Sistema FMUSP-HC: Faculdade de Medicina da Universidade de São Paulo (FMUSP) e Hospital das Clínicas da FMUSPGAUDERETO, Juliana JanuarioPERDIGAO NETO, Lauro VieiraLEITE, Gleice CristinaSANCHEZ, EvelynMARTINS, Roberta Cristina RuedasPRADO, Gladys Villas BoasROSSI, FlaviaGUIMARAES, ThaisLEVIN, Anna SaraCOSTA, Silvia Figueiredo2020-06-012020-06-012020BMC MICROBIOLOGY, v.20, n.1, article ID 97, 7p, 20201471-2180https://observatorio.fm.usp.br/handle/OPI/35980Background 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. Results 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. Conclusions The DA and MIC:MIC ratiometric methods are easier to perform and might be a more viable tool for clinical microbiology laboratories.engopenAccessSynergyTime-killDisk approximationMICMIC ratioGram-negativeMultidrug-resistantklebsiella-pneumoniaeacinetobacter-baumanniipolymyxin-bcolistin-resistantpseudomonas-aeruginosaantimicrobial agentsbeta-lactamasesrapid detectiondisk diffusioncombinationarticleCopyright BMC10.1186/s12866-020-01756-0Microbiology