NOVELTY - A Caulobacter host cell for expression and secretion of a heterologous polypeptide, is new.    USE - The host cell is used for the expression and secretion of a heterologous polypeptide (claimed). The modified Caulobacter cells may be used to treat sewage, waste water etc. The cells could also be grown in wood pulp suspensions and then used in the pulping process. They can also be used as fish vaccines.    ADVANTAGE - As Caulobacter has natural bio-film forming characteristics, using Caulobacter overcomes the problems of polysaccharide slimes that quickly plug filtration systems that are produced with other bacteria.    DETAILED DESCRIPTION - The host cell is a Caulobacter comprising at least one surface layer transport protein having an amino acid sequence homologous to the RasD or RasE sequence given in the specification. The host further comprises a DNA construct comprising DNA encoding a polypeptide heterologous to a surface layer protein of the cell 5' from and operably linked to a DNA encoding a Caulobacter surface layer protein secretion signal, with the proviso that when the cell comprises transport proteins having the same sequence as both the RasD and RasE proteins, the secretion signal is not from C. crescentus.    INDEPENDENT CLAIMS are also included for the following:    (1) a method for identifying a Caulobacter suitable for use as a host cell for expression and secretion of a heterologous polypeptide comprising:    (a) extracting DNA from a candidate Caulobacter;    (b) contacting the DNA with an oligonucleotide capable of selective hybridization to a nucleotide sequence encoding RasD or RasE; and    (c) determining whether the oligonucleotide hybridizes to the DNA;    (2) a DNA construct comprising one or more restriction sites for facilitating insertion of DNA into the construct, where the construct further comprises DNA encoding a Caulobacter surface layer protein secretion signal not present in C. crescentus;    (3) a DNA construct comprising DNA encoding a polypeptide not present in Caulobacter surface layer protein 5' from and operatively linked to DNA encoding a Caulobacter surface layer protein secretion signal not present in C. crescentus;    (4) a bacterial cell comprising a DNA construct of (2) or (3); and    (5) a secreted protein obtained from a cell of (4), where the protein comprises one or more portions heterologous to a surface layer protein of the cell and where the protein has a C-terminal portion comprising a surface layer protein secretion signal not present in C. crescentus.