Assessment of predatory bacteria and prey interactions using culture-based methods and EMA-qPCR

Traditional culture-based enumeration methods were compared with the ethidium monoazide quantitative polymerase chain reaction (EMA-qPCR) technique to assess Bdellovibrio-and-like-organisms (BALOs) predator-prey interactions. Gram-negative [Pseudomonas spp. and Klebsiella pneumoniae (K. pneumoniae)] and Gram-positive [Staphylococcus aureus (S. aureus) and Enterococcus faecium (E. faecium)] organisms were employed as prey cells, while a Bdellovibrio bacteriovorus strain (PF13) was used as the predator. The co-culture experiments were also compared in diluted nutrient broth (DNB) and HEPES buffer. In both media, K. pneumoniae (maximum log reduction of 5.13) and Pseudomonas fluorescens (P. fluorescens) (maximum log reduction of 4.21) were sensitive to predation by B. bacteriovorus PF13 as their cell counts and gene copies were reduced during all the co-culture experiments, while the concentration of B. bacteriovorus PF13 increased. The concentration of B. bacteriovorus PF13 also increased in the presence of S. aureus (HEPES buffer) and E. faecium (DNB), indicating that the predator interacted with these Gram-positive prey in order to survive. Moreover, as no predator plaques were produced in the co-culture experiments with P. aeruginosa (DNB and HEPES buffer), S. aureus (DNB and HEPES buffer) and E. faecium (HEPES buffer), EMA-qPCR proved to be beneficial in monitoring the concentration of B. bacteriovorus. In conclusion, the cell counts and/or EMA-qPCR analysis for the HEPES buffer and DNB assays were successfully employed to monitor the predation of P. fluorescens and K. pneumoniae by B. bacteriovorus, while E. faecium was sensitive to predation in DNB and S. aureus was sensitive to predation in HEPES buffer.