Effect of Coexistence of Antimicrobial-susceptible Gram-negative Bacilli on the Resistant Gram-negative Bacilliin the Absence of Antimicrobial Agents / 감염

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.

Isolation Trend of Enterobacter, Citrobacter, and Serratia for the Recent Five Years and Antimicrobial Susceptibility of Derepressed AmpC beta-lactamase Producing Strains / 감염

BACKGROUND: Derepressed AmpC beta-lactamase producing Enterobacter cloacae, Citrobacter f reundii, and Serratia marcescens are important nosocomial pathogens and the infections are difficult to treat, because they are multi-drug resistant. The aim of this study was to determine the isolation rate and trend, and antimicrobial susceptibility of derepressed strains isolated from clinical specimens. METHODS: E. cloacae, S. marcescens, and C. f reundii isolated from 1996 through 2000 were enrolled in the study. Antimicrobial susceptibility was tested by NCCLS disk diffusion method. Derepressed strain was defined as strain non-susceptible to third generation cephalosporin. The isolation patterns of important gram-negative bacilli with the derepressed strains were analyzed with respect to years, patient's locations and specimens. RESULTS: Among the clinical isolates, the derepressed strains of E. cloacae, S. marcescens, and C. f reundii were 65%, 70%, and 56%. The proportion of the derepressed strains : E. cloacae increased from 68% in 1996 to 71% in 1998, however, decreased to 59% in 2000, S. marcescens increased from 68% in 1996 to 73% in 2000, C. f reundii decreased from 69% in 1996 to 41% in 2000. The proportion of the derepressed strains were high among the isolates from blood and respiratory specimens of inpatient and intensive care patient. The resistance rates of the depressed strains were 47~62% to third generation cephalosporin and aztreonam, 15~85% to aminoglycoside, 68% to cotrimoxazole, and 31% to levofloxacin. CONCLUSION: Among the clinical isolates of E. cloacae, S. marcescens, and C. f reundii, the derepressed strains were as high as 56~70%, and they were commonly isolated from blood and sputum specimens of inpatient and intensive care patient, and showed high resistance rates to the most antimicrobial agents.