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

Abstract

Blood cultures drawn from a patient with clinically diagnosed invasive meningococcal disease, who had been previously administered benzylpenicillin, had -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 diVusion 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 -lactamases showed the same eVect of false penicillin resistance, owing to carry over of suYcient -lactamase from blood culture bottles during inoculation of direct susceptibility plates to inactivate the penicillin in the disks. Direct susceptibility tests to -lactam agents should not be carried out on positive blood cultures to which -lactamases have been added. (J Clin Pathol 2001;54:893-895)

Enteric Fever in a 6-Year-Old Traveler Caused by Salmonella enterica Serotypes Typhi and Paratyphi A: Laboratory Detection Strategies and Treatment Options

“…This procedure not only allowed the identification of a mixed culture but also gave a preliminary indication of significant antimicrobial resistance. While susceptibility results obtained by disk diffusion performed directly from blood culture broth generally agree with those obtained by testing isolated bacterial colonies from subculture (8,27), certain organism/antimicrobial combinations may display false resistance or susceptibility when tested by this method (25,31), and these results must be confirmed using standardized approaches.…”