[Ampicillin-resistant "Haemophilus": their detection and occurrence in Brussels area (author's transl)]

Three hundred and five strains of Haemophilus (129 H. influenzae, 55 H. parainfluenzae, 97 H. parahaemolyticus, 5 H. haemolyticus, 10 H. paraphrophilus and 9 H. paraphrohaemolyticus) isolated from pathological material over the year 1976, were systematically tested for beta-lactamase production. Only 2 strains of H. parainfluenzae produced this enzyme. Both were able to transfer ampicillin resistance to Escherichia coli K12. All strains but the two beta-lactamase producers were susceptible to penicillin G, ampicillin and cephalotin. However, the correct interpretation of the susceptibility tests needed the microscopic observation of prints of the inhibition zones surrounding the disks: all sensitive strains presented a hazy growth around the disks which corresponded to the presence of spheroplasts; this phenomenon was not observed with the 2 beta-lactamase producing strains of H. parainfluenzae which grew up to the disks but presented a typical bacillary form.