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BackgroundWe wanted to compare growth differences between 13 Escherichia coli strains exposed to various concentrations of the growth inhibitor lactoferrin in two different types of broth (Syncase and Luria-Bertani (LB)). To carry this out, we present a simple statistical procedure that separates microbial growth curves that are due to natural random perturbations and growth curves that are more likely caused by biological differences.Bacterial growth was determined using optical density data (OD) recorded for triplicates at 620 nm for 18 hours for each strain. Each resulting growth curve was divided into three equally spaced intervals. We propose a procedure using linear spline regression with two knots to compute the slopes of each interval in the bacterial growth curves. These slopes are subsequently used to estimate a 95% confidence interval based on an appropriate statistical distribution. Slopes outside the confidence interval were considered as significantly different from slopes within. We also demonstrate the use of related, but more advanced methods known collectively as generalized additive models (GAMs) to model growth. In addition to impressive curve fitting capabilities with corresponding confidence intervals, GAM’s allow for the computation of derivatives, i.e. growth rate estimation, with respect to each time point.ResultsThe results from our proposed procedure agreed well with the observed data. The results indicated that there were substantial growth differences between the E. coli strains. Most strains exhibited improved growth in the nutrient rich LB broth compared to Syncase. The inhibiting effect of lactoferrin varied between the different strains. The atypical enteropathogenic aEPEC-2 grew, on average, faster in both broths than the other strains tested while the enteroinvasive strains, EIEC-6 and EIEC-7 grew slower. The enterotoxigenic ETEC-5 strain, exhibited exceptional growth in Syncase broth, but slower growth in LB broth.ConclusionsOur results do not indicate clear growth differences between pathogroups or pathogenic versus non-pathogenic E. coli.

Summary-This article reports the existing knowledge about the proteolytic system of Propionibacterium and its ability to degrade amino acids. Propionibacterium contains at least 2 weak proteinases, 1 cell wall-associated and 1 intracellular or membrane-bound. A wide variety of peptidases, such as amino peptidases, proline iminopeptidase, proline imidopeptidase, X-prolyl-dipeptidyl-aminopeptldase, endopeptidases and carboxypeptidase, has been described and characterized. A wide variety of amino acids, especially aspartic acid, alanine, serine and glycine, were easily degraded by Propionibacterium, but large strain and species variations were observed.

Eight laboratories participated in a collaborative study to evaluate an enzyme-linked immunosorbent assay (ELISA) to determine soy, pea, and wheat proteins in pasteurized or ultra-high temperature (UHT) milk powders. To perform this assay, polyclonal antibodies for soy, pea, and wheat proteins were obtained from rabbit sera. Collaborators received calibration standards composed of milk powder containing 0–8% (w/w) vegetal protein in total protein and blind test samples containing approximately 1, 2, and 5% (w/w) vegetal protein. An indirect competitive ELISA was performed with a kit prepared by a participating laboratory; the kit contained plates coated with soy, pea, or wheat proteins, the corresponding specific antisera, enzyme-labeled second antibody, and substrate solution. Test samples and calibrants were extracted with phosphate-buffered saline, pH 7.4, containing 0.05% Tween and assayed with the ELISA kits. The degree of adulteration was affected by the type of heat treatment applied to the samples. The estimated percentage of vegetal protein addition was close to the theoretical value for pasteurized samples but much lower for UHT samples. For pasteurized samples, intralaboratory relative standard deviations ranged from 5 to 22% and interlaboratory relative standard deviations ranged from 14 to 34%.

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