Isotope dilution analysis and rates of 32P incorporation into phospholipid as a measure of microbial growth rates in biofilms

The method described represents an alternative to the 3H thymidine incorporation technique in the determination of bacterial growth rates within biofilms. The effect of increasing incubation duration was investigated, and incorporation rates found to approximate towards linearity for ca 1 h. Isotope dilution analyses were carried out in an attempt to measure dilution of added 32P by phosphate that was being cycled internally within the biofilm. Statistically significant differences (P < 0.001) were found in rates of phospholipid synthesis in biofilms from the River Conwy during different seasons. A close correlation was found in a comparison of 3H thymidine incorporation and 32P incorporation techniques (r = 0.82; P < 0.001) in data collected from the rivers Conwy, Clywedog and Aber in North Wales. That close correlation suggested that the isotope dilution analysis technique was successful in overcoming potential problems caused by the pool of internally cycled phosphate. The alterantive 3H thymidine incorporation approach cannot be used with sulphate reducing bacteria. And so finally, it was proposed that the 32P incorporation technique could prove of value in measurements of the growth of sulphate reducing bacteria in biofilms causing metal corrosion.