Bacterial Adsorption to Suspended Particles in Urban Stormwater.

Abstract

Bacterial adsorption to particles in stormwater is presently poorly understood, although it may be an important disappearance mechanism for the removal of bacteria from polluted surface waters. Adhesion to particles is also likely to protect bacteria from a number of environmental stresses. A large stormwater outfall (Allen Drain) was sampled seventeen times throughout the study. Experiments were performed with a fractional filtration apparatus and with static settling to determine whether natural populations of fecal coliforms were similar to the opportunistic pathogens Pseudomonas aeruginosa and Klebsiella pneumoniae and to total gram negative bacteria in their adsorption characteristics. Pure cultures of E. coli, P. aeruginosa, K. aerogenes, K. pneumoniae and S. typhimurium were also used in laboratory clay flocculation experiments and in stormwater sediment spiking experiments. Electron microscopy and hemagglutination assays were used to define cell surface characteristics. The filtration and settling tests showed significant numbers of bacteria (15 to 40%) to be associated with stormwater particulates (mostly in size-fractions > 30 (mu)m). Significant differences were observed between degree of adsorption of fecal coliforms (16%) and Klebsiella sp. and P. aeruginosa (26%) as determined by retention on Nitex screens > 5 (mu)m mesh-size. Static settling results were similar. Stormwater chemistry and solids data were not highly correlated with adsorption, although some effects of total solids, percent organic matter and bacterial numbers were suggested as contributing factors by regression analyses. Methods of growing laboratory cultures were found to markedly affect bacterial adsorptive abilities in the clay flocculation and sediment spiking experiments. In all cases, broth-grown cells were more effectively adsorbed than were agar-grown cells. The presence of abundant fimbriae (pili) on the outer surfaces of the cells appeared to promote nonspecific adsorption to bentonite and to stormwater sediments. These observations demonstrate that extrapolation of laboratory adhesion experimental results to natural waters is presently of questionable validity. Also, judging from this study, fecal coliforms may not be ideal indicators of adsorption and settleability. They should be viewed as conservative indicators, however, because other bacteria and pathogens may be more highly associated with suspended particles in stormwater.