An efficient method for enumerating oral spirochetes using flow cytometry

Abstract Spirochetes, such as Treponema denticola , are thin walled, helical, motile bacteria. They are notoriously difficult to enumerate due to their thinness and the difficulties associated with culturing them. Here we have developed a modified oral bacterial growth medium (OBGM) that significantly improves the cultivation of T. denticola compared with a previously published growth medium. Three methods for the enumeration of T. denticola , semi-solid growth medium colony-forming unit (CFU) counts, DNA analysis and flow cytometry, are described and compared. Enumeration of T. denticola using the semi-solid agar method resulted in a positive linear relationship with absorbance of the culture ( R 2  = 0.9423). However, the semi-solid agar method was found to consistently underestimate (by 50 fold) the T. denticola cell density compared to previously published data. DNA analysis of T. denticola cultures reliably and consistently resulted in a positive linear relationship with absorbance ( R 2  = 0.9360), giving a calculated cell density of 6.9 × 10 8  cells/mL at an absorbance of 0.2 at 650 nm. Flow cytometry was also found to result in a positive linear relationship with absorbance ( R 2  = 0.9874), giving a calculated cell density of 6.6 × 10 8  cells/mL at an absorbance of 0.2 at 650 nm. In comparing all of these enumeration methods, the flow cytometry method was found to have distinct advantages, as it is accurate, rapid, and could distinguish between live and dead bacteria. Thus flow cytometry is a recommended means for the rapid and reliable enumeration of viable spirochetes from culture.