Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
Staphylococcus aureus is a serial killer with an attitude. S. aureus lurks mainly on nasal mucous membrane and skin - hair follicles. It produces highly pathological exotoxins, including those that cause toxic shock syndrome, with resulting skin rash, boils, cellulitis, septicemia and kidney, liver and central nervous system diseases. That's not all. Staph enterotoxin wreaks food poisoning, pneumonia, osteomyelitis, endocarditis - among other pathologies.
Infectious diseases are the second leading cause of death worldwide, and the acquisition of antibiotic resistance by many pathogenic bacteria has spurred interest in generating vaccines to cure or prevent disease. For the design of potent and universally applicable subunit vaccines, it's necessary to identify those antigens that are recognized as nonself by the immune systems of many individuals of a wide patient population during infection.
One such strategy is reported in the Proceedings of the National Academy of Sciences (PNAS), dated May 14, 2002, but released online May 7. Its title: "Identification of in vivo expressed vaccine candidate antigens from Staphylococcus aureus." The paper's co-authors are at the Biocenter Campus in Vienna, Austria, seconded by genomicist Claire Fraser, president of TIGR - The Institute for Genomic Research in Rockville, Md.
As a vehicle for delivering antigens in mice, they displayed S. aureus peptides on the surface of Escherichia coli bacteria, by fusing one of two outer membrane proteins. Those were probed with sera selected for high antibody titer and opsonic activity (that is, target organisms or cells altered to be more efficiently engulfed by phagocytes). The co-authors identified a total of 60 antigenic proteins, mainly located on S. aureus' cell surface. Their reactivity with individual sera from 40 patients and 30 healthy individuals as controls facilitated the selection of promising vaccine candidates. Human serum samples were obtained from patients suffering mostly from Staph wound- and catheter-related infections, causing soft-tissue infection and septicemia.
The co-authors generated small, diversely sized peptide libraries, encoded by randomly fragmented genomic DNA, to ensure that all potential antigens encoded by the S. aureus genome could be identified. Screening of those libraries determined the profile of antigens expressed in vivo and elicited an immune response in humans. They applied the procedure to the methicillin-resistant Staphylococcus aureus, one of the most common causes of hospital-acquired infections.
"This approach," the PNAS paper pointed out, "which makes use of whole genome information, has the potential to greatly accelerate and facilitate the formulation of novel vaccines, and is applicable to any pathogen that induces antibodies in humans and/or experimental animals."
Quinolone, Head Of An Antibiotic Family, Proved Vulnerable After All To Bacterial Drug Resistance
During last fall's anthrax scare (five people dead, seven others infected), the prime protection against that bacterial attack was the antibiotic Cipro - ciprofloxacin. It's one of four closely related antibacterial compounds - with norfloxacin, ofloxacin and nalidixic acid. All belong to a family of antibiotics called quinolones, best known pre-anthrax for treating urinary tract infections.
Since the introduction of nalidixic acid in the 1960s, quinolones have been increasingly prescribed to treat a variety of infectious diseases. This widespread clinical reliance has duly led to increasing bacterial resistance to these antibiotics. Two main genetic events fuel this bacterial backlash: One is mutations in the chromosomal genes for two enzymes, topoisomerase and DNA gyrase - key targets of quinolone action. The other altered environment: changes in expression of two critical bacterial proteins - porins and efflux pumps.
Porins are minuscule pores or apertures studding the outer membranes of bacteria. They serve as ports of entry - now really traps - that give antibiotics access to their targets. Efflux pumps, as the name implies, expel the antibacterial drugs. And that resistance strategy can be transmitted from one bacterial species to another.
It was long thought that transmissible resistance to quinolones simply did not exist. But the resistance factor was discovered four years ago on a plasmid from a clinical isolate in the microbe Klebsiella pneumoniae. That extrachromosomal loop of DNA confers fourfold to eightfold resistance to all four quinolone antibiotics, from its ciprofloxacine flagship on down.
A paper in the Proceedings of the National Academy of Sciences (PNAS) dated April 16, 2002, but released April 8, bears the title: "Mechanism of plasmid-mediated quinolone resistance." Its co-authors are in the infectious disease department of the Lahey Clinic in Burlington, Mass. They report cloning qnr - the novel plasmid-encoded quinolone-resistance gene - as well as amplification and purification of Qnr, its gene product. Qnr, they demonstrated, directly protects E. coli DNA gyrase (which generates supercoils of DNA) from quinolone inhibition - proof of the bug's drug-resistance strategy.
Novel Gamma-Delta T Cell Emerges From Obscurity To Fix Skin, Lung, Gut Wounds
Don't pick that scab! Your body has a slew of cell types that take care of healing wounds. The latest addition to this repair squad is the gamma-delta T cell, a mysterious immune system cell that resides mainly in skin and gut. A paper in Science dated April 26, 2002, reports this discovery under the title: "A role for skin gd T cells in wound repair." Its co-authors are immunologists at the Scripps Research Institute in La Jolla, Calif. Their findings, they propose, should interest scientists involved in treating diseases that arise from epithelial cell disorders - such as asthma, psoriasis, cancers and inflammatory bowel disease.
Unlike the familiar white blood cells, gamma-delta T cells don't circulate through the bloodstream. Rather, they are the main T-cell component of skin, lung and intestine, where they reside and monitor the neighboring epithelial cells for damage and disease.