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Genetic (in)stability of 2,6-dichlorobenzamide (BAM)-catabolism in Aminobacter sp. MSH1 biofilms under carbon starved conditions.

Research paper by Benjamin B Horemans, Bart B Raes, Hannelore H Brocatus, Jeroen J T'Syen, Caroline C Rombouts, Lynn L Vanhaecke, Johan J Hofkens, Dirk D Springael

Indexed on: 02 Apr '17Published on: 02 Apr '17Published in: Applied and environmental microbiology



Abstract

Aminobacter sp. MSH1 grows on and mineralizes the groundwater micropollutant 2,6-dichlorobenzamide (BAM) and is of interest for BAM removal in drinking water treatment plants (DWTPs). The BAM-catabolic genes in MSH1 are located on plasmid pBAM1 carrying bbdA encoding the conversion of BAM to 2,6-dichlorobenzoic acid (2,6-DCBA) (BbdA(+) phenotype) and plasmid pBAM2 carrying gene clusters encoding the conversion of 2,6-DCBA to TCA cycle intermediates (Dcba(+) phenotype). Indications exist that MSH1 loses easily its BAM-catabolic phenotype. Evidence was obtained that MSH1 rapidly develops a population that lacks the ability to mineralize BAM when grown on non-selective (R2B medium) and semi-selective (R2B with BAM) medium. Lack of mineralization was explained by loss of the Dcba(+) phenotype and corresponding genes. The ecological significance of this instability for the use of MSH1 for BAM removal in the oligotrophic environment of DWTPs was explored in lab and pilot systems. A higher incidence of BbdA(+) Dcba(-) MSH1 cells was also observed when MSH1 was grown as biofilms in flow chambers under C and N starved conditions due to growth on non-selective residual assimilable organic carbon. Similar observations were done in a pilot sand filter reactor bioaugmented with MSH1. BAM conversion to 2,6-DCBA was though not affected by loss of the DCBA catabolic genes. Our results show that MSH1 is prone to BAM-catabolic instability under the conditions occurring in a DWTP. While conversion of BAM to 2,6-DCBA remains unaffected, BAM-mineralization activity is at risk and monitoring of metabolites is warranted.Importance Bioaugmentation of dedicated biofiltration units in DWTPs with bacterial strains that grow on and mineralize micropollutants was suggested as an alternative for treating micropollutant-contaminated water in a drinking water treatment plant (DWTP). Organic pollutant catabolic genes in bacteria are often easily lost especially under non-selective conditions which will affect the bioaugmentation success. In this study, we provide evidence that Aminobacter sp. MSH1 that uses the common groundwater micropollutant 2,6-dichlorobenzamide (BAM) as a C-source, shows a high frequency of loss of its BAM-mineralizing phenotype due the loss of genes that convert 2,6-DCBA to Krebs cycle intermediates when non-selective conditions occur. Moreover, we show that catabolic gene loss also occurs in the oligotrophic environment of DWTPs where growth of MSH1 mainly depends on the high fluxes of low concentration of assimilable organic carbon and hence show the ecological relevance of catabolic instability for using strain MSH1 for BAM removal in DWTPs.

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