LOW-FREQUENCY HORIZONTAL TRANSFER OF AN ELEMENT CONTAINING THE CHLOROCATECHOL DEGRADATION GENES FROM PSEUDOMONAS SP. STRAIN B13 TO PSEUDOMONAS-PUTIDA F1 AND TO INDIGENOUS BACTERIA IN LABORATORY-SCALE ACTIVATED-SLUDGE MICROCOSMS

The possibilities for low-frequency horizontal transfer of the self-tr ansmissible chlorocatechol degradative genes (clc) from Pseudomonas sp . strain B13 were investigated in activated-sludge microcosms. When th e de genes mere transferred into an appropriate recipient bacterium su ch as Pseudomonas putida F1 a new metabolic pathway for chlorobenzene degradation was formed by complementation which could be selected far by the addition of mono-or 1,4-dichlorobenzene (CB), Under optimized c onditions with direct donor-recipient filter matings, very low transfe r frequencies were observed (approximately 3.5 x 10(-8) per donor per 24 h), In contrast, in matings on agar plate surfaces, transconjugants started to appear after 8 to 10 days, and their numbers then increase d during prolonged continuous incubation with CB, In activated-sludge microcosms, CB-degrading (CB+) transconjugants of strain F1 which had acquired the de genes were detected but only when strain B13 cell dens ities of more than 10(5) CFU/ml could be maintained by the addition of its specific growth substrate, 3-chlorobenzoate (3CBA). The CB+ trans conjugants reached final cell densities of between 10(2) and 10(3) CFU /ml, When strain B13 was inoculated separately (without the designated recipient strain F1) into an activated-sludge microcosm, CBC transcon jugants could not be detected. However, in this case a new 3CBA-degrad ing strain appeared which had acquired the clc genes from strain B13, The effects of selective substrates on the survival and growth of and gene transfer between bacteria degrading aromatic pollutants in a wast ewater ecosystem are discussed.