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Background Although households of tuberculosis (TB) cases represent a setting for intense transmission of Mycobacterium tuberculosis, household exposure accounts for Methods We performed a cross-sectional study in Kampala, Uganda, to delineate social networks of TB cases and matched controls without TB. We estimated the age-stratified prevalence difference of TB infection between case and control networks, partitioned as household and extra-household contacts. Results We enrolled 123 index cases, 124 index controls, and 2415 first-degree network contacts. The prevalence of infection was highest among household contacts of cases (61.5%), lowest among household contacts of controls (25.2%), and intermediary among extra-household TB contacts (44.9%) and extra-household control contacts (41.2%). The age-adjusted prevalence difference between extra-household contacts of cases and their controls was 5.4%. The prevalence of infection was similar among the majority of extra-household case contacts and corresponding controls (47%). Conclusions Most first-degree social network members of TB cases do not have adequate contact with the index case to experience additional risk for infection, but appear instead to acquire infection through unrecognized exposures with infectious cases in the community.

Afipia felis, a Gram-negative alphaproteobacterium, has been implicated as one of the causative agents of cat scratch disease. To identify and begin to examine the virulence traits of this organism, we developed and tested a highly efficient transposon delivery system and a stable plasmid vector expressing green fluorescent protein. The transposome system is based on a Tn5-derived transposon and a phage restriction endonuclease type I inhibitor. Electroporation of this construct produced a library of >2600 mutants, which were screened for flagella biosynthesis mutants using a monoclonal antibody to Afipia flagellin. Insertion loci for two selected mutants were located in the genes for flagellin and flagellin biosynthesis FlhA, confirming the validity of the approach.

We developed an in vitro tissue-culture model to analyze the process involved in mycobacterial spread through lung epithelial cell monolayers. A549 cells were infected with low numbers of viable Mycobacterium tuberculosis bacilli expressing the gfp gene. Subsequent addition of a soft agarose overlay prevented the dispersal of the bacilli from the initial points of attachment. By fluorescence microscopy the bacteria were observed to infect and grow within the primary target cells; this was followed by lysis of the infected cells and subsequent infection of adjacent cells. This process repeated itself until an area of clearing (plaque formation) was observed. The addition of amikacin after initial infection did not prevent intracellular growth; however, subsequent plaque formation was not observed. Plaque formation was also observed after infection with Mycobacterium bovis BCG bacilli, but the plaques were smaller than those formed after infection with M. tuberculosis. These observations reinforce the possibility that cell-to-cell spreading of M. tuberculosis bacilli, particularly early in the course of infection within lung macrophages, pneumocytes, and other cells, may be an important component in the infectious process.

Eight-day-old embryonated hen's eggs were used as a model to study Mycobacterium avium virulence. Strains isolated from human patients caused 20-90% mortality when eggs were infected by injection of bacterial suspensions into the amniotic sac. Virulence of examined strains subsequently decreased with passage through eggs to between 0 and 40% mortality in four passages. Virulence of the egg-attenuated strains could be restored by passage through human peripheral blood mononuclear cells. The site of infection in the egg was usually the mesodermal layer of the chorioallantoic membrane. A few small granulomas containing acid-fast bacteria were seen in the liver, but not in other organs. Death of chicken embryos may have resulted from destruction of the mesodermal layer of the chorioallantoic membrane with consequent respiratory failure. PBMCs infected with less virulent egg-passaged strains of M. avium produced higher levels of tumor necrosis factor-alpha than did peripheral blood mononuclear cells infected with more virulent nonpassaged strains.

The role of Mycobacterium avium isolates in modulating human immunodeficiency virus type 1 (HIV-1) replication was examined by use of an in vitro, resting T cell system. Two human clinical isolates (serotypes 1 and 4) but not an environmental M. avium isolate (serotype 2) enhanced HIV-1 replication. The M. avium-induced HIV-1 replication was not associated with cell activation or differential cytokine production or utilization. Addition of matrix metalloproteinase (MMP) inhibitors and their in vivo regulators, tissue inhibitors of metalloproteinases-1 and -2, abrogated M. avium-induced HIV-1 replication 80%-95%. The MMP inhibitors did not have any effect on the HIV-1 protease activity, suggesting that they may affect cellular processes. Furthermore, MMP-9 protein was differentially expressed after infection with clinical M. avium isolates and paralleled HIV-1 p24 production. Collectively, these data suggest that M. avium-induced HIV-1 replication is mediated, in part, through the induction of MMP-9.

The chick embryo model was evaluated as a method to compare virulence between selected strains of Neisseria meningitidis. Inoculation of 13-day-chick embryos via the egg yolk distinguished strains having an LD50 of 10(3) colony forming units (CFU) or greater (low virulence) from those having an LD50 of approximately 10(1) or less (high virulence). A strain of serogroup B and a spontaneous nonpiliated strain of group C were found to be of relatively high virulence while a strain of N. lactamica, a serogroup A carrier strain, and certain nongroupable strains were found to be of low virulence. Strains having an LD50 of 10(2) were not differentiated from either of these. Alternatively, inoculation of the chorioallantoic membrane (CAM) of 9-day-old chick embryos statistically differentiated most strains of N. meningitidis although inoculation via this route was less sensitive.