In vitro activity of cefoxitin and imipenem against Mycobacterium abscessus complex

The in vitro activity of cefoxitin and imipenem was compared for 43 strains of the Mycobacterium abscessus complex, mostly isolated from cystic fibrosis patients. The MICs of imipenem were lower than those of cefoxitin, although the number of imipenem-resistant strains was higher according to the CLSI breakpoints. Strain comparisons indicated that the MICs of cefoxitin were significantly higher for Mycobacterium bolletii than for M. abscessus. The MICs of both beta-lactams were higher for the rough morphotype than for the smooth morphotype. The clinical impact of the in vitro difference between the activity of imipenem and that of cefoxitin remains to be determined.