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It is difficult to imagine undertaking today's surgical procedures, transplantations, cancer chemotherapy, or care of the critically ill or HIV-infected without effective antimicrobial agents. Bacteria are champions of evolution, and a few microbes have adapted to a point where they pose serious clinical challenges for humans. Among the gram-positive organisms, methicillin-resistant Staphylococcus aureus (MRSA) and E. faecium represent the biggest therapeutic hurdles (see table and slide presentation). The evolution of MRSA exemplifies the genetic adaptation of an organism into a first-class multidrug-resistant pathogen. After the introduction of penicillin and, later, methicillin, S. aureus quickly developed resistance to these β-lactam compounds, and by 2003, more than 50% of S. aureus isolates recovered in U.S. hospitals were MRSA.
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MRSA has also recently emerged as an important cause of infections outside hospitals. Community-associated MRSA is now the leading cause of identifiable skin and soft-tissue infections seen in U.S. emergency rooms. Such MRSA frequently causes severe infections resembling spider bites, as well as severe necrotizing fasciitis and pneumonia, and it often produces toxins such as the Panton–Valentine leukocidin and cytolytic peptides. It has also acquired genes that may increase its ability to survive. A single clone, USA300, is responsible for most community-associated MRSA infections in the United States.3 Although such MRSA is commonly susceptible to oral antibiotics such as clindamycin, fluoroquinolones, trimethoprim–sulfamethoxazole, tetracyclines, and rifampin, some multidrug-resistant strains are emerging.
Though less virulent than MRSA, enterococci have long represented therapeutic problems, initially because of their "tolerance" to penicillin and vancomycin (which inhibit but don't kill them). Enterococci are the third most common cause of infective endocarditis, and the effect of penicillin tolerance on therapeutic outcomes was apparent by the late 1940s, when it became routine to add an aminoglycoside to penicillin in treating this disease. High-level resistance to all aminoglycosides is increasing, however, so that the synergistic and bactericidal activity of the combination of a cell-wall agent and an aminoglycoside is no longer effective against some isolates of enterococci from endocarditis.
More worrisome is the increased occurrence of E. faecium infections, since the majority of E. faecium isolated in U.S. critical care units is now resistant to vancomycin (more than 90% of VRE isolates in the United States are E. faecium) and to ampicillin (almost 100% of isolates are resistant), with some strains having developed resistance to the newer antibiotics as well. No appropriate therapy for VRE endocarditis has been defined,4 and no agent has been approved by the Food and Drug Administration for this indication. The emergence of multidrug-resistant E. faecium correlates with the predominance of a single genetic lineage worldwide; members of this lineage have acquired genetic determinants that appear to increase their success in the hospital environment, and some have developed resistance to practically all available antibiotics.
Despite the recent dramatic reduction in antibiotic research by pharmaceutical companies, several compounds have been developed or resurrected to treat gram-positive infections. However, the available agents have important limitations: none have been shown to work better than vancomycin against MRSA; quinupristin–dalfopristin and linezolid have important toxic effects, and resistance to each has been observed (including linezolid-resistant VRE in patients who have never received the drug); daptomycin has sometimes failed against MRSA and enterococci, and resistance to it has emerged; and there are few data regarding tigecycline for enterococcal infections, and its low blood levels raise concern about its use in bacteremia. Among the agents in late stages of clinical development, the new cephalosporins (ceftobiprole and ceftaroline) will not be clinically useful against ampicillin-resistant E. faecium; dalbavancin, telavancin, and oritavancin will have important limitations for the treatment of vancomycin-resistant organisms; and although iclaprim may have a role in MRSA infections, its clinical usefulness against enterococci has not been demonstrated.
The situation is even more dire when it comes to nosocomial gram-negative infections, since no new antibiotics against these multidrug-resistant organisms are in advanced stages of clinical development. Though multidrug-resistant Pseudomonas aeruginosa and acinetobacter are the best-known therapeutic challenges among the gram-negative bacteria (e.g., multidrug-resistant acinetobacter species are causing enormous challenges in soldiers returning from Iraq and Afghanistan), resistance to the most potent antibiotics has recently extended to members of the Enterobacteriaceae family, including hospital-associated strains of klebsiella, Escherichia coli, and enterobacter. Equally worrisome is the fact that multidrug-resistant gram-negative organisms have been found in otherwise healthy patients outside of hospitals — for instance, urinary tract infections caused by E. coli that is resistant to trimethoprim–sulfamethoxazole, fluoroquinolones, or both and that produce extended-spectrum β-lactamases (enzymes capable of destroying the most potent cephalosporins),5 and recent major outbreaks of food poisoning caused by multidrug-resistant salmonella.
Until recently, carbapenems, such as imipenem, were almost uniformly active against resistant gram-negative organisms, but some strains have now developed very effective ways to deal with the carbapenems, including the production of β-lactamases (designated carbapenemases) that demolish the carbapenems; changes in outer-membrane porins that block the entry of these antibiotics; and active pumping of the antibiotic out of the cell using complex "efflux pumps." The situation is further complicated by the fact that the "permeability" barrier and efflux mechanisms also affect other classes of antibiotics (e.g., quinolones, aminoglycosides, and tigecycline). Moreover, the common presence of these β-lactamase genes of gram-negative bacteria in transferable mobile elements means that these genes could reach virtually any gram-negative bacterium and become a major threat in the future. Recognition of the presence of a carbapenemase in a gram-negative organism is of paramount importance, since strict infection-control measures are required to avert hospital epidemics and the dissemination of these genes to other gram-negative species.
Faced with this gloomy picture, 21st-century clinicians must turn to compounds developed decades ago and previously abandoned because of toxicity — or test everything they can think of and use whatever looks active. The resurrected polymixins (e.g., colistin with or without rifampin) are often the only available alternative for some pan-resistant gram-negatives, particularly acinetobacter, although toxicity (mainly renal) is still a problem, and reports of resistance are emerging.
It is more difficult than ever to eradicate infections caused by antibiotic-resistant "superbugs," and the problem is exacerbated by a dry pipeline for new antimicrobials with bactericidal activity against gram-negative bacteria and enterococci. A concerted effort on the part of academic researchers and their institutions, industry, and government is crucial if humans are to maintain the upper hand in this battle against bacteria — a fight with global consequences.
Dr. Arias reports receiving a lecture fee from Merck and grant support from Pfizer. Dr. Murray reports receiving grant support from Johnson & Johnson, Astellas, and Intercell and serving as a consultant for Astellas Pharma and Theravance, Cubist, Targanta Therapeutics, Johnson & Johnson, Pfizer, AstraZeneca, and Wyeth-Ayerst. No other potential conflict of interest relevant to this article was reported.
Source Information
Dr. Arias is an assistant professor of medicine at the University of Texas Medical School, Houston, and director of the Molecular Genetics and Antimicrobial Resistance Unit, Universidad El Bosque, Bogotá, Colombia. Dr. Murray is a professor and the vice-chair for research in the Department of Internal Medicine and the director of the Division of Infectious Diseases, University of Texas Medical School, Houston.
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