Increasing trends in antimicrobial resistance among clinically important anaerobes and Bacteroides fragilis isolates causing nosocomial infections: emerging resistance to carbapenems

Abstract

This study reports data on the susceptibilities to five commonly used antianaerobic agents of five clinically frequently encountered anaerobes from 2000 to 2007 and to Bacteroides fragilis isolates causing nosocomial infections from 1990 to 2006. There was a trend of decreasing susceptibilities of these anaerobes to ampicillin-sulbactam, cefmetazole, chloramphenicol, and clindamycin with time during the study period. The rates of susceptibility to clindamycin and cefmetazole for all clinical isolates of Bacteroides fragilis isolates were higher than those of isolates associated with nosocomial infections. The MICs of 207 anaerobic blood isolates collected in 2006 to 14 antimicrobial agents were determined by the agar dilution method. The rates of nonsusceptibility to imipenem and meropenem were 7% and 12% for B. fragilis isolates (n = 60), 7% and 3% for Bacteroides thetaiotamicron isolates (n = 30), 4% and 4% for Fusobacterium species (n = 27), 6% and 0% for Prevotella species (n = 16), 15% and 0% for Clostridium species (n = 28), and 0% and 0% for Peptostreptococcus species (n = 32). The rates of susceptibility to moxifloxacin were 90% for B. fragilis isolates, 87% for B. thetaiotaomicron isolates, 81% for Fusobacterium species, 75% for Prevotella species, 93% for Clostridium species, and 78% for Peptostreptococcus species. Thirty-six percent of Clostridium species and 12% of Peptostreptococcus species were not susceptible to metronidazole. Comparison of the data with the data from a previous survey from the same institute in 2002 revealed higher rates of nonsusceptibility to carbapenems, especially for B. fragilis, Fusobacterium species, and Prevotella species isolates. The high rates of nonsusceptibility to commonly used antianaerobic agents mandate our attention, and periodic monitoring of the trend of the resistance is crucial.

FIG. 1.

Trends of susceptibility of clinical isolates of common anaerobic pathogens from 2000 to 2007 at National Taiwan University Hospital. Bacteroides fragilis (A), Bacteroides species other than B. fragilis (B), Prevotella species (C), Fusobacterium species (D), and gram-positive cocci other than Peptostreptococcus anaerobius (E). Susceptibilities of the isolates were determined by the breakpoint agar dilution method. An asterisk denotes a significant decreasing trend (P < 0.05) of susceptibility to the indicated antimicrobial agent from 2000 to 2007.

FIG. 2.

Trends of susceptibility of all B. fragilis isolates associated with nosocomial infections from 1990 to 2006 at National Taiwan University Hospital. An asterisk denotes a significant decreasing trend (P < 0.05) of susceptibility to the indicated antimicrobial agent from 1996 to 2006.

FIG. 3.

Differences in rates of nonsusceptibility to imipenem and meropenem for selected clinical isolates of anaerobes isolated between 2002 and 2006 at National Taiwan University Hospital. The numbers of isolates are shown above the bars.