Our results showed that B. abortus

Our results showed that B. abortus strain RB51 was not iso-lated from any of the milk samples tested during nine weeks(63 days) post-vaccination. The use of selective media for direct plating of suspected samples could reduce the number of Brucellaspp. colonies, and consequently could result in reduction of sensi-tivity of the diagnosis (Marin et al., 1996). In order to overcome that and improve the sensitivity of the culture, reducing false-negative results, enrichment in tryptose broth supplemented with antimicrobials (Farrell’s supplement) was used before plating. This approach is reported to enhance the rate of isolation of B. abortus from infected animals by 50% (Minharro, 2009) and has an analyt-ical sensitivity of 200 B. abortus CFUs/mL of milk (data not shown).
Additionally, to further increase the sensitivity of detection ofB. abortus, samples were also analyzed in parallel with a moresensitive technique, the PCR as described by Baily et al. (1992).The analytical sensitivity of this PCR technique was evaluated byRichtzehain et al. (2002) using two different extraction protocols,and was confirmed to be between 2 and 20 CFU/mL. This detectionthreshold was confirmed in our pre-experiments using the DNAextraction protocol described by Pitcher et al. (1989) with milksamples, in which we could detect 10 CFU/mL (data not shown).