Use of species distribution data in the determination of bacterial viability by extinction culture of aquatic bacteria

Effect of trophic status on the culturability and activity of bacteria from a range of lakes in the English Lake District

The bacterioplankton from a number of lakes that differed in nutrient status in the English Lake District was examined with a number of techniques for enumeration and activity assessment. Natural water samples showed a clear correlation between total counts and trophic status. Esterase activity measurements with Chemchrome B were able to distinguish high- and low-nutrient-status lakes, whereas tetrazolium salt (5-cyano-2,3-ditoyltetrazolium chloride) reduction, the direct viable count-cell elongation assay, and culturability measurements could not. Tetrazolium salt reduction and esterase activity measurements labeled a significant number of cells from water of all nutrient levels, whereas the direct viable count-cell elongation method was of use only in oligotrophic waters. Size fractionation of samples showed that the culturable cells were retained by the larger filters, especially in nutrient-rich waters. Esterase activity measurements also favored the larger cells. The differences observed between assays using water that differed in trophic status raise questions about the use of these tests as a definitive measure of viability.

Survival of faecal indicator bacteria in tropical estuarine waters (Bangpakong River, Thailand)

AIMS

To investigate the survival of cultivable bacteria in the tropical Bangpakong estuary (Eastern Thailand) under different salinities and light conditions.

METHODS AND RESULTS

Dark and light microcosm experiments using membrane diffusion chambers were carried out under three different experimental conditions, namely (i) low salinity, (ii) progressive mixing with brackish water and (iii) fast mixing with high salinity water spiked with raw urban sewage. Faecal coliforms declined faster than faecal enterococci, as shown by survival T90 values ranging from 82.2 +/- 4.2 to 14.5 +/- 0.8 h and 97.5 +/- 0.4-20.6 +/- 1.2 h, respectively. The survival of freshwater heterotrophic bacteria was higher but variable (121.2 +/- 5.0-30.1 +/- 14.3 h), whereas that of heterotrophic marine bacteria was rather stable (81.5 +/- 4.2-44.6 +/- 2.5 h).

CONCLUSIONS

Overall survival was higher in low salinities. Light had a further deleterious effect, since it accelerated the decay of faecal indicators, particularly in high salinities. Faecal enterococci had a higher resistance to environmental conditions compared with faecal coliforms.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is relevant to the understanding of the behaviour of different faecal indicator bacteria and the optimization of sewage treatment plants aimed at the reduction and/or elimination of faecal load discharged into estuarine waters submitted to salinity variations.

Determination of DNA content of aquatic bacteria by flow cytometry

The distribution of DNA among bacterioplankton and bacterial isolates was determined by flow cytometry of DAPI (4',6'-diamidino-2-phenylindole)-stained organisms. Conditions were optimized to minimize error from nonspecific staining, AT bias, DNA packing, changes in ionic strength, and differences in cell permeability. The sensitivity was sufficient to characterize the small 1- to 2-Mb-genome organisms in freshwater and seawater, as well as low-DNA cells ("dims"). The dims could be formed from laboratory cultivars; their apparent DNA content was 0.1 Mb and similar to that of many particles in seawater. Preservation with formaldehyde stabilized samples until analysis. Further permeabilization with Triton X-100 facilitated the penetration of stain into stain-resistant lithotrophs. The amount of DNA per cell determined by flow cytometry agreed with mean values obtained from spectrophotometric analyses of cultures. Correction for the DNA AT bias of the stain was made for bacterial isolates with known G+C contents. The number of chromosome copies per cell was determined with pure cultures, which allowed growth rate analyses based on cell cycle theory. The chromosome ratio was empirically related to the rate of growth, and the rate of growth was related to nutrient concentration through specific affinity theory to obtain a probe for nutrient kinetics. The chromosome size of a Marinobacter arcticus isolate was determined to be 3.0 Mb by this method. In a typical seawater sample the distribution of bacterial DNA revealed two major populations based on DNA content that were not necessarily similar to populations determined by using other stains or protocols. A mean value of 2.5 fg of DNA cell(-1) was obtained for a typical seawater sample, and 90% of the population contained more than 1.1 fg of DNA cell(-1).