Biofilms as Reservoirs of Pathogenic Bacteria

A review of: Shikuma NJ, Hadfield MG (2010) Marine biofilms on submerged surfaces are a reservoir for Escherichia coli and Vibrio cholerae. Biofouling, 26 (1): 39-46.

Biofilms are complex in their composition and structure, and are commonly found on damp and aquatic surfaces. Pathogenic bacteria thrive in biofilms which are able provide protection from the external environment. The abundance and occurrence of pathogenic bacteria in coastal areas is usually determined by water filtration and/or using biomonitor species, such as shellfish that also filter the surrounding medium. Marine biofilms on ship’s hulls can serve as pathogenic bacteria reservoirs and are ideal routes for long distance dissemination. Escherichia coli and Vibrio cholerae are known to persist in marine environments and are ideal model organisms in biofilm research. Because monitoring for pathogenic bacteria in harbours and coastal areas are mostly done using water samples, the authors set out to determine whether marine biofilms on abiotic surfaces such as glass and ship’s hulls are ideal E. coli and V. cholera reservoirs. Furthermore, the aim was to compare the bacteria in the water column with those on nearby biofilms and biofilms in different locations to elucidate whether microbial communities varied between locations that have different local and international boat traffic as well as fresh water input.

To collect biofilm samples from the Hawaiian waters of Ala Wai Yacht Harbour (AWYH), Kewalo Basin (KB) and Pearl Harbour, microscope glass slides were submerged one metre below sea level in each location for one week, while ship samples were taken directly from hulls. The results revealed E. coli abundance within biofilms from all harbours sampled (ranging from 6.43 to 1.25 x 10⁵ genome copies cm^-2), while water column samples were less abundant (ranging from 2.3 x 10^-1 to 1.4 x 10⁴ genome copies ml^-1). E. coli was also abundant within biofilms on the hulls of ships from each harbour (ranging from 9.61 x 10² to 2.52 x 10³ genome copies). V. cholerae was detected in two harbours (AWYH and KB), with AWYH biofilms being the most abundant (mean 3.15 x 10³ genome copies) compared to KB and the water column samples (mean 1.56 x 10¹ genome copies). V. cholerae was also detected in one replicate (n=3) in one ship (n=3) in both AWYH and KB. None of the V. cholerae samples were deemed pathogenic but an E. coli isolate from AWYH was determined to be enteropathogenic. Multiple bacterial community fingerprints were found to be exclusive to either biofilm or water column communities, and biofilm communities were more similar to other biofilms across all sampling sites than to water column communities from the same harbour. Furthermore, biofilm communities from two sites in the same harbour were more similar to each other than biofilms from different harbours.

E. coli thrive in tropical soils and the observed abundance may be a result of soil being washed through sewage systems that empty out into the harbours during rainfall, thus affecting water quality standards. The distribution of V. cholerae was predominantly in brackish parts of AWYH, possibly due to an ability to tolerate low salinity unlike other Vibrio species. The study underscores the limitations of current water testing protocols and suggests biofilms on abiotic surfaces may facilitate the transport and introduction of bacterial species to new locations, and might be an overlooked reservoir of pathogenic bacteria. Biofilms protect bacteria from stress, osmotic pressure, dehydration, UV radiation, grazing etc., making them suitable microenvironments for overseas transport. The seventh cholera pandemic that encompassed four continents, starting in Indonesia in 1961 and more recently in South America in 1991, was conjectured to be caused by large ships which take on ballast water from the coast prior to heading out to open water, which is subsequently emptied out in destination ports. However, the results of this investigation indicate another avenue of pathogen dissemination that must be considered in cholera pandemic control and prevention.