OBJECTIVES:In Australia, resistance to β-lactam-class antibiotics in Enterobacterales has increased. This study aimed to identify potential oral treatments for these multi-drug-resistant infections by investigating four β-lactam (mecillinam, tebipenem, sulopenem and faropenem) and three non-β-lactam (fosfomycin, omadacycline and delafloxacin) antibiotics, and to elucidate the genetic resistance mechanisms.
METHODS:Seventy-four E. coli and twenty-four K. pneumoniae, with extended-spectrum β-lactamase or plasmid-mediated AmpC β-lactamases genes, were isolated from New South Wales hospital patients in 2021. Minimum inhibitory concentrations (MIC) were determined using Clinical and Laboratory Standards Institute guidelines with agar dilution for fosfomycin and mecillinam, and broth microdilution for the remaining antibiotics. Multiplex polymerase-chain-reaction and whole genome sequencing (Illumina) confirmed bacterial resistance mechanisms.
RESULTS:The E. coli were mostly susceptible to mecillinam (97%), and had low MIC90 to tebipenem, sulopenem and faropenem (0.125mg/L, 0.5mg/L and 4mg/L). K. pneumoniae were mostly susceptible to mecillinam (96%), and had low MIC90 to tebipenem and sulopenem, but higher MIC90 to faropenem (2mg/L, 2mg/L and 8mg/L). Moreover, the E. coli were mostly susceptible to fosfomycin and omadacycline, but not delafloxacin (100%, 95% and 16%); K. pneumoniae susceptibilities to these antibiotics were 92%, 50% and 17%, respectively. Resistance mechanisms included tet(A) and ramR mutations for omadacycline; gyrA, parC and qnr mutations for delafloxacin; and ompK36 gene deletions for fosfomycin, mecillinam, tebipenem, sulopenem and faropenem.
CONCLUSIONS:The favourable results support the use of mecillinam, tebipenem, sulopenem, faropenem, fosfomycin and omadacycline against multi-drug resistant E. coli. Mecillinam, sulopenem and fosfomycin may be useful for multi-drug resistant K. pneumoniae in Australia.