Anne H. Bates

Detection on surfaces and in Caco-2 cells of Campylobacter jejuni cells transformed with new gfp, yfp, and cfp marker plasmids.

ABSTRACT We have developed two sets of Campylobacter shuttle vectors containing either the gfp (green fluorescent protein), yfp (yellow fluorescent protein), orcfp (cyan fluorescent protein) reporter gene. In one set, the reporter gene is fused to a consensus Campylobacterpromoter sequence (Pc). The other set contains a pUC18 multicloning site upstream of the reporter gene, allowing the construction of transcriptional fusions using known promoters or random genomic fragments. C. jejuni cells transformed with the Pc fusion plasmids are strongly fluorescent and easily visualized on chicken skin, on plant tissue, and within infected Caco-2 cells. In each C. jejuni strain tested, these plasmids were maintained over several passages in the absence of antibiotic selection. Also, in many C. jejuni strains, >91% of the cells transformed with the Pc fusion plasmids remained fluorescent after several days. Experiments with yellow fluorescent and cyan fluorescent C. jejuni transformants suggest that aggregates containing two or more strains of C. jejuni may be present in an enrichment broth culture. Colonies arising from these aggregates would be heterologous in nature; therefore, isolation of a pure culture of C. jejuni, by selecting single colonies, from an environmental sample may not always yield a single strain.

Production of Autoinducer 2 in Salmonella enterica Serovar Thompson Contributes to Its Fitness in Chickens but Not on Cilantro Leaf Surfaces

ABSTRACT Food-borne illness caused by Salmonella enterica has been linked traditionally to poultry products but is associated increasingly with fresh fruits and vegetables. We have investigated the role of the production of autoinducer 2 (AI-2) in the ability of S. enterica serovar Thompson to colonize the chicken intestine and the cilantro phyllosphere. A mutant of S. enterica serovar Thompson that is defective in AI-2 production was constructed by insertional mutagenesis of luxS. The population size of the S. enterica serovar Thompson parental strain was significantly higher than that of its LuxS− mutant in the intestine, spleen, and droppings of chicks 12 days after their oral inoculation with the strains in a ratio of 1:1. In contrast, no significant difference in the population dynamics of the parental and LuxS− strain was observed after their inoculation singly or in mixtures onto cilantro plants. Digital image analysis revealed that 54% of S. enterica serovar Thompson cells were present in large aggregates on cilantro leaves but that the frequency distributions of the size of aggregates formed by the parental strain and the LuxS− mutant were not significantly different. Carbon utilization profiles indicated that the AI-2-producing strain utilized a variety of amino and organic acids more efficiently than its LuxS− mutant but that most sugars were utilized similarly in both strains. Thus, inherent differences in the nutrients available to S. enterica in the phyllosphere and in the chicken intestine may underlie the differential contribution of AI-2 synthesis to the fitness of S. enterica in these environments.

Distinct acid resistance and survival fitness displayed by Curli variants of enterohemorrhagic Escherichia coli O157:H7.

ABSTRACT Curli are adhesive fimbriae of Enterobacteriaceae and are involved in surface attachment, cell aggregation, and biofilm formation. Here, we report that both inter- and intrastrain variations in curli production are widespread in enterohemorrhagic Escherichia coli O157:H7. The relative proportions of curli-producing variants (C+) and curli-deficient variants (C−) in an E. coli O157:H7 cell population varied depending on the growth conditions. In variants derived from the 2006 U.S. spinach outbreak strains, the shift between the C+ and C− subpopulations occurred mostly in response to starvation and was unidirectional from C− to C+; in variants derived from the 1993 hamburger outbreak strains, the shift occurred primarily in response to oxygen depletion and was bidirectional. Furthermore, curli variants derived from the same strain displayed marked differences in survival fitness: C+ variants grew to higher concentrations in nutrient-limited conditions than C− variants, whereas C− variants were significantly more acid resistant than C+ variants. This difference in acid resistance does not appear to be linked to the curli fimbriae per se, since a csgA deletion mutant in either a C+ or a C− variant exhibited an acid resistance similar to that of its parental strain. Our data suggest that natural curli variants of E. coli O157:H7 carry several distinct physiological properties that are important for their environmental survival. Maintenance of curli variants in an E. coli O157:H7 population may provide a survival strategy in which C+ variants are selected in a nutrient-limited environment, whereas C− variants are selected in an acidic environment, such as the stomach of an animal host, including that of a human.

ELISA Analysis of Soybean Trypsin Inhibitors in Processed Foods

Soybean proteins are widely used in human foods in a variety of forms, including infant formulas, flour, protein concentrates, protein isolates, soy sauces, textured soy fibers, and tofu. The presence of inhibitors of digestive enzymes in soy proteins impairs the nutritional quality and possibly the safety of soybeans and other legumes. Processing, based on the use of heat or fractionation of protein isolates, does not completely inactivate or remove these inhibitors, so that residual amounts of inhibitors are consumed by animals and humans. New monoclonal antibody-based immunoassays can measure low levels of the soybean Kunitz trypsin inhibitor (KTI) and the Bowman-Birk trypsin and chymotrypsin inhibitor (BBI) and the Bowman-Birk foods. The enzyme-linked immunosorbent assay (ELISA) was used to measure the inhibitor content of soy concentrates, isolates, and flours, both heated and unheated; a commercial soy infant formula; KTI and BBI with rearranged disulfide bonds; browning products derived from heat-treatment of KTI with glucose and starch; and KTI exposed to high pH. The results indicate that even low inhibitor isolates contain significant amounts of specific inhibitors. Thus, infants on soy formula consume about 10 mg of KTI plus BBI per day. The immunoassays complement the established enzymatic assays of trypsin and chymotrypsin inhibitors, and have advantages in (a) measuring low levels of inhibitors in processed foods; and (b) differentiating between the Kunitz and Bowman-Birk inhibitors. The significance of our findings for food safety are discussed.

Monoclonal Antibody to Fenbendazole: Utility in Residue Studies

A monoclonal antibody-based ELISA was developed for fenbendazole, a widely used benzimidazole anthelmintic, with approved uses in cattle and other food animals. The antibody was elicited using as hapten 2-succinamido-5(6)-phenylthiobenzimidazole, which was conjugated with bovine serum albumin to produce an immunogen and with horseradish peroxidase to prepare a labeled ligand. The ELISA was performed on aqueous extracts of bovine liver or diluted milk samples. In bovine liver, the limit of detection was 200 ppb; in the milk matrix, the limit of detection was 3 ppb. The ELISA method is a simple approach to screen food samples for residues of fenbendazole.

Competitive ELISA of Thiabendazole Residues in Produce Using Indirectly Immobilized Monoclonal Antibodies

An enzyme‐linked immunosorbent assay (ELISA), developed originally for analysis of thiabendazole (2‐[4‐thiazolyl] 1H‐benzimidazole, TBZ) residues in bovine liver, was used to analyze residues of this post‐harvest fungicide in the peel of apple, potato, orange, grapefruit and banana. Residues were extracted by soaking peels overnight in 80% methanol and filtering the decanted supernatants. A 20‐fold dilution eliminated significant matrix effects. The ELISA has a limit of detection of 0.1 ppm in peel samples, corresponding to 10–40 ppb in the whole fruit or tuber. The ELISA employs a monoclonal antibody layer, immobilized indirectly via a rabbit anti‐mouse IgG secondary antibody, and horseradish peroxidase‐conjugated 5‐succinamido‐TBZ. Assay wells coated in this way could be dried and stored for at least 6 months without detectable changes in assay parameters. The method exceeds the required sensitivity for regulatory monitoring of TBZ residues in the US, and offers greater speed and specificity than previo...