A Novel, Rapid, and Simple PMA-qPCR Method for Detection and Counting of Viable Brucella Organisms

Abstract

Introduction: The plate counting method widely used at present to discern viable from non-viable Brucella in the host or cell is time-consuming and laborious. Therefore, it is necessary to establish a rapid, simple method for detecting and counting viable Brucella organisms.

Material and methods: Using propidium monoazide (PMA) to inhibit amplification of DNA from dead Brucella, a novel, rapid PMA-quantitative PCR (PMA-qPCR) detection method for counting viable Brucella was established. The standard recombinant plasmid with the target BCSP31 gene fragment inserted was constructed for drawing a standard curve. The reaction conditions were optimised, and the sensitivity, specificity, and repeatability were analysed.

Results: The optimal exposure time and working concentration of PMA were 10 min and 15 μg/mL, respectively. The correlation coefficient (R²) of the standard curve was 0.999. The sensitivity of the method was 10³ CFU/mL, moreover, its specificity and repeatability also met the requirements. The concentration of B. suis measured by the PMA-qPCR did not differ significantly from that measured by the plate counting method, and the concentrations of viable bacteria in infected cells determined by the two methods were of the same order of magnitude.

Conclusion: In this study, a rapid and simple PMA-qPCR counting method for viable Brucella was established, which will facilitate related research.

Keywords: BCSP31gene; Brucella; propidium monoazide; quantitative PCR; viable bacteria.

Fig. 1

The target BCSP31 fragment of PCR amplification from Brucella suis S2 strain. M – DL2000 DNA Marker (TaKaRa Bio); lane 1 – DNA fragment of PCR amplification

Fig. 2

The standard curve for calculating Brucella concentration according to CT of qPCR based on the template of recombinant standard plasmid pMD-18T-BCSP31

Fig. 3

Optimisation of PMA treatment conditions. A – Effect of different exposure time on qPCR amplification; B – Different concentrations of PMA treating dead bacteria; C – Different concentrations of PMA treating viable bacteria; * – P < 0.05; Yellow columns – optimal PMA treatment time of 10 min; Yellow columns with ☆ – optimal PMA treatment concentration of 15 μg/mL; × – not detected

Fig. 4

The sensitivity of detecting B. suis S2 strain. A – sensitivity by the PMA-qPCR; B – sensitivity by the conventional PCR, comprising M – DL2000 DNA Marker; lanes 1–10 – 10⁸ – 10^-1 CFU/mL

Fig. 5

Analysis of the specificity of PMA-qPCR. A – amplification plot; B – melting curves of the qPCR amplification product; C – agarose gel electrophoresis of different Brucella species detected by the normal PCR, comprising M – DL5000 DNA Marker; lane 1 – B. suis S2; lane 2 – B. abortus 2308; lane 3 – B. abortus A19; lane 4 – B. melitensis M5; lane 5 – B. melitensis 16M; and lane 6 – B. ovis; D – agarose gel electrophoresis of species other than Brucella amplified by the conventional PCR, comprising M – DL2000 DNA Marker; lane 1 – B. suis S2; lane 2 – E. coli; lane 3 – S. typhimurium; lane 4 – Y. enterocolitica; lane 5 – V. parahaemolyticus; and lane 6 – RNase-free water

Fig. 6

Analysis of different ratios of viable to dead B. suis S2 strain using the PMA-qPCR method and the qPCR method. * – P < 0.05

Fig. 7

Determination of the amount of the viable B. suis S2 strain by PMA-qPCR and plate counting methods. Pure bacterial liquid – B. suis cultured in TSB; A – B. suis harbouring in mouse macrophage RAW 264.7 cells infected with 1.28 × 10¹² CFU/mL of B. suis S2 strain; B – B. suis harbouring in mouse macrophage RAW 264.7 cells infected with 0.64 × 10¹² CFU/mL of B. suis S2 strain; * P < 0.05