By David N. Leff
Antibiotics are now shooting themselves in their second foot.
Foot One was wounded decades ago, as the antibacterial "miracle drugs" against bacterial infection began succumbing to microbial resistance factors, deployed by the wily germs against the growing arsenal of antibiotics.
At the same time, cancer chemotherapy severely compromised the body's immune defenses, leaving patients prey to antimicrobial and fungal infections.
As for Foot Two, consider this scenario outlined by pediatrician Julia Köhler, an infectious-disease researcher at the Whitehead Institute for BioMedical Research at the Massachusetts Institute of Technology, in Cambridge:
A child undergoing chemotherapy for leukemia begins to run a fever. The clinician's routine reaction is to prescribe antibiotics. Both drug and cancer wipe out the body's normal microbial flora and further impair the small patient's immune defenses. This leaves the field wide open for a ubiquitous fungus, Candida albicans, to multiply out of control and spread its own nosocomial and iatrogenic infection.
Fungal infections in hospitalized patients doubled during the 1980s.
C. albicans is no stranger to the human body. "In about 30 percent of healthy people," Köhler pointed out, "the fungus is commensal, living harmlessly in mucus membranes from the mouth to the digestive tract to the genitalia. Unlike [Escherichia] coli," she pointed out, "which inhabits the intestinal flora of [Homo] sapiens 100 percent [see BioWorld Today, Sept. 5, 1997], C. albicans offers no metabolic pay-back to its human host."
Candida Fights Phagocytes To Finish
From the oral cavity to the anal and vaginal passages and urinary tract, from newborn infancy to old age, C. albicans lies low in the mucus membranes, ready to pounce whenever the immune system lets down its guard. In the very young, the fungus lays down soft, creamy itchy spots — known by the mellifluous, innocuous-sounding name of "thrush" — on the tongue, inner cheeks, and tonsils.
In adults, thrush is best known as afflicting the vaginal tissues and urinary tract with an itchy, irritating torment that yields only grudgingly to drastic antifungal treatment. Women with diabetes, or on the contraceptive pill, are especially vulnerable if receiving antibiotics.
But if the sometimes-lethal C. albicans infection enters the bloodstream and infects inner organs of the body — notably, liver, spleen, even brain — it's called, more grimly, invasive candidiasis or moniliasis, which is sometimes fatal.
"A lot of people nowadays," Köhler pointed out, "have survived serious illnesses, such as burn injury, that might have killed them in earlier decades. But their immune systems remain defective, and highly susceptible to Candida."
Just how C. albicans takes on and overwhelms the immune system has only lately come into focus. Typically, a smooth, rounded fungal cell encounters a phagocyte — macrophage or neutrophil — that blocks its progress to the bloodstream. In a healthy person," she continued, "both such immune-defense cells will be on hand to receive the fungus. In a person on chemotherapy, the neutrophils are lacking, which allows the fungus to progress pretty much unimpeded."
If a smoothly rounded C. albicans cell invades the bloodstream, a patrolling macrophage will gobble it up, with intent to disassemble it, and expel the debris. Instead, the threatened fungus swiftly switches hats, and starts to extrude virulent filaments. These ever-lengthening projections penetrate the macrophage's cell from within, lay it low, and exit to carry on their mayhem in new phagocytic victims.
As a crowning insult to molecular biologists , the unrelenting fungus has no known sexual reproduction cycle, which makes it difficult for researchers to follow and clone its mutations.
That's more or less where things stood until last Friday, when a paper appeared in the journal Cell, dated Sept. 5, 1997, bearing the title: "Nonfilamentous C. albicans mutants are avirulent." Its senior author is molecular geneticist and biologist Gerald Fink, director of Whitehead.
"Back in 1992," Köhler recounted, "a graduate student in Fink's lab observed that a mutant strain of baker's yeast, Saccharomyces cerevisiae, was putting out filaments. Where anyone else might have thrown the culture away as aberrant, the student showed it to Fink, a pioneer in yeast molecular biology."
Until then it had been common knowledge and received wisdom that no way could yeast fungi generate filaments. Wine-growers had noted the phenomenon early in this century, but never brought it to the attention of biologists.
Fink's unexpected laboratory finding had momentous repercussions, not only on S. cerevisiae, but on C. albicans research. Fink played an educated hunch that both forms of fungus, though separated by 300 million years of evolution, being eukaryotic, might share enough genomic sequences to let yeast serve as a genetic stand-in for the pathogenic fungus.
"Their genomes," Köhler pointed out, "are the same order of magnitude, but C. albicans' life style is a little more complex. Yeast evolved to grow on vegetation, and not fight back."
Completing Yeast Sequence Helped Effort
A contributing factor to the team's developing strategy was the complete sequencing of the yeast genome's 12,057,500 base pairs early last year (see BioWorld Today, April 25, 1996, p.1)
Recognizing filamentation as the key to C. albicans' virulence, molecular biologist Hsiu-Jung Lo, the paper's first author, inserted suspect segments of C. albicans' eight-chromosome genome — as yet unsequenced — into that of S. cerevisiae. "Nobody knew of the two signaling pathways involved in the filamentous process," Lo told BioWorld Today. "When we knocked out the first one, our injected fungi still killed mice, to which candidiasis is invariably fatal.
"After we found and removed mutants from a second pathway," Lo went on, "thereby creating a double mutant, the mice survived happily."
Three of the Cell paper's six co-authors are at the Schering-Plough Research Institute, in Kenilworth, N.J. It was they who injected the tail veins of mice with the experimental mutant fungi and knockout strains of C. albicans, and counted the survivors.
"We hope," Lo observed, "that inhibiting this double-mutant mechanism, provided the genes are not found in humans, might lead to a therapeutic drug target. This work is in progress," she concluded.
"If we could design drugs that inactivate or block Candida's filamentation pathway," Whitehead's Fink observed, "we might be able to fight the organism's insidious and devastating effects on patients with weak immune systems." *