Isolation from food sources, of lactic acid bacteria that produced antimicrobials

Antarctic bacteria inhibit growth of food-borne microorganisms at low temperatures

The aim of this study was to identify Antarctic microorganisms with the ability to produce cold-active antimicrobial compounds with potential for use in chilled food preservation. Colonies (4496) were isolated from 12 Antarctic soil samples and tested against Listeria innocua, Pseudomonas fragi and Brochothrix thermosphacta. Thirteen bacteria were confirmed as being growth-inhibitor producers (detection rate 0.29%). When tested against a wider spectrum of eight target organisms, some of the isolates also inhibited the growth of L. monocytogenes and Staphylococcus aureus. Six inhibitor producers were psychrotrophic (growth optima between 18 and 24 degrees C), halotolerant (up to 10% NaCl) and catalase-positive; all but one were Gram-positive and oxidase-positive. The inhibitors produced by four bacteria were sensitive to proteases, suggesting a proteinaceous nature. Four of the inhibitor-producers were shown to be species of Arthrobacter, Planococcus and Pseudomonas on the basis of their 16S rRNA gene sequences and fatty acid compositions. It was concluded that Antarctic soils represent an untapped reservoir of novel, cold-active antimicrobial-producers.

Microbial antagonists of Escherichia coli O157:H7 on fresh-cut lettuce and spinach

Fresh-cut lettuce and spinach can become contaminated with pathogens at numerous points from the field to the retail market. Natural microflora present on fresh produce may help reduce the pathogen load. The objective of this study was to isolate natural microflora from fresh-cut iceberg lettuce and baby spinach and to determine whether these bacteria were antagonistic toward Escherichia coli O157:H7. Samples were collected under conditions that mimicked actual practices between production and retail sale. Evidence of naturally occurring microorganisms on fresh lettuce (295 isolates) and spinach (200 isolates) and of possible antagonistic activity toward E. coli O157:H7 was documented. Inhibitory activity by several isolates was due to either acid production or antimicrobial peptides. Bacteria with inhibitory activity were isolated from every step in the processing and handling of the fresh-cut iceberg lettuce and baby spinach.

Antifungal lactic acid bacteria with potential to prolong shelf-life of fresh vegetables

AIM

The aim of this study was to isolate and identify antifungal lactic acid bacteria from fresh vegetables, and evaluate their potential in preventing fungal spoilage of vegetables.

METHODS AND RESULTS

Lactic acid bacteria from fresh vegetables were enriched in MRS (de Man Rogosa Sharpe) broth and isolated by plating on MRS agar. All the isolates (359) were screened for activity against Aspergillus flavus of which 10% showed antifungal activity. Potent antifungal isolates were identified by phenotypic characters and confirmed by partial 16S rRNA gene sequencing. These were screened against additional spoilage fungi viz. Fusarium graminearum, Rhizopus stolonifer, Sclerotium oryzae, Rhizoctonia solani, Botrytis cinerea and Sclerotinia minor by overlay method. Most of the isolates inhibited wide range of spoilage fungi. When fresh vegetables were inoculated with either cell suspension (10(4) cells ml(-1)) or cell-free supernatant of Lact. plantarum, followed by application of vegetable spoilage fungi (A. flavus and F. graminearum, R. stolonifer, B. cinerea each with 10(4) conidia ml(-1)) the vegetable spoilage was significantly delayed than control.

CONCLUSIONS

Fresh vegetables constitute a good source of lactic acid bacteria with ability to inhibit wide range of spoilage fungi. Such bacteria can be applied to enhance shelf-life of vegetables. In the present study, we report for the first time the antifungal activity of Weissella paramessenteroides and Lact. paracollinoides isolated from fresh vegetables, against wide range of food spoilage fungi.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fresh vegetables can be used as a source of antifungal lactic acid bacteria. Their exploitation as biopreservative will help in prolonging shelf-life of fresh vegetables.

Atividade antimicrobiana de bactérias ácido-lácticas isoladas de queijos de coalho artesanal e industrial frente a microrganismos indicadores

Quatro cepas de Lactobacillus spp. e duas cepas de Lactococcus spp. isoladas de queijos de coalho artesanal e industrial foram testadas quanto às suas atividades antimicrobianas. Observou-se atividade antagonista dessas bactérias ácido-lácticas frente a elas, a outras bactérias ácido-lácticas isoladas de queijo de coalho, aos patógenos isolados dos mesmos queijos e a cepas de patógenos de referência. Verificou-se diferença (P<0,05) entre as atividades antagonistas, exceto quando as próprias bactérias ácido-lácticas foram utilizadas como reveladoras. Lactobacillus spp. apresentaram as atividades antagonistas mais potentes.

Microbial antagonists of foodborne pathogens on fresh, minimally processed vegetables

On many types of raw or minimally processed foods, the bacterial microbiota is often composed of mixed species. The activities of one bacterial species may influence the growth and activities of others that are present. The objective of this project was to evaluate the microbial composition of fresh and minimally processed vegetables to determine if naturally occurring bacteria on produce are competitive with or antagonistic to potentially encountered pathogens. Naturally occurring bacteria were obtained from ready-to-eat salad vegetables on four occasions to allow for seasonal variation. Minimally processed vegetables were sampled at various stages in their processing from raw vegetables to packaged products. Some portions were analyzed microbiologically within 24 h, while other portions were stored refrigerated and analyzed after 72 h. Microbiological analysis was conducted for bacterial enumeration and to obtain isolates. An agar spot method was used to screen isolates for antimicrobial activity against Staphylococcus aureus ATCC 27664, Escherichia coli O157:H7 E009, Listeria monocytogenes LCDC 81-861, and Salmonella Montevideo. Of the 1,180 isolates screened for inhibitory activity, 37 (3.22%) were found to have various degrees of inhibitory activity against at least one test pathogen. Many isolates showed inhibitory activity against all four pathogens. The isolates with the most extensive inhibition were removed from finished lettuce piece shreds. Of the 37 inhibitory isolates, 34 (91.9%) were gram negative. All isolates with inhibitory activity are able to multiply at both 4 and 10 degrees C.

Pre-inoculation enrichment procedure enhances the performance of bacteriocinogenic Lactococcus lactis meat starter culture

Sodium nitrite and sodium chloride may inhibit growth and bacteriocinogenesis of protective starter cultures. To reduce sensitivity of a lacticin 3147-producing starter culture to nitrite, prior to production of salami, Lactococcus lactis DPC 4275 was placed in a number of pre-inoculation treatments, containing (a) 1% glucose, (b) 2.5 ppm manganese (Mn), (c) 250 ppm magnesium (Mg), (d) 2.5 ppm manganese + 250 ppm magnesium (Mn + Mg), and held at ambient temperature for 30 min and 4 degrees C for 2 h. The growth, pH reduction, and bacteriocin production was monitored in beaker sausage over a period of 10 days at 28 degrees C, corresponding to typical salami production time, and compared to untreated starter culture. The effect of 1% tryptone and inoculum level on growth and bacteriocin production was also determined. Challenge tests were performed using Listeria innocua DPC 1770 and Staphylococcus aureus MMPR3 as target strains. All treatments gave a significantly higher (P < 0.05) initial starter level than the untreated starter. Beaker sausage inoculated with either low (10(7)) or high (10(9)) levels of starter culture, treated with Mn + Mg reached significantly (P < 0.05) higher levels by day 10 than other treatments. Trends indicate that Mn + Mg also gave best pH reduction in sausage containing the low-level starter culture, sausage and significantly lower (P < 0.05) values for sausage produced with higher inoculum. Bacteriocin production was also higher in starter culture treated with Mn, or glucose. Pre-treatment with Mg gave a 2-fold increase in bacteriocin, the addition of Mn augmenting this increase further. The incorporation of tryptone gave no additional effect. In beaker sausage, both L. innocua and S. aureus populations showed significant reductions (P < 0.05) in the presence of the bacteriocinogenic strain compared to a non-bacteriocinogenic control strain.

The effects of cultivating lactic starter cultures with bacteriocin-producing lactic acid bacteria

The effects of bacteriocins produced by six strains of lactic acid bacteria on 9 mesophilic and 11 thermophilic commercial starter cultures were investigated in mixed cultures of commercial starters with bacteriocin-producing strains in milk. The bacteriocins produced by the test organisms were nisin A, nisin Z, lacticin 481, enterocin AS-48, a novel enterocin, and a novel plantaricin. Mesophilic commercial starters were in most cases tolerant of bacteriocins, with only two of the starters being partially inhibited, one by four and the other by two bacteriocins. The aminopeptidase activities of mesophilic starters were generally low, and only one of the combinations of mesophilic starter-bacteriocin producer gave double the aminopeptidase activity of the starter culture without the bacteriocin producer. Thermophilic commercial starters were more sensitive to bacteriocins than mesophilic starters, with six thermophilic starters being partially inhibited by at least one of the bacteriocins. Their aminopeptidase activities were generally higher than those of the mesophilic starters. The aminopeptidase activities of seven thermophilic starters were increased in the presence of bacteriocins, by factors of up to 9.0 as compared with the corresponding starter cultures alone. Bacteriocin-producing strains may be used as adjunct cultures to mesophilic starters for the inhibition of pathogens in soft and semihard cheeses, because mesophilic starters are rather tolerant of bacteriocins. Bacteriocin producers may also be used as adjunct cultures to thermophilic starters of high aminopeptidase activity, more sensitive to lysis by bacteriocins than mesophilic starters, for the acceleration of ripening in semihard and hard cheeses.

Atividade antimicrobiana de bactérias lácticas de embutidos curados frente a Listeria monocytogenes

Cepas de bactérias lácticas recuperadas de 336 colônias isoladas e selecionadas foram submetidas ao teste de atividade antimicrobiana direta, que identificou as produtoras de substâncias antimicrobianas capazes de inibir in vitro o desenvolvimento de duas cepas indicadoras de Listeria monocytogenes. As 108 cepas que inibiram diretamente pelo menos uma das cepas indicadoras receberam a denominação DTEI e foram selecionadas para o teste de atividade antimicrobiana indireta contra as mesmas cepas de L. monocytogenes, assim como frente a outras cepas de bactérias lácticas de origens diversas. Essa atividade inibidora indireta foi avaliada por meio de sobrenadantes isentos de células, esterilizados por meio de microfiltração, eliminando-se os principais compostos responsáveis por ela, como por exemplo os ácidos orgânicos e o peróxido de hidrogênio, mediante o ajuste do pH e a liofilização dos sobrenadantes. Oito cepas de bactérias lácticas apresentaram atividade antimicrobiana indireta frente a pelo menos um dos microrganismos indicadores utilizados, sugerindo terem produzido substâncias semelhantes a bacteriocinas. Três destas cepas foram caracterizadas e identificadas como pertencentes ao gênero Lactobacillus sp.

Partial characterization and plasmid linkage of a non-proteinaceous antimicrobial compound in a Lactobacillus casei strain of vegetable origin

Lactobacillus casei IMPC LC34 of vegetable origin produces a non-proteinaceous inhibitory compound with a broad spectrum of activity towards Gram-positive and Gram-negative bacteria, including pathogens. The active substance, mainly produced in the stationary phase of growth, is insensitive to proteolytic enzymes, lipase and catalase, and is stable at 121 degrees C for 30 min. The inhibitory activity was detected either at 8 degrees C or at 37 degrees C. The active compound does not contain glucidic groups, is inactivated by Na-metaperiodate, and its molecular mass is between 2000 and 5000 Da. Plasmid curing experiments showed that both antimicrobial compound immunity and production determinants were encoded by an 8.8 kbp plasmid. The effectiveness of the active agent was verified on ready-to-use vegetables, using either the Lact. casei strain or its culture supernatant fluid as inoculant, compared with cured clone. The application potential of the Lact. casei strain or its culture supernatant fluid for assuring the microbiological safety of ready-to-use vegetables is discussed.

Molecular mechanisms of bacteriocin evolution

Microorganisms are engaged in a never-ending arms race. One consequence of this intense competition is the diversity of antimicrobial compounds that most species of bacteria produce. Surprisingly, little attention has been paid to the evolution of such extraordinary diversity. One class of antimicrobials, the bacteriocins, has received increasing attention because of the high levels of bacteriocin diversity observed and the use of bacteriocins as preservatives in the food industry and as antibiotics in the human health industry. However, little effort has been focused on evolutionary questions, such as what are the phylogenetic relationships among these toxins, what mechanisms are involved in their evolution, and how do microorganisms respond to such an arsenal of weapons? The focus of this review is to provide a detailed picture of our current understanding of the molecular mechanisms involved in the process of bacteriocin diversification.

Antibacterial activity of Lactobacillus species against Vibrio species

Forty-one Lactobacillus strains were tested for antagonistic activity against nine strains of Vibrio. L. plantarum and L. casei were the most effective, and L. brevis was the least effective in inhibiting the growth of Vibrio species. L. gasseri and L. helveticus strains showed higher activity, while L. reuteri and L. fermentum showed lower inhibitory activity against Vibrio species. L. acidophilus strains exhibited various degrees of antagonistic activities against Vibrio species. However, none of the Lactobacillus species were able to inhibit the growth of Salmonella enteritidis, S. typhimurium, Escherichia coli, and Staphylococcus aureus. Inhibition of the Vibrio species was probably due to the production of organic acids by the Lactobacillus species.

Isolation and characterization of bacteriocin-producing lactic acid bacteria from ready-to-eat food products

Lactic acid bacteria isolated from a range of foods sold in ready-to-eat form were screened for bacteriocin production. Twenty-two bacteriocin-producing cultures were isolated from 14 of the 41 foods sampled. Bacteriocin-producing isolates from meat, fish and dairy products were Lactobacillus and Leuconostoc species typically found associated with these products. Most of these isolates gave only a narrow inhibitory spectrum although two showed activity against Listeria monocytogenes. Fruit and vegetable products gave a broader range of organisms but most of the bacteriocin-producing cultures were found to be strains of Lactococcus. Several lactococci produced a nisin-like activity, and showed a broad inhibitory spectrum against the indicator strains tested. The ease with which bacteriocin-producing strains could be isolated implies that they are already being safely consumed in food, and highlights the potential for using bacteriocin-producing cultures for biopreservation, especially in association with minimally processed products.

Application of antimicrobial-producing lactic acid bacteria to control pathogens in ready-to-use vegetables

Five psychrotrophic strains of lactic acid bacteria (Lactobacillus casei, Lact. plantarum and Pediococcus spp.) were isolated from 22 samples of commercial salads. These strains were shown to inhibit Aeromonas hydrophila, Listeria monocytogenes, Salmonella typhimurium and Staphylococcus aureus on MRS agar, in salads and in juice prepared from vegetable salads. Lactobacillus casei IMPCLC34 was most effective in reducing total mesophilic bacteria and the coliform group; Aer. hydrophila, Salm. typhimurium and Staph. aureus disappeared after 6 d of storage, while the counts for L. monocytogenes remained constant. The potential application of antimicrobial-producing lactic acid bacteria as biopreservatives of ready-to-use vegetables is suggested.

Antibacterial activity of Lactobacillus plantarum UG1 isolated from dry sausage: characterization, production and bactericidal action of plantaricin UG1

Lactobacillus plantarum UG1 isolated from dry sausage produced an antimicrobial substance that inhibited other strains of the genera Lactobacillus and Lactococcus, and some foodborne pathogens including Listeria monocytogenes, Bacillus cereus, Clostridium perfringens and Clostridium sporogenes. This antibacterial substance was inactivated by proteolytic enzymes and showed a bactericidal mode of action. Consequently, it was characterized as a bacteriocin, and was designated plantaricin UG1. This bacteriocin was stable in the pH range 4.5 to 7.0, partially inactivated by amylolytic enzymes and relatively thermostable. It was not affected by organic or lipolytic enzymes. Production of plantaricin UG1 was pH- and temperature-dependant and maximum yields were obtained in MRS broth cultures maintained at initial pH 6.5, and incubated at 25 degrees C to 30 degrees C, in the exponential to the early stationary growth phase of the producer organism. Ultrafiltration studies indicated that plantaricin UG1 has a molecular weight between 3 and 10 KDa. Curing experiments with L. plantarum UG1 resulted in the appearance of variants that lost bacteriocin production ability but were still immune to the bacteriocin. Plantaricin UG1 production appeared to be chromosomal encoded. Sensitive and insensitive Gram-positive bacteria adsorbed plantaricin UG1 irrespective of their susceptibility to it. In contrast, Gram-negative bacteria did not adsorb plantaricin UG1. The bactericidal action of plantaricin UG1 did not depend on the physiological state of the indicator culture and did not cause cell lysis. The resistance of two indicator strains to plantaricin UG1 has been studied.