp65 gene, and pulsed-field gel electrophoresis) were evaluated. The same strain was found in all the environmental isolates, 21 out of 23 (91.3%) of the isolates cultured from urine samples, and 5 out of 19 (26.3%) isolates from respiratory specimens. This strain did not cause disease in the patients. Testing best characterized the strain as M. avium subsp. hominissuis, with the unusual feature that 81.4% of these isolates lacked the IS1245 element. Contamination of certain clinical samples with an environmental strain was the most likely event; therefore, characterization of the environmental mycobacteria present in health care facilities should be performed to discard false-positive isolations in nonsterile samples, mainly urine samples. Molecular techniques applied in this study demonstrated their usefulness for this purpose.

Mycobacterium avium with the bird type IS1245 RFLP profile is commonly found in wild and domestic animals, but rarely in humans

Cervical lymphadenitis is the main manifestation of Mycobacterium avium infection in immunocompetent children. Exposure to birds has been discussed as a source of infection. To clarify from where children acquire the infection, M. avium isolates from different origins were analysed with restriction fragment length polymorphism (RFLP) on insertion sequence IS1245, and compared by computer cluster correlation analysis. This molecular epidemiological tool has previously revealed a distinction between multiband profiles found mainly in strains from humans, and a 3-band/bird type profile in strains isolated mainly from birds. 32 isolates from children were compared with 28 isolates from adults and 45 isolates from animals. We found that 67% of the animal isolates had the bird type profile, also found in 1 sputum isolate from an adult. Strains from children showed only multiband profiles that did not differ significantly from profiles of isolates from adults. All but 2 bird isolates showed the bird type profile. Neither of the remaining 2, which had multiband profiles, clustered with the isolates from children. Our results indicate that the true reservoir of M. avium is unknown. Thus the question of whether or not M. avium can be incriminated as a zoonotic disease remains unanswered.

Most human isolates of Mycobacterium avium Mav-A and Mav-B are strong producers of hemolysin, a putative virulence factor

Hemolysin was quantified in 58 isolates of Mycobacterium avium from human, animal, and environmental sources. Human Mav-A and Mav-B isolates were the strongest producers; in contrast, animal and environmental Mav-A isolates and human, animal, and environmental Mav-C organisms were low-level producers. Hemolysin production was not restricted to isolates causing invasive infections.

Novel mycobacterium related to Mycobacterium triplex as a cause of cervical lymphadenitis

The Mycobacterium avium complex (MAC) is an important cause of cervical lymphadenitis in children, and its incidence appears to be increasing in the United States and elsewhere. In areas where Mycobacterium tuberculosis is not prevalent, M. avium causes the vast majority of cases of mycobacterial lymphadenitis, although several other nontuberculous mycobacterial species have been reported as etiologic agents. This report describes the case of a child with cervical lymphadenitis caused by a nontuberculous mycobacterium that could not be identified using standard methods, including biochemical reactions and genetic probes. Direct 16S ribosomal DNA sequencing showed greater than 99% homology with Mycobacterium triplex, but sequence analysis of the 283-bp 16S-23S internal transcribed spacer (ITS) sequence showed only 95% identity, suggesting that it is a novel species or subspecies within a complex of organisms that includes M. triplex. Mycolic acid high-performance liquid chromatography analysis also identified this isolate as distinct from M. triplex, and differences in susceptibility to streptomycin and rifampin between this strain and M. triplex were also observed. These data support the value of further testing of clinical isolates that test negative with the MAC nucleic acid probes and suggest that standard methods used for the identification of mycobacteria may underestimate the complexity of the genus Mycobacterium. ITS sequence analysis may be useful in this setting because it is easy to perform and is able to distinguish closely related species and subspecies. This level of discrimination may have significant clinical ramifications, as closely related organisms may have different antibiotic susceptibility patterns.