Epidemiology of Mycobacterium tuberculosis strains in San Francisco that do not contain IS6110.

IS6110 is commonly used as the basis for molecular epidemiologic and diagnostic studies of Mycobacterium tuberculosis. However, strains that do not contain IS6110 have been reported. If common, such strains would pose a limitation for molecular studies of M. tuberculosis. Analysis of a population-based sample from San Francisco of 1569 specimens submitted for fingerprinting demonstrated that the proportion of strains that lack IS6110 is less than 1%. While this low percentage permits IS6110 fingerprinting in San Francisco, it may be problematic in other settings.

Use of multiple markers in population-based molecular epidemiologic studies of tuberculosis.

SETTING: Many epidemiologic studies of tuberculosis are being conducted worldwide. Fingerprinting with a secondary marker in strains with fewer than six IS6110-hybridizing bands enhances the tracking of strains, but its impact on population-level inferences has not been well studied. OBJECTIVE: To investigate the effects of secondary genotyping for low-copy Mycobacterium tuberculosis isolates with polymorphic guanine-cytosine-rich repetitive sequence (PGRS) on epidemiologic inferences in population-based research settings. DESIGN: For San Francisco tuberculosis cases (1991-1996), clusters were defined by IS6110 alone and by PGRS/IS6110 to 1) estimate recent transmission, 2) evaluate the theoretical influence of bacterial population parameters on these estimates, and 3) assess risk factors for recent transmission. RESULTS: Secondary typing on low-copy strains (20.3% of all isolates) decreased the estimate of recent transmission from 29.1% to 25.3% (P = 0.03). The most influential parameters in determining whether supplemental genotyping results in different estimates were the proportion of low-copy strains and the amount of clustering. Risk factors for recent transmission were identical for both definitions of clustering. CONCLUSION: The statistical and inferred effects of secondary genotyping of M. tuberculosis seem to depend on the proportion of low-copy strains in the population. When this proportion is low or when few secondary patterns match, supplemental genotyping may yield minimal insight into population-level investigations.

Stability of Mycobacterium tuberculosis DNA genotypes.

To assess genotype stability in Mycobacterium tuberculosis, DNA genotypes were compared in sequential isolates from 49 patients who had sputum cultures separated by at least 90 days that grew M. tuberculosis. By use of IS6110 and the polymorphic GC-rich sequence (PGRS) as markers, it was found that paired isolates from 14 (29%) of 49 patients showed changes in their DNA genotypes between isolates (12 in IS6110 genotypes and 2 in PGRS genotypes). Changed IS6110 genotypes were confined to strains with 8-14 bands and were not related to the bacterial drug susceptibility, the patients' human immunodeficiency virus serostatus, or adherence to therapy. Although this rate of change complicates the interpretation of molecular epidemiologic studies, it can be exploited to gain additional insight into disease transmission. Furthermore, IS6110-related mutations may be a major source of genetic plasticity in M. tuberculosis and provide insights into the organism's evolution and virulence.

Dissemination of Mycobacterium tuberculosis across the San Francisco Bay Area.

The propensity of Mycobacterium tuberculosis genotypes to spread across geographic boundaries was investigated by comparing the IS6110 and polymorphic GC-rich sequence patterns of M. tuberculosis isolates from San Francisco and the East Bay, two distinct regions separated by San Francisco Bay. Of 724 isolates from incident tuberculosis patients during 1992 and 1993, only 53 (7.3%) had patterns matching > or = 1 isolates from the other region. In the multivariable analysis of patient risk factors, an AIDS diagnosis (odds ratio [OR], 1.89; 95% confidence interval [CI], 1.00-3.57) and non-Asian race (OR, 3.43; 95% CI, 1.59-7.42) were associated with having an isolate with a matching pattern. Of 375 unique IS6110 patterns among San Francisco isolates, only 9 (2.4%) matched patterns of East Bay isolates. These population-based data suggest that in the San Francisco Bay Area, M. tuberculosis does not rapidly spread across geographic boundaries, and tuberculosis control efforts should focus on transmission within defined areas.

Differences in contributing factors to tuberculosis incidence in U.S. -born and foreign-born persons.

To determine the factors contributing to tuberculosis incidence in the U.S.-born and foreign-born populations in San Francisco, California, and to assess the effectiveness of tuberculosis control efforts in these populations, we performed a population-based molecular epidemiologic study using 367 patients with strains of Mycobacterium tuberculosis recently introduced into the city. IS6110-based and PGRS-based restriction fragment length polymorphism (RFLP) analyses were performed on M. tuberculosis isolates. Patients whose isolates had identical RFLP patterns were considered a cluster. Review of public health and medical records, plus patient interviews, were used to determine the likelihood of transmission between clustered patients. None of the 252 foreign-born cases was recently infected (within 2 yr) in the city. Nineteen (17%) of 115 U. S.-born cases occurred after recent infection in the city; only two were infected by a foreign-born patient. Disease from recent infection in the city involved either a source or a secondary case with human immunodeficiency virus (HIV) infection, homelessness, or drug abuse. Failure to identify contacts accounted for the majority of secondary cases. In San Francisco, disease from recent transmission of M. tuberculosis has been virtually eliminated from the foreign-born but not from the U.S.-born population. An intensification of contact tracing and screening activities among HIV-infected, homeless, and drug-abusing persons is needed to further control tuberculosis in the U.S.-born population. Elimination of tuberculosis in both the foreign-born and the U.S. -born populations will require widespread use of preventive therapy.