Influence of type of culture medium on characterization of Mycobacterium avium subsp. paratuberculosis subtypes

We evaluated the effects of culture and/or enrichment methods on the selection of Mycobacterium avium subsp. paratuberculosis subtypes. M. avium subsp. paratuberculosis isolates from bovine fecal samples processed using a centrifugation protocol were investigated in both liquid (MGIT ParaTb tubes) and solid (Herrold's egg yolk medium) culture media. For this evaluation, M. avium subsp. paratuberculosis subtyping was based on the sequence variation in two of the most discriminatory short sequence repeat loci, i.e., mononucleotide G and trinucleotide GGT, in isolates from liquid and solid cultures. This study identified the existence of one major predominating fingerprint (>13G-5GGT) in bovine fecal samples, regardless of the type of medium used for isolation. Matched-pair analysis of subtypes showed that 69% of samples presented unique subtypes in the two culture media used, while 31% shared the same G-GGT allele. Furthermore, the liquid culture method appeared to select for a more genotypically diverse subtype population than the solid culture method. The variety of subtypes observed between liquid and solid cultures obtained from the same fecal samples suggests that the culture method could provide a "microbiological" bias and lead to a discrepancy in the growth of M. avium subsp. paratuberculosis strains. In conclusion, this study identified that these two types of culture media determined differential growth of M. avium subsp. paratuberculosis strains and that this should be considered in evaluating detection capabilities of diagnostic tests or interpreting data from molecular epidemiological studies performed using conventional solid fecal culture or automated liquid medium culture methods.