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Conjugal transfer of chromosomal DNA between strains of Mycobacterium smegmatis occurs by a novel mechanism. In a transposon mutagenesis screen, three transfer-defective insertions were mapped to the lsr2 gene of the donor strain mc(2)155. Because lsr2 encodes a nonspecific DNA-binding protein, mutations of lsr2 give rise to a variety of phenotypes, including an inability to form biofilms. In this study, we show that efficient DNA transfer between strains of M. smegmatis occurs in a mixed biofilm and that the process requires expression of lsr2 in the donor but not in the recipient strain. Testing cells from different strata of standing cultures showed that transfer occurred predominantly at the biofilm air-liquid interface, as other strata containing higher cell densities produced very few transconjugants. These data suggest that the biofilm plays a role beyond mere facilitation of cell-cell contact. Surprisingly, we found that under standard assay conditions the recipient strain does not form a biofilm. Taking these results together, we conclude that for transfer to occur, the recipient strain is actively recruited into the biofilm. In support of this idea, we show that donor and recipient cells are present in almost equal numbers in biofilms that produce transconjugants. Our demonstration of genetic exchange between mycobacteria in a mixed biofilm suggests that conjugation occurs in the environment. Since biofilms are considered to be the predominant natural microhabitat for bacteria, our finding emphasizes the importance of studying biological and physical processes that occur between cells in mixed biofilms.

Mycobacterial Biofilms Facilitate Horizontal DNA Transfer between Strains of Mycobacterium smegmatis