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Friday, 27 February 2009 13:08

In contrast to viruses and protozoa, many bacteria have the ability to actively form biofilms by excretion of exopolysaccharides and intra- and interspecies communication (quorum sensing). Biofilms form by attachment of cells to a wide spectrum of different surfaces, which finally results in a slimy coat. In addition, bacteria attach to each other. Viruses and protozoa, which do not have the ability to form biofilms, in turn can get attached to these aggregates which ultimately leads to the formation of complex microbial communities. In respect to pathogenic bacteria, biofilm formation in aqueous environments acquires a special meaning as biofilm clusters can harbor a great number of cells and the uptake of detached biofilm clumps through consumption of contaminated water by humans and animals might be an efficient way to deliver an infective dose large enough to cause disease ( Faruque et al. 2006). This has been suspected for Vibrio cholerae as a landmark organism ( Faruque et al. 2006). Huq et al. assumed that this epidemiological role of environmental biofilms might also apply to other waterborne bacterial pathogens ( Huq et al. 2008). In addition, biofilms can also develop in patients and protect pathogens from the human immune system, harsh pH conditions and antibiotic treatment ( Huq et al. 2008). Interestingly, a study reported that Vibrio cholerae are shed from patients not only as individual planktonic cells, but also as biofilm-like clumps that are formed in vivo ( Faruque et al. 2006). It was speculated that the formation of such structures might contribute to enhanced infectivity and improved environmental persistence.

In the aqueous environment, biofilms serve an important ecological role by providing a safe environment under adverse conditions offering protection from physiochemical stresses like UV exposure, dehydration, oxidative and osmotic stress, and biocidal agents ( Huq et al. 2008). Again with V. cholerae serving as an example, environmental biofilms might harbor NC cells that can survive over extended periods of time. Culturability of non-cultivatible V. cholerae cells from biofilms that were harvested from microcosms up to 495 days after seeding could be restored by animal passage ( Alam et al. 2007). The only disadvantage for being in a biofilm state might be that the aggregation of a large number of cells makes them more susceptible to predation by protozoa and infection by phages. However, this close proximity also has positive sides. First, it allows internalization into protozoa. There is increasing evidence that internalization events (in contrast to digestion) does not affect bacterial viability, but enhances environmental persistence and resistance to adverse conditions. Secondly, in case of dissemination of genetic material, infection by phages is evolutionary beneficial by increasing the probability of gene transfer events resulting in increased fitness and higher pathogenicity.

It is important to realize that factors like interaction with protozoa and viruses are not accounted for in studies looking at survival of specific bacteria in seeded sterile water. The same holds true for competition with naturally occurring microorganisms. Whereas some of the interactions with other microorganisms negatively impact survival, others can be of beneficial nature and can have a positive effect on persistence of certain pathogens. Generalizations are dangerous due to the diversity of pathogens and the microbial complexity. An example is the study of the naturally present flora on the survival of allochthonous pathogens in water. The effect cannot be generalized and varies according to the pathogen studied. In bottled mineral water, the autochthonous flora was reported to enhance the survival of E. coli O157:H7 ( Kerr et al. 1999) or of Klebsiella pneumoniae ( Moreira et al. 1994), to have an inhibitory effect of E. coli ( Ducluzeau et al. 1984), and finally to have no effect on survival of Campylobacter jejuni ( Tatchou-Nyamsi-König et al. 2007). This shows that pathogens respond to water composition very differently. It also implies that the presence of a single indicator organism does not allow conclusions on microbial safety.

Last Updated on Saturday, 31 October 2009 04:14
 

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