The effects of submerged aquatic vegetation on the persistence of environmental populations of Enterococcus spp.

Enterococcus spp. are utilized worldwide as faecal indicator bacteria, but certain strains exhibit extended survival in environmental habitats and the factors influencing their persistence are poorly understood. We used flowing freshwater mesocosms to explore the effect of submerged aquatic vegetation (SAV) on the persistence of natural enterococci populations from a subtropical lake. The highest mean densities of culturable enterococci over 2 weeks occurred in SAV [8.6 x 10(2) colony-forming units (cfu) per 100 g wet weight], followed by sediments (1.3 x 10(2) cfu per 100 g) and water (18 cfu per 100 ml). However, due to relative differences in the total mass of each substrate in the entire system (water > sediments > SAV), SAV-associated enterococci represented only a minor proportion of the total population. Vegetated mesocosms harboured significantly higher mean cfu per mesocosm and cfu densities in sediments compared with their unvegetated counterparts, suggesting that SAV indirectly facilitates persistence in aquatic habitats. Populations were dominated (> 96%) by a single Enterococcus casseliflavus strain according to BOX-PCR genotyping, which did not change over the 10-month study and strongly suggests bacterial replication in the lake. The presence of such strains in the environment may represent highly competitive, naturalized and reproducing indicator bacteria populations that are not directly related to pollution events.