Intertidal sediments as reservoirs for hippurate negative campylobacters, salmonellae and faecal indicators in three EU recognised bathing waters in North West England

Intertidal sediment samples from three EU recognised bathing waters in Morecambe Bay, Lancashire, U.K. (Morecambe North, Morecambe South and Heysham) were analysed for thermophilic campylobacters, salmonellae, faecal coliforms and faecal streptococci over a 12-month period. Campylobacters show a strong seasonality. They are generally absent in the summer months but are consistently present in the winter. The main Campylobacter species isolated were C. lari and urease-positive thermophilic campylobacters (UPTC), both of which are associated with avian hosts. C. jejuni and C. coli and Salmonella were not isolated from the sediments. Faecal coliforms and faecal streptococci were isolated throughout the year with no obvious seasonal trends in their numbers. Higher numbers were found at Morecambe North and South than at Heysham. There was no discernible relationship between the numbers of campylobacters and faecal indicators. Faecal indicators were found predominantly in the surface layers of the sediments and declined in number with depth. Campylobacters were restricted to the surface layer. Experimental results for the in situ deposition of bacteria onto clean, sterile surfaces from the water column during tidal cover showed deposition rates equivalent to approximately 0.1% of the total population of faecal coliforms, 0.01% of the faecal streptococci and 1% of the campylobacters in the sediments. These rates of accretion were not high enough to be detected during routine sampling carried out before and after high water, using standard methods. The results show that the sediments act as a reservoir for bacteria, especially the faecal indicators. In rough weather resuspension can contribute significantly to bacterial numbers in the surface waters.