Relationship between in vitro susceptibility test results for chloramphenicol and production of chloramphenicol acetyltransferase by Haemophilus influenzae, Streptococcus pneumoniae, and Aerococcus species

Abstract

Haemophilus influenza, Streptococcus pneumoniae, and Aerococcus species were tested for susceptibility to chloramphenicol by standard broth microdilution and disk-diffusion methods. MICs and zone diameter breakpoints were correlated with production of chloramphenicol acetyltransferase (CAT). A comparison of MICs and zone diameters indicated that the interpretative criteria for H. infiuenzae and S. pneumoniae should be an MIC of-4 ,ug/ml or a zone diameter-25 mm for susceptible strains and an MIC of-8 ,ug/ml or a zone diameter of-20 mm for resistant strains; for Aerococcus species, interpretative criteria should be an MIC of c8 ,ug/ml or a zone diameter of-20 mm for susceptible strains and an MIC of .32 ,ug/ml or a zone diameter of s 12 mm for resistant strains. Ail but four strains of H. infiuenzae and one strain of S. pneumoniae that were resistant to chloramphenicol by these criteria produced CAT. For Aerococcus species, however, chloramphenicol-resistant strains were negative for CAT as determined by a commercially available disk test. When comparing susceptibility results with CAT production, thiamphenicol was a better indicator of the presence of the enzyme than chloramphenicol and may be useful in assaying resistance to chloramphenicol. Detecting chloramphenicol resistance in Haemophilus influenzae and Streptococcus pneumoniae when trying to relate results of in vitro susceptibility tests to chloramphenicol acetyltransferase (CAT) production has been a problem which has been of increasing relevance since chloramphenicol resistance has occurred in these species (2-5, 7). Recently, the National Committee for Clinical Laboratory Standards (NCCLS) revised the breakpoints for H. influenzae to better correlate CAT production with chloramphenicol-resistant strains of H. influenza (12). The NCCLS did not, however, change the breakpoints for S. pneumoniae, an organism also known to produce CAT (2, 6, 7). Even though chloramphenicol is not a drug of first choice

Use of commercially available rapid chloramphenicol acetyltransferase test to detect resistance in Salmonella species

“…The d-CAT and the t-CAT detection methods yielded identical results, with no difficulty in their interpretation. Similar results were observed by Matthews et al (12) for 91 H. influenza and 113 S. pneumoniae isolates, although the only two chloramphenicol-resistant (MIC, 32 ,ug/ml) Aerococcus spp. in their study were negative for CAT, as determined by the d-CAT assay.…”

Development of interpretive criteria and quality control limits for broth microdilution and disk diffusion antimicrobial susceptibility testing of Streptococcus pneumoniae

“…Chloramphenicol resistance in pneumococci is most often due to production of the inactivating enzyme chloramphenicol acetyltransferase (19,25). Chloramphenicol MICs for such strains are 8 ,ug/ml or greater (19).…”

Multicenter evaluation of the use of Haemophilus test medium for broth microdilution antimicrobial susceptibility testing of Streptococcus pneumoniae and development of quality control limits

“…Since pneumococci for which chloramphenicol MICs are 8 to 16 ,g/ml have been shown to produce the inactivating enzyme chloramphenicol acetyltransferase in prior studies (5,11), the Haemophilus interpretive breakpoints (Table 4) are probably appropriate for use with pneumococci. Therefore, it may not be possible to reduce the incidence of minor interpretive errors between testing with LHB and HTM without creating very major or major errors.…”