Australian evaluation of Autobac I with suggested interpretive and technical modifications

Abstract

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

Examination of Pseudomonas aeruginosa-gentamicin discrepancies encountered in an Autobac I-disk diffusion comparison

“…Thus, a "false" reading of susceptible could be obtained with the Autobac I system, depending on the ratio of susceptible strains to resistant strains selected for testing. An extended period of incubation of the Autobac I cuvette has been suggested as a remedy for the erroneous results obtained with S. aureus and the penicillins (3). Our results also indicate that a decrease in the LSI upon extended incubation of the cuvette could serve as a marker for detecting such a mixture.…”

“…In an initial collaborative study, the performance of this system was shown to have a high overall level of agreement (91.5%) with the disk diffusion method (7). However, a number of discrepancies have been reported for several organism-antibiotic combinations (1,3,4,6,7).…”

“…Funnell and Guinness (3) showed that their staphylococci that were methicillin susceptible, as determined by disk diffusion, invariably had LSIs of >0.9, whereas their staphylococci that were methicillin resistant had LSIs of <0.70. Six of our seven discrepant pseudomonads had initial LSIs between 0.6 and 0.97, whereas 152 (96%) of our truly gentamicinsusceptible Pseudomonas sp.…”

“…These organisms were rarely seen in this country at that time. Subsequent reports have shown that the Autobac I system indeed had difficulty detecting methicillin-resistant Staphylococcus aureus (1,3). In one study (1) Susceptibility test methods.…”

Two-hospital study of Staphylococcus aureus susceptibility to penicillin and ampicillin by Autobac I

“…However, some organisms when tested with certain antimicrobial agents produced agreements of less than 90%, including Staphylococcus aureus tested with penicillin G (85.3%). Other workers noted similar disagreements with ampicillin and penicillin (4)(5)(6). Testing of the same strains on subsequlent days results in some strains demonstrating a susceptible result one day and a resistant result the next (9).…”