Procedure for Expediting Determinations of Antibiotic Susceptibility of Gram-Negative, Urinary Tract Pathogens

Abstract

Standardized direct disk diffusion antibiotic susceptibility testing on monomicrobial urine specimens is compared with the Food and Drug Administration method. The direct procedure yields acceptable data and may conserve 24 h in reporting results.Disk diffusion antibiograms of bacterial urinary tract pathogens are most appropriately determined by the Food and Drug Administration (FDA) recommended procedure (3,5,6,7), which requires a single isolate to prepare inocula for susceptibility testing. The necessity to await pure cultures causes 36 to 48 h to elapse before antibiotic susceptibility data become available. This period has been shortened in clinically urgent situations by using the urine specimen itself as inoculum for disk diffusion testing (1, 4). Because the urine sometimes contains inhibitory factors and varying quantities of organisms, sensitivity data so obtained may differ from information collected by the accepted method.Previous studies in this laboratory (9) showed that when bacteremia is caused by a single, rapidly growing pathogen, 24 h may be conserved by inoculating a small volume of blood culture fluid into a relatively large volume of nutrient broth and growing it to standard density. This results in an inoculum for disk diffiusion testing that is standardized and in which toxic factors are eliminated by dilution. The present study shows that the principle is also applicable to the susceptibility testing of urine specimens containing a single gram-negative organism.Urine specimens from patients with suspected urinary tract infection were inoculated onto MacConkey agar plates, colistin-nalidixic acid sheep blood agar, and 10% sheep blood agar plates. One drop of each urine specimen (ca. 0.01 ml) was inoculated into 2 ml of Columbia broth with a Pasteur capillary pipette.The Columbia broth suspensions were incubated for 4 to 6 h at 350C. Those were measured to the nearest whole millimeter with a linear scale. Urine specimens shown to be polymicrobic by stain or culture, those containing gram-positive organisms, and specimens-containing slow-growing organisms were not considered.Isolated colonies from the blood agar plates were used to prepare inocula for the FDA standardized Bauer-Kirby procedure. Zone sizes for each isolate and drug tested by the direct and standard procedure were recorded and compared to interpretive standards established by the FDA method (5).Seventy-two urine specimens, containing single, rapidly growing, gram-negative pathogens, were evaluated.

Comparison of direct and standard antimicrobial disk susceptibility testing for bacteria isolated from blood

“…We found only 1 minor discrepancy (a shift from R by direct to I by standard testing with ampicillin) among the 15 enterococci studied. Penicillin and methicillin accounted for 22 and 15, respectively, of the 38 minor discrepancies seen with gram-positive cocci ( Table 1). Table 2 summarizes the results of comparisons of direct and standard susceptibility tests with all strains of Enterobacteriaceae that were isolated more than 10 times from blood cultures.…”

“…Direct susceptibility testing of wound exudates (6) and mixed urine cultures (9) also have been shown to provide clinically misleading information. Conversely, pure urine cultures (9,22) and positive blood cultures (7, 10,21) have yielded accurate results by direct testing when care has been taken to provide an adequate inoculum density.…”