Clinical microbiology laboratories are faced with the challenge of accurately detecting emerging antibiotic resistance among several important bacterial pathogens. Certain of these are fastidious organisms that require enriched media and modified growth conditions for reliable susceptibility testing, whereas some other organisms have subtle or inducible resistance mechanisms that may require special screening tests for reliable recognition (e.g., vancomycin-resistant enterococci, staphylococci with reduced susceptibility to vancomycin, methicillin-resistant staphylococci, and gram-negative bacilli that produce extended-spectrum β-lactamases [ESBLs]). Clinical laboratories may not be able to rely on a single susceptibility testing method or commercial system to detect all of these emerging resistant organisms. For reliable detection, it may be necessary to employ conventional broth or agar dilution MIC procedures, special fixed-drug-concentration screening tests, and modified antibiotic interpretive breakpoints that increase the opportunity for recognizing resistant strains.

Facultative bacteria, The standard broth microdilution, agar dilution, and disk diffusion tests can be adapted to test several nutritionally fastidious bacterial species by appropriate modification of the test medium and the incubation conditions influenzae can be tested with use of a supplemented Mueller-Hinton medium referred to as haemophilus test medium. HTM includes hematin, nicotinamide adenine dinucleotide, and yeast extract as supplements for growth of Haemophilus species, and it requires incubation in a 5% CO2 atmosphere when used in the agar formulation. Despite these additives, the broth and agar versions of HTM are transparent and only slightly darker than Mueller-Hinton medium.

Quality control and interpretive criteria have been published for a large number of parenteral and oral agents useful in treating infections caused by these organisms, although routine testing of agents other than ampicillin and trimethoprim/sulfamethoxazole may not be needed to manage most respiratory infections due to Haemophilus species. Indeed, H. influenzae isolates are predictably susceptible to a number of combinations of b-lactam/b-lactamase inhibitors, later-generation cephalosporins and fluoroquinolones.

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Microbiology: Current Research
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