The yeasts constitute a large group of microorganisms characterized by the ability to grow and survive in different and stressful conditions and then to colonize a wide range of environmental and human ecosystems. The competitive traits against other microorganisms have attracted increasing attention from scientists, who proposed their successful application as bioprotective agents in the agricultural, food and medical sectors. These antagonistic activities rely on the competition for nutrients, production and tolerance of high concentrations of ethanol, as well as the synthesis of a large class of antimicrobial compounds, known as killer toxins, which showed clearly a large spectrum of activity against food spoilage microorganisms, but also against plant, animal and human pathogens. This review describes the antimicrobial mechanisms involved in the antagonistic activity, their applications in the processed and unprocessed food sectors, as well as the future perspectives in the development of new bio-drugs, which may overcome the limitations connected to conventional antimicrobial and drug resistance.

/pdf/bioprotective-role-of-yeasts-2tuwqown4z.pdf

Bioprotective Role of Yeasts.

The effect that yeast research and development on yeasts has had on the brewing industry is traced from the time of Pasteur to the present time.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1986.tb04453.x

One hundred years of yeast research and development in the brewing industry

Many bacteria are known to inhibit food pathogens, such as Listeria monocytogenes, by secreting a variety of bactericidal and bacteriostatic substances. In sharp contrast, it is unknown whether yeast has an inhibitory potential for the growth of pathogenic bacteria in food. A total of 404 yeasts were screened for inhibitory activity against five Listeria monocytogenes strains. Three hundred and four of these yeasts were isolated from smear-ripened cheeses. Most of the yeasts were identified by Fourier transform infrared spectroscopy. Using an agar-membrane screening assay, a fraction of approximately 4% of the 304 red smear cheese isolates clearly inhibited growth of L. monocytogenes. Furthermore, 14 out of these 304 cheese yeasts were cocultivated with L. monocytogenes WSLC 1364 on solid medium to test the antilisterial activity of yeast in direct cell contact with Listeria. All yeasts inhibited L. monocytogenes to a low degree, which is most probably due to competition for nutrients. However, one Candida intermedia strain was able to reduce the listerial cell count by 4 log units. Another four yeasts, assigned to C. intermedia (three strains) and Kluyveromyces marxianus (one strain), repressed growth of L. monocytogenes by 3 log units. Inhibition of L. monocytogenes was clearly pronounced in the cocultivation assay, which simulates the conditions and contamination rates present on smear cheese surfaces. We found no evidence that the unknown inhibitory molecule is able to diffuse through soft agar.

Inhibition of Listeria monocytogenes by Food-Borne Yeasts

When the domains of individual microorganisms overlap, it is likely that interactions will occur (Boddy and Wimpenny 1992) The outcome of these interactions is evaluated on the basis of the effect they have on population size (Odum 1953) regardless of whether the interactions are detrimental, neutral or beneficial The types of interaction found in mixed populations of microorganisms are classified on the basis of these effects as direct or indirect interactions (Bull and Slater 1982) Indirect interaction refers to competition, commensalism, mutualism, amensalism or antagonism and neutralism (Linton and Drozd 1982), and direct interaction to predation and parasitism (Frederickson 1977; Bull and Slater 1982) However, fermented foods and beverages develop their nutritional and organoleptic qualities as a result of the metabolic activity of a succession of different microorganisms and it is unlikely that the interactions will separate into these discrete groups since more than one type of interaction occurs simultaneously (Verachtert et al 1990) Present understanding of the positive, negative or neutral role of interactions between yeasts, bacteria and fungi has its origins the first time fermentation was employed The fermentation of many products includes interaction both within and between different microbial groups (eg yeast–yeast, yeast–bacteria, yeast–moulds), the physiological activity of which brings about desirable changes which decisively determine the character of a product and stabilise the population in a specific ecological niche (Wood and Hodge 1985; Leroi and Pidoux 1993; Geisen et al 1992; Rossi 1978; Challinor and Rose 1954) However, interaction does not necessarily only imply the positive or negative attributes within fermentation but it also involves the antagonistic activity of yeasts against other microorganisms by means of the production of microcins (Baquero and Moreno 1984; Golubev and Boekhout 1992), secretion of antibacterial and antifungal compounds, co-fermentation, and their role as in biological control Chapter 4

Yeast Ecological Interactions. Yeast'Yeast, Yeast'Bacteria, Yeast'Fungi Interactions and Yeasts as Biocontrol Agents

Screening for yeast with antibacterial properties from an ethanol distillery.

A general screening test for the expression of antibacterial activity was performed on over 400 cultures belonging to 31 yeast genera. Of these cultures, only two, Kluyveromyces thermotolerans and Kloeckera apiculata, were found to produce zones of inhibition of bacterial growth on Diagnostic Sensitivity Test Agar medium supplemented with 0.002% methylene blue. Of nine bacteria used as test organisms, only Lactobacillus plantarum and Bacillus megaterium were inhibited. No antibacterial activity was evident against four gram-negative bacteria used in this study. Optimal activities were found to be expressed after yeasts were grown at pH 6. A requirement for cultivation in the presence of methylene blue added to culture media for the expression of apparent antibacterial activity was demonstrated.

G Innamorato

Papers

Identification and characterization of antimicrobial activity in two yeast genera.