False penicillin resistance in Neisseria meningitidis following direct susceptibility tests from blood cultures

Blood cultures drawn from a patient with clinically diagnosed invasive meni ngococcal disease, who had been previously administered benzylpenicillin, h ad beta -lactamases added to increase the probability of recovery of the ca usative organism. The blood cultures subsequently yielded Neisseria meningi tidis but direct susceptibility tests by the comparative disk diffusion met hod demonstrated greatly reduced zones of inhibition to penicillin (I 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 beta -lactamases showed the same effect of f alse penicillin resistance, owing to carry over of sufficient beta -lactama se from blood culture bottles during inoculation of direct susceptibility p lates to inactivate the penicillin in the disks. Direct susceptibility test s to beta -lactam agents should not be carried out on positive blood cultur es to which beta -lactamases have been added.