DNA from Slow-Growing Mycobacteria in Culture Negative Sputa Reveal Common Exposure to Mycobacteria

Mycobacteria isolated only once in repeated sputum cultures are deemed colonizers that are not normally present in the lungs despite their ubiquitous presence in the environment. We have developed a sensitive and specific PCR assay to detect slow-growing mycobacteria in culture negative sputum and have found them in 30% of the samples obtained from individuals with clinical suspicion of tuberculosis or mycobacteriosis (n = 50) and in 45% of the samples from individuals with no suspicion (n = 49). As a negative control we have analyzed infections from an extra-pulmonary location, the urinary tract (n = 21), in which mycobacterioses are rare, and did not detect them. The load of bacteria in the lungs is kept low by the defense mechanisms of the host, and mycobacteria are not normally detected in sputum cultures. Their incidental isolation would not be a consequence of a new exposure to the microorganism but a sign of susceptibility to mycobacteriosis.