Revista de terapia y enfermedades infecciosas

Abstracto

Effect of Antibiotics Combination and Comparison of Methods for Detection of Synergism in Multiresistant Gram-Negative Bacteria

Gleice Cristina Leite, Lauro Vieira Perdigão Neto, Juliana Januário Gaudereto, Cláudia Maria Dantas de Maio Carrilho, Flavia Rossi, Anna Sara Levin and Silvia Costa

Background: Because of the rapid spread of antimicrobial resistance and the slow development of novel antimicrobials, Gram-negative infections treatments are challenging clinicians and the Public Health. We aimed to evaluate the activities of antimicrobial combination against MDR Gram-negative bacteria.

Methods: Twenty-eight P. aeruginosa, 20 A. baumannii and 17 K. pneumoniae carbapenem-resistant clinical isolates had MIC determined by broth microdilution. Synergistic effect was investigated using checkerboard method and time-kill assay, the gold standard method. PCR for carbapenemase genes and PFGE clonality assessment were done.

Results: All P. aeruginosa isolates were resistant to meropenem, but susceptible to colistin; the genes blaSPM and blaKPC were found in 82% and 25% of them, respectively; synergistic effect was seen only in combinations of colistin with meropenem (43%), meropenem with amikacin (36%) and colistin with amikacin (7%), by time-kill. Twenty-five P. aeruginosa isolates belonged to the same clonal profile. All 20 isolates of A. baumannii were resistant to meropenem, rifampicin, fosfomycin and harbored blaOXA-51-like; the genes blaOXA-23-like, blaOXA-143-like and blaIMP were found in 50%, 35% and 15% of them, respectively. Eleven in thirteen colistin-susceptible A. baumannii isolates showed distinct profiles and six in seven colistin-resistant isolates belonged to the same clone; synergistic effect was observed in almost all combinations by time-kill. Resistance to polymyxin B and resistance to imipenem were found in 100% and 91% of K. pneumoniae isolates. The blaKPC gene was found in 82% of them; five clusters were identified (each one with two isolates) and other seven different isolates; synergistic effect occurred in most of threedrug combinations.

Conclusion: We demonstrated that time kill assay must be considered the gold standard method to detect synergism in vitro, as it allows greater dynamic assessment and higher sensitivity, when compared to the other methods. We detected that colistin combinations are frequently synergic against A. baumannii and K. pneumoniae. For P. aeruginosa, the results were not that optimistic.