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New Technologies for Infection

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| 1276NewTechnologiesfor InfectionDiagnosis andControlhree major options exist for the controlof bacterial diseases: 1) disrupt or halttransfer of bacteria from person to per-son and from the environment to people,2) treat cases of disease with antibiotics, and3) prevent disease through vaccination. Thischapter describes diagnostic methods that guidethe selection and use of antibiotics, the use ofvaccines, methods for delivery of high concen-trations of antibiotics to areas of localized infec-tions, devices and materials designed to reducethe transfer of bacteria in the hospital, and sometreatment methods used before the antibiotic age.The cartoon, which is adapted from one thatoriginally appeared in Science, is a humorouslook at the serious problem posed by bacteriaresistant to all available antibiotics. Some bacte-ria are expected to develop resistance to any anti-biotic introduced into medical practice.Therefore, continued improvement in infectiondiagnosis and control is necessary to optimizethe use of antibiotics and slow the spread ofresistant bacteria.DIAGNOSTIC METHODSIn the future, science may develop a smalldevice, such as the “tricorder” used in the TVseries Star Trek, that physicians can pass over thebody of a sick person to identify the cause of adisease. Such methods are far in the future, andcurrent techniques used to identify bacteria andsusceptibility patterns are “traditional methods”that have been developed over the last century.Newer methods that involve techniques frommolecular biology and modern instrumenta-tion—not immediately at the level of Star Trek—promise to make identification and characteriza-tion faster and more certain.❚ Traditional Methods for the Identification of BacteriaSome experts estimate that there may be a mil-lion different bacteria and that scientists haveidentified only one percent (10,000 species) ofthat total. Of those 10,000, only a fraction havebeen associated with human diseases.When seeing a patient, a physician will askquestions, make observations, and perform teststo determine which bacteria are likely to be asso-ciated with an illness and to choose an antibiotictreatment. The physician may swab the throat inthe case of a sore throat or obtain a sample ofurine in the case of a urinary tract infection. Thecollected material on the swab or the urine can bestained with diagnostic dyes, such as the Gramstain (see chapter 2), and examined under aT128 Impacts of Antibiotic-Resistant BacteriaCopyright @ 1995 Sidney Harris. Reprinted with permission,microscope. Distinctive shapes and stainingproperties facilitate reliable preliminary rapididentification of the bacteria causing infection.Collected samples may contain such low num-bers of bacteria as to make finding them underthe microscope difficult. The staining propertiesand shapes of the bacteria may not be unique andtherefore not identifiable. The sample may con-tain a mixture of bacteria, as is common in faecalsamples. To identify the bacteria in those cases,the physician sends a biological sample of somekind—a volume of blood or pus or other exudate,a scraping or swab from the throat or otherorifice, a sample of urine or feces—to a micro-biology laboratory.In the laboratory, the sample is transferred toculture media specifically designed to encouragethe growth of certain pathogenic bacteria and toprevent the growth of others such as commensalbacteria that may be present in samples fromboth healthy and sick individuals. The bacteriathat are able to grow form visible colonies onagar-based media in a Petri dish or grow in brothso that the broth becomes turbid, as apple ciderdoes when yeast grow in it. In both the collectionand handling of the sample, health care personnelmust be careful to avoid contamination with thebacteria that grow literally everywhere, on thepatient’s and physician’s skin, on the surfaces offurniture and unsterilized devices in the examin-ing room, and on apparatus in the diagnostic lab-oratory.Microbiologists can sometimes look at andsmell the colonies or liquid cultures and, basedChapter 6 New Technologies for lnfection Diagnosis and Control 129on their knowledge and experience, identify thebacteria in the sample. They may be able to dis-miss some bacteria from further consideration byrecognizing them as contaminants. Iterative testswith more selective media and biochemical testsmay be used for more specific identification.Culturing and identification take time. Theshigella that might be present in a fecal sample,or the Escherichia coli that frequently cause uri-nary tract infections, grow quickly, forming col-onies in 24 hours or so, and a laboratory wouldprobably identify them in one or two days. Myco-bacterium tuberculosis grows far more slowly,and six weeks may pass before traditional meth-ods can be used to identify it.Identifying the bacteria is often critical forchoosing the most appropriate antibiotic therapybecause some antibiotics work better against cer-tain bacteria. But identification does not provideinformation about whether the bacteria are resis-tant to the antibiotic or susceptible to it. “Suscep-tibility tests” are used to determine that.Traditional Susceptibility TestsInformation about antibiotic-resistance/suscepti-bility is developed by testing the bacteria isolatedfrom the infection against six to 12 different anti-biotics, or more if necessary. The results fromthese tests may support the use of the antibioticthat was empirically selected by the physician,indicate that other antibiotics would work aswell, or show that the disease-causing bacteriaare resistant to the antibiotic empirically chosen.Jorgensen (1995) describes four methods thatare currently used to determine the antibioticsusceptibility or resistance of bacteria: 1) diskdiffusion tests, 2) broth dilution tests, 3) agardilution tests, and 4) agar gradient methods.Disk diffusion testsDisk diffusion tests measure the size of a cleararea of no bacterial growth around a sterile paperdisk containing antibiotic. The size of this area,called the “zone of inhibition,” can be measuredand reported directly, or the measurement can becompared to criteria established by the NationalCommittee for Clinical Laboratory StandardsPhoto of a disk diffusion (Kirby-Bauer) susceptibility test plate.Courtesy of James H. Jorgensen, University of Texas HealthScience Center, San Antonio, TX, 7995.(NCCLS) to classify the bacteria as susceptible,intermediate or resistant


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