Comparison of the inhibitory and bactericidal activity of aztreonam and amikacin against gram negative aerobic bacilli

Aztreonam has been compared both in vitro and in clinical trials to aminoglycosides in its activity against aerobic gram-negative bacteria. The inhibitory and killing abilities of aztreonam and amikacin were examined against five gram-negative bacillary strains. Time kill analysis was carried out at serum-achievable concentrations (25 micrograms per ml amikacin, 100 and 200 micrograms per ml aztreonam) and levels found three to five hours post-infusion (8 micrograms per ml amikacin and 25 micrograms per ml aztreonam). Amikacin killed all five strains faster than aztreonam at all the concentrations tested. Regrowth and the presence of persisters were observed in aztreonam-treated cultures. In the presence of amikacin, there was no detectable increase in cell mass, as measured by optical density. However, following aztreonam treatment, bacterial cell mass increased in the first two to three hours before decreasing. Long, filamentous cells were observed in aztreonam-containing cultures. Though amikacin and aztreonam are bactericidal drugs, prolonged bacterial survival, continued cell growth, regrowth, and persisters were observed only in aztreonam-treated cultures.