BACKGROUND: Carrying antimicrobial resistance genes is a burden to bacteria. Therefore, in the absence of antimicrobial selective pressure, susceptible bacteria are expected to replace resistant ones. The cost was reported to decrease with time, but the effect of different species of susceptible bacteria on extended-spectrum -lactamase (ESBL)-, AmpC beta-lactamase-, and VIM-2 metallo-beta-lactamase-producing gram-negative bacilli are not known. The aim of this study was to determine the effect in vitro. METHODS: Antimicrobial-susceptible and -resistant strains of Escherichia coli, Enterobacter aerogenes, Klebsiella p neumoniae, and Acinetobacter baumannii were subcultured daily in glucose limited minimal salt medium at 30degrees C and 37degrees C, and the numbers of cells (CFU/mL) were determined by culturing on Mueller-Hinton agar and MacConkey agar plates. RESULTS: Continued incubation without subculture of both individual and mixed cultures at 37degrees C showed higher counts of a ESBL-producing K. p neumoniae than a susceptible E. coli. Daily subcultures of two strains in a tube showed the counts were : ESBL-producing K. pneumoniae >susceptible E. coli; susceptible E. aerogenes >ESBL-producing K. p neumoniae; susceptible E. aerogenes >VIM-2-beta-lactamase-producing Acinetobacter baumannii. The counts were similar for susceptible K. p neumoniae and AmpC beta-lactamasehyperproducing E. aerogenes. Initial low count of a susceptible E. coli and an ESBL-producing K. p neumoniae at 30degrees C gradually increased with continued subculture. CONCLUSION: Growth of not all resistant bacteria are slower and the growth improves with continued subculture. Coexistence of a susceptible bacteria with resistant bacteria in GLMS medium both at 30degrees C and 37degrees C does not reduce the number of resistant bacteria.