Prophylactic antibiotics significantly decrease the incidence of urinary tract infection following endoscopic prostate resection. They are ineffective in reducing the incidence of postoperative fevers, or in reducing the frequency of positive blood cultures during or after operation. A majority of prostate glands harbour colonies of potentially pathogenic organisms, and these are the commonest source of infection of the urinary tract following release by prostatic surgery.

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Autobac I, a recently introduced semiautomated method for rapid antibiotic susceptibility testing, has been evaluated by comparison with the calibrated dichotomous sensitivity disk diffusion technique, which is routinely used in many Australian hospitals. Only the most common clinical isolates, Staphylococcus aureus, Escherichia coli, Klebsiella sp., and Proteus mirabilis, were included in this evaluation, and an overall interpretive agreement of 93% was obtained. However, an unusually high rate of discrepancy was noted in several organismantibiotic combinations, in particular E. coli and P. mirabilis with ampicillin, S. aureus with penicillin, and methicillin-resistant S. aureus with methicillin, erythromycin, and clindamycin. The discrepancies associated with ampicillin have been reduced from 29 and 24% for E. coli and P. mirabilis, respectively, to less than 5% after the utilization of commercial 10-jug diffusion disks, in preference to the lower antibiotic content disks supplied by the Autobac manufacturer. Furthermore, modifications in the interpretive procedure have eliminated discrepancies associated with S. aureus and penicillin.The manufacturers of Autobac I, Pfizer Diagnostics Inc., claim that their product offers high interpretive agreement (average, 90%; range, 80 to 100%) with the results of the BauerKirby agar diffusion method. These figur6s were confirmed in a collaborative study performed at several United States hospitals (7). Since the Autobac in the Division of Microbiology, Repatriation General Hospital, Concord, was the first to be installed in Australia, a similar evaluation was undertaken to compare Autobac I susceptibility results with those obtained by the calibrated dichotomous sensitivity (CDS) disk diffusion technique. Organism-antibiotic combinations which showed poor correlation were studied in greater detail.MATERILS AND METHODS OrganiSm8. Clinical isolates of S. aureus, E. coli, Klebsiella sp., and P. mirabilis were used. The staphylococci were differentiated into three groups according to their susceptibility to methicillin as determined by the disk diffusion technique, and to their ability to produce penicillinase, this being determined by direct observation of the inhibitory zone surrounding the penicillin disk. A sharp well-defined border or large, discreet colonies at the zone edge indicated penicillinase production (1,5,8