Virulence for guinea pigs of tubercle bacilli isolated from the sputum of participants in the BCG trial, Chingleput District, South India

Genomic interrogation of ancestral Mycobacterium tuberculosis from south India

Mycobacterium tuberculosis is a very important global pathogen. One quarter of the world's TB cases occur in India. The tuberculosis strains isolated from south Indian patients exhibit certain phenotypic characteristics like low virulence in guinea-pigs, resistance to isoniazid, thiophene-2-carboxylic acid hydrazide (TCH) and para-amino salicylic acid (PAS), and enhanced susceptibility to H2O2. Besides this, a large percentage of the isolates harbor only a single copy of IS 6110 which makes these strains distinct. Hence, we have studied the genotypic characteristics of these strains by using advanced techniques like Deletion Micro array, deletion PCR, allelic discrimination RT-PCR using several lineage specific markers and KatG G1388T (non-synonymous) polymorphism along with spoligotyping. The analysis of 1215 tuberculosis patient isolates from south India revealed that 85.2% belonged to the ancestral lineage of M. tuberculosis. Comparative whole-genome hybridization identified six new genomic regions within this lineage that were variably deleted.

The detection of airborne transmission of tuberculosis from HIV-infected patients, using an in vivo air sampling model

BACKGROUND

Nosocomial transmission of tuberculosis remains an important public health problem. We created an in vivo air sampling model to study airborne transmission of tuberculosis from patients coinfected with human immunodeficiency virus (HIV) and to evaluate environmental control measures.

METHODS

An animal facility was built above a mechanically ventilated HIV-tuberculosis ward in Lima, Peru. A mean of 92 guinea pigs were continuously exposed to all ward exhaust air for 16 months. Animals had tuberculin skin tests performed at monthly intervals, and those with positive reactions were removed for autopsy and culture for tuberculosis.

RESULTS

Over 505 consecutive days, there were 118 ward admissions by 97 patients with pulmonary tuberculosis, with a median duration of hospitalization of 11 days. All patients were infected with HIV and constituted a heterogeneous group with both new and existing diagnoses of tuberculosis. There was a wide variation in monthly rates of guinea pigs developing positive tuberculin test results (0%-53%). Of 292 animals exposed to ward air, 159 developed positive tuberculin skin test results, of which 129 had laboratory confirmation of tuberculosis. The HIV-positive patients with pulmonary tuberculosis produced a mean of 8.2 infectious quanta per hour, compared with 1.25 for HIV-negative patients with tuberculosis in similar studies from the 1950s. The mean monthly patient infectiousness varied greatly, from production of 0-44 infectious quanta per hour, as did the theoretical risk for a health care worker to acquire tuberculosis by breathing ward air.

CONCLUSIONS

HIV-positive patients with tuberculosis varied greatly in their infectiousness, and some were highly infectious. Use of environmental control strategies for nosocomial tuberculosis is therefore a priority, especially in areas with a high prevalence of both tuberculosis and HIV infection.

An assay to compare the infectivity of Mycobacterium tuberculosis isolates based on aerosol infection of guinea pigs and assessment of bacteriology

The aim of this study was to establish an assay to compare Mycobacterium tuberculosis strains, and cells grown under different growth conditions, in terms of their ability to cause a lung infection and disseminate to the spleen. M. tuberculosis strains H37Rv, Erdman, South Indian (TMC120, SI) and H37Rv cells grown aerobically or under low oxygen/iron limitation in a chemostat were assayed for infectivity. Groups of 8 animals were challenged with 3 different doses of each strain. Lung and spleen bacteriology was assessed at 16 days post-infection for all strains. Bacteriology and lung pathology at day 56 was studied for H37Rv, Erdman and SI. Strains H37Rv and Erdman had a statistically significantly higher pathogenic potential than SI and this was confirmed by analysis of lung pathology performed at 8 weeks post-infection, although the Erdman strain caused more extensive caseation without calcification and little encapsulation. The model could discriminate between cells grown under different growth conditions; low-oxygen/iron-limited cells had a significantly higher infectivity than those grown aerobically. This study presents a quick and reliable method for comparing with statistical confidence, the pathogenic potential of M. tuberculosis strains and the impact of in vitro growth conditions on the infectivity of M. tuberculosis in vivo.

An analysis of some hypotheses related to the Chingelput bacille Calmette-Guérin trial

Investigation of several hypotheses related to the outcome of the Chingelput Bacille Calmette-Guérin (BCG) Trial suggests at least 2 factors that might explain the major scientific puzzle of a protective effect expected of 80% and a protective effect observed of 0% (i.e., equivalent protection for BCG and placebo). One factor that explains some of the low efficacy observed for BCG in this trial is the virtual saturation level of exposure to environmental mycobacteria (EM). Studies in animal models demonstrated that the protection afforded by infection with EM was equivalent to the protection that resulted from BCG vaccination. The second factor, pathogenetic pathway, explains why there was still a high case rate for tuberculosis, even though the population was fully vaccinated by EM. This hypothesis states that tuberculosis in India, as well as in most developing countries, results primarily from exogenous reinfection, a pathway against which BCG (or EM) exerts no protective effect beyond that induced by the first episode of infection with Mycobacterium tuberculosis.

Pathogenesis of tuberculosis: pathway to apical localization

We have examined the published work of investigators which dealt with the pathogenesis of tuberculosis, especially the following: the infective dose, the yield of bacilli from the primary lesion and primary complex, the predominant location of the minimal lesion, the hypotheses of a vulnerable region in the lung and the specific pathways (endogenous or exogenous) by which tubercle bacilli cause disease. More knowledge of the pathogenic pathway to tuberculosis would provide clues to the development of new vaccines and drug regimens that can intervene at a specific stage in the pathogenesis. Based on the examination of the literature on pathogenesis of human tuberculosis and our findings in a guinea-pig model of experimental airborne tuberculosis, we have proposed an hypothesis which integrates the endogenous and exogenous pathways to tuberculosis. This hypothesis is based on the following observations: 1. The infectious dose is very low, usually 1-5 tubercle bacilli. 2. The first implant can occur anywhere in the lungs. 3. The cavitary lesion, characteristic of tuberculous disease, is often located in the apical regions in the lungs. 4. Whereas the primary implant can occur anywhere in the lungs, for the progression from infection to disease, the tubercle bacilli must gain access to the 'vulnerable' regions in the apex of the lungs. Our hypothesis states that in areas of the world where there is a low risk of infection with tubercle bacilli low incidence of vaccination or sensitization to environmental mycobacteria, or high incidence of high virulent isolates, the virulent tubercle bacilli reach the vulnerable region via a bacillemia during the first infection. In areas of the world where there is a high risk of infection with tubercle bacilli, high incidence of vaccination or sensitization to environmental mycobacteria or a high incidence of low virulent isolates, the tubercle bacilli reach the vulnerable region via the airway, which requires repeated episodes of infection as the probability of a first implant occurring in the vulnerable regions is low.

Growth characteristics of recent sputum isolates of Mycobacterium tuberculosis in guinea pigs infected by the respiratory route

The consideration of virulence must distinguish between infectivity and the ability to cause progressive disease once the infection is established. Several investigators have reported the presence of naturally occurring isolates which differ in virulence for guinea pigs. Isolates from south India which differed with respect to gross disease and number of bacilli recovered from spleen after an intramuscular infection also differed in their efficiencies to initiate an infection, once inhaled and retained. Also, this difference was correlated with differences in the rate of multiplication at the site of implantation and rate of multiplication at sites of hematogenous seeding, as well as the extent of hematogenous seeding. The number of metastatic foci was identified as a quantitative measure of hematogenous seeding, which was not confounded by the rate of multiplication of bacilli. Even allowing for the fourfold-reduced efficiency of low-virulence tubercle bacilli to produce a lesion, this measure clearly revealed a significantly reduced ability of the low-virulence tubercle bacilli to disseminate via the bloodstream.

Virulence of Mycobacterium tuberculosis for guinea pigs: a quantitative modification of the assay developed by Mitchison

One of the more accepted methods of assay of virulence of tubercle bacilli is one developed by Mitchison in which guinea pigs were infected by the intramuscular route with 1.0 mg of tubercle bacilli freshly harvested from Lowenstein Jensen medium and in which virulence was based on a subjective score of the extent of gross disease in the animal 6 weeks after infection. Due to the practical difficulties involved in such an assay when routinely performed, the following modifications were made: frozen stocks of the culture were prepared prior to the date of infection, the inoculum was quantitated by means of colony forming units, and virulence was based on the number of tubercle bacilli recovered from the spleen of infected animals 6 weeks after infection. A significant correlation was obtained between the findings from the Mitchison assay of virulence and the modification. The latter assay is recommended for its quantitative and objective features.

Guinea-pig alveolar macrophages kill Mycobacterium tuberculosis in vitro, but killing is independent of susceptibility to hydrogen peroxide or triggering of the respiratory burst

Alveolar macrophages from the lungs of guinea-pigs that had been vaccinated, boosted and then intravenously challenged with Mycobacterium microti or Mycobacterium bovis BCG killed tubercle bacilli phagocytosed in vitro. The killing was modest, about 40% of phagocytosed bacilli were killed in a day, but alveolar macrophages from animals that had been vaccinated and boosted but had not received the intravenous challenge did not kill bacilli. Different strains of tubercle bacilli had different degrees of susceptibility to these activated macrophages but there was no correlation between killing by macrophages and mycobacterial susceptibility to killing by hydrogen peroxide. The different strains of tubercle bacilli triggered peroxide release from these macrophages but there was no correlation with susceptibility to killing by macrophages or with virulence in the guinea-pig. However, phagocytic uptake of these strains by the activated macrophages was inversely correlated with virulence, and uptake by activated macrophages was less than uptake by normal macrophages.

Immunity to tuberculosis from the perspective of pathogenesis

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