Sorbitol-fermenting bifidobacteria as indicators of diffuse human faecal pollution in estuarine watersheds

Sorbitol fermenting bifidobacteria were evaluated as indicators of non-point source human faecal pollution to three sub-estuaries with elevated faecal coliform densities. Human-specific bifidobacteria correlated with identifiable human sanitary deficiencies in feeder streams to estuarine creeks in two of three watersheds examined, one rural and one moderately developed. Sorbitol-fermenting bifidobacteria were recovered at densities ranging from 1 to 90 colony-forming-units 100 ml-1 in 11 of 258 water samples but were undetected in sediment (n = 68) and scat from resident wildlife (deer, muskrat and raccoon, n = 20). Failure to detect sorbitol-fermenting bifidobacteria in water samples during the summer months was consistent with laboratory microcosm results showing non-recoverability of Bifidobacterium adolescentis after 5-9 d in membrane-filtered estuarine water at 23 and 30 degrees C, but persistence for 4 weeks at 10 degrees C. Persistence of sewage-derived bifidobacteria in membrane-filtered freshwater at 15 degrees C was also observed. Recovery of sorbitol-fermenting bifidobacteria was complicated by high background levels of Gram-positive rods and cocci. Use of propionic acid and reduced pH (pH = 5.0), or use of a two-step resuscitation protocol using non-selective and selective media, did not improve recovery. Although human specific bifidobacteria hold promise as indicators of diffuse faecal contamination, methodological constraints now limit its application to situations of gross contamination, or sampling potential sources during environmental conditions conducive to bifid persistence.