Comparison of methods for the detection of in vitro synergy in multidrug-resistant gram-negative bacteria

• Published in: BMC microbiology 2020-04, Vol.20 (1), p.97-97, Article 97
• Main Authors: Gaudereto, Juliana Januario, Neto, Lauro Vieira Perdigão, Leite, Gleice Cristina, Espinoza, Evelyn Patricia Sanchez, Martins, Roberta Cristina Ruedas, Villas Boa Prado, Gladys, Rossi, Flavia, Guimarães, Thais, Levin, Anna Sara, Costa, Silvia Figueiredo
• Format: Article
• Language: English
• Subjects: Agreements; Amikacin; Amikacin - pharmacology; Antibiotics; Antimicrobial agents; Assaying; Bacteria; Clinical isolates; Clinical microbiology; Colistin; Colistin - pharmacology; Comparative analysis; Disk approximation; Disk Diffusion Antimicrobial Tests; Drug Combinations; Drug Resistance, Multiple, Bacterial; Drug Synergism; Fosfomycin; Gram-negative; Gram-negative bacteria; Gram-Negative Bacteria - drug effects; Gram-Negative Bacteria - genetics; Health aspects; In vitro methods and tests; Infection; Infections; Laboratories; Mathematical analysis; Meropenem; Methods; MIC:MIC ratio; Microbial drug resistance; Microbial Sensitivity Tests - methods; Microbial Viability - drug effects; Microbiology; Minimum inhibitory concentration; Mortality; Multidrug resistance; Multidrug resistant organisms; Multidrug-resistant; Organisms; Pathogens; Pseudomonas aeruginosa; Statistical analysis; Studies; Synergism; Synergy; Time-kill
• ISSN: 1471-2180; 1471-2180
• DOI: 10.1186/s12866-020-01756-0

The use of combined antibiotic therapy has become an option for infections caused by multidrug-resistant (MDR) bacteria. The time-kill (TK) assay is considered the gold standard method for the evaluation of in vitro synergy, but it is a time-consuming and expensive method. The purpose of this study was to evaluate two methods for testing in vitro antimicrobial combinations: the disk diffusion method through disk approximation (DA) and the agar gradient diffusion method via the MIC:MIC ratio. The TK assay was included as the gold standard. MDR Gram-negative clinical isolates (n = 62; 28 Pseudomonas aeruginosa, 20 Acinetobacter baumannii, and 14 Serratia marcescens) were submitted to TK, DA, and MIC:MIC ratio synergy methods. Overall, the agreement between the DA and TK assays ranged from 20 to 93%. The isolates of A. baumannii showed variable results of synergism according to TK, and the calculated agreement was statistically significant in this species against fosfomycin with meropenem including colistin-resistant isolates. The MIC:MIC ratiometric agreed from 35 to 71% with TK assays. The kappa test showed good agreement for the combination of colistin with amikacin (K = 0.58; P = 0.04) among the colistin-resistant A. baumannii isolates. The DA and MIC:MIC ratiometric methods are easier to perform and might be a more viable tool for clinical microbiology laboratories.