"Listeriosis is a food-borne pathogen," recounts microbiologist/immunologist Mary O'Riordan at the University of Michigan in Ann Arbor. So generally, people get it through contamination of soft cheeses or deli meats. And the reason one occasionally sees outbreaks of listeriosis is that it has a very interesting property of being able to grow in cold storage conditions. Low temperatures won't stop Listeria from thriving in the household fridge. However, the one reason you don't see more listeriosis - only a couple of thousand cases in the U.S. every year - is because healthy, immunocompetent adults are not susceptible to listeriosis. They can generally fight off the infection, so there are no clinical syndromes. However, it is a problem for people who are specifically immunocompromised to Listeria, and for infected pregnant women, for whom it can cause spontaneous miscarriages.
"The genome of L. monocytogenes," O'Riordan went on, "was sequenced by the Pasteur Institute consortium in 2001. Listeria's genome has 2.94 million base pairs - and 2,853 genes.
"One reason listeriosis continues to be a serious disease in the U.S," O'Riordan explained, "is because when L. monocytogenes causes disease, it does not inflict a very characteristic symptom. So when people catch this infection they get flu-like symptoms, possibly some hepatitis, meningitis, and various features that can be symptomatic of other bacterial infections.
"So when a suffering patient first goes to see a doctor, and since listeriosis is relatively uncommon, it's not diagnosed right away. And what's bad about that is when you get a significant systemic infection with listeria, then it's very hard to treat without any untoward consequences. Once you get sick enough to end up in the hospital, then it has something like a 25 percent fatality range."
Besides Folks & Bugs, Beasts, Wild & Domestic
O'Riordan is lead author of an article in Science, dated Oct. 17, 2003. It's titled: "Listeria intracellular growth and virulence require host-derived lipoic acid." The paper's senior author is molecular and cell biologist Daniel Portnoy at the University of California at Berkeley.
"The substance of the Science paper, our general finding," O'Riordan told BioWorld Today, "is that we have found that Listeria has evolved specifically to allow the bacterium to grow in environments inside cells. We found that Listeria has a similar gene that allows it to grow in an extracellular environment, too. The role of the enzyme is very much conserved among mammals, birds and other organisms, besides humans and Listeria.
"The gene is a lipoic acid ligase. This puts lipoic acid onto an important metabolic enzyme called pyruvate dehydrogenase. People actually shop for the lipoic acid cofactor as an antioxidant," O'Riordan commented. "There's a huge amount of media reporting to that effect. But, in fact, lipoic acid is important for aerobic metabolism and episodically to treat mushroom poisoning. People buy it in health food stores because antioxidants are thought to protect against cancer. L. monocytogenes is sold, because they like to take it, thinking it's prophylactic against oxidative stress, which can lead to cancer. I don't know how good the evidence is for its clinical function in vivo.
"The gene itself is one of two similar sequences in the Listeria genome. And that gene is expressed both outside and inside of host cells. Its function is to steal lipoic acid from some complex host source - possibly even the enzymes the host uses for the same purpose, that is, metabolism. Outside in the environment, Listeria can use any free lipoic acid that's floating around, but inside the host cell the host macrophages use up all the lipoic acid for its own metabolism.
"The enzyme is the protein product of the gene," O'Riordan continued. "It involved a very simple and beautiful genetic selection process. It's based on the properties of well-known antimicrobials - the penicillins - which select against rapidly growing bacteria. The way the selection worked is that the penicillin family interferes with bacterial cell-wall synthesis. The bacteria need to synthesize more cell wall when they're actively growing. Otherwise they burst. So if one interferes with cell-wall synthesis, actively growing bacteria will die. But bacteria that are not growing very well, and therefore not making a lot of cell wall, will not be affected by the antibiotic drug."
Cerus Inc. Licensed Gene For Vaccine Development
"Consequently, we used a library of mutant Listeria monocytogenes genes to infect macrophages. A few hours after infection, we added the antibiotic drug to select against any bacteria that grew just like the wild-type (WT) strain. That is, growing poorly, and would be selected against by penicillins. Having a listeriosis mutation in a bacterium, which allows it to grow a little bit but not very well, makes it a very good candidate for a vaccine strain," O'Riordan pointed out. "And that's because we've shown in mice that this lipoate protein lipase-A1 deletion strain is 250 to 300 times less virulent in mice than the WT strains. Therefore, it's very good at stimulating host immunity. Which is what we want to see in a vaccine.
"A company called Cerus Corp., in Concord, Calif.," O'Riordan recounted, "has licensed the patent for this bacteria. They are developing it in addition to other strains. I'm one of the co-inventors. Cerus would like to be in a clinic with a Listeria-based vaccine by the year 2005. From what they have told me, they have had very good results with the Listeria vaccine. One of the modifications is deletion of this lipoic acid ligase. They are testing it on mice and perhaps other animal models.
"We now know more about it as an agent of disease," O'Riordan said. "What I would like to do next is dissect the contributions of these two different lipoic acid ligases. So we knocked out one of their genes, which is what the paper is about, and saw this clear phenotype in host cells but not in lab culture. We are now actually in the process of deleting the second gene.
"The principle I like to stress," O'Riordan explicated, "is that bacterial metabolism has very important consequences for pathogenesis. Therefore, understanding the immune responses is essential for developing many kinds of clinical applications," she concluded, "either vaccines or antimicrobials."