Increased resistance to amikacin in a neonatal unit following intensive amikacin usage

Gram-negative isolates from blood and cerebrospinal fluid were monitored for 1 year before and for 1 year after the first-line aminoglycoside in a busy pediatric department was changed from gentamicin to amikacin. In the general pediatric wards, the switch to amikacin resulted in no change in resistance of nosocomial gram-negative infections to either amikacin (0% before and after) or gentamicin (23.9% [before] versus 26.5% [after]). In the neonatal unit, the switch to amikacin was followed by an outbreak of Serratia spp. that were commonly resistant to amikacin but susceptible to gentamicin. This outbreak abated spontaneously. In the year after the change in aminoglycoside usage, the resistance to amikacin of nosocomially acquired gram-negative infections increased from 7.6 to 27.7% (P less than 0.001), and the resistance to gentamicin decreased from 71.2 to 60.2% (P = 0.07). The increase in amikacin resistance of gram-negative bacilli other than Serratia spp. has persisted for more than a year after the introduction of amikacin as the sole aminoglycoside. The different effects observed in the two sections of the pediatric department may be related to the more intensive usage of aminoglycosides in the neonatal unit.