June 2008 Feature: Fighting the Latest Public Health Threat

by Bill Shepard

When you think of major health threats -- cancer, AIDS and heart disease immediately come to mind.

But these days, health officials are worried about another phenomenon looming on the horizon: antibiotic-resistant bacteria.

In the fall of 2007, this trend made headlines when six different states reported the spread of MRSA (methicillin-resistant staphylococcus aureus), a dangerous drug-resistant staphylococcus bacteria. According to the Centers for Disease Control and Prevention, MRSA killed nearly 19,000 Americans in 2005, and affects more than 90,000 annually. Spread by skin-to-skin contact or through items that an infected person has used, this “superbug” does not respond to antibiotics such as penicillin and methicillin.

While public health officials across the country are calling for more aggressive measures by the federal government to curb the spread of drug-resistant bacteria, researchers are working hard to find a way to track and stop their spread.

And some of the leading research and discoveries in the fight against antibiotic-resistant bacteria originate at the Creighton University School of Medicine, within the department of medical microbiology and immunology.

“We’re researching the molecular mechanics of antibiotic resistance,” says Professor Richard Goering, chair of the department of medical microbiology and immunology. “We try to determine how and why certain bacteria become resistant to antibiotics by examining their molecular structures.”

One of the department’s more promising discoveries involves using polymerase chain reaction (PCR) tests for diagnostic purposes, and for controlling the spread of infection in patients.

These tests can amplify a gene inside a cell, to detect specific genes in antibiotic-resistant bacteria. Using this genetic code, clinicians can trace linkages between different infections stemming from different types of bacteria that may share the same pieces of DNA.

A multiplex PCR test can identify more than one genetic “marker” in the bacteria, and therefore is more effective in identifying the common genetic “family ties” between bacteria.

Goering and his team are working on the multiplex PCR diagnostic technology to identify the genetic code underlying MRSA, so it can be tracked and hopefully stopped through the development of more potent antibiotics. 

“The multiplex PCR can give us three different products or tests that allow us to look for a kind of ‘fingerprint’ -- signature sequences of DNA that will help us identify the MRSA bacteria,” says Goering.

In a limited sense, it’s analogous to using highly specialized DNA testing to investigate a series of crimes committed by a family of criminals. The crime scenes, like infections, may be different in nature. The individuals perpetrating the crimes may be different as well, just as antibiotic-resistant bacteria may differ from infection to infection.

But traces of each perpetrator’s DNA found at different crime scenes and on various victims can help the criminal investigator establish a pattern, linking the individuals in the crime family to various crimes. Armed with this information, the criminal investigator can try to predict where the next crime might occur, limit the crime family’s activities, and hopefully capture members of the crime family.

Likewise, by identifying the DNA in the antibiotic-resistant bacteria and in the patients’ infections, clinicians can establish patterns, too, linking different types of bacteria with one another. They can use this information to develop intelligent strategies for anticipating where infections may spread, limiting the spread of infections, and, hopefully developing new classes of antibiotics to kill the “crime family” of bacteria causing the infections.

But there are a few troublesome aspects of antibiotic-resistant bacteria that are incongruous with the “crime family” analogy.

For one thing, pieces of DNA may not necessarily cause a person to commit criminal acts. Additionally, DNA from one person doesn’t merge into the genetic makeup of another person.

But components of bacterial DNA, known as plasmids, can replicate on their own and spread from one bacterium to another – even between dead and live bacteria. These plasmids house the genes that can block antibiotics, and therefore can spread antibiotic-resistance to different kinds of bacteria.

And there are even more sinister differences, according to Nancy Hanson, associate professor of microbiology and immunology, and director of molecular biology in the Center for Research in Anti-Infectives and Biotechnology.

“Once a bacterium dies, part of its DNA can move into another bacterium,” explains Hanson. “So, you might think that you’ve gotten rid of the resistance by killing a certain bacterium with antibiotics. But the DNA from the dead bacterium can move into another kind of bacterium, which will continue the spread of the infection. This is yet another way that resistance can easily spread from one part of the body to another, or from person to person.”

Hanson’s research focuses mainly on bacteria resistant to beta-lactam antibiotics such as penicillin and amoxicillin. These antibiotics share in common a ring of four atoms, known as the beta-lactam molecule.

Bacteria that resist these beta-lactam antibiotics are known as beta-lactamase producing bacteria. That’s because they produce an enzyme called beta-lactamase, which breaks open the ring of four molecules, rendering the beta-lactam molecule useless in fighting bacteria.

Creighton has patented a particularly effective form of the multiplex PCR test, developed by Hanson and her team, that targets beta-lactamase genes. This novel diagnostic test is capable of detecting six different genes underlying AmpC beta-lactamase bacteria, which are resistant to a wide variety of beta-lactam drugs. Additionally, other PCR-based tests can be used to determine the genetic roots of extended-spectrum beta-lacatamases (ESBLs), a mutant form of beta-lactamase bacteria that can resist not only penicillin and other beta-lactam drugs, but also cephalosporin and aztreonam drugs. Cephalosporins are a broad class of bacteria-killing antibiotics often used to prevent post-surgery infections, while aztreonam drugs target infections of the skin, bone, blood, stomach, respiratory tract, sinuses and kidneys.

Three separate patented versions of Hanson’s PCR diagnostic testing technology have been exclusively licensed to one company, which is in the process of developing kits that can be used in any lab setting.

“It was a real team effort in discovering and developing the multiplex PCR diagnostic technology,” Hanson says. “We’re very fortunate to have such talented staff and grad students that contributed to the discovery.”

What does the future hold for this promising technology?

“We plan to continue to modify our multiplex PCR diagnostic tests to adapt to advances in medical technology,” Hanson says.

Armed with the multiplex PCR test developed at Creighton Medical School, along with other diagnostic tools from other institutions, researchers hopefully will be able to limit the spread of “superbugs.”

“It’s very gratifying when you can take basic scientific research tests used in the molecular biology lab here at Creighton, and give it a clinical application that can help people,” Goering says. “It’s actually presenting answers to people in real time.”