Methodological implications of testing anaerobe susceptibility to cephalosporins (cefazolin, cefamandole, cefoxitin)

Krasemann, C.; Werner, H.; Benning, Matthew M.; Brandt, U.; Ungerechts, J.; Hildenbrand, G.

doi:10.1007/bf01639888

Cited by 1 publication

(1 citation statement)

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“…The Alamar method tended to produce lower MICs than did agar dilution if the organism was very susceptible to the ,B-lactam agent but higher MICs if the organism produced an inducible or extended-spectrum r-lactamase. Other investigators have reported this dichotomous effect when agar dilution and broth dilution were compared (5,7,10,13,19). Our additional testing of ampicillin, ampicillinsulbactam, gentamicin, and imipenem suggests that we might have obtained better agreement of results between the Alamar system and the reference method if we had chosen broth microdilution as the reference method for all of the agents except imipenem.…”

Section: Discussionmentioning

confidence: 97%

Evaluation of Alamar colorimetric MIC method for antimicrobial susceptibility testing of gram-negative bacteria

1994

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The Alamar (Alamar Biosciences, Inc., Sacramento, Calif.) colorimetric antimicrobial susceptibility testing method is a new approach to the determination of broth microdilution MICs. The method uses a color indicator to detect growth of microorganisms within the wells of a microdilution tray. The color changes can be read visually or with a fluorometer. The system contains growth and sterility control wells and 20 antimicrobial agents per MIC tray with eight twofold dilutions for each antimicrobial agent. We tested 186 multiresistant, gram-negative bacterial isolates against 33 antimicrobial agents and compared the results to those obtained by agar dilution. Categorical agreement for all agents was 90.9% and ranged from 78.2% for ampicillin-sulbactam to 98.1% for amikacin. Percent agreement for MIC results (within ± 1 log2 dilution) was 91.0% for all agents and ranged from 69.1% for gentamicin to 97.9% for ciprofloxacin. Most of the disagreements were with the penicillins and cephalosporins for jI-lactamase-producing strains. The Alamar MIC system is very easy to read visually and appears to be a satisfactory addition to currently used MIC determination methods. The method most commonly used to determine MICs in clinical laboratories is the broth microdilution method described by the National Committee for Clinical Laboratory Standards (NCCLS) (17). This method is convenient to use, since the reagents can be prepared in house or purchased from commercial manufacturers. The reagents may be stored frozen or lyophilized, require only a small amount of storage space, and can be removed from storage as needed. A modification of this technique has been developed by Alamar Biosciences, Inc., Sacramento, Calif. This manufacturer has used the conventional broth microdilution method and added a color indicator to enhance the detection of growth within the microdilution wells. The colorimetric growth indicator is based on detection of metabolic activity. Specifically, the system incorporates an oxidation-reduction indicator that changes color in response to chemical reduction of growth medium resulting from bacterial growth. The indicator can be read visually or with a fluorometer. Other manufacturers have included fluorogenic compounds (3, 16, 21) for detection of growth in a 5to 10-h period, but the results must be read with a fluorometer. The results of the Alamar method, on the other hand, can be read visually, and the indicator used makes reading easy because the change from blue (no growth) to red (growth) is obvious, even for wells with reduced or small amounts of growth. This overcomes problems frequently encountered in conventional broth microdilution tests, such as inoculum sedimentation or very scant or transparent growth, which occur with some species of bacteria. This study assessed the reliability of results obtained by the Alamar colorimetric method, compared with agar dilution results, by using a well-characterized set of difficult-to-test, multiresistant, gram-negative bacteria (the Centers for Disease Cont...

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Section: Discussionmentioning

confidence: 97%