Escherichia coli has long been used as an indicator organism for water quality assessment. Recently there has been an accumulation of evidence that suggests some strains of this organism are able to proliferate in the environment, a characteristic that would detract from its utility as an indicator of faecal pollution. Phenotypic and genotypic characterization of E. coli isolated from blooms in two Australian lakes, separated by a distance of approximately 200 km, identified that the blooms were dominated by three E. coli strains. A major phenotypic similarity among the three bloom strains was the presence of a group 1 capsule. Genetic characterization of a conserved region of the cps gene cluster, which encodes group 1 capsules, identified a high degree of genetic variation within the bloom isolates. This differs from previously described encapsulated E. coli strains which are highly conserved at the cps locus. The phenotypic or genotypic profiles of the bloom strains were not identified in 435 E. coli strains isolated from vertebrates. The occurrence of these encapsulated strains suggests that some E. coli have evolved a free-living lifestyle and do not require a host in order to proliferate. The presence of the same three strains in bloom events in different geographical regions of a temperate climate, and at different times, indicates that free-living E. coli strains are able to persist in these water reservoirs. This study provides further evidence of circumstances where caution is required in using E. coli as an indicator organism for water quality.

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A novel Escherichia coli outer membrane protein A (OmpA) was discovered through a proteomic investigation of cell surface proteins. DNA polymorphisms were localized to regions encoding the protein's surfaceexposed loops which are known phage receptor sites. Bacteriophage sensitivity testing indicated an association between bacteriophage resistance and isolates having the novel ompA allele.Outer membrane protein A (OmpA) is a major, two-domain, heat-modifiable membrane protein in bacteria. The Nterminal domain is comprised of antiparallel ␤-strands that cross the membrane eight times, producing four large surfaceexposed hydrophilic loops and three short periplasmic turns. The C terminus, located in the periplasm, is connected to the outer membrane via interactions with peptidoglycan (6). It has been proposed that OmpA is involved in the structural integrity of the outer membrane (1, 2). OmpA also acts as a phage and colicin receptor (1,3,7), and a number of ompA mutants with alterations near residues 25, 70, and 110 have been found to be resistant to bacteriophage (6, 7). The residues involved in phage resistance occur in the large surface-exposed loops of the protein, the same loops that act as phage receptors.Outer membrane proteins similar to OmpA have been identified in 17 species of gram-negative bacteria (1). Similarities in the structure of OmpA and the high degree of similarity within the nucleotide and amino acid sequences of several enteric species indicate a high degree of evolutionary conservation. Further, a comparison of five closely related genera have shown that the ␤-strands are highly conserved, whereas the surface-exposed loops are highly variable (9).An investigation to identify differences in the outer membrane proteins of Escherichia coli from different animal sources resulted in the identification of a novel ompA allele (ompA2). Here we describe the genetic characteristics of the novel ompA allele, its frequency in isolates from a range of vertebrate hosts, and an evolutionary advantage of organisms possessing the novel allele.Outer membrane protein identification and characterization. Outer membrane proteins from three vertebrate E. coli isolates (H474 from a human, TA024 from a Tasmanian devil, and B194 from a varied honeyeater) were isolated using a carbonate extraction method combined with two-dimensional gel electrophoresis (8). Differential display of protein profiles from each of the three isolates showed a distinct shift in the isoelectric point of an integral outer membrane protein for isolate B194. Matrixassisted laser desorption ionization-time of flight mass spectrometry was used to obtain mass fingerprints for each protein spot (8). Peptide analysis using appropriate databases (Profound and TrEMBL) indicated that the proteins were most similar to the E. coli OmpA protein.Nucleotide analysis of the ompA allele from isolates characterized using proteomics revealed two sequences (ompA1 and ompA2). A BLASTN search identified ompA1 as being the ompA sequence of E. coli (GenBank acce...

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