TB, Still a Potent Threat|
According to the Centers for Disease Control and Prevention, tuberculosis is still the leading cause of death from a single infectious disease. And that is due in part to its ability to foil antibiotics.
The first TB therapy, streptomycin, was discovered in 1947 by Selman A. Waksman, PhD, at Rutgers University, and it won him the Nobel Prize. By the 1950s, a combination of strep, para-aminosalicyclic acid and isoniazid was needed. The disease was then thought to be under control - at least in developed countries.
But along with the AIDS epidemic came multidrug-resistant strains of TB, some that can require an arsenal of seven drugs. Nancy Connell, PhD, at UMDNJ-New Jersey Medical School, is studying the problem of resistance - both in general and as it relates to TB.
An assistant professor of microbiology and molecular genetics, she has an NIH grant to study drug resistance at the molecular level. Her work focuses on how substances get transported through the cell wall - those like amino acids, sugars and peptides that the cell requires in order to function, as well as drugs directed at pathogens.
One of these is a TB therapy called cycloserine. "It's a 'last-resort drug," Connell says, "because the TB it's used against is resistant to everything else. Cycloserine can be used only for a short time since it has serious neurological side effects. But it is very effective against multidrug-resistant TB. We're trying to understand just how it works, so that pharmacologists could then design modifications or address the same target of the drug in a different way."
A cell has two layers of protection, its wall and its membrane. Connell explains that proteins called porins allow substances to diffuse through the outer wall. Then permeases on the membrane admit them into the cell.
"We are trying to clone the genes that encode these permeases," she says. "The permeases of each bacterium and virus have similarities and differences. Sometimes it's just one protein that allows the substance to enter and other times maybe four or five are involved - with each having a specific task. In the case of cycloserine, it's a single protein."
Her group is also studying the transport of peptides, strings of amino acids. She notes that studies done some 40 years ago on a peptide associated with E. coli found that if one end of the substance was recognized by the permease, the other end could carry any kind of toxin and still be pulled along into the cell.
Mycobacterium tuberculosis is an interesting organism. Connell compares its cell wall to a steel-belted radial - tough and thick, difficult for a drug to permeate. Once the germ is breathed into the lungs, it is engulfed by a macrophage, but is not always destroyed. "Sometimes the bacterium holes up in the macrophage," explains Connell.
She's interested in how it survives during this period: "It's a very important stage. People who have positive TB tests, but not active disease, harbor TB bacteria that are in this phase. What keeps the germs from becoming active? There are events going on then, and if we can identify them we might be able to develop drugs that will target the bacteria then. It might be easier to eradicate TB at this point."
Connell says one of the big concerns clinicians have is the number of people who may have been infected with multidrug-resistant TB during outbreaks in the past decade: "All they need is a case of HIV, bad nutrition or a couple of debilitating events, and it can become activated."
One basis for mutation is instability in the chromosomes, Connell points out, "but that's not the case with TB. We're lucky that it is not like some organisms - staph for instance - that exchange DNA. Because then you could have wild proliferation and exchange of markers for resistance."
The entire TB genome - four megabases - has been cloned and sequenced, a project supported in part by the Wellcome Trust, notes Connell. The data is available on a web page. She has already used the information from the database to clone the genes involved in cycloserine resistance (not cycloserine uptake), and will use the sequences to characterize these genes from clinical strains of TB known to be resistant to cycloserine.