Coliform bacteria have been considered as a model for studies on the deposition and survival of microorganisms in estuaries. These bacteria were deposited in bottom muds of an estuarine system once the salinity exceeded a specific conductivity of 2.5 mS cm‐1. Survival of the bacteria appeared to be enhanced in the sediments. Studies of bacterial survival in specially constructed chambers immersed in an estuary indicated that sediment particulates have a protective effect, prolonging the survival of the bacteria compared with that in seawater. A similar protection of the bacteria was observed in the presence of a montmorillonitic clay. The interaction of microorganisms with both colloidal and larger particulates is considered in relation to such protective effects. The role of salinity in microbial sorption‐desorption phenomena, as well as the role of particulates in inhibiting biological control of alien bacteria, must be of general significance in the geomicrobiology of sediments in estuaries.