Iron is a signal for Stenotrophomonas maltophilia biofilm formation, oxidative stress response, OMPs expression, and virulence.

Stenotrophomonas maltophilia is an emerging nosocomial pathogen. In many bacteria iron availability regulates, through the Fur system, not only iron homeostasis but also virulence. The aim of this work was to assess the role of iron on S. maltophilia biofilm formation, EPS production, oxidative stress response, OMPs regulation, quorum sensing (QS), and virulence. Studies were done on K279a and its isogenic fur mutant F60 cultured in the presence or absence of dipyridyl. This is the first report of spontaneous fur mutants obtained in S. maltophilia. F60 produced higher amounts of biofilms than K279a and CLSM analysis demonstrated improved adherence and biofilm organization. Under iron restricted conditions, K279a produced biofilms with more biomass and enhanced thickness. In addition, F60 produced higher amounts of EPS than K279a but with a similar composition, as revealed by ATR-FTIR spectroscopy. With respect to the oxidative stress response, MnSOD was the only SOD isoenzyme detected in K279a. F60 presented higher SOD activity than the wt strain in planktonic and biofilm cultures, and iron deprivation increased K279a SOD activity. Under iron starvation, SDS-PAGE profile from K279a presented two iron-repressed proteins. Mass spectrometry analysis revealed homology with FepA and another putative TonB-dependent siderophore receptor of K279a. In silico analysis allowed the detection of potential Fur boxes in the respective coding genes. K279a encodes the QS diffusible signal factor (DSF). Under iron restriction K279a produced higher amounts of DSF than under iron rich condition. Finally, F60 was more virulent than K279a in the Galleria mellonella killing assay. These results put in evidence that iron levels regulate, likely through the Fur system, S. maltophilia biofilm formation, oxidative stress response, OMPs expression, DSF production and virulence.

Stenotrophomonas maltophilia interferes via the DSF-mediated quorum sensing system with Candida albicans filamentation and its planktonic and biofilm modes of growth / Stenotrophomonas maltophilia interfiere a través del sistema de "quorum sensing" mediado por DSF con la filamentación y el crecimiento planctónico y en biofilm de Candida albicans

Stenotrophomonas maltophilia is a nosocomial pathogen of increasing importance. S. maltophilia K279a genome encodes a diffusible signal factor (DSF) dependent quorum sensing (QS) system that was first identified in Xanthomonas campestris pv. campestris. DSF from X. campestris is a homologue of farnesoic acid, a Candida albicans QS signal which inhibits the yeast-to-hyphal shift. Here we describe the antagonistic effects of S. maltophilia on C. albicans on filamentation as well as on its planktonic and biofilm modes of growth. To determine the role of the DSF-mediated quorum sensing system in these effects, C. albicans ATCC 10231 and C. albicans tup1 mutant, locked in the filamentous form, were grown with K279a or with its rpfF deletion mutant (DSF-). A significant reduction in viable counts of C. albicans was observed in planktonic cocultures with K279a as well as in mixed biofilms. Furthermore, no viable cells of C. albicans tup1 were recovered from K279a mixed biofilms. Fungal viability was also assessed by labeling biofilms with SYTO 9 and propidium iodide. Confocal images showed that K279a can kill hyphae and also yeast cells. Light microscopic analysis showed that K279a severely affects hyphae integrity. On the other hand, the presence of K279a rpfF did not affect fungal morphology or viability. In conclusion, we report for the first time that S. maltophilia interferes with two key virulence factors of C. albicans, the yeast-to-hyphal transition and biofilm formation. DSF could be directly responsible for these effects or may induce the gene expression involved in antifungal activity.

Stenotrophomonas maltophilia es un patógeno nosocomial de importancia creciente. El genoma de S. maltophilia K279a codifica un factor de señalización difusible (DSF), autoinductor de "quorum sensing" (QS), identificado previamente en Xanthomonas campestris pv. campestris. El DSF de X. campestris es homólogo del ácido farnesoico, señal de QS de Candida albicans, que inhibe la transición levadura-hifa. En este trabajo se describe el efecto antagónico de S. maltophilia sobre la filamentación y el crecimiento planctónico y en biofilms de C. albicans. Para determinar la participación del sistema de QS mediado por el DSF en dichos efectos, C. albicans ATCC 10231 y la mutante C. albicans tup1, que solo crece en forma filamentosa, fueron cultivadas en presencia de K279a o de su mutante K279a rpfF (DSF-). Se observó una reducción significativa del número de viables de C. albicans en cultivos planctónicos y biofilms desarrollados en presencia de K279a. Es de señalar que no se recuperaron células viables de C. albicans tup1 a partir de biofilms mixtos en presencia de K279a. Las imágenes de microscopía confocal revelaron que K279a produce la muerte de hifas y levaduras en biofilms mixtos teñidos con ioduro de propidio y SYTO 9. El análisis por microscopía óptica mostró que K279a afecta la integridad de las hifas. En cambio, la presencia de K279a rpfF no afectó la morfología ni la viabilidad fúngica. En conclusión, informamos por primera vez que S. maltophilia interfiere con dos factores de virulencia de C. albicans, la transición levadura-hifa y la formación de biofilms. Estos efectos pueden ser mediados por el DSF en forma directa o a través de la inducción de genes involucrados en la actividad antifúngica.

Stenotrophomonas maltophilia interferes via the DSF-mediated quorum sensing system with Candida albicans filamentation and its planktonic and biofilm modes of growth / Stenotrophomonas maltophilia interfiere a través del sistema de "quorum sensing" mediado por DSF con la filamentación y el crecimiento planctónico y en biofilm de Candida albicans

Stenotrophomonas maltophilia is a nosocomial pathogen of increasing importance. S. maltophilia K279a genome encodes a diffusible signal factor (DSF) dependent quorum sensing (QS) system that was first identified in Xanthomonas campestris pv. campestris. DSF from X. campestris is a homologue of farnesoic acid, a Candida albicans QS signal which inhibits the yeast-to-hyphal shift. Here we describe the antagonistic effects of S. maltophilia on C. albicans on filamentation as well as on its planktonic and biofilm modes of growth. To determine the role of the DSF-mediated quorum sensing system in these effects, C. albicans ATCC 10231 and C. albicans tup1 mutant, locked in the filamentous form, were grown with K279a or with its rpfF deletion mutant (DSF-). A significant reduction in viable counts of C. albicans was observed in planktonic cocultures with K279a as well as in mixed biofilms. Furthermore, no viable cells of C. albicans tup1 were recovered from K279a mixed biofilms. Fungal viability was also assessed by labeling biofilms with SYTO 9 and propidium iodide. Confocal images showed that K279a can kill hyphae and also yeast cells. Light microscopic analysis showed that K279a severely affects hyphae integrity. On the other hand, the presence of K279a rpfF did not affect fungal morphology or viability. In conclusion, we report for the first time that S. maltophilia interferes with two key virulence factors of C. albicans, the yeast-to-hyphal transition and biofilm formation. DSF could be directly responsible for these effects or may induce the gene expression involved in antifungal activity.(AU)

Stenotrophomonas maltophilia es un patógeno nosocomial de importancia creciente. El genoma de S. maltophilia K279a codifica un factor de señalización difusible (DSF), autoinductor de "quorum sensing" (QS), identificado previamente en Xanthomonas campestris pv. campestris. El DSF de X. campestris es homólogo del ácido farnesoico, señal de QS de Candida albicans, que inhibe la transición levadura-hifa. En este trabajo se describe el efecto antagónico de S. maltophilia sobre la filamentación y el crecimiento planctónico y en biofilms de C. albicans. Para determinar la participación del sistema de QS mediado por el DSF en dichos efectos, C. albicans ATCC 10231 y la mutante C. albicans tup1, que solo crece en forma filamentosa, fueron cultivadas en presencia de K279a o de su mutante K279a rpfF (DSF-). Se observó una reducción significativa del número de viables de C. albicans en cultivos planctónicos y biofilms desarrollados en presencia de K279a. Es de señalar que no se recuperaron células viables de C. albicans tup1 a partir de biofilms mixtos en presencia de K279a. Las imágenes de microscopía confocal revelaron que K279a produce la muerte de hifas y levaduras en biofilms mixtos teñidos con ioduro de propidio y SYTO 9. El análisis por microscopía óptica mostró que K279a afecta la integridad de las hifas. En cambio, la presencia de K279a rpfF no afectó la morfología ni la viabilidad fúngica. En conclusión, informamos por primera vez que S. maltophilia interfiere con dos factores de virulencia de C. albicans, la transición levadura-hifa y la formación de biofilms. Estos efectos pueden ser mediados por el DSF en forma directa o a través de la inducción de genes involucrados en la actividad antifúngica.(AU)

Stenotrophomonas maltophilia interferes via the DSF-mediated quorum sensing system with Candida albicans filamentation and its planktonic and biofilm modes of growth.

Stenotrophomonas maltophilia is a nosocomial pathogen of increasing importance. S. maltophilia K279a genome encodes a diffusible signal factor (DSF) dependent quorum sensing (QS) system that was first identified in Xanthomonas campestris pv. campestris. DSF from X. campestris is a homologue of farnesoic acid, a Candida albicans QS signal which inhibits the yeast-to-hyphal shift. Here we describe the antagonistic effects of S. maltophilia on C. albicans on filamentation as well as on its planktonic and biofilm modes of growth. To determine the role of the DSF-mediated quorum sensing system in these effects, C. albicans ATCC 10231 and C. albicans tup1 mutant, locked in the filamentous form, were grown with K279a or with its rpfF deletion mutant (DSF-). A significant reduction in viable counts of C. albicans was observed in planktonic cocultures with K279a as well as in mixed biofilms. Furthermore, no viable cells of C. albicans tup1 were recovered from K279a mixed biofilms. Fungal viability was also assessed by labeling biofilms with SYTO 9 and propidium iodide. Confocal images showed that K279a can kill hyphae and also yeast cells. Light microscopic analysis showed that K279a severely affects hyphae integrity. On the other hand, the presence of K279a rpfF did not affect fungal morphology or viability. In conclusion, we report for the first time that S. maltophilia interferes with two key virulence factors of C. albicans, the yeast-to-hyphal transition and biofilm formation. DSF could be directly responsible for these effects or may induce the gene expression involved in antifungal activity.

Stenotrophomonas maltophilia interferes via the DSF-mediated quorum sensing system with Candida albicans filamentation and its planktonic and biofilm modes of growth. / Stenotrophomonas maltophilia interferes via the DSF-mediated quorum sensing system with Candida albicans filamentation and its planktonic and biofilm modes of growth.

Stenotrophomonas maltophilia is a nosocomial pathogen of increasing importance. S. maltophilia K279a genome encodes a diffusible signal factor (DSF) dependent quorum sensing (QS) system that was first identified in Xanthomonas campestris pv. campestris. DSF from X. campestris is a homologue of farnesoic acid, a Candida albicans QS signal which inhibits the yeast-to-hyphal shift. Here we describe the antagonistic effects of S. maltophilia on C. albicans on filamentation as well as on its planktonic and biofilm modes of growth. To determine the role of the DSF-mediated quorum sensing system in these effects, C. albicans ATCC 10231 and C. albicans tup1 mutant, locked in the filamentous form, were grown with K279a or with its rpfF deletion mutant (DSF-). A significant reduction in viable counts of C. albicans was observed in planktonic cocultures with K279a as well as in mixed biofilms. Furthermore, no viable cells of C. albicans tup1 were recovered from K279a mixed biofilms. Fungal viability was also assessed by labeling biofilms with SYTO 9 and propidium iodide. Confocal images showed that K279a can kill hyphae and also yeast cells. Light microscopic analysis showed that K279a severely affects hyphae integrity. On the other hand, the presence of K279a rpfF did not affect fungal morphology or viability. In conclusion, we report for the first time that S. maltophilia interferes with two key virulence factors of C. albicans, the yeast-to-hyphal transition and biofilm formation. DSF could be directly responsible for these effects or may induce the gene expression involved in antifungal activity.

Siderophores of Stenotrophomonas maltophilia: detection and determination of their chemical nature.

Stenotrophomonas maltophilia is an emerging nosocomial pathogen. Despite the broad spectrum of syndromes associated with S. maltophilia infections, little is known about its virulence factors, including siderophore production. The aims of this work were to detect S. maltophilia siderophores and to determine their chemical nature. We studied 31 S. maltophilia isolates from device-associated infections, recovered over the period 2006-2011 at Hospital de Clínicas José de San Martín, Buenos Aires, Argentina, and the strain K279a, whose genome has been fully sequenced. The production of siderophores was screened by the chrome azurol S (CAS) agar assay, previously modified to detect siderophores in this species. When grown on modified CAS agar plates, all the clinical isolates and K279a were CAS-positive for siderophore production. In order to determine the chemical nature of siderophores, the Csáky (hydroxamate-type) and Arnow (catechol-type) assays were used. All S. maltophilia isolates produced catechol-type siderophores, but hydroxamate-type siderophores were not detected.

Comparative in vitro efficacies of ethanol-, EDTA- and levofloxacin-based catheter lock solutions on eradication of Stenotrophomonas maltophilia biofilms.

The aim of this study was to compare the in vitro activity of ethanol, EDTA and levofloxacin (Levo), alone or in combination, on biofilms of Stenotrophomonas maltophilia recovered from patients with catheter-related bloodstream infections (CRBSIs) at a university hospital in Argentina. First, 24 and 48 h biofilms were formed in microtitre plates and challenged with 25 or 40 % ethanol for 1 h. Biofilms, of the 14 local isolates and from the reference strain K279a, were eradicated after both treatments as shown by plate counts and the regrowth technique. Second, 24 h biofilms of all isolates were established in silicone catheter segments and challenged with 25 or 40 % ethanol, Levo (2.5 mg ml(-1)), EDTA (30 mg ml(-1)), 25 % ethanol-EDTA or Levo-EDTA for 1, 3 and 24 h. Viable counts of biofilms treated for 1 h with 25 or 40 % ethanol or 25 % ethanol-EDTA were under the limit of detection. Killing of biofilms by Levo or Levo-EDTA was gradual and it was only after 24 h of treatment that no differences could be seen between the effects of these catheter lock solutions (CLSs) and those of ethanol (P>0.05). Levo-EDTA, in combination, did not act synergistically against biofilms. After 24 h of exposure, EDTA did not eradicate biofilms but reduced biofilm survival rates to 1-5 %. The effect of the different CLSs on biomass reduction, estimated by crystal violet staining, was highly dependent on the isolate, and the most effective agents were 25 and 40 % ethanol. Our results suggest that when used as a CLS for short periods, ethanol at low concentrations, alone or in combination with a chelator, can decontaminate the line from S. maltophilia in cases of CRBSI and help, in conjunction with systemic antibiotics, in the retention of precious vascular catheters.

Fourier transform infrared spectroscopy for rapid identification of nonfermenting gram-negative bacteria isolated from sputum samples from cystic fibrosis patients.

The accurate and rapid identification of bacteria isolated from the respiratory tract of patients with cystic fibrosis (CF) is critical in epidemiological studies, during intrahospital outbreaks, for patient treatment, and for determination of therapeutic options. While the most common organisms isolated from sputum samples are Pseudomonas aeruginosa, Staphylococcus aureus, and Haemophilus influenzae, in recent decades an increasing fraction of CF patients has been colonized by other nonfermenting (NF) gram-negative rods, such as Burkholderia cepacia complex (BCC) bacteria, Stenotrophomonas maltophilia, Ralstonia pickettii, Acinetobacter spp., and Achromobacter spp. In the present study, we developed a novel strategy for the rapid identification of NF rods based on Fourier transform infrared spectroscopy (FTIR) in combination with artificial neural networks (ANNs). A total of 15 reference strains and 169 clinical isolates of NF gram-negative bacteria recovered from sputum samples from 150 CF patients were used in this study. The clinical isolates were identified according to the guidelines for clinical microbiology practices for respiratory tract specimens from CF patients; and particularly, BCC bacteria were further identified by recA-based PCR followed by restriction fragment length polymorphism analysis with HaeIII, and their identities were confirmed by recA species-specific PCR. In addition, some strains belonging to genera different from BCC were identified by 16S rRNA gene sequencing. A standardized experimental protocol was established, and an FTIR spectral database containing more than 2,000 infrared spectra was created. The ANN identification system consisted of two hierarchical levels. The top-level network allowed the identification of P. aeruginosa, S. maltophilia, Achromobacter xylosoxidans, Acinetobacter spp., R. pickettii, and BCC bacteria with an identification success rate of 98.1%. The second-level network was developed to differentiate the four most clinically relevant species of BCC, B. cepacia, B. multivorans, B. cenocepacia, and B. stabilis (genomovars I to IV, respectively), with a correct identification rate of 93.8%. Our results demonstrate the high degree of reliability and strong potential of ANN-based FTIR spectrum analysis for the rapid identification of NF rods suitable for use in routine clinical microbiology laboratories.

Polymorphisms in the pertussis toxin promoter in Bordetella bronchiseptica veterinary isolates from Argentina.

Five restriction patterns (including a novel one) could be defined by polymerase chain reaction-restriction fragment length polymorphism on the pertussis toxin (PT) promoter region in local veterinary isolates, suggesting that PT gene analysis is a potential molecular marker for Bordetella bronchiseptica detection and typing.

Vir90, a virulence-activated gene coding for a Bordetella pertussis iron-regulated outer membrane protein.

Bordetella pertussis undergoes phenotypic changes modulated by the bvgAS locus, which regulates the expression of many genes related to virulence and immunogenicity. We previously reported the N-terminal sequence of a 90 kDa bvg-regulated outer membrane protein (OMP) of B. pertussis (SWISS-PROT accession No. p81549), a novel potential virulence factor that we named Vir90. The open reading frames (ORFs) which potentially code for Vir90 in B. pertussis, B. parapertussis and B. bronchiseptica were identified by computer analysis of the genomic sequences available for the three Bordetella species. Nucleotide sequence analysis of the vir90 upstream region revealed the presence of a putative promoter, a BvgA binding site and a putative Fur binding site. The B. pertussis Vir90 protein showed significant homology with ferrisiderophore receptors from Gram-negative bacteria. An antiserum raised against Vir90His recombinant protein recognized the 90-kDa protein in immunoblots of OMPs from these three virulent Bordetella species. The accumulation of the Vir90 protein increased 4-fold under low iron growth conditions. Therefore, the vir90 gene is expressed in the tested species and its expression is regulated positively by the BvgAS system and negatively under high iron concentration, likely by Fur.

Characterization of intermediate phenotypes induced by chemically undefined laboratory media in virulent Bordetella bronchiseptica strains.

The expression of many virulence factors of Bordetella bronchiseptica is regulated by the bvgAS locus and reduced in response to environmental signals called modulators. Virulent strains can alternate between virulent (Bvg(+)), intermediate (Bvg(i)), and modulated (Bvg(+)mod) phenotypes. Potential vaccine antigens can be expressed by Bvg1 strains grown only in the absence of modulators. In the present study we evaluated filamentous hemagglutinin (FHA) and outer membrane protein (OMP) expression in Bvg(+) B. bronchiseptica strains grown in chemically undefined media: nutrient agar (NA), tryptic soy agar (TSA), tryptose phosphate broth (TPB), and brain-heart infusion (BHI). Our results suggest that TSA and TPB usually induce semimodulation, since Bvg(+) strains cultured in these media retained the expression of FHA and virulence-associated OMPs in the 30 kDa region, but failed to express other virulence markers such as OMPs in the regions of 90 and 200 kDa, though they expressed flagellin (avirulence marker). On the other hand, NA and BHI usually induce modulation. Thus the assayed chemically undefined media should not be used in vaccine production. Semimodulation induced by TSA and TPB can be accurately detected by SDS-PAGE Sarkosyl-insoluble OMP-enriched profiles. The reduction or absence of OMPs in the regions of 90 and 200 kDa is the most sensitive marker, and in some cases the presence of flagellin in intermediate profiles is another trait of the Bvg(i) phenotypes. Therefore these markers could be useful for selecting media for vaccine production. We also characterized the phenotype of Bvg(+) strains grown in Stainer-Scholte broth, an expensive medium, with and without glutathione, and we have detected no differences; this is the first attempt to reduce the cost of a Bordetella growth medium for veterinary vaccine production.

Effect of (2, 6-0-Dimethyl) Beta-Cyclodextrin (heptakis) on cell mass, pertussis toxin and filamentous hemagglutinin production in three bordetella pertussis vaccinal strains

Estudou-se a produçäo de toxina pertussis (PT) e hemaglutininas filamentosas (FHA) para vacinas do vírus Bortella pertussis em cultura supernadante e em supernadante de células lavadas com soluçöes iônicas. O vírus cresceu em meio Stainer-Scholte (SS) e na modificaçäo chamou-se CL-basal (CL-b) e desse meio com heptakis (2,6-0-Dimethyl Beta-Cyclodextrin (MeBCD). FHA e PT foram estimados pelo total de hemaglutinaçäo, hemaglutinaçäo diferencial (com colesterol) e imunoensaio dot-blot. O meio CL-b decresce a produçäo de massa celular e também decresceria a estabilidade e liberaçäo de hemaglutininas. A adiçäo of MeBCD para SS ou meio CL-b estimularia a produçäo, exportaçäo e estabilidade de PT e FHA. Esses efeitos säo maiores para meio CL-b e mais importante para FHA. Esses efeitos säo maiores par meio CL-b e mais importante para FHA que para PT. Obteve-se resultados similares com a adiçäo de 0,5 ou 1g/l de MeBCD para CL-b. Cultura supernadante contém FHA e PT mas supernadantes de células em soluçäo única contém basicamente FHA e seeria usada para prover esse campo

Viabilidad de Bacillus subtilis auxotrofos en ausencia de sus metabolitos esenciales / Viability of Bacillus subtilis auxotrophs in the absence of their essential metabolites

Se estudió el efecto de la privación de triptofano y uracilo sobre la viabilidad del tansformante Bacillus subtilis BSA trp ura y de las cepas parentales Bacillus subtilis PB 168 trp-C y Bacillus subtilis PB 3308 ura. Los ensayos se llevaron a cabo en las condiciones descriptas para el desarrollo de competencia para transformación, durante 16 h. Los resultados obtenidos demostraron que Bacillus subtilis BSA 170 trp ura es resistente a la muerte por carencia de triptofano durante el tiempo del ensayo y a la muerte por carencia de uracilo durante 3h. Luego ocurre una disminución de la recuperación de UFC, acompañada por una reduccción de las absorbancias de los cultivos. La muerte por carencia de uracilo es prevenida en ausencia de triptofano, sugiriendo que reuiere síntesis de proteínas. Las cepas parentales mostraron un comportamiento semejante. Bacillus subtilis PB 168 trp-C mostró ser resistente a la muerte por carencia de triptofano y Bacillus subtilis PB 3308 uta sufrió una reducción de la capacidad para formar colonias, debida a la privación de uracilo, comparable a la de la cepa transformante

Viabilidad de Bacillus subtilis auxotrofos en ausencia de sus metabolitos esenciales / Viability of Bacillus subtilis auxotrophs in the absence of their essential metabolites

Se estudió el efecto de la privación de triptofano y uracilo sobre la viabilidad del tansformante Bacillus subtilis BSA trp ura y de las cepas parentales Bacillus subtilis PB 168 trp-C y Bacillus subtilis PB 3308 ura. Los ensayos se llevaron a cabo en las condiciones descriptas para el desarrollo de competencia para transformación, durante 16 h. Los resultados obtenidos demostraron que Bacillus subtilis BSA 170 trp ura es resistente a la muerte por carencia de triptofano durante el tiempo del ensayo y a la muerte por carencia de uracilo durante 3h. Luego ocurre una disminución de la recuperación de UFC, acompañada por una reduccción de las absorbancias de los cultivos. La muerte por carencia de uracilo es prevenida en ausencia de triptofano, sugiriendo que reuiere síntesis de proteínas. Las cepas parentales mostraron un comportamiento semejante. Bacillus subtilis PB 168 trp-C mostró ser resistente a la muerte por carencia de triptofano y Bacillus subtilis PB 3308 uta sufrió una reducción de la capacidad para formar colonias, debida a la privación de uracilo, comparable a la de la cepa transformante (AU)

Viabilidad y multiplicación de Listeria monocytogenes cepa murray frente a antimicrobianos de uso en medios selectivos / Viability and multiplication of Listeria monocytogenes strain Murray facing antimicrobial agents used in selective media

Se han efectuado estudios comparativos de velocidad de crecimiento, cosecha máxima y duración de la fase de adaptación de poblaciones de Listeria monocytogenes (CCMA 29454, ATCC 15313) a 37-C, en medios de cultivo líquidos conteniendo concentraciones variables de antimicrobianos con diferente espectro de acción. Concentraciones de 160microng/ml de ácido nalidíxico, 80microng/ml de ácido pipemédico, 40microng/ml de ácido oxolínico, 0,8U/ml de bacitracina y 200U/ml de nistatina, constituyen límites confiables que permiten la sobrevivencia y multiplicación de Listeria monocytogenes y ejercen una importante acción inhibitoria sobre el crecimiento de gran parte de la flora habitual de materiales normalmente sépticos. Es de importancia la potencial capacidad selectiva de diversas asociaciones de los agentes ensayados en la formulación de medios de cultivo destinados a un enriquecimiento de Listeria monocytogenes, que posibilite su aislamiento a partir de cultivos mixtos