Rapid detection of chloramphenicol resistance in Haemophilus influenzae

We compared a rigid (1-h) screening method for the detection of chloramphenicol acetyltransferase (CAT) activity with the standard spectrophotometric CAT assay to determine whether CAT-mediated chloramphenicol resistance in Haemophilus influenzae could be determined upon primary isolation. Of 58 H. influenzae cell sonicates, 28 had detectable CAT activity when the chloramphenicol-dependent production of free coenzyme A from acetyl coenzyme A was measured spectrophotometrically (standard method). These 28 strains were identified as producing CAT by the rapid method which uses lysed cell suspensions and a color change to detect CAT. The remaining 30 strains did not have CAT activity detectable by either method. This 1-h test for CAT should prove to be useful for the early presumptive identification of chloramphenicol resistance in H. influenzae.