Production of laccase from Pleurotus florida using agro‐wastes and efficient decolorization of Reactive blue 198

Pleurotus florida NCIM 1243 produced laccase as the dominant lignolytic enzyme during the dye decolorization. Banana peel was the best substrate for extracellular laccase production under solid state fermentation when compared to mandarin peel and cantaloupe peel. The maximum activity of laccase (5.4 U/g) was detected on the 10 day. The ratio of banana peel: mandarin peel: cantaloupe peel (5:2:3) showed increased production of laccase (6.8 U/g). P. florida produced two extracellular laccase isoenzymes (L1 and L2). The half life of laccase at 60 °C was 2 h and at 4 h it retained 25% residual activity. P. florida laccase showed high thermostability and an interesting difference was noticed in the behavior of laccase isoenzymes at different temperature. The L1 isoenzyme of laccase showed remarked thermostability at 60 °C in the native PAGE when compared to L2 isoenzyme. The optimum pH, temperature and enzyme concentration for maximum decolorization was found to be 4.5, 60 °C and 1.2 U/ml, respectively. Partially purified laccase enzyme showed excellent decolorization activity to Reactive blue 198. The maximum decolorization (96%) was observed at lower dye concentrations (50–100 ppm) which decreased markedly when the dye concentration was increased beyond 150 ppm. The thermostable laccase of P. florida could be effectively used to decolorize the synthetic dyes in the textile effluent and other biotechnological applications. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancing bioremoval of textile dyes by eight fungal strains from media supplemented with gelatine wastes and sucrose

Eight Aspergillus strains were found to be successful in removing textile dyes from liquid media. These fungal strains were grown on medium containing: gelatine wastes and sucrose, as sources of nitrogen and carbon to test the possible speed up of the dyes removing while fungus biomass is building up in the media. The growth of fungal strains ranged from 10 to 110 mg biomass dry weight/100 ml medium. This growth induced high decolorization percentages, which ranged 33-95% within eight days. Two textile dyes Direct brown and Polar red were included in the study. The growth of the fungal strains as well as decolorization percentage of the dyes increased after 5, 6, and 8 days from incubation time with most tested strains. With Direct brown dye the strains number 2, 5, 31 and 37 recorded the highest percentage of decolorization (91, 92, 93 and 95 respectively) after incubation for 6 days. Fungal strains Aspergillus 5 and 31 gave the highest mycelium dry weight being 110 mg. Most of fungal strains induced 86 to 95 percentage of decolorization after 6 days incubation with Polar red dye. The possible toxicity of the remaining supernatant media after fungal biomass removal was tested by Ames test to assess the residual mutagenic agents remaining after dye removal, using three strains of Salmonella typhimurium (TA 1535, TA 1537, TA 1538). The results showed that the toxicity of the dyes, measured by Ames test could be removed by the dye absorption on the fungal biomass.     

Decolorization of diazo-dye Reactive Blue 172 by Pseudomonas aeruginosa NBAR12

A novel bacterial strain capable of decolorizing textile dyes was isolated from dye contaminated soil obtained from industrial estate of Ahmedabad, Gujarat, India. The bacterial isolate Pseudomonas aeruginosa NBAR12 was capable of decolorizing 12 different dyes tested with decolorization efficiency varying in the range of 80 to 95%. Maximum extent as well as rate of Reactive Blue 172 (RB 172) decolorization was observed when glucose (2 g x l(-1)) and yeast extract (2.5 g x l(-1)) were supplemented in the medium. The optimum dye pH and temperature for dye decolorization was found to be 7 and 40 degrees C, respectively. The decolorizing activity was found to increase with increasing the dye concentration from 50 to 400 mg x l(-1). The dye decolorization was strongly inhibited at 500 mg dye l(-1) in the medium. High performance thin layer chromatography analysis indicated that dye decolorization occurred due to the breakdown of dye molecules into colorless end products.     

Production of laccase from Pleurotus florida NCIM 1243 using Plackett–Burman Design and Response Surface Methodology

Statistically‐based experimental designs were applied to optimize the fermentation for the production of laccase by Pleurotus florida NCIM 1243. Eleven components were screened for their significant effect on laccase production using Plackett–Burman factorial design. Glucose (carbon source), asparagine (nitrogen source), CuSO₄(inducer) and incubation period were found to have highest positive influence on the laccase production. The combined effect of these factors on laccase production was studied using central composite design of Response surface methodology. The optimal point of variables for maximum laccase production using Response surface methodology are glucose (15.21 g/l), asparagine (6.40 g/l), CuSO₄ (91.78 μM) and incubation period (178.55 h), respectively. The maximum enzyme activity predicted by the model was 5.0 U/ml which was in perfect agreement with the actual experimental value (4.8 U/ml). Further, partially purified laccase from the optimized cultural condition was used for the decolorization of reactive dyes, Reactive Blue 198 and Reactive Red 35. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Superoxide dismutase activity of Mycobacterium avium, M. intracellulare, and M. scrofulaceum.

Superoxide dismutase (EC 1.15.1.1) (SOD) activity has been detected in crude cell extracts of representative strains of the Mycobacterium avium, M. intracellulare, and M. scrofulaceum (MAIS) group. Polyacrylamide gel electrophoresis demonstrated a single SOD activity band for each of the MAIS strains, though there were differences in mobility. All M. avium and M. intracellulare and two of five M. scrofulaceum strains demonstrated a single activity band of identical mobility (Rf = 0.83), while the SOD activity band for the three remaining M. scrofulaceum strains migrated farther (Rf = 0.85). The differences in mobility correlated with differences in sensitivity to NaN3 and H2O2. The SOD activities of the majority of the MAIS strains which displayed the slower-migrating activity band were inhibited 22 to 81% after 15 min of exposure to 5 mM H2O2, suggesting that both iron and manganese may be present in a single enzyme. The SOD activities of the three M. scrofulaceum strains which had the faster-migrating activity band were inhibited 100% after only 5 min of exposure to 5 mM H2O2 and exhibited greater sensitivity to 5 and 10 mM NaN3, characteristics of an iron-containing SOD. A concentration of 1 mM KCN did not cause inhibition of enzyme activity in any of the MAIS strains tested. Extracellular SOD activity was detected in four of six MAIS strains and was shown to be identical in mobility to the SOD activity of the crude extracts.     

Production of laccase as the sole phenoloxidase by a Brazilian strain of Pleurotus pulmonarius in solid state fermentation

The production of laccase by a Brazilian strain of Pleurotus pulmonarius was studied in solid state fermentation using wheat bran as substrate. Among oxidative and hydrolytic enzymes tested (laccase, aryl alcohol oxidase, lignin peroxidase, Mn peroxidase, xylanase and cellulase), laccase was the main enzyme produced by P. pulmonarius. The most suitable condition for maximum production of laccase (8,600 U/g substrate) was initial moisture content of 75% and 5 days of cultivation at 30 degrees C. The optimum pH and temperature for laccase activity were found to be 6.5 and 50 degrees C, respectively. P. pulmonarius laccase was stable at 50 degrees C for more than 6 hours, and it retained about 73% and 18% of its activity when heated for 1 h at 55 and 60 degrees C, respectively. The enzyme was greatly stable at alkaline pH, but not at acidic pH. The laccase activity appear to be correlated with the ability of crude extract to decolourize several industrial dyes.     

Laccase production by Pycnoporus sanguineus under different culture conditions

Pycnoporus sanguineus is a white‐rot fungus that produces ligninolytic enzymes such as laccases. These enzymes can endure temperatures as high as 60 °C and are useful for pulp bleaching, dye decolorization and phenolic degradation. Laccase production by fungi depends not only on the carbon and nitrogen sources but also on the nitrogen concentration of the culture medium. In this work, we examined the effect of four carbon sources (maltose, glucose, fructose and sucrose) and four nitrogen sources (ammonium tartrate, sodium nitrate, asparagine and yeast extract) on the activity of laccase from Pycnoporus sanguineus. All carbon and nitrogen sources exhibited a strong influence on laccase activity, a sucrose–asparagine medium providing the best results (320 mU/ml). Moreover, using an asparagine concentration 5 times higher than the reference level increased laccase activity to 820 mU/ml. Higher asparagine concentrations, however, resulted in no further increase in activity. Consistent with previous results, the carbon and nitrogen sources, and the nitrogen concentration, had a strong impact on laccase activity, the optimum conditions depending on the particular fungus. The conditions of the culture medium had a marked effect on laccase activity, which increased up to 820 mU/ml. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of P fimbriae on Escherichia coli O2, O4, and O6 strains by immunoprecipitation.

P fimbriae on Escherichia coli O2, O4, and O6 strains were analyzed by immunoprecipitation. Fimbrial extracts were prepared from a total of 35 strains and tested for precipitation with four anti-P-fimbria sera. The overall fimbrial composition of the strains was related to the O:K:H serotype, and two to three P fimbrial variants per strain were found on most of the O4 and some of the O6 strains. The O2 strains, in contrast, showed only one antigenic variant of P fimbriae per strain, which was serologically unrelated to those of the O4 and O6 strains. The results stress the multiplicity and serological complexity of E. coli P fimbriae.     

Mycobacterial growth and sensitivity to H2O2 killing in human monocytes in vitro.

The intracellular growth and susceptibilities to killing by H2O2 in cultured human monocytes of a number of mycobacterial species including laboratory strains and clinical isolates of Mycobacterium tuberculosis, and Mycobacterium bovis bacillus Calmette-Guerin (BCG) and a clinical isolate of Mycobacterium avium-M. intracellulare were examined. The clinical isolate of M. avium-M. intracellulare did not replicate in freshly explanted monocytes (generation time of >400 h); BCG replicated with a generation time of 95 h, and M. tuberculosis strains CDC551, H37Rv, and H37Ra replicated with generation times of 24, 35, and 37 h, respectively, during the 4-day growth assay. When cultured in monocytes for 4 days, the mycobacteria were variably sensitive to H2O2-induced killing. A positive correlation between the generation time and percent killing of intracellular bacilli was observed. By comparison, mycobacterial strains were similarly sensitive to H2O2 treatment in cell-free culture media and in sonicated cell suspensions. Using a number of inhibitors of reactive oxygen intermediates we determined that other than catalase the inhibitors tested did not affect H2O2-induced killing of intracellular mycobacteria. Our studies suggest that the killing of mycobacteria growing in human monocytes in vitro by the addition of exogenous H2O2 is dependent on the susceptibility to a peroxide-induced killing pathway as well as on the intracellular growth rate of the mycobacteria.     

过氧化氢对大鼠血小板线粒体功能的影响

目的 探讨血小板能否用来观察氧化应激过程中线粒体退变.方法 将30只SD大鼠随机分为MTT组、ATP组和SOD组,各组血小板分别用50、100、200n mol/L H2O2处理30 min后,检测相应指标.结果 50 nmol/L H2O2中的血小板SOD活性(U/mgprot)为(3.4571±1.0340),A...     

Decreased adherence of enterohemorrhagic Escherichia coli to HEp-2 cells in the presence of antibodies that recognize the C-terminal region of intimin

Antiserum raised against intimin from enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain 86-24 has been shown previously by our laboratory to inhibit adherence of this strain to HEp-2 cells. In the present study, we sought to identify the region(s) of intimin important for the effect of anti-intimin antisera on EHEC adherence and to determine whether antisera raised against intimin from an O157:H7 strain could reduce adherence of other strains. Compared to preimmune serum controls, polyclonal sera raised against the histidine-tagged intimin protein RIHisEae (intimin(O157)) or against His-tagged C-terminal fragments of intimin from strain 86-24 reduced adherence of this strain. Furthermore, an antibody fraction purified from the anti-RIHisEae serum that contained antibodies to the C-terminal third of intimin, the putative receptor-binding domain, also reduced adherence of strain 86-24, but a purified fraction containing antibodies to the N-terminal two-thirds of intimin did not inhibit adherence. The polyclonal anti-intimin(O157) serum raised against RIHisEae inhibited, to different degrees, the adherence of another O157:H7 strain, an EHEC O55:H7 strain, one of two independent EHEC O111:NM isolates tested, and one of two EHEC O26:H11 strains tested. Adherence of the other O26:H11 and O111:NM strains and an EPEC O127:H6 strain was not reduced. Finally, immunoblot analysis indicated a correlation between the antigenic divergence in the C-terminal third of intimins from different strains and the capacity of anti-intimin(O157) antiserum to reduce adherence of heterologous strains. Taken together, these data suggest that intimin(O157) could be used as an immunogen to elicit adherence-blocking antibodies against O157:H7 strains and closely-related EHEC.     

Production of laccase from Trametes trogii TEM H2: a newly isolated white‐rot fungus by air sampling

This work represents the first report of isolation of potential laccase producers by air sampling using media supplemented with 2,2‐azinobis (3‐ethylbenzothiazoline‐6‐sulfonate) and guaiacol for laccase production and secretion indicators. Nine fungal isolates showed positive reactions with 2,2‐azinobis (3‐ethylbenzothiazoline‐6‐sulfonate) and guaiacol. The isolate named TEM H2 exhibited the largest and intensive oxidation zones with 2,2‐azinobis (3‐ethylbenzothiazoline‐6‐sulfonate) (85 mm) and guaiacol (66 mm) and therefore it was selected for detailed investigations. The strain was identified as Trametes trogii TEM H2 due to the morphological characteristics and the comparison of internal transcribed spacer ribosomal DNA gene sequences. The laccase production was screened in different liquid cultures. The best laccase production medium was determined as soluble starch yeast extract medium in which laccase production was reached to a maximum level (989.6 U l^–1) on the 8^th day of cultivation. Effects of different initial pH values on laccase production were tested. Optimum pH value for laccase production in soluble starch yeast extract medium was determined as pH 3.0 with 15425.0 U l^–1laccase production at 12^th day of cultivation. In addition, effects of eight inducers (veratryl alcohol, ferulic acid, 1‐Hydroxybenzotriazole, syringic acid, 2,2‐azinobis (3‐ethylbenzothiazoline‐6‐sulfonate), 1 mmol l^–1 CuSO₄, 3% ethanol, guaiacol) were examined. Only cultures with 2,5‐xylidine exhibited 1.9 fold increase in laccase activity reaching to 28890.0 U l^–1. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

NOD8对H2O2诱导的人肝细胞凋亡的影响

 目的:探讨NOD8对H2O2诱导的人肝细胞L02凋亡的影响。 方法: pEGFP-C2 及pEGFP-NOD8重组质粒经JetPRIME介导转染L02细胞;用H2O2诱导细胞凋亡。实验分为pEGFP-C2组、pEGFP-C2+H2O2组和pEGFP-NOD8+H2O2组。采用MTT法检测细胞活性,Western blotting检测细胞NOD8的蛋白表达,Hoechst 33342染色检测细胞凋亡情况,流式细胞术检测细胞凋亡率,比色法检测细胞caspase-3活性。 结果: 通过MTT检测不同浓度（0.2～2 mmol/L）H2O2刺激6 h后的细胞活性,确定1 mmol/L H2O2为诱导细胞凋亡的剂量。Western blotting检测结果显示,转染pEGFP-NOD8质粒的细胞NOD8蛋白表达明显增加。Hoechst 33342染色法观察发现,pEGFP-C2+H2O2组有较多细胞出现强蓝色荧光细胞核,细胞凋亡较多,而pEGFP-NOD8+H2O2组细胞凋亡明显减少。流式细胞术分析显示,pEGFP-C2+H2O2组的细胞凋亡率明显升高,pEGFP-NOD8+H2O2组的细胞凋亡率则显著下降。pEGFP-C2+H2O2组细胞的caspase-3活性明显升高,而pEGFP-NOD8+H2O2组细胞的caspase-3活性显著下降。 结论: NOD8可抑制H2O2诱导的L02细胞凋亡,其作用机制可能与NOD8抑制细胞的caspase-3活性有关。     

Effect of O2, H2 and redox potential on the activity and synthesis of hydrogenase 2 in Escherichia coli.

The aim of this work was to study the influence of O2 with special emphasis on low oxygen tension, the effect of H2 under various conditions of oxygen tension and the influence of the redox potential in the growth medium on hydrogenase 2 of Escherichia coli. The hydrogenase activity and the content of the large (HybC) and small (HybO) subunits of hydrogenase 2 were compared during turbidostat cultivation in a wild strain and mutant HDK103 lacking hydrogenases 1 and 3. No hydrogenase 2 activity in the mutant HDK103 was observed under aerobic conditions, but it was maximal under anaerobic conditions and half-maximal at an oxygen tension of approximately 4 mbar as is common for enzymes of anaerobic respiration. The content of hydrogenase 2 in both the strains was maximal under anaerobic conditions. In the wild strain, H2 addition enhanced hydrogenase activity and the HybO content under microaerobic conditions only. Under anaerobic conditions endogenous H2 production hindered this effect. Under aerobic conditions, the 02-related negative effect seemed to dominate over the H2-related positive effect. By contrast, in the mutant HDK103, hydrogen influenced neither hydrogenase 2 activity nor its content. A possible role of hydrogenase I in the response of hydrogenase 2 to hydrogen is discussed. Under conditions of different O2 tension, hydrogenase activity in both strains correlated inversely with the value of the redox potential of the medium. The presence of H2 changed this dependence. Thus, the value of the redox potential itself is not a controlling factor for hydrogenase 2.     

Isogenic Strain of Escherichia coli O157:H7 That Has Lost both Shiga Toxin 1 and 2 Genes

An Escherichia coli O157:H7 strain isolated from a patient with hemorrhagic colitis was found to exhibit two slightly different colony morphology types on differential medium. Each morphological type, designated TT12A and TT12B, was isolated, and serological testing using various assays confirmed that both strains carried the O157 and the H7 antigens. Biochemical testing showed that the strains had identical profiles on AP120E analysis and, like typical O157:H7 strains, did not ferment sorbitol or exhibit β-glucuronidase activity. Analysis with a multiplex PCR assay showed that TT12B did not carry the gene for either Shiga toxin 1 (Stx1) or Stx2, whereas these genes were present in TT12A and the toxins were produced. Apart from that, both strains carried the +93 gusA mutation, the cluster I ehxA gene for enterohemolysin, and the eae gene for γ-intimin, which are all characteristics of the O157:H7 serotype. Phenotypic assays confirmed that both strains exhibited enterohemolysin activity and the attachment and effacing lesion on HeLa cells. Multilocus enzyme electrophoresis analysis showed that the strains are closely related genetically and belong in the same clonal group. Pulsed-field gel electrophoresis (PFGE) typing of XbaI-digested genomic DNA revealed that the two strains differed by two bands but shared 90% similarity and clustered in the same clade. All other non-Stx-producing O157:H7 strains examined clustered in a major clade that was distinct from that of Stx-producing O157:H7 strains. The findings that TT12B was identical to TT12A, except for Stx production, and its PFGE profile is also more closely related to that of Stx-producing O157:H7 strains suggest that TT12B was derived from TT12A by the loss of both stx genes.     

Isogenic strain of Escherichia coli O157:H7 that has lost both Shiga toxin 1 and 2 genes.

An Escherichia coli O157:H7 strain isolated from a patient with hemorrhagic colitis was found to exhibit two slightly different colony morphology types on differential medium. Each morphological type, designated TT12A and TT12B, was isolated, and serological testing using various assays confirmed that both strains carried the O157 and the H7 antigens. Biochemical testing showed that the strains had identical profiles on AP120E analysis and, like typical O157:H7 strains, did not ferment sorbitol or exhibit beta-glucuronidase activity. Analysis with a multiplex PCR assay showed that TT12B did not carry the gene for either Shiga toxin 1 (Stx1) or Stx2, whereas these genes were present in TT12A and the toxins were produced. Apart from that, both strains carried the +93 gusA mutation, the cluster I ehxA gene for enterohemolysin, and the eae gene for gamma-intimin, which are all characteristics of the O157:H7 serotype. Phenotypic assays confirmed that both strains exhibited enterohemolysin activity and the attachment and effacing lesion on HeLa cells. Multilocus enzyme electrophoresis analysis showed that the strains are closely related genetically and belong in the same clonal group. Pulsed-field gel electrophoresis (PFGE) typing of XbaI-digested genomic DNA revealed that the two strains differed by two bands but shared 90% similarity and clustered in the same clade. All other non-Stx-producing O157:H7 strains examined clustered in a major clade that was distinct from that of Stx-producing O157:H7 strains. The findings that TT12B was identical to TT12A, except for Stx production, and its PFGE profile is also more closely related to that of Stx-producing O157:H7 strains suggest that TT12B was derived from TT12A by the loss of both stx genes.     

Antibiosis between bacteria isolated from the vagina of women with and without signs of bacterial vaginosis.

Lactobacilli from women with and without bacterial vaginosis (BV) were tested for H2O2 production. Thirty-seven (79%) of the 47 strains of lactobacilli isolated from the women without BV produced H2O2, while only nine (23%) of the 39 strains of lactobacilli obtained from women with BV did so. Five of 20 H2O2-producing and two of 26 non-producing strains of Lactobacillus exhibited antibiosis against four of 12 strains of peptostreptococci and two of 10 strains of Mobiluncus spp. None of a further 41 different anaerobic and facultative anaerobic bacterial strains were inhibited by any of the isolates of lactobacilli tested. Some strains of Gardnerella vaginalis, Bacteriodes spp., Mobiluncus spp. and Peptostreptococcus spp. inhibited the growth of three strains of lactobacilli belonging to different species. When the pH of the culture medium was increased from 6.0 to 6.5 this led to a decrease in the number of strains inhibited and/or the size of the growth-inhibitory zones. Different concentrations of H2O2 did not inhibit any of the strains tested. The growth-inhibitory effect of lactobacilli could not be related to their bacteriocin production. Increasing the iron content of the medium by adding FeCl3 (0.01 mM-1 mM) decreased or completely abolished the antibiosis.     

Laccase protects Cryptococcus neoformans from antifungal activity of alveolar macrophages.

While laccase of Cryptococcus neoformans is implicated in the virulence of the organism, our recent studies showing absence of melanin in the infected mouse brain has led us to a search for alternative roles for laccase in cryptococcosis. We investigated the role of laccase in protection of C. neoformans against murine alveolar macrophage (AM)-mediated antifungal activity by using a pair of congenic laccase-positive (2E-TUC) and laccase-deficient (2E-TU) strains. The laccase-positive cells with laccase derepression were more resistant to the antifungal activity of AM than a laccase-deficient strain ([28.9 +/- 1.2]% versus [40.2 +/- 2.6]% killing). Addition of L-dopa to Cryptococcus to produce melanin in a laccase-positive strain resulted in a slight increase in protection of C. neoformans from the antifungal activity of macrophages ([25.4 +/- 3.4]% versus [28.9 +/- 1.2]% killing). Recombinant cryptococcal laccase exhibited iron oxidase activity in converting Fe(II) to Fe(III). Moreover, recombinant laccase inhibited killing of C. neoformans by hydroxyl radicals catalyzed by iron in a cell-free system. Addition of the hydroxyl radical scavenger mannitol or dimethyl sulfoxide to AMs prior to the introduction of cryptococcal cells decreased killing of both strains and reduced the difference in susceptibility between the laccase-positive and laccase-deficient strains. Furthermore, laccase-mediated protection from AM killing was inhibited by the addition of Fe(II), presumably by overcoming the effects of the iron oxidase activity of cryptococcal laccase. These results suggest that the iron oxidase activity of laccase may protect C. neoformans from macrophages by oxidation of phagosomal iron to Fe(III) with a resultant decrease in hydroxyl radical formation.     

Virulence of enterohemorrhagic Escherichia coli O91:H21 clinical isolates in an orally infected mouse model.

Escherichia coli K-12 strains producing high levels of Shiga-like toxin type II (SLT-II) but not SLT-I were previously shown to be virulent in an orally infected, streptomycin-treated mouse model. In this investigation, we tested the virulence of several SLT-II-producing enterohemorrhagic E. coli (EHEC) isolates from patients with hemorrhagic colitis or hemolytic uremic syndrome. All of the strains tested were able to colonize the mouse intestine. However, only two strains were consistently virulent for mice: O91:H21 strain B2F1 (Strr), which was previously shown to carry two copies of slt-II-related toxins, and O91:H21 strain H414-36/89 (Strr), which was found in this study to contain three genes from the slt-II group. The oral 50% lethal doses of strains B2F1 (Strr) and H414-36/89 (Strr) when fed to streptomycin-treated mice were less than 10 bacteria. Histological sections from moribund mice fed the O91:H21 strains demonstrated extensive renal tubular necrosis; however, hematological results were not consistent with a diagnosis of hemolytic uremic syndrome. The central role of SLT in the virulence of the O91:H21 EHEC strains was supported by the finding that streptomycin-treated mice preinoculated with monoclonal antibody specific for SLT-II survived oral challenge with either B2F1 (Strr) or H414-36/89 (Strr). The basis for the variation in virulence among the SLT-II-producing EHEC strains tested was not determined. However, a correlation between the capacity of an EHEC strain to grow in small intestinal mucus and lethality in the streptomycin-treated mice was observed.