Case report and seeded blood culture study of Brucella bacteremia

Diagnosis of brucellosis requires prompt detection and identification of the coccobacillus for appropriate patient management, as the organism is associated with a potentially severe outcome. In a recent experience, an 18-year-old migrant farm worker presented at a local hospital with nonspecific symptoms. A significant Brucella titer of 2,560 was followed by the recovery of a gram-negative coccobacillus, subsequently identified as Brucella abortus, from subcultured 5-day-old BACTEC NR730 negative blood cultures. The organism proved to be susceptible to a variety of antimicrobial agents and resistant to nitrofurantoin. The patient was administered antimicrobial therapy for Brucella spp. consisting of tetracycline and streptomycin for 21 days. During the course of therapy the patient experienced defervescence and was discharged with the recommendation for periodic follow-up examinations. Seeded culture studies of this isolate with fresh human blood and target inocula of 5 and 500 CFU/ml indicated that the larger (500-CFU/ml) inoculum produced positive instrument detection within 2 days, whereas the smaller (5-CFU/ml) inoculum required 5.5 to 7.5 days for detection, depending on the medium used. These findings underscore the potential for Brucella bacteremia to escape instrument detection given a low bacterial inoculum.