Screening for antibiotic combinationsagainst carbapenem-resistant Gram-negative bacteria

Abstract: Carbapenem-resistant Enterobacteriaceae is a growing problem. Not only are they resistant against carbapenems but the genes that encode for carbapenemase production, are often situated amongst other resistance genes making these bacteria resistant to multiple antibiotic classes. This project aimed to screen for new antibiotic combinations with synergistic effects that could be used to combat carbapenem-resistant Enterobacteriaceae. Carbapenem-resistant E. coli and K. pneumonia was tested and the oCelloScope instrument was also evaluated for these strains combined with the antibiotics used. Four antibiotics, aztreonam, meropenem, minocycline and rifampicin were combined with polymyxin B against E. coli. Aztreonam and fosfomycin were combined with polymyxin B against K. pneumoniae. Screening experiments included time-kill experiments, time-lapse microscopy with the oCelloScope instrument and a dot test which combined the oCelloScope with viable counts at 24h. Results showed minocycline combined with polymyxin B to be the best combination with synergistic effects against 9 out of 20 E. coli strains. This was closely followed by rifampicin combined with polymyxin B with synergistic effects against 7 out of 20 E. coli. Meropenem in combination with polymyxin B resulted in synergistic effects against only 1 out of 20 E. coli. The oCelloScope instrument could not detect synergistic effects with aztreonam due to the presence of dead filamented cells, which the instrument detected as a relatively high concentration of cells, but could be determined with the dot test method showing synergistic effects against 2 out of 20 E. coli with this combination.