Direct Urease Test on BACTEC Blood Cultures: Early Presumptive Diagnosis of Brucellosis in an Area of Endemicity

Brucellosis is endemic in the Kingdom of Saudi Arabia. In our area, blood cultures from suspected cases performed with the BACTEC 9240 yield a positive signal, generally within 5 days (2). Gram stain of the broth medium reveals either gram-negative coccobacilli or no definite bacteria. Subculture of the broth to an appropriate medium yields Brucella species in 2 to 3 days. By analogy with screens of biopsy specimens for Helicobacter pylori (1) (both Helicobacter and Brucella are strong urease producers), we reasoned that the direct detection of urease activity in the above-mentioned blood culture broth might provide an early marker for the presence of Brucella. This would permit a presumptive report of Brucella bacteremia 2 to 3 days sooner than that provided by conventional laboratory protocols.

Thirty-three signal-positive BACTEC broths containing gram-negative coccobacilli and 32 containing no visible organisms were subcultured (0.5 ml) onto urea slants. Tubes were incubated at 35 to 37°C in carbon dioxide and examined for the red color that is characteristic of urease activity every 2 h for 8 h and again after overnight incubation. Of the 44 Brucella species eventually isolated, 37 gave a positive urease reaction within 4 h and 7 were positive after overnight incubation. Two Haemophilus influenzae isolates were also positive after overnight incubation. Negative urease tests were obtained for 19 specimens (18 with no visible bacteria and one with gram-negative coccobacilli) which eventually yielded one strain each of group C streptococcus, Haemophilus influenzae, and a Bacteroides species and two isolates of Campylobacter jejuni. No early (4-h) positive cultures were noted with broths which subsequently yielded other organisms or no organisms.

The present study demonstrates that 100% of 4-h urease tests on signal-positive blood culture broths (BACTEC 9240) containing gram-negative coccobacilli or no visible organisms were indicative of Brucella bacteremia. A positive and early direct urease test resulting from blood culture broths of the type described above and in an area endemic for brucellosis may provide a presumptive diagnosis of brucellosis 2 to 3 days earlier than conventional laboratory protocols.