By David N. Leff
If an infectious-diseases special prosecutor were to haul Staphylococcus aureus before a grand jury for indictment, the bacterium's rap sheet (more literally RAP — see below) might read like this:
"S. aureus — Gram-positive serial killer; profile, spherical; diameter, 0.5 to 1.5 microns. Heavily armed with 20 or more different toxins. Wanted for food poisoning, cellulitis, pyemia, pneumonia, osteomyelitis, endocarditis, furunculitis, septic shock. Favorite killing field — hospitals. Resists arrest by antibiotics."
More than half a million hospitalized patients in the U.S. suffer such life-threatening infections every year, and every year S. aureus resistance to antibiotics increases.
As anti-bacterial measures to take no pathogenic prisoners falter, an alternative strategy proposes to tame the bacterium rather than execute it.
It's spelled out in the current issue of Science, dated April 17, 1998, under the title: "Autoinducer of virulence as a target for vaccine and therapy against Staphylococcus aureus."
The paper's lead author is infectious diseases specialist Naomi Balaban at the University of California at Davis.
"S. aureus," Balaban told BioWorld Today, "does its infectious damage not directly, but by the toxins it produces. Every bacterium," she pointed out, "makes many different toxins, and thereby can cause many different diseases."
The score of toxins secreted by S. aureus are regulated by one bacterial gene locus, the agr (accessory gene regulon). "So if you take away its agr genes," she observed, "in principle you eliminate production of most of its toxins."
The pathogen turns toxin secretion on and off to meet changing survival needs, she pointed out.
"So if S. aureus is not under survival stress, it's a complete waste of energy for it to produce the toxin. In other words, if the pathogen is happy, it has no need for it."
Balaban and her co-authors tackled the question: "What stress signal makes the bacterium suddenly feel that it has to produce these toxins? People used to think," she recalled, "that that stress was some kind of an environmental condition. But what we found out is that S. aureus has its own stress alarm system.
"That system is what we call RAP — RNA-III-activating protein. The bacterium always produces it, spits it out into its environment all the time. When there's a little bit of RAP, S. aureus knows 'I'm okay,'" she said.
Bugs Don't Kill People; Toxins Kill People
People think of toxins as disease-causing molecules. But the co-authors wondered how the pathogen 'thinks' of them.
"Typically," Balaban pointed out, "toxins can be proteases. When S. aureus runs out of food, it produces these enzymes, which break down host tissue and whatever else it can find, to make more nutrients for itself.
"Another family of staph toxins, for example," she added, "toxic shock syndrome toxins, simply interferes with the host's immune responses."
From these observations, Balaban made the point that "the bacterium doesn't need to cause its host's death; it just wants to avoid being attacked by its host. So if it is unable to produce the toxin, it would run out of food and die, or be eaten up by the host's immune responses. Either way, if it doesn't produce toxins, eventually it will not survive."
RAP is virtually synonymous with the agr gene product, of which it is a part.
"So when there is little RAP around the bacterium," Balaban observed, "it knows, 'Oh, I'm alone in space. I'm the only one producing RAP. This means I'm not running out of food, so I have nothing to worry about.'
"But when there's a lot of RAP around," she went on," it means that more of the bacterium's kind have moved into its neighborhood. They are also producing RAP, and starting to compete for food. So it had better start secreting those host-eating toxins. The Davis co-authors' strategy is to take this RAP stress-alert system away.
"When RAP is not there," Balaban explained, "the bacterium has no way to sense its situation in the environment. It feels safe because the alarm system did not go off. If it feels safe it doesn't produce toxin. If it doesn't produce toxin, it doesn't cause disease. And also its survival is jeopardized."
The in vivo experiments reported in Science led the co-authors to two non-lethal, staph-subduing approaches — a vaccine and an inhibitor. Balaban laid out the rationale behind these proposed measures:
"We know that our bodies are always in equilibrium with various microorganisms," she began. "If you interrupt this balance, you allow another pathogenic population to move in and create another kind of infection.
"Say a woman takes antibiotics against a bacterial infection. A week or two later she will contract a yeast infection, because the drug has taken away the bacterial population. The yeast says, 'Wow! wonderful! Here's a new neighborhood for me to move into.' So it moves in and creates a different infection.
"Our approach," Balaban summed up, "stabilizes this equilibrium, because it does not kill the bacteria. It just disables them from proliferating, and surviving and multiplying, etcetera —all the things that would make it become infectious. Instead, it just sits there."
Vaccine Lets Mice Beat RAP
The co-authors' vaccine method injected RAP into mice, which raised antibodies against it. So when infected with S. aureus, the antibodies eliminated the pathogen's RAP, leaving it without an alarm system. Thus, although not dead, it was no longer virulent. "The mice had no disease whatsoever," Balaban recounted.
She went on: "Because directed suppression of virulence would not kill the bacteria, but only interfere with their pathogenicity, this would likely decrease selective pressures for emergence of S. aureus strains resistant to our mode of therapy."
As children are most vulnerable to staph infection, the Davis team would recommend vaccinating this age-group widely as well as elderly and immune-deficient individuals.
Alternatively, Balaban foresees employing a small molecule called RIP — RAP inhibiting protein — to "coat surgical instruments and other articles that end up in the body, such as tampons, catheters, artifical joints, heart valves, sutures, even soap."
Meanwhile, she and her colleagues are planning to treat widespread S. aureus infection in dairy cows. The bug infects their udders with mastitis, which they suggest, would respond to a RAP vaccine and RIP teat-coating drug.
Moreover, Balaban pointed out, "Cows are also pumped full of antibiotics, which are transmitted to the milk we drink And bacterial toxins are not eliminated by pasteurization.
"So with regulatory approval," she concluded, "we could already help the human population by having it drink safer cow's milk." *