Co-incubation of Bacillus spp. alleviated the growth and gas production of potentially harmful bacteria in different in vitro methodologies

A series of experiments evaluated the effects of a Bacillus-based direct-fed microbial (DFM) on the growth of potentially harmful bacteria. Experiment 1 evaluated the effects of co-incubating Bacillus licheniformis 809 or Bacillus subtilis 810 with C. perfringens type A on in vitro gas production and Clostridium counts. This methodology was performed using the in vitro gas production Ankom RF system bottle. Bottles were incubated with C. perfringens type A (CPF) or with the addition of individual DFM strains (B. licheniformis 809 [BL] or B. subtilis 810 [BS]). Gas production of the different treatments was recorded for 24 h, and then enumeration of the strains was performed. A treatment × hour interaction was observed, as CPF grew faster than either BL or BS from 7 to 24 h, whereas BS grew faster than BL at 7 h. Adding BL or BS reduced gas production versus CPF from 18 to 24 h (treatment × hour interaction). Moreover, the co-incubation of BL or BS with C. perfringens type A significantly reduced the counts of these potentially harmful bacteria by 63% and 69%, respectively. Experiment 2 was performed by using an agar-diffusion assay to evaluate C. perfringens types A, B, C, D, and E; Fusobacterium necrophorum; Trueperella pyogenes; and L. monocytogenes. After incubation, the zone of decreased growth was measured. The Bacillus-based DFM addition resulted in efficacy against C. perfringens that ranged from 10.5 mm (type C) to 15.5 mm (types A and E). Moreover, for T. pyogenes and F. necrophorum, the clearance zone was in the order of 11.5 and 8.7 mm, respectively. Three strains of L. monocytogenes were assessed following 4 h of incubation with BL and BS, and zones between 10.5 and 11.5 mm were reported. In summary, this series of in vitro experiments demonstrated the inhibitory activity of BL and BS against selected potentially harmful bacteria of interest for livestock.