PubMed Health⌕ Search

Biomedical subjects

M A Daeschel

Publications and source records attributed to M A Daeschel.

14 recordsLinked to original sources
PubMed1999

Resistance responses of microorganisms in food environments.

Food borne microorganisms display a broad spectrum of resistance responses to naturally occurring and intentionally added antimicrobial agents. Resistance may be conferred by innate structural features of the bacterial strain such as an impermeable outer membrane or a mechanism for antibiotic-inactivation. Bacteria previously susceptible to an antimicrobial compound can acquire resistance through mutation or through genetic transfer processes such as transformation, transduction, and conjugation. Resistance can also be conferred by biofilm formation on food processing surfaces as an adaptive response to protect colonies from cleaning and sanitation. Resistant pathogens are a global problem, facilitated by international trade of raw and processed foods. Cross resistance between clinical and nonclinical antimicrobials can exist and is of concern. The development of resistant foodborne pathogens has been attributed to increased antibiotic use in hospitals, outpatient facilities, and veterinary applications. Resistant microorganisms can also develop as a result of physical processes used in food preservation, such as acid treatments and irradiation processes. Strategies to effectively counter resistance development include: changing current practices of antibiotic usage, developing new antibiotics, applying hurdle preservation approaches, preventing bacterial adhesion, and utilizing competitive exclusion. This paper presents an overview of problems arising from the development of microbial resistance, and explores possible solutions for detecting and defeating the adaptive changes of microorganisms.

Drug Resistance, Microbial↗
PubMed1998

Protein antimicrobial barriers to bacterial adhesion.

The ability of microorganisms to adhere to solid surfaces is a problem of high visibility and has been the focus of numerous investigations because these organisms can cause disease and food spoilage. During the last several years, considerable attention has been focused on the development of food-grade antimicrobial barriers to adhesion in order to inhibit the initial adhesion of microbial contaminants by application of an antimicrobial agent to the surface rather than trying to remove these contaminants once they are adhered. The premise is that, if both the presence of the agent and its antimicrobial activity are maintained at the interface, sensitive bacterial cells or spores that attempt to attach would be killed. Nisin has been used in foods as a direct additive to inhibit the growth of Gram-positive cells and spores. Similarly, hen lysozyme is a commercially available antimicrobial protein that offers application in food processing systems, but the mode of action of this enzyme differs from that of nisin. We have shown that nisin can adsorb to surfaces, maintain activity, and kill cells that have adhered. In addition, we have addressed questions relating to the short- and long-term stability of adsorbed nisin, the degree to which immobilized nisin can resist exchange with dissolved solution components, and the surface concentrations that are necessary to inhibit biofilm formation. More recently, we have focused on basic questions relating to molecular influences on antimicrobial activity at interfaces using synthetic mutants of bacteriophage T4 lysozyme and hen lysozyme in addition to nisin.

Adsorption↗
PubMed1995

Influences on the antimicrobial activity of surface-adsorbed nisin.

The efficacy of the antimicrobial peptide nisin was examined after adsorption to silica surfaces. Three protocols were used to evaluate nisin's activity against adhered cells of Listeria monocytogenes: bioassay using Pediococcus pentosaceous FBB 61-2 as the sensitive indicator strain; visualization and enumeration of cells by microscopic image analysis; and viability of adhered cells as determined by lodonitrotetrazolium violet uptake and crystallization. The activity of adsorbed nisin was highly dependent upon conditions of adsorption. The highest antimicrobial activity of adsorbed nisin occurred with high concentrations of nisin (1.0 mg ml-1) and brief contact times (1 h) on surfaces of low hydrophobicity. Sequential adsorption of a second protein (beta-lactoglobulin or bovine serum albumin) onto surfaces consistently resulted in decreased nisin activity. These data provide direction for the development of applications to limit microbial attachment on food contact surfaces through the use of adsorbed antimicrobial peptides.

Adsorption↗
PubMed1995

Suppression of Listeria monocytogenes colonization following adsorption of nisin onto silica surfaces.

Nisin is an antimicrobial peptide proven to be an effective inhibitor of gram-positive bacteria. It is known that nisin can adsorb to various surfaces and still retain much of its original activity (M. A. Daeschel, J. McGuire, and H. Al-Makhlafi, J. Food Prot. 55:731-735, 1992). In this study, nisin films were allowed to form on silanized silica surfaces and then exposed to medium containing Listeria monocytogenes. Representative areas were selected from each surface, and images of resident listeriae were obtained at 4-h intervals for 12 h. During this time, cells on surfaces that had been in contact with a high concentration of nisin (1.0 mg/ml) exhibited no signs of growth and many displayed evidence of cellular deterioration. Surfaces treated with a lower concentration of nisin (0.1 mg/ml) had a smaller degree of inhibition. In contrast, both protein-free surfaces and those with films of heat-inactivated nisin allowed attached L. monocytogenes cells to grow and reproduce. These studies, when repeated with a nisin-resistant strain of L. monocytogenes, resulted in no inhibition of growth on surfaces with adsorbed nisin. The bactericidal effect of adsorbed nisin was also studied with iodonitrotetrazolium violet, a tetrazolium salt, which is reduced to a red formazan crystal by viable bacteria. Crystals were visible in 95% of the cells adhered to control surfaces but were present in less than 20% of the cells on surfaces with adsorbed nisin. These data indicate that adsorbed nisin may have potential for use as a food grade antimicrobial agent on food contact surfaces.

Adsorption↗
PubMed1992

Influence of fat and emulsifiers on the efficacy of nisin in inhibiting Listeria monocytogenes in fluid milk.

The recent FDA affirmation of nisin, an antimicrobial peptide, as a GRAS (generally recognized as safe) additive in pasteurized cheese spreads has renewed interest in its potential application in US dairy products. Fluid milks were prepared with varying concentrations of milk fat (0 to 12.9%) and of nisin (0 to 50 U/ml). Biological activity assays using a sensitive indicator microorganism in a well diffusion system indicated that initial nisin activity (50 U/ml) decreased by about 33% when it was added to skim milk and by more than 88% when added to milk containing 12.9% fat. Nisin activity decreased by ca. 50% in milk containing 1.29% fat. Milks containing 0, 10, or 50 U/ml of nisin and varying fat percentages were challenged with approximately log10 7 to 7.5 cfu/ml of log phase Listeria monocytogenes Scott A or Jalisco. At 2 h after inoculation, the viable count of L. monocytogenes Scott A decreased to log10 .30 cfu/ml in skim milk with 50 U/ml of nisin, decreased to log10 2.90 cfu/ml in skim milk with 10 U/ml of nisin, and increased slightly (log10 7.8 cfu/ml) in skim milk without nisin. In half-and-half (12.9% milk fat), nisin was far less effective in inhibiting Listeria with populations decreasing to log10 6.57 cfu/ml for 10 U/ml of nisin and log10 5.87 cfu/ml for 50 U/ml. Similar results were obtained with L. monocytogenes Jalisco. The nonionic emulsifier, Tween 80, partially counteracted decreases of nisin activity in milks, whereas the anionic emulsifier, lecithin, had no effect. Addition of Tween 80 significantly increased the activity of nisin against L. monocytogenes in milk regardless of fat content.

Animals↗
PubMed1991

Controlling Wine Malolactic Fermentation with Nisin and Nisin-Resistant Strains of Leuconostoc oenos.

The polypeptide nisin (100 U/ml) prevented malolactic fermentation in wines by indigenous or intentionally added lactic acid bacteria. Nisin (100 U/ml)-resistant mutants of Leuconostoc oenos were obtained and used with nisin in wine to carry out a pure-culture malolactic fermentation in the presence or absence of other lactic acid bacteria. Nisin degradation by mutants was not observed, and residual nisin was detectable in wines 4 months after it was added. Results indicated that nisin or nisin with resistant bacterial starter cultures can be used to control malolactic fermentation in wines.

Journal Article↗
PubMed1988

Antibacterial activity of plantaricin SIK-83, a bacteriocin produced by Lactobacillus plantarum.

Lactobacillus plantarum SIK-83 produces a bacteriocin, designated plantaricin SIK-83, which inhibits 66 of 68 lactic acid bacteria from the genera Lactobacillus, Leuconostoc, Pediococcus and Streptococcus. A 500-fold dilution of L. plantarum SIK-83 MRS culture supernatant with phosphate buffer was sufficient to kill 10(5) cells/ml of Pediococcus pentosaceus within 120 s. The killing of a sensitive population followed exponential kinetics. It was shown that the bacteriocin binds specifically to sensitive cells but not to nonsensitive lactic acid bacteria, the producer strain or Gram-negative bacteria. Sensitive cells, after exposure to the bacteriocin, could be rescued by treatment with proteolytic enzymes. In buffer, plantaricin SIK-83 was adsorbed to the cell surface almost immediately, and morphological lesions were observed within 2 h after the cells were exposed to the bacteriocin. The lethal mode of action appeared to be due to damage to the cell membrane, resulting in cell lysis, which was detected by electron microscopy and by determination of released intracellular components.

Adsorption↗
PubMed1988

A pH Control System Based on Malate Decarboxylation for the Cultivation of Lactic Acid Bacteria.

Most species of lactic acid bacteria decarboxylate l-malate to lactate and CO(2) if an energy source such as glucose is present. A proton is taken up in the reaction, which prevents pH decreases in the growth medium caused by lactic acid production from glucose fermentation. MRS broth (pH 7.0) (Difco Laboratories) containing 10 mM glucose and various concentrations of l-malate (0, 25, 50, 75, and 100 mM) was used to cultivate Lactobacillus plantarum. After 72 h at 37 degrees C, all malate was decarboxylated and all glucose was fermented, with resultant final pH values of 4.5, 6.3, 6.9, 7.3, and 7.5, respectively. When d-malate (which cannot be decarboxylated) was substituted for l-malate, the final pH values were 4.5, 5.2, 5.6, 5.8, and 5.9. By varying the ratios of glucose to l-malate in the growth medium, it was possible to obtain pH values which were lower, the same, or higher than the initial pH values. In contrast, buffers such as phosphate only retard decreases in pH. l-Malate, when compared with K(2)PO(4) on an equal molar basis, provided greater resistance to decreases in pH. Higher specific growth rates were observed for L. plantarum and Leuconostoc mesenteroides when l-malate rather than K(2)PO(4) was incorporated into the growth medium.

Journal Article↗
PubMed1987

A differential medium for the enumeration of homofermentative and heterofermentative lactic Acid bacteria.

A medium was developed for the differential enumeration of homofermentative and heterofermentative lactic acid bacteria. Essential components of the medium included fructose (14 mM), KH(2)PO(4) (18 mM), bromcresol green (as a pH indicator), and other nutrients to support growth. In agar medium, homofermentative colonies were blue to green, while heterofermentative colonies remained white. A total of 21 Lactobacillus, Pediococcus, Leuconostoc, and Streptococcus species were correctly classified with the medium.

Journal Article↗
PubMed1985

Association of a 13.6-Megadalton Plasmid in Pediococcus pentosaceus with Bacteriocin Activity.

Pediococcus pentosaceus FBB61 and L7230, isolated from cucumber fermentations, produced a bacteriocin, designated pediocin A, which had identical activity spectra against species of Pediococcus, Clostridium, Staphylococcus, and Streptococcus. Both strains possessed a 13.6-megadalton plasmid (pMD136). Plasmid curing experiments suggested that both bacteriocin immunity and production determinants were encoded by pMD136. Use of pediocin-producing strains in food fermentations is discussed.

Journal Article↗
PubMed1984

Mutation and Selection of Lactobacillus plantarum Strains That Do Not Produce Carbon Dioxide from Malate.

A differential medium was developed to distinguish between malate-decarboxylating (MDC) and -non-decarboxylating (MDC) strains of Lactobacillus plantarum. MDC strains produced a visible acid reaction in the medium, whereas MDC strains did not. Use of the medium allowed for rapid screening and isolation of mutagenized cells that had lost the ability to produce CO(2) from malate.

Journal Article↗
PubMed1981

Microbial Changes in Sweet Sorghum (Sorghum bicolor) Juices.

Juice freshly expressed from Sorghum bicolor for making sweet sorghum syrup contained 10 microorganisms per ml. The dominant bacterium was Leuconostoc mesenteroides, followed by gram-negative rods. Lactobacilli, yeasts, and nonfecal coliform bacteria each comprised about 1% of the microbial population. Spoilage of juice, manifested by a sour odor, discoloration, and foaming, occurred between 5 and 12 h at ambient temperatures. Spoilage was correlated with a drop in pH from 4.9 to 4.5 L. mesenteroides was the dominant spoiling agent at 20 degrees C, and Lactobacillus plantarum was the dominant spoiling agent at 32 degrees C, as determined by pure culture studies. Juice may be stored for 14 days at 4 degrees C if promptly refrigerated.

Journal Article↗
PubMed1981

Entrance and growth of lactic Acid bacteria in gas-exchanged, brined cucumbers.

Entrance of lactic acid bacteria into the interior of brined cucumbers was found to be greatly influenced by gas composition of the cucumbers before brining. Exchange of the internal gas of fresh cucumbers with O(2) resulted in absorption of bacteria into the subsequently brined fruit within a few hours. Bacteria were absorbed into nonexchanged cucumbers to a lesser extent. Little bacterial absorption occurred in N(2)-exchanged cucumbers. Stomata of the cucumber skin appeared to be a likely port for bacterial entry. When Pediococcus cerevisiae or Lactobacillus plantarum cells were added to the brine of O(2)-exchanged cucumbers, the respective cell types colonized in large numbers within intercellular spaces and vascular elements of mesocarp tissue during fermentation of the cucumbers. Implications of these observations, particularly with regard to bloater formation in brined cucumbers, are discussed.

Journal Article↗