"In this study, we wanted to see if the two SecAs do the same thing or have different functions. We had a hunch that SecA2 was involved in the bacteria's virulence, that it might be dedicated to secreting a specific subset of proteins involved in virulence," Braunstein said. To test that hypothesis, she and her colleagues genetically engineered a mutant strain of M. tuberculosis that did not have secA2. This gene deletion meant it could not produce the protein SecA2.
"We tested the strain for virulence by infecting mice with it and observed how long the mice survived over time," Braunstein said. "And we found that the mice infected with mutant TB survived longer than 'wild-type' mice infected with TB having a functioning secA2 gene. This told us the mutant strain was not as virulent."
To further examine virulence, the researchers also examined the extent of bacterial growth in the lung, liver and spleen. "Those experiments showed the same thing," she said. "When TB is missing SecA2, it is less virulent. There is less bacterial growth, particularly in the lung."
Having demonstrated that SecA2 is required for virulence, the next step was to identify the virulence factor secreted by the protein. Two of the proteins dependent on SecA2 that were identified were antioxidant molecules: superoxide dismutase-A and catalase-peroxidase.
When M. tuberculosis is inhaled and enters the lungs via the small air sacs called alveoli, the bacteria becomes aggressively attacked and engulfed by macrophages, immune system scavenger cells. But where other bacteria succumb to the attack, TB survives in macrophage, having evolved over the millennia a mechanism to overcome the "oxidative burst" leveled at it.
"These SecA2-dependent secretions, superoxide dismutase and catalase-peroxidase are enzymes that actually scavenge the oxygen radicals that are shot at the bacteria," Braunstein said.