Phenotypic changes contributing to Enterobacter gergoviae biocide resistance.

UNLABELLED: Enterobacter gergoviae is a recurrent contaminant of cosmetic and hygiene products. To understand how this bacterium adapts to biocides, we studied Ent. gergoviae CIP 76.01 and its triclosan and Methylisothiazolinone-chloromethylisothiazolinone (MIT-CMIT) tolerant isogenic mutants. They were compared with others also isolated from contaminated cosmetics. Phenotypic differences were noted and these included changes in the bacterial envelope and flagella along with differences in motility, and biofilm growth rates. Triclosan and MIT-CMIT derivatives expressed flagella and other MIT-CMIT derivatives exhibited some external appendages. Those bacteria expressing a high-level minimal inhibitory concentration to MIT-CMIT, expressed a strong biofilm formation. No differential phenotypes were noted for carbon source utilisation. Enterobacter gergoviae demonstrated a diverse response to both of these preservatives contained in cosmetic preparations, depending on their concentrations. Interestingly, this adaptive response is associated with modifications of filament structure-related proteins contributing to increase the organism motility and the production of biofilm. SIGNIFICANCE AND IMPACT OF THE STUDY: Recurrent contaminations of cosmetics products by Ent. gergoviae, needed a better understanding concerning the bacterial adaptation to preservative agents, with particular concern to triclosan and MIT-CMIT. We demonstrated that bacteria response is associated to various mechanisms represented by expression of external appendages (pili or fimbriae) that control cell motility and biofilm formation and evolving as the concentration of biocides adaptation increased. Such mechanisms which are not chemical specific can also promote a cross-resistance to other biocidal agents. The characterization of Ent. gergoviae adaptability to biocides allows industry to adjust the ranges of concentrations and composition of preservatives in formula.

How to minimize toxic exposure to pyridine during continuous infusion of ceftazidime in patients with cystic fibrosis?

Ceftazidime is particularly efficient against Pseudomonas aeruginosa in cystic fibrosis patients. Thus, the spontaneous production of pyridine, which is a toxic product, raises some concern. Our aim was to examine the kinetics of degradation of ceftazidime in portable infusion pumps either at 4°C, 22°C, or 33°C and to propose some recommendations in order to reduce the pyridine exposure. Two administration models were studied in vitro. In model 1, we administered 12 g of ceftazidime infused over 23 h (once-daily infusion) compared to 6 g infused over 11.5 h in model 2 (twice-daily regimen). Samples were collected at 0 h and then every 4 and 2 h after the shaping of portable infusion pumps in models 1 and 2, respectively. Both ceftazidime and pyridine were analyzed using an ultraviolet high-performance liquid chromatograph. Production of pyridine is highly depending on the temperature. The in situ production of pyridine per day of treatment decreases at a ratio close to 1/6 and 1/3 between 33°C and 4°C in models 1 and 2, respectively. Regardless of the conditions, the production of pyridine is significantly lower in model 2, whereas the total delivery amount of ceftazidime is significantly higher at 4°C and 33°C compared to that in model 1. According to a the precautionary principle, these findings lead to three major recommendations: (i) exposing a solution of ceftazidime to over 22°C should be strictly avoided, (ii) a divided dose of 6 g over 11.5 h instead of a once-daily administration is preferred, and (iii) infusion should be administered immediately after reconstitution.

Natural extracts stimulate membrane-associated mechanisms of resistance in Gram-negative bacteria.

UNLABELLED: Several mechanisms are involved in the bacterial resistance towards antimicrobial agents. The membrane-associated mechanisms of resistance were studied in Escherichia coli strains after incubation with Thymus maroccanus essential oil, its major components (carvacrol and thymol) or with certain antibiotics. The minimum inhibitory concentration (MIC) and the expression of membrane proteins, porins and efflux pumps were determined in wild type and derivative strains. Derivative strains adapted to different compounds displayed a high level of resistance to all tested antibiotics. The MIC increase is associated with an overexpression of an efflux pump immunorelated to AcrAB-TolC in various variants. Interestingly, the expression of outer membrane proteins slightly decreases in these strains. We demonstrate that the increase in antibiotic resistance correlates with membrane changes observed in the variants. This type of bacterial adaptation to natural compounds can occur in vivo providing the emergence/selection of bacteria less susceptible to clinically used antibiotics. SIGNIFICANCE AND IMPACT OF THE STUDY: Thymus maroccanus essential oil and some major components are able to select variants that modify the expression of transporters involved in the influx (porins) and in the efflux (AcrAB family) of various drugs. Importantly, these membrane proteins are involved in the transport of natural compounds and several antibiotic families. This special 'membrane adaptation' can explain the persistence of less susceptible/tolerant bacteria in the environment where natural compounds are present and the continuous stimulation of efflux systems in these bacteria.

Enterobacter gergoviae adaptation to preservatives commonly used in cosmetic industry.

OBJECTIVE: The aim of this study was to obtain a better understanding regarding the origin of recurrent contamination by Enterobacter gergoviae in diverse cosmetic formula. We studied 65 isolates collected from various sources (clinical, food, cosmetics). METHODS: RAPD analysis using AP12H, REP and ERIC-PCR was carried out for epidemiological typing. Evaluation of susceptibility to preservatives currently used in cosmetics for a representative panel of collection strains was measured. Preservative efficacy was evaluated by minimum inhibitory concentrations and minimum bactericidal concentrations (MBCs). RESULTS: Eighty per cent of isolates was unrelated. E. gergoviae showed significant levels of resistance to preservatives. MBC was higher than maximum permitted concentrations imposed by European Commission (EC). Association of preservatives showed in rare case additive effects, and no synergic effects were observed. CONCLUSION: Most of the cosmetic formulations are contaminated with unrelated E. gergoviae strains. Maximum allowed concentrations for sodium benzoate are inefficient to limit proliferation and control adaptability to this bacterium in cosmetic products. Efflux mechanisms should be involved in methylisothiazolinone-chloromethylisothiazolinone and triclosan adaptation.

Effect of low-frequency KRAS mutations on the response to anti-EGFR therapy in metastatic colorectal cancer.

BACKGROUND: Only patients with wild-type (WT) KRAS tumors benefit from anti-epidermal growth factor receptor (EGFR) monoclonal antibodies (Mabs) in metastatic colorectal cancer (mCRC). Pyrosequencing is now widely used for the determination of KRAS mutation burden and a conservative cut-off point of 10% has been defined. Up until now, the impact of low-frequency KRAS mutations (<10%) on the response to anti-EGFR Mabs has yet to be evaluated. PATIENTS AND METHODS: Tumors from patients receiving anti-EGFR Mabs based on a WT genotype for KRAS, as determined using direct sequencing, have been retrospectively analyzed by pyrosequencing. Patients were categorized as WT (no KRAS mutation) or low-frequency mutation when KRAS mutation was <10% (KRAS low MT). RESULTS: A total of 168 patients treated by anti-EGFR Mabs for mCRC were analyzed. According to pyrosequencing, 138 tumors remained KRAS WT, while 30 tumors were KRAS low MT. In the KRAS low MT and KRAS WT groups, the response rates were 6.7% and 37.0%, respectively, while stabilization amounted to 23.3% versus 32.6% and progression to 70% versus 29% (P < 0.01). Progression-free survival (PFS) was 2.7 ± 0.5 months for KRAS low MT and was 6.0 ± 0.3 months for KRAS WT (P < 0.01). CONCLUSIONS: These results appear to validate consideration of low-frequency KRAS mutation tumors as positive, and justify a large-scale prospective study.

Thymus maroccanus essential oil, a membranotropic compound active on Gram-negative bacteria and resistant isolates.

AIMS: The effects of Thymus maroccanus essential oil (EO) on the integrity of the cell membranes and the permeability of the outer membrane (OM) and inner membrane (IM) of Escherichia coli, Enterobacter aerogenes and Salmonella enterica Typhimurium were investigated. METHODS AND RESULTS: The bacterial release of intracellular proteins, cytoplasmic ß-galactosidase and periplasmic ß-lactamase induced by T. maroccanus EO was compared to the membranotropic activity of polymyxin B (PB) known as an effective permeabilizer of the membrane of Gram-negative bacteria. Results showed that T. maroccanus EO increased the permeability of the OM and IM of studied bacteria and induced the release of intracellular proteins into the external medium. CONCLUSIONS: The effect of T. maroccanus EO on the outer membrane was comparable to that of PB, and both T. maroccanus EO and PB induce similar levels of ß-lactamase release. In addition, it also promoted the release of the cytoplasmic ß-galactosidase. Moreover, the lipopolysaccharide molecules and the overexpression of efflux pumps seem to play a crucial role in the level of susceptibility of studied bacteria to the permeabilizing effect of T. maroccanus EO. SIGNIFICANCE AND IMPACT OF STUDY: These results demonstrate that T. maroccanus EO can restore antibiotic activity by targeting the two bacterial membranes and would be attractive candidates for developing new adjuvants for combating resistant Gram-negative bacteria.

Cross-resistance between biocides and antimicrobials: an emerging question.

The widespread use of biocides, and their resulting dissemination in the environment, can contribute to adaptations in bacteria leading to the development of low-level susceptibility to antibacterial agents. The mechanisms of resistance in bacteria are similar for both antimicrobials and biocides, and exposure to biocides can result in cross-resistance to antibacterial agents. Resistance mechanisms altering the activity of biocide and antibiotic molecules are discussed with regard to regulation and mode of action in the light of laboratory studies of induced resistance. It is clear that in order to preserve their activity and avoid the development of possible cross-resistance, prudent use of antibacterial agents is to be strongly recommended, not only in clinical settings but also in veterinary and agricultural and other applications.

Evoked response activity eigenmode analysis in a convoluted cortex via neural field theory.

Neural field theory of the corticothalamic system is used to explore evoked response potentials (ERPs) caused by spatially localized impulse stimuli on the convoluted cortex and on a spherical cortex. Eigenfunctions are calculated analytically on the spherical cortex and numerically on the convoluted cortex via eigenfunction expansions. Eigenmodes on a convoluted cortex are similar to those of the spherical cortex, and a few such modes are found to be sufficient to reproduce the main ERP features. It is found that the ERP peak is stronger in spherical cortex than convoluted cortex, but in both cases the peak decreases monotonically with increasing distance from the stimulus point. In the convoluted case, cortical folding causes ERPs to differ between locations at the same distance from the stimulus point and spherical symmetries are only approximately preserved.

[Improvement of carbohydrate deficient transferrin measurement by capillary zone electrophoresis using immunosubtraction of immunoglobulins and transferrin]. / Intérêt de l'immunosoustraction des immunoglobulines et de la transferrine dans le dosage de la transferrine désialylée en électrophorèse capillaire.

CDT (Carbohydrate Deficient Transferrin) is considered as the most efficient biomarker of alcohol abuse available for routine use. Among the various methods developed for its measurement, capillary zone electrophoresis (CZE) on the multicapillary analyzer Capillarys2 provides high quality results at high throughput. However, the non CDT specific measurement of protein absorbance at 200 nm may bring abnormal profiles in samples from patients with high polyclonal immunoglobulin level or monoclonal component. We evaluated the automated immunosubtraction procedure developed by the manufacturer in 48 samples with abnormal electrophoretic profiles that potentially could interfere with CZE measurement of CDT. Elimination of the serum immunoglobulins raised the number of interpretable profiles from 19 (40%) to 37 (77%). The immunosubtraction procedure failed in samples with a monoclonal component present at a concentration > 60 g/L and in some samples harbouring a partially degraded C3 fraction. Six samples identified as genetic BC transferrin variants were also included in the study and submitted to an automated transferrin subtraction procedure to ascertain whether the additional peak were actually transferrin glycoforms. After treatment, two samples were classified as homozygote C for transferrin due to the persistence of one of the supposed transferrin peak. In conclusion, immunoglobulin and transferrin subtraction allow a better CDT measurement in most samples with interfering monoclonal components and avoid misclassification of suspected transferrin BC or CD variants.

A multicentric retrospective study of serum/plasma urea and creatinine concentrations in dogs using univariate and multivariate decision rules to evaluate diagnostic efficiency.

BACKGROUND: Urea and creatinine are the most frequently used indirect markers in plasma and serum of glomerular filtration rate in dogs. Both have been shown to lack sensitivity but their diagnostic efficiency for the diagnosis of kidney disease has been minimally investigated. OBJECTIVE: The purpose of this retrospective study was to investigate the influence of possible factors of variation on both analytes and to determine whether specific decision rules should be drawn up for subpopulations of dogs. METHODS: The results of urea and creatinine measurements, breed, sex, age, and health status (healthy, renal disease, or nonrenal disease) of 3822 dogs were collected from the archives of 5 veterinary clinics. Data were analyzed with univariate and multivariate decision rules with and without adjustment. RESULTS: There were significant effects and interactions of almost all of the sources of variation. Slight improvements in diagnostic efficiency were obtained by adjusting the decision rules to these sources of variations. Univariate decision rules gave approximately the same diagnostic efficiency for urea and creatinine concentrations, with sensitivity and specificity in the range of 70% and 90%, respectively, using the upper limit of the reference interval as the threshold value. Multivariate decision rules provided only minor improvements in diagnostic efficiency. CONCLUSION: Simultaneous measurement of both urea and creatinine is of limited diagnostic value over the analysis of a single variable. Creatinine is the preferred analyte as it is affected by fewer extrarenal factors of variation.

[Lentiviral vectors]. / Lentivecteur: l'anti-vecteur rétroviral ?

The recent advent of RNA interference has strongly stimulated the growing interest toward lentiviral vectors. This, plus the occurrence of several adverse effects in a clinical trial in which oncoretroviral vectors were employed, refocused the need for efficient tools allowing a stable in vivo gene transfer. The lentivectors were first developed in 1996 to address the poor efficiency of murine retroviral vectors to transduce arrested cells. Taking advantage of the accumulated experience in retroviral vector design, a rapid evolution of the structural form of the lentivectors converted this gene transfer agent into a very simple and popular tool. This rapidly led to several commercially available solutions. In the present review, more than a simple comparison to oncoretroviral vectors, we aimed at emphasizing the specific points that distinguish lentiviral vectors and confirm them as good and safe candidates for the clinical delivery of therapeutic genes.

Neural field theory of perceptual echo and implications for estimating brain connectivity.

Neural field theory is used to predict and analyze the phenomenon of perceptual echo in which random input stimuli at one location are correlated with electroencephalographic responses at other locations. It is shown that this echo correlation (EC) yields an estimate of the transfer function from the stimulated point to other locations. Modal analysis then explains the observed spatiotemporal structure of visually driven EC and the dominance of the alpha frequency; two eigenmodes of similar amplitude dominate the response, leading to temporal beating and a line of low correlation that runs from the crown of the head toward the ears. These effects result from mode splitting and symmetry breaking caused by interhemispheric coupling and cortical folding. It is shown how eigenmodes obtained from functional magnetic resonance imaging experiments can be combined with temporal dynamics from EC or other evoked responses to estimate the spatiotemporal transfer function between any two points and hence their effective connectivity.

Results from the Loire-Ardèche-Drôme-Isère-Puy-de-Dôme (LADIP) trial on atrial flutter, a multicentric prospective randomized study comparing amiodarone and radiofrequency ablation after the first episode of symptomatic atrial flutter.

BACKGROUND: There is no published randomized study comparing amiodarone therapy and radiofrequency catheter ablation (RFA) after only 1 episode of symptomatic atrial flutter (AFL). The aim of the Loire-Ardèche-Drôme-Isère-Puy-de-Dôme (LADIP) Trial of Atrial Flutter was 2-fold: (1) to prospectively compare first-line RFA (group I) versus cardioversion and amiodarone therapy (group II) after only 1 AFL episode; and (2) to determine the impact of both treatments on the long-term risk of subsequent atrial fibrillation (AF). METHODS AND RESULTS: From October 2002 to February 2006, 104 patients (aged 78+/-5 years; 20 women) with AFL were included, with 52 patients in group I and 52 patients in group II. The cumulative risk of AFL or AF was interpreted with the use of Kaplan-Meier curves and compared by the log-rank test. Clinical presentation, echocardiographic data, and follow-up were as follows: age (78.5+/-5 versus 78+/-5 years), history of AF (27% versus 21.6%); structural heart disease (58% versus 65%), left ventricular ejection fraction (56+/-14% versus 54.5+/-14%), left atrial size (43+/-7 versus 43+/-6 mm), mean follow-up (13+/-6 versus 13+/-6 months; P=NS), recurrence of AFL (3.8% versus 29.5%; P<0.0001), and occurrence of significant AF beyond 10 minutes (25% versus 18%; P=0.3). Five complications (10%) were noted in group II (sick sinus syndrome in 2, hyperthyroidism in 1, and hypothyroidism in 2) and none in group I (0%) (P=0.03). CONCLUSIONS: RFA should be considered a first-line therapy even after the first episode of symptomatic AFL. There is a better long-term success rate, the same risk of subsequent AF, and fewer secondary effects.

ICE inhibitor YVADcmk is a potent therapeutic agent against in vivo liver apoptosis.

In liver, apoptosis is a physiological process involved in the clearance of injured cells and in homeostatic control [1]. However, in patients with viral fulminant hepatitis or with nonacute liver diseases [2], dramatic liver failure or secondary cirrhosis results from the death of hepatocytes, which express the cell-surface receptor Fas, by apoptosis. To date, treatment of fulminant hepatitis relies mainly on orthotopic liver transplantation, which is limited by immunological complications and graft availability. Unravelling the molecular mechanisms that underlie acute liver failure could allow the design of an appropriate therapy. Ligand-bound Fas and tumour necrosis factor alpha (TNF-alpha) induce hepatic apoptosis in mice [3-6]. In various cell types, Fas- or TNF-alpha-induced apoptosis is blocked by viral proteins (such as p35 and CrmA) as well as by a decoy peptide (YVADcmk) [7-11], suggesting that these mechanisms of apoptosis involve ICE (interleukin-1 beta converting enzyme)-like proteases. Here, we report that, in vivo, pre-treatment of mice with YVADcmk protects them from the lethal effect of anti-Fas antibody and from liver failure induced by injection of TNF-alpha. Remarkably, YVADcmk administration is also highly effective in rescuing mice that have been pretreated with anti-Fas antibody from rapid death, despite extensive hepatic apoptosis. This dramatic curative effect could be of clinical benefit for the treatment of viral and inflammatory liver diseases.

[Alphavirus derived vector, still a relevant system through new mobilisation procedure]. / Les vecteurs dérivés des alphavirus, un système toujours pertinent grâce à de nouveaux modes de vectorisation.

Among eukaryotic cell-expression-systems, the one derived from alphaviruses, including Semliki forest virus (SFV), offers an efficient method for protein production in mammalian cells. Despite this efficacy, twenty years after their discovery alphaviruses vectors remain poorly used. Alphavirus vectors exist as naked RNA vectors or as recombinant particles. The use of costly RNA-based replicons, and the fact that production of recombinant particles is a complex process to carry out, have hampered the attractiveness of the methods. Lastly, the apoptotic signals induced by alphavirus vectors replication leads to a rapid death of the producing cells. This feature, which can be detrimental in vitro, is advantageously exploited for in vivo applications. Besides laboratory applications, alphavirus vectors have been explored in rare phase I clinical trials, for vaccine development and cancer gene therapy, therefore, alphavirus vector will benefit from the advent of new, biosafety-efficient, methods for particles production. Most of the recent advances in the field proposed an heterologous mobilisation of alphavirus replicon. While increasing biosafety aspects, new methods are also simpler regarding the genesis of recombinant particles. In the present review, we overview the alphavirus life cycle with a special attention to the features influencing vector design and utility.

The orientation of porin OmpF in the outer membrane of Escherichia coli.

The in vivo orientation of the channel forming porin OmpF from the outer membrane of Escherichia coli was assessed by immunological, biochemical and structural techniques. Porin OmpF exists as a trimer of channels formed by 16 antiparallel beta-strands. These are connected by long hydrophilic loops on one side of the bilayer and short loops or beta-turns on the other. The former constitute the rough side of the porin channel, the latter the smooth side. Epitopes at the cell surface have all been mapped within the long loops, suggesting a rough-side-out orientation of OmpF in the membrane. We analyzed detergent solubilized OmpF trimers, reconstituted 2-D OmpF crystals, OmpF containing outer membranes (sacculi) and intact cells of an E. coli strain overexpressing OmpF. Both solubilized OmpF and OmpF containing sacculi were exposed to proteases, and distinct cleavage sites were identified by protein sequencing. Solubilized OmpF, reconstituted 2-D OmpF crystals and detergent extracted sacculi were tested for their capacity to adsorb colicin N. We used antibodies directed against surface exposed epitopes for immunogold labeling of reconstituted 2-D OmpF crystals and sacculi. The surfaces of intact cells and extracted sacculi were analyzed by electron microscopy and image processing. Finally, a full 3-D reconstruction of negatively stained OmpF containing sacculi revealed the OmpF trimer in its native conformation within the outer membrane. Colicin N and antibody experiments, as well as the 3-D map of the sacculi demonstrated that OmpF exposes the long loops to the extracellular space. In contrast, reconstituted crystalline OmpF vesicles and double layered sheets were found to be in an inside-out conformation, hence hiding colicin or antibody binding epitopes. Two proteinase K cleavage sites were identified, one on a protruding loop and the other inside the channel on the loop penetrating the pore.

Translocations involving the short arm of chromosome 17 in chronic B-lymphoid disorders: frequent occurrence of dicentric rearrangements and possible association with adverse outcome.

Unbalanced translocations involving chromosome arm 17p, where the TP53 tumor suppressor gene localizes, are rarely described in chronic lymphocytic leukemia and small lymphocytic lymphoma (CLL/SLL), but recent use of molecular cytogenetic techniques have indicated a significant incidence of TP53 deletions, suggesting the involvement of chromosome 17p in these disorders. By conventional karyotype, we have identified unbalanced translocations involving 17p in 14 out of 123 (11%) CLL/SLL patients with clonal abnormalities. Cases were characterized by resistance to chemotherapy and a poor clinical outcome. The karyotypes presented a high incidence of complex rearrangements and 17p translocations were characterized by various partners. In 10 cases a centric fusion was assessed by fluorescent in situ hybridization (FISH) experiments using specific centromeric probes. The incidence of dicentric translocations in these series is therefore significantly higher than usually described, arising in up to 71% (10 out of 14 cases). In all cases, translocations led to a monosomy 17p and to a TP53 monoallelic deletion. The adverse clinical outcome confirms that structural abnormalities involving chromosome 17p are associated with disease progression in patients with chronic lymphoproliferative disorders.

[An anicteric cholectasis and nephrotic syndrome: AL amyloïdosis in a 51 years-old man]. / Une cholestase anictérique associée à un syndrome néphrotique chez un homme de 51 ans: découverte d'une amylose AL.

Amyloidosis is a multiple-organ disease for which the diagnosis is often confusing and thereby delayed. Here, we present an archetypal case illustrating such difficulties. A 51 years-old man presented a mixed dyslipemia in November 2002, in June 2004 he has finally been diagnosed with a primary AL-amyloidosis. Within these two years, the arising of a non-icteric cholestasis and a nephrotic syndrome have triggered the search for a disease related to a multiple-organ protein deposition. Confirmation of the AL-amyloidosis was obtained through an histological examination, including direct immuno-fluorescence. Amyloidosis is a life threatening disease that need to be diagnosed at an early stage, in order to maximise the therapeutic expectations. The average survival after the diagnosis of AL-amyloidosis is 5% at 10 years. Often, treatments are initiated late in the course of the disease, at a time when organ lesion are constituted, severely affecting the prognosis.

Immunological evidence for differences in the exposed regions of OmpF porins from Escherichia coli B and K-12.

Nine monoclonal antibodies (MoF 0-8) directed against the native form (trimeric) of outer membrane protein OmpF of Escherichia coli B were obtained and characterized. All these antibodies bind to OmpF porin in intact E. coli B cells but not OmpF from E. coli K-12 cells which only differ at positions 66, 117 and 262 in the sequence. These antibodies exhibit a specificity to the native form, failing to recognize the denatured form in a liquid immunorecognition assay. Four tested antibodies are able to protect against colicin A, a bacteriotoxin using OmpF as receptor. One monoclonal antibody (MoF 0) is specific to the external topology of native porin in the outer membrane and three antibodies could recognize epitopes present in each conformation of subunits of trimer form. It is concluded that the region around the 66th and more probably around the 262nd amino acids are involved in cell-surface exposed epitopes. Moreover, these results support the assumption that the conformation of protruding regions of OmpF from E. coli B and K-12 are different.

Immunological analysis of porin polymorphism in Escherichia coli B and K-12.

Two sets of monoclonal antibodies (MoF type I and MoF type II) directed against the OmpF protein were used to analyze the immunological reactivity of the major outer membrane porins of E. coli B and K-12. All these antibodies present a specificity to the native OmpF protein. In addition, among the type II antibodies, MoF 18, 19 and 20 could recognize an epitope present on both monomeric and trimeric forms of the porin as demonstrated by immunoblotting analyses. The use of two different screening methods led to the isolation of two different sets of MoF, one specific for a native conformation accessible only on E. coli B strain and the second directed against epitopes present on OmpF of the two strains, B and K-12. These various responses are discussed in relation to the lipopolysaccharide binding to OmpF and with respect to the screening test used.