Estimation of plaque forming unit (pfu count)

Serial 10 fold dilutions of the purified phage stock up to 10⁻¹² were prepared in sterile SM buffer. Equal quantity of each phage dilution was mixed in 3 ml semisolid NZCYM agar at 47°C and seeded with the log phase cultures of Brucella abortus strain S19 and Brucella abortus strain RB51. After mixing properly, the soft agar in each tube was plated on hard Brucella agar plates and left for solidifying. All the plates were then incubated at 37°C for 48 hours. The plaques produced were counted and phage count was determined. The phage titer was expressed in pfu/ml after multiplying with the dilution factor.