Sorbitol-fermenting Bifidobacteria are indicators of very recent human faecal pollution in streams and groundwater habitats in urban tropical lowlands

Abstract

Sorbitol-fermenting Bifidobacteria (SFB) proved to be an excellent indicator of very recent human faecal pollution (hours to days) in the investigated tropical stream and groundwater habitats. SFB were recovered from human faeces and sources potentially contaminated with human excreta. SFB were undetectable in animal faeces and environmental samples not contaminated with human faeces. Microcosm studies demonstrated a rapid die-off rate in groundwater (T90 value 0.6 days) and stream water (T90 value 0.9-1.7 days). Discrimination sensitivity analysis, including E. coli, faecal coliforms, total coliforms and Clostridium perfringens spores, revealed high ability of SFB to distinguish differing levels of faecal pollution especially for streams although high background levels of interfering bacteria can complicate its recovery on the used medium. Due to its faster die-off, as compared to many waterborne pathogens, SFB cannot replace microbiological standard parameters for routine water quality monitoring but it is highly recommendable as a specific and complementary tool when human faecal pollution has to be localized or verified. Because of its exclusive faecal origin and human specificity it seems also worthwhile to include SFB in future risk evaluation studies at tropical water resources in order to evaluate under which situations risks of infection may be indicated.

Figure 1

The study area in Tanzania showing the locations of the 112 and 15 environmental faecal and water sampling sites, respectively: ○, Low-risk boreholes (BL1 to BL3); ●, Medium-risk boreholes (BH1 to BH3); ▼, High-risk borehole (OP1 to OP3); □, Upstream (US1 to US3); ■, Downstream (DS1 to DS 3); + , Human faeces; ◆, Sewage; △, Animal faeces; , Polluted water; ◐ Tap water; DRC, Democratic Republic of Congo.

Figure 2

Grouping of sampling sites by cluster analysis using within-group average linkage, square Euclidean distance and physicochemical data set. Cluster I = Groundwater sites, Cluster II = Stream sites, n = 75-90 single samples per parameter. For abbreviations see Figure 1.

Figure 3

Survival of sorbitol-fermenting Bifidobacteria in comparison to coliform bacteria (E. coli, FC, TC) in microcosms using water from stream- and groundwater habitats incubated at room temperature for 120 h. Values plotted are the mean of three samples per faecal indicator and site. SFB were not detected in samples from site BL3 and BH2 throughout the sampling period and during the survival determination. For BL3, BH2, OP2, US2 and DS2 abbreviations see Figure 1; ◆, Total coliform (TC) (BH2, OP2, DS2 sites); ■, Fecal coliform (FC) (BH2, OP2, DS2 sites); ▲, E. coli (BH2, OP2, DS2 sites); ●, Sorbitol-fermenting Bifidobacteria (SFB) (OP2, DS2 sites); ◇, TC (BL3, US2 sites); □, FC (BL3, US2 sites); △, E. coli (BL3, US2 sites) and ○, SFB (US2 sites).