Enumeration of faecal coliforms from recreational coastal sites: evaluation of techniques for the separation of bacteria from sediments

Aims: To identify the most efficient techniques for the separation of micro‐organisms from coastal sediments and, using these techniques, to determine the concentration of faecal indicator organisms in recreational coastal water and sediment. Methods and Results: Sediment samples were taken from a range of recreational coastal sites and subjected to various physical techniques to separate micro‐organisms from sediment particles. Techniques investigated included manual shaking, treatment by sonication bath for 6 and 10 min, respectively, and by sonication probe for 15 s and 1 min, respectively. The use of the sonication bath for 10 min was the most successful method for removing micro‐organisms from sediment particles where sediments consisted mainly of sand. When sediments contained considerable proportions of silt and clay, however, manual shaking was most successful. Faecal coliforms were then enumerated by membrane filtration in both water and sediment from three recreational coastal sites, chosen to represent different physical sediment characteristics, over a 12‐month period. Faecal coliform concentrations were generally greater in sediment compared with overlying water for all samples. This was most evident in sediment consisting of greater silt/clay and organic carbon content. Conclusions: This study demonstrated the importance of sediment characteristics in determining the most efficient method for the separation of micro‐organisms from coastal sediments. Sediment characteristics were also found to influence the persistence of micro‐organisms in coastal areas. Significance and Impact of the Study: Recreational coastal sediments can act as a reservoir for faecal coliforms; therefore, sampling only overlying water may greatly underestimate the risk of exposure to potentially pathogenic micro‐organisms in recreational waters.