Does a positive M. hyopneumoniae PCR simply detect DNA from dead bacteria?

Problem: A positive PCR result could theoretically represent residual DNA from nonviable M. hyopneumoniae rather than active infection.

Methods: Eight three-week-old pigs received 10 mL of inactivated bacteria and eight received 10 mL of viable bacteria, with sampling from 6 hours through 14 days and multiple respiratory tissues tested by PCR.

1. No ante-mortem or post-mortem sample from pigs receiving inactivated organisms tested PCR-positive for M. hyopneumoniae at any sampling time.

2. In pigs receiving viable organisms, M. hyopneumoniae was detected beginning at day 2 in tracheal secretions and BALF and at day 3 in bronchial secretions, tracheal scrapings, and thoracic lymph nodes, with detection continuing through day 14.

3. In this healthy-pig model, DNA from inactivated organisms did not persist at detectable levels, reducing—without eliminating in every clinical setting—the likelihood that a positive PCR simply reflects residual nonviable DNA.