Inhibition of Listeria Monocytogenes by Human Gut Bacteria

Listeria monocytogenes is a foodborne pathogen, which continues to be problematic to the food industry due to its ability survive within food products and persist within food processing environments. The growing trend towards natural food preservatives and antimicrobial agents increases the need for the development of novel, natural anti-Listeria agents. Evidence presented in the literature suggests that the human gut microbiota is a reservoir of novel antimicrobial agents. It was therefore hypothesized that novel anti-Listeria agents are produced by human gut-derived bacteria. The objective of this study was to isolate and characterize gut bacteria with anti-Listeria monocytogenes activity. A total of 23 fecal banks of human origin were screened for antagonistic activity against L. monocytogenes 10403S. The results of the initial screening have shown that 1,569 gut-derived isolates have demonstrated various levels of anti-Listeria activity. Following an extensive screen 59 gut-derived isolates were initially shortlisted and were identified mostly as Enterococcus spp. By a combination of MALDI-TOF MS analysis and 16S rRNA gene sequencing. The data collected allowed a further shortlisting of strains from 59 to 16 based on differences observed in the colony morphology, the size and appearance of the zones of inhibition observed in the deferred antagonism assays and well diffusion assays as well as spectrum of activity of each isolate against a number of indicator strains. The antimicrobials produced by 11 out of 16 shortlisted gut derived isolates were identified as peptides using a proteinase K assay. The anti-Listeria activity of the shortlisted isolates was examined in a model broth co-culture experiment. Based on the results obtained the antimicrobials produced by two Enterococcus faecium isolates were selected for further investigation. The activity of the antimicrobials was investigated in dairy food homogenate models and stainless steel biofilm assays. The findings of the food trials have shown a reduction of Listeria numbers by ~0.6 Log CFU/mL in natural yogurt treated with cell free supernatant (CFS) and a reduction of ~1.5 log CFU/mL observed in cheddar cheese treated with whole cell extract (WCE) following a 3 hr incubation. The anti-biofilm assays demonstrated a reduction in biofilm formation of stainless steel by 70% when treated with CFS and 92.42% when treated with WCE. v The experiments presented in this thesis confirm that bacteria from the human gut produce anti-Listeria compounds which can control or reduce the numbers of Listeria in model co-cultures and in food homogenates. The antimicrobials isolated in this study have also shown the ability to reduce Listeria biofilm formation. Future efforts should focus on identifying the antimicrobials produced by all of the short-listed strains (through a combination of genome sequencing and peptide purifications). It is hoped that any novel antimicrobial that is discovered have an application in the food industry to reduce the significant threat posed by L. monocytogenes. These antimicrobials could be used as a natural preservative in food products or packaging or incorporated into a spray to control biofilm in the food processing environment

Sums of totally positive matrices

AbstractIt is shown that an arbitrary m×n positive matrix can be written as a sum of at most min{m,n} totally positive matrices, and that this is in general the best possible value for the number of summands. Sufficient conditions are given under which fewer than min{m,n} totally positive summands are needed

Sharpness of generalized Gerschgorin disk

AbstractThe generalized Gerschgorin disks of a partitioned matrix A, as introduced by D. G. Feingold and R. S. Varga (Pacif. J. Math. 12 1241–1250 (1962)), are shown to give the smallest region covering all eigenvalues of all matrices from a certain class related to A

Z-Pencils

The matrix pencil (A,B) = {tB-A | t \in C} is considered under the assumptions that A is entrywise nonnegative and B-A is a nonsingular M-matrix. As t varies in [0,1], the Z-matrices tB-A are partitioned into the sets L_s introduced by Fiedler and Markham. As no combinatorial structure of B is assumed here, this partition generalizes some of their work where B=I. Based on the union of the directed graphs of A and B, the combinatorial structure of nonnegative eigenvectors associated with the largest eigenvalue of (A,B) in [0,1) is considered.Comment: 8 pages, LaTe

An Experimental Investigation of the Burn Rates of Naturally Aspirated Spark Assisted Compression Ignition Combustion in a Single Cylinder Engine with Negative Valve Overlap.

The implementation of homogeneous charge compression ignition (HCCI) in an engine remains a challenge due to the limited operating range and lack of a direct ignition timing control mechanism. Spark assisted compression ignition (SACI) has been shown by several research groups, including the work presented here, to provide such a mechanism, helping to control the phasing and stability of a primarily auto-igniting charge, as well as provide a means of extending the high load limit of HCCI while maintaining high thermal efficiency. The approach used in this study is unique in that flexible engine valve timing allowed for independent control of the thermal/compositional stratification associated with a large internal residual fraction, allowing its effect to be isolated from other thermophysical parameters. In these experiments, a single-cylinder engine equipped with fully-flexible valve actuation was used to explore the effects of spark assist in controlling peak heat release rates. With spark assist, a small portion of the heat release occurred via flame propagation, increasing the overall duration of the combustion event and dramatically reducing peak rates of heat release. At constant engine load and combustion phasing, peak heat release rates were reduced by 40% by controlling spark timing and unburned gas temperature via changes in internal and external EGR rates. Internal EGR was adjusted by varying the duration of negative valve overlap (NVO); for the range of NVO investigated, potential variations in in-cylinder mixing and thermal/compositional stratification were found to have a weak effect on burn characteristics, confirming the notion that temperature and spark timing are the primary variables affecting SACI burn rates for a fixed mixture composition. In the experiments, heat release analysis showed that the behavior of SACI was consistent with the theoretical kinetics associated with turbulent flame propagation and auto-ignition, supporting the hypothesis that SACI is essentially two distinct energy release events coupled by compression heating from an expanding flame front. The results of this work provide new insights into the physical and chemical mechanisms important during low temperature combustion. The results confirm proposed representations of SACI, and thereby provide direction for developing new advanced low temperature engine strategies.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99979/1/manofsky_1.pd

Inertially arbitrary sign patterns with no nilpotent realization

AbstractAn n by n sign pattern S is inertially arbitrary if each ordered triple (n1,n2,n3) of nonnegative integers with n1+n2+n3=n is the inertia of some real matrix in Q(S), the sign pattern class of S. If every real, monic polynomial of degree n having a positive coefficient of xn−2 is the characteristic polynomial of some matrix in Q(S), then it is shown that S is inertially arbitrary. A new family of irreducible sign patterns G2k+1(k⩾2) is presented and proved to be inertially arbitrary, but not potentially nilpotent (and thus not spectrally arbitrary). The well-known Nilpotent-Jacobian method cannot be used to prove that G2k+1 is inertially arbitrary, since G2k+1 has no nilpotent realization. In order to prove that Q(G2k+1) allows each inertia with n3⩾1, a realization of G2k+1 with only zero eigenvalues except for a conjugate pair of pure imaginary eigenvalues is identified and used with the Implicit Function Theorem. Matrices in Q(G2k+1) with inertias having n3=0 are constructed by a recursive procedure from those of lower order. Some properties of the coefficients of the characteristic polynomial of an arbitrary matrix having certain fixed inertias are derived, and are used to show that G5 and G7 are minimal inertially arbitrary sign patterns

Elementary bidiagonal factorizations

AbstractAn elementary bidiagonal (EB) matrix has every main diagonal entry equal to 1, and exactly one off-diagonal nonzero entry that is either on the sub- or super-diagonal. If matrix A can be written as a product of EB matrices and at most one diagonal matrix, then this product is an EB factorization of A. Every matrix is shown to have an EB factorization, and this is related to LU factorization and Neville elimination. The minimum number of EB factors needed for various classes of n-by-n matrices is considered. Some exact values for low dimensions and some bounds for general n are proved; improved bounds are conjectured. Generic factorizations that correspond to different orderings of the EB factors are briefly considered