Empirical monotherapy with meropenem or combination therapy: the microbiological point of view.

The increase in the number of clinical isolates of multiresistant Enterobacteriaceae and Pseudomonas aeruginosa raises problems in decision-making on empirical treatments for severe Gram-negative bacilli-associated infections. The aim of our study is to determine the resistance of meropenem in our setting and the co-resistance of a combination of this compound with two antibiotics from different families: amikacin and ciprofloxacin. Between 2009 and 2013, a total of 81,310 clinical isolates belonging to the main species of Enterobacteriaceae and 39,191 clinical isolates of P. aeruginosa isolated in 28 hospitals in the Valencian Community on the South East Mediterranean Coast of Spain were analyzed using data provided by RedMiva (microbiological surveillance network of the Valencian Community). Meropenem resistance in Enterobacteriaceae increased from 0.16 % in 2009 to 1.25 % in 2013. Very few Enterobacteriaceae strains resistant to meropenem were sensitive to ciprofloxacin; in contrast, the combination of meropenem and amikacin led to a marked decrease in the risk of the microorganisms being resistant to both drugs (RR = 34 in 2013). In the case of P. aeruginosa, meropenem resistance also increased (from 14.32 % in 2009 to 24.52 % in 2013). Most meropenem-resistant P. aeruginosa isolates were also resistant to fluoroquinolones. However, the addition of amikacin led to a more than three-fold decrease in the risk of resistance. In our setting, empirical treatment with meropenem is adequate in enterobacterial infections, but poses difficulties when infection due to P. aeruginosa is suspected, in which case a combination of meropenem and amikacin has been shown to have a higher microbiological success rate.