Although the incidence of recently transmitted TB as well as reactivation disease has been decreasing in Arkansas, the major fraction of the decrease has been due to a decrease in the incidence of reactivated infections [7
]. These findings highlight the need to know more about the factors involved in active transmission. Host risk factors for clustering identified in this study included non-Hispanic black race and age < 65 years. Furthermore, a significantly higher proportion of large cluster cases were non-Hispanic black, homeless, less than 65 years old, male sex, smear-positive sputum, excessive use of alcohol, smear-positive, and HIV sero-positive, compared to cases in small clusters all diagnosed within one year. However, being non-Hispanic black and homeless within the past year were the only two host characteristics that were identified as independent risk factors for being in large clusters, representing current transmission. Comparing the genomic content of one of the large cluster strains to that of a non-clustered strain from the same community identified 25 genes that differed between the two strains, potentially contributing to the observed differences in transmission.
Two characteristics are identified as risk factors for clustering of M. tuberculosis
genotypes in our study, both of which, the non-Hispanic black race and younger age are among the previously known risk factors [8
]. It is interesting to observe, in the current study, the differences in host risk factors when comparing clustered cases to non-clustered cases and large cluster cases to small cluster cases. Homelessness, a commonly known risk factor for clustering, was not significantly associated with small clusters; however, it was significantly associated with large clusters. It was found previously that M. tuberculosis
genotype clusters in our study population represent both clusters resulted from current ongoing tuberculosis transmission, such as the large clusters, and clusters resulted from the reactivation of clusters of cases involved in remote tuberculosis transmission, which are more likely to be seen as small clusters [7
]. Our finding suggest that while being non-Hispanic black is a risk factor for both tuberculosis transmission and reactivation, being homeless mainly affect the chance for tuberculosis transmission. Our observation that social/demographic factors (e.g. being non-Hispanic black and homeless) are associated with large clusters, but clinical characteristics (sputum smear positivity, pulmonary cavitary disease, and HIV sero-positivity) are not, despite the inclusion of some large clusters resulting from ongoing outbreaks that might be caused by highly infectious TB cases in the analysis, suggests that social behavioral factors have a more important role in transmission of tuberculosis than does the infectiousness of the source.
Although previously documented and confirmed in this study that host risk factors can play an important role in TB transmission, the ability of M. tuberculosis
to be transmitted from host to host is not well understood. Epidemiologic studies have observed that some strains are more successful in transmission than others [9
]. A large cluster of TB could be explained if the infecting strain has a higher probability of transmission or a higher probability of infection progressing to disease. The mycobacterial cell envelope contains immunomodulatory molecules that are important determinants of intracellular survival and virulence [18
]. Two of the genes (MT1800 and MT1802) affected by an LSP in strain SA201, the more widely transmitted strain, but not SA178, the less successful strain encode proteins that influence properties of the cell envelope [19
]. MT1802 encodes a membrane protein of the MmpL family. Although the function of this member of the MmpL family has not been studied, these membrane proteins are thought to have a function in the transport of lipids across the cell membrane, affecting the structure of the cell envelope [20
]. Six of the 16 genes absent in strain SA201, but present in SA178, were of the PE/PPE gene family. PE/PPE family genes are thought to play a role in maintenance of the latent state through antigenic variation [21
]. Disruption of these genes may decrease the available repertoire of antigens available to the M. tuberculosis
strain, decreasing its ability to remain in a latent state. The other important LSP found in this study was the presence of one of the M. tuberculosis
lipase-encoding genes, lipR
(Rv3084) in strain SA201 and absent in strain SA178.
lipases comprise a diverse class of enzymes that are involved in lipid metabolism and may, therefore, have an important role in tuberculosis pathogenesis. Recently, as a follow-up to our microarray findings, Sheline and coworkers explored the association of LSP in lipR
with patient characteristics using a population-based sample of 665 clinical isolates and found that DNA fingerprinting-clustered cases infected with a lipR
LSP isolate were more often epidemiologically linked than clustered cases infected with a lipR
wild-type isolate [22
]. This finding suggests the usefulness of the genomic comparison conducted in the present study. Further studies are needed to investigate whether the presence or absence of any of these 25 genes is associated with large clusters of tuberculosis cases. This will require a larger number of large cluster strains than is present in ourM. tuberculosis collection.
The 25 genes that differ between the large cluster strain SA201 and the non-clustered strain SA178 identified in this study can serve as a basis for additional functional studies or population-based molecular epidemiologic studies that examine the association of these genetic changes with the ability of M. tuberculosis
to cause persistent clusters of disease.