The occurrence of unilateral flagellar phase variation was previously demonstrated in Escherichia coli strains carrying the non-fliC flagellin-specifying locus flk. In this study, we investigated the mechanism involved in this process. By using sequencing and sequence analysis, the flk region between the chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains, including the H35 strain identified in this study (the other strains used are H3, H36, H47, and H53 strains), and this region was found to contain a putative integrase gene and flanking direct repeats in addition to the flk flagellin-specifying gene flkA and a fliC repressor gene, flkB, indicating that there is a typical genomic islet (GI), which was designated the flk GI. The horizontal transfer potential of the flk GI was indicated by detection of the excised extrachromosomal circular form of the flk GI. By generating fliC-expressing variants of H3 and H47 strains, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and a complementation test. The potential integration sites of the flk GI were identified. A general model for flagellar phase variation in flk-positive E. coli strains can be expressed as fliC off ؉ flkA on 3 fliC on ؉ flkA none . This is the first time that a molecular mechanism for flagellar phase variation has been reported for E. coli.Flagella are best known for conferring motility to bacteria, which allows the bacteria to swim toward attractants and away from repellents (5). Flagella also play a variety of other roles in many bacterial lifestyles, including bacterial pathogenesis and biofilm formation (27,45). Pathogenic bacteria specifically produce flagella to promote colonization and invasion of mucosa (27). Flagellar antigen, also known as H antigen, is one of the major antigens in gram-negative bacteria. The serological variety of flagella is important for intrageneric differentiation of bacteria (12). Flagellin is the protein subunit of the flagellar filament and determines the specificity of the flagellar antigen.Phase variation of antigenic expression, especially expression of surface structures, such as flagella, fimbria, capsular polysaccharide, and lipopolysaccharide, is a common strategy used by many bacteria for adaptation to particular environments (38). Phase variation of flagellar antigens was first described in Salmonella enterica serovar Typhimurium, in which two different flagellin genes, fljB and fliC, are alternatively expressed, giving rise to two different H phases (18, 50). Flagellar phase variation is thought to be related to bacterial survival in the presence of host defense systems and therefore is linked to bacterial virulence (19). The molecular mechanism for flagellar phase variation in S. enterica has been well studied (1,20,22,40,47,51). Alternating expression of fljB and fliC is controlled by site-specific inversion of an approximately ...

In this study, Lactobacillus pentosus expressing porcine lactoferrin (pLF) was tested for in vitro antibacterial activity and for its ability to enhance immunity induced by an orally administered Aujeszky's disease virus (ADV) vaccine. The cDNA encoding N-terminus of pLF was cloned into a Lactobacillus-specific plasmid to produce L. pentosus pLF expressing transformants (pPG612.1-pLF-N/L. pentosus). The antimicrobial activity of the recombinant pLF protein inhibited bacterial growth in vitro. The supernatant of pPG612.1-pLF-N/L. pentosus had an inhibitory effect on Staphylococcus aureus strain CVCC26003, Bacillus subtilis strain CVCC63501, Escherichia coli strain CVCC10141 and Salmonella enterica ssp. enterica Choleraesuis strain CVCC79102, while it did not inhibit the growth of Lactobacillus casei strain ATCC393. A mouse model was established to test the effectiveness of the orally administered probiotic L. pentosus recombinant strain in the gastrointestinal tract. Mice were immunised with an attenuated porcine Aujeszky's disease virus (ADV) vaccine. Serum antibody levels determined using a mouse Aujeszky's disease IgG ELISA showed that IgG levels were significantly higher in the pPG612.1-pLF N /L. pentosus group than in the PBS and Lactobacillus pentosus groups at days 7 and 21 (P < 0.01) and at day 14 (P < 0.05), indicating that this oral recombinant strain can improve the effectiveness of the vaccine and play a role in immune enhancement through humoral immunity. These results suggest that the recombinant Lactobacillus pentosus not only has the beneficial characteristics of lactic acid bacteria but also produces biologically functional lactoferrin.Key words: Porcine lactoferrin, recombinant Lactobacillus pentosus, antibacterial activity, immune enhancement Lactoferrin (LF) is an 80-kDa iron-binding glycoprotein. It is abundantly found in exocrine secretions of mammals and especially in milk and fluids of the digestive tract released by mucosal epithelia and neutrophils during inflamma-

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