Multicenter Evaluation of Colistin Broth Disk Elution and Colistin Agar Test: a Report from the Clinical and Laboratory Standards Institute

Abstract

Susceptibility testing of the polymyxins (colistin and polymyxin B) is challenging for clinical laboratories. The Clinical and Laboratory Standards Institute (CLSI) Antimicrobial Susceptibility Testing Subcommittee evaluated two methods to enable accurate testing of these agents. These methods were a colistin broth disk elution (CBDE) and a colistin agar test (CAT), the latter of which was evaluated using two inoculum volumes, 1 μl (CAT-1) and 10 μl (CAT-10). The methods were evaluated using a collection of 270 isolates of Enterobacterales, 122 Pseudomonas aeruginosa isolates, and 106 Acinetobacter spp. isolates. Overall, 94.4% of CBDE results were in essential agreement and 97.9% in categorical agreement (CA) with reference broth microdilution MICs. Nine very major errors (VME; 3.2%) and 3 major errors (ME; 0.9%) were observed. With the CBDE, 98.6% CA was observed for Enterobacterales (2.5% VME, 0% ME), 99.3% CA was observed for P. aeruginosa (0% VME, 0.7% ME), and 93.1% CA was observed for Acinetobacter spp. (5.6% VME, 3.3% ME). Overall, CA was 94.9% with 6.8% VME using CAT-1 and improved to 98.3% with 3.9% VME using CAT-10. No ME were observed using either CAT-1 or CAT-10. Using the CAT-1/CAT-10, the CA observed was 99.4%/99.7% for Enterobacterales (1%/0.5% VME), 98.7%/100% for P. aeruginosa (8.3%/0% VME), and 88.5%/92.3% for Acinetobacter spp. (21.4%/14.3% VME). Based on these data, the CLSI antimicrobial susceptibility testing (AST) subcommittee endorsed the CBDE and CAT-10 methods for colistin testing of Enterobacterales and P. aeruginosa.

Evaluation of Calcium-Enhanced Media for Colistin Susceptibility Testing by Gradient Agar Diffusion and Broth Microdilution

“…Study isolates. Study isolates were comprised of 87 challenge isolates from the ahWG collection (14) and 140 isolates collected from CUIMC ( Table 1). All isolates were obtained from patient samples that had undergone clinical laboratory testing for colistin or polymyxin susceptibility as part of routine care and were selected to represent balanced proportions that were susceptible and resistant to polymyxins.…”

“…Frozen BMD panels were made by Accelerate Diagnostics with polystyrene trays containing rows of wells with doubling concentrations of colistin (Sigma-Aldrich, St. Louis, MO) ranging from 0.25 to 16 g/ml without polysorbate 80. Two of these rows were used to determine the reference MIC by reference BMD (rBMD) (14). An additional row, which consisted of Sigma Mueller-Hinton broth (MHB) in which the calcium content was adjusted to a final concentration of 5 mM, was used to assess the CE-BMD as previously described (13).…”

“…CLSI breakpoints were used for determination of categorical agreement (CA; the percentage of isolates that have the same categorical interpretation). Given the lack of a susceptible category, intermediate results were considered susceptible for the performance calculations (14). Accordingly, isolates with a MIC of Յ2 g/ml were considered susceptible and those with a MIC of Ն4 g/ml were considered nonsusceptible.…”

“…The colistin testing ad hoc working group (ahWG) was convened by CLSI to address the challenges of polymyxin susceptibility testing and assess promising novel testing methods. We completed a multicenter study to evaluate the colistin broth disk elution (CBDE) and colistin agar test (CAT), which were both provisionally approved in June 2019 for Enterobacterales and P. aeruginosa by the CLSI Antimicrobial Susceptibility Testing (AST) Subcommittee (14). Clinical isolates gathered from this multicenter study were made available to members of the ahWG to assess the performance of other novel methods.…”

Bacterial Contaminants and Antibiogram of Ghana paper currency notes in circulation and its associated Health risks in Asante-Mampong, Ghana

“…The study tested 10 antibiotic discs of the most commonly used drugs to treat human and animal infections caused by bacteria. These included Erythromycin (ERY) [5µg], Ciprofloxacin (CIP) [5µg], Gentamycin (GEN) [10µg], Ampicillin (AMP) [10µg], Amoxicillin (AMX) [5µg], Vancomycin (VAN) [30µg], Tetracycline (TET) [30µg], Chloramphenicol (CHL) [30µg], Amikacin (AMK) [30µg], and Cotrimoxazole (COT)[25µg][49,50].…”