Ampicillin resistance in Haemophilus influenzae—test methods for the activity of acylureidopenicillins, cephamycins and new cephalosporins

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Abstract
Tests for ampicillin susceptibility and β-lactamases (confirmed by clavulanic acid inhibition) divided nine Haemophilus influenzae strains into three groups:- susceptible, resistant β-lactamase producers, and resistant non β-lactamase producers. Susceptibility of the strains to the acylureidopenicillins, cefotaxime, thienamycin, moxalactam, ceftazidime, cefoperazone, and cefotetan, was determined for stationary-phase inocula with high and low osmotic media to detect spheroplasts. The effect of inoculum size and metabolic state was tested on the resistant non β-lactamase producers, which formed a definite group, comparatively resistant to most β-lactamase stable antibiotics. Even low inocula were ampicillin-resistant. The β-lactamase producers were relatively resistant to cefoperazone, and the acylureidopenicillins. Osmolarity affected ampicillin, cefuroxime, and thienamycin. Cefotetan and cefoxitin, although β-lactamase stable, were relatively weak, while ceftazidime and cefotaxime were very active on all strains, and unaffected by osmolarity, inoculum size or metabolic state.