Relapse, re-infection and mixed infections in tuberculosis disease

Tuberculosis (TB) disease can be characterized by genotypic and phenotypic complexity in Mycobacterium tuberculosis bacilli within a single patient. This microbiological heterogeneity has become an area of intense study due its perceived importance in drug tolerance, drug resistance and as a surrogate measure of transmission rates. This review presents a descriptive analysis of research describing the prevalence of mixed-strain TB infections in geographically distinct locations. Despite significant variation in disease burden and a rampant human immunodeficiency virus (HIV)-TB co-epidemic, there was no difference in the prevalence range of mixed infections reported in African countries when compared to the rest of the world. The occurrence of recurrent TB was associated with a higher prevalence of mixed-strain infections, but this difference was not reported as statistically significant. These interpretations were limited by differences in the design and overall size of the studies assessed. Factors such as sputum quality, culture media, number of repeated culture steps, molecular typing methods and HIV-infection status can affect the detection of mixed-strain infection. It is recommended that future clinical studies should focus on settings with varying TB burdens, with a common sample processing protocol to gain further insight into these phenomena and develop novel transmission blocking strategies.