Abstract

Blood cultures drawn from a patient with clinically diagnosed invasive meningococcal disease, who had been previously administered benzylpenicillin, had β-lactamases added to increase the probability of recovery of the causative organism. The blood cultures subsequently yielded Neisseria meningitidis but direct susceptibility tests by the comparative disk diffusion method demonstrated greatly reduced zones of inhibition to penicillin (1 unit disk). Repeat testing from subcultures showed full penicillin sensitivity. Inoculation of blood culture bottles with a variety of penicillin sensitive bacteria with the addition of β-lactamases showed the same effect of false penicillin resistance, owing to carry over of sufficient β-lactamase from blood culture bottles during inoculation of direct susceptibility plates to inactivate the penicillin in the disks. Direct susceptibility tests to β-lactam agents should not be carried out on positive blood cultures to which β-lactamases have been added.