Structural and genetic evidence for the close relationship between Escherichia coli O71 and Salmonella enterica O28 O-antigens

Optical detection and enumeration of Escherichia coli and Salmonella enterica using a low-magnification light microscope

A rapid and cost-effective method for detecting bacterial cells from surfaces is critical to food safety, clinical hygiene, and pharmacy quality. Herein, we established an optical detection method based on a gold chip coating with 3-mercaptophenylboronic acid (3-MPBA) to capture bacterial cells, which allows for the detection and quantification of bacterial cells with a standard light microscope under low-magnification (10×) objective lens. Then, integrate the developed optical detection method with swab sampling to detect bacterial cells loading on stainless-steel surfaces. Using Salmonella enterica (SE1045) and Escherichia coli (E. coli OP50) as model bacterial cells, we achieved a capture efficiency of up to 76.0 ± 2.0 % for SE1045 cells and 81.1 ± 3.3 % for E. coli OP50 cells at 103 CFU/mL upon the optimized conditions, which slightly decreased with the increasing bacterial concentrations. Our assay showed good linear relationships between the concentrations of bacterial cells with the cell counting in images in the range of 103 -107 CFU/mL for SE1045, and 103 -108 CFU/mL for E. coli OP50 cells. The limit of detection (LOD) was 103 CFU/mL for both SE1045 and E. coli OP50 cells. A further increase in sensitivity in detecting E. coli OP50 cells was achieved through a heat treatment, enabling the LOD to be reduced as low as 102 CFU/mL. Furthermore, a preliminary application succeeded in assessing bacterial contamination on stainless-steel surfaces following integration with the approximately 40 % recovery rate, suggesting prospects for evaluating the bacteria from surfaces. The entire process was completed within around 2 h, costing merely a few dollars per sample. Considering the low cost of standard light microscopes, our method holds significant potential for practical industrial applications in bacterial contamination control on surfaces, especially in low-resource settings.

Rapid Dot-Blot Immunoassay for Detecting Multiple Salmonella enterica Serotypes

Salmonella, a major contributor to foodborne infections, typically causes self-limiting gastroenteritis. However, it is frequently invasive and disseminates across the intestinal epithelium, leading to deadly bacteremia. Although the genus is subdivided into >2,600 serotypes based on their antigenic determinants, only few serotypes are responsible for most human infections. In this study, a rapid dot-blot immunoassay was developed to diagnose multiple Salmonella enterica serotypes with high incidence rates in humans. The feasibility of 10 commercial antibodies (four polyclonal and six monoclonal antibodies) was tested using the 18 serotypes associated with 67.5% Salmonella infection cases in the United States of America (U.S.A) in 2016. Ab 3 (polyclonal; eight of 18 serotypes), Ab 8 (monoclonal; 13 of 18 serotypes), and Ab 9 (monoclonal; 10 of 18 serotypes) antibodies exhibited high detection rates in western blotting and combinations of two antibodies (Ab 3+8, Ab 3+9, and Ab 8+9) were applied to dot-blot assays. The combination of Ab 3+8 identified 15 of the tested 18 serotypes in 3 h, i.e., S. Enteritidis, S. Typhimurium, S. Javiana, S. I 4,[5],12:i:-, S. Infantis, S. Montevideo, S. Braenderup, S. Thompson, S. Saintpaul, S. Heidelberg, S. Oranienburg, S. Bareilly, S. Berta, S. Agona, and S. Anatum, which were responsible for 53.7% Salmonella infections in the U.S. in 2016. This cost-effective and rapid method can be utilized as an on-site colorimetric method for Salmonella detection.

Specific Bacterial Cell Wall Components Influence the Stability of Coxsackievirus B3

Enteric viruses infect the mammalian gastrointestinal tract and lead to significant morbidity and mortality worldwide. Data indicate that enteric viruses can utilize intestinal bacteria to promote viral replication and pathogenesis. However, the precise interactions between enteric viruses and bacteria are unknown. Here, we examined the interaction between bacteria and coxsackievirus B3, an enteric virus from the picornavirus family. We found that bacteria enhance the infectivity of coxsackievirus B3 (CVB3) in vitro. Notably, specific bacteria are required, as Gram-negative Salmonella enterica, but not Escherichia coli, enhanced CVB3 infectivity and stability. Investigating the cell wall components of both S. enterica and E. coli revealed that structures in the O-antigen or core of lipopolysaccharide, a major component of the Gram-negative bacterial cell wall, were required for S. enterica to enhance CVB3. To determine if these requirements were necessary for similar enteric viruses, we investigated if S. enterica and E. coli enhanced infectivity of poliovirus, another enteric virus in the picornavirus family. We found that while E. coli did not enhance the infectivity of CVB3, E. coli enhanced poliovirus infectivity. Overall, these data indicate that distinct bacteria enhance CVB3 infectivity and stability, and specific enteric viruses may have differing requirements for their interactions with specific bacterial species. IMPORTANCE Previous data indicate that several enteric viruses utilize bacteria to promote intestinal infection and viral stability. Here, we show that specific bacteria and bacterial cell wall components are required to enhance infectivity and stability of coxsackievirus B3 in vitro. These requirements are likely enteric virus specific, as the bacteria for CVB3 differ from poliovirus, a closely related virus. Therefore, these data indicate that specific bacteria and their cell wall components dictate the interaction with various enteric viruses in distinct mechanisms.

Characterization of a New and Efficient Polyvalent Phage Infecting E. coli O157:H7, Salmonella spp., and Shigella sonnei

Ongoing outbreaks of foodborne diseases remain a significant public health concern. Lytic phages provide promising attributes as biocontrol agents. This study characterized KFS-EC3, a polyvalent and lytic phage, which was isolated from slaughterhouse sewage and purified by cesium chloride density centrifugation. Host range and efficiency of plating analyses revealed that KFS-EC3 is polyvalent and can efficiently infect E. coli O157:H7, Salmonella spp., and Shigella sonnei. KFS-EC3 had a latent time of 20 min and burst size of ~71 phages/infected cell. KFS-EC3 was stable and infectious following storage at a pH range of 3 to 11 and a temperature range of -70 °C to 60 °C. KFS-EC3 could inhibit E. coli O157:H7 growth by 2 logs up to 52 h even at the lowest MOI of 0.001. Genomic analysis of KFS-EC3 revealed that it consisted of 167,440 bp and 273 ORFs identified as functional genes, without any genes associated with antibiotic resistance, virulence, allergenicity, and lysogenicity. This phage was finally classified into the Tequatrovirus genus of the Myoviridae family. In conclusion, KFS-EC3 could simultaneously infect E. coli O157:H7, S. sonnei, and Salmonella spp. with the lowest MOI values over long periods, suggesting its suitability for simultaneous pathogen control in foods.

Development of a fluorescent distance-based paper device using loop-mediated isothermal amplification to detect Escherichia coli in urine

The highly sensitive and selective determination of Escherichia coli (E. coli) in urine was achieved using a SYBR™ safe loop-mediated isothermal amplification (LAMP) method with a distance-based paper device. New primers set specific to multi-copy the 16s rRNA gene of E. coli were designed and used in this study. The detection sensitivity of these primers was higher than in related work and they could be incorporated with a low-cost paper-based device to quantify E. coli in urine at a concentration lower than 101 CFU mL-1. Regarding standard artificial urine, a linear range of a 10-fold dilution of E. coli concentration (105-100 CFU mL-1) with an R-square value (R2) = 0.9823 was observed directly using a fluorescent migratory distance of the 4 μL reaction mixture in the detection zone under blue light without the need for postreaction staining process. Based on the device, E. coli infection could be significantly categorized into 3 groups; none, light, and heavy levels, which is beneficial for UTI diagnosis. Hence, this paper-based device is suitable for use with the SYBR™ Safe-LAMP assay to semi-quantify E. coli, especially in resource-limited settings due to advantages of low cost, simple fabrication and operation, and no requirement for sophisticated instruments, as well as its disposability and portability.

Lactoferrin as an antimicrobial against Salmonella enterica and Escherichia coli O157:H7 in raw milk

Improper storage conditions or processing of milk leads to potential spoilage and illness, due in part to temperature abuse, allowing bacteria present to grow and spoil the product. However, certain proteins naturally found in raw milk, such as lactoferrin, have reported antibacterial properties. The levels of lactoferrin required to effectively inhibit growth of pathogens have not been investigated thoroughly. This study aimed to examine various concentrations of lactoferrin as a potential biopreservative and as an antimicrobial against the common dairy pathogens Salmonella enterica and Escherichia coli O157:H7. Minimum inhibitory concentration assays were conducted on raw bovine milk in which the bacteria were exposed to varying concentrations of lactoferrin. In the raw milk system, the growth of E. coli O157:H7 was significantly decreased at levels greater than 14.05 mg/mL lactoferrin based on the reduction of tetrazolium salts. For S. enterica, only lactoferrin concentrations at or above 112.5 mg/mL in the milk resulted in reduced growth. Taken together, these results indicate that lactoferrin may have biopreservative potential. To fully examine the practicality and effectiveness of lactoferrin as an antimicrobial additive, a similar study should be conducted using additional (gram-positive) pathogens, such as Bacillus cereus and Listeria monocytogenes. If effective, lactoferrin could prolong the shelf life of dairy products and help reduce the incidence of foodborne illnesses in developing countries with limited refrigeration capability.

A Targeted Sequencing Assay for Serotyping Escherichia coli Using AgriSeq Technology

The gold standard method for serotyping Escherichia coli has relied on antisera-based typing of the O- and H-antigens, which is labor intensive and often unreliable. In the post-genomic era, sequence-based assays are potentially faster to provide results, could combine O-serogrouping and H-typing in a single test, and could simultaneously screen for the presence of other genetic markers of interest such as virulence factors. Whole genome sequencing is one approach; however, this method has limited multiplexing capabilities, and only a small fraction of the sequence is informative for subtyping or identifying virulence potential. A targeted, sequence-based assay and accompanying software for data analysis would be a great improvement over the currently available methods for serotyping. The purpose of this study was to develop a high-throughput, molecular method for serotyping E. coli by sequencing the genes that are required for production of O- and H-antigens, as well as to develop software for data analysis and serotype identification. To expand the utility of the assay, targets for the virulence factors, Shiga toxins (stx₁, and stx₂) and intimin (eae) were included. To validate the assay, genomic DNA was extracted from O-serogroup and H-type standard strains and from Shiga toxin-producing E. coli, the targeted regions were amplified, and then sequencing libraries were prepared from the amplified products followed by sequencing of the libraries on the Ion S5™ sequencer. The resulting sequence files were analyzed via the SeroType Caller™ software for identification of O-serogroup, H-type, and presence of stx₁, stx₂, and eae. We successfully identified 169 O-serogroups and 41 H-types. The assay also routinely detected the presence of stx_1a,c,d (3 of 3 strains), stx_2c−e,g (8 of 8 strains), stx_2f (1 strain), and eae (6 of 6 strains). Taken together, the high-throughput, sequence-based method presented here is a reliable alternative to antisera-based serotyping methods for E. coli.

Structure and genetics of Escherichia coli O antigens

Escherichia coli includes clonal groups of both commensal and pathogenic strains, with some of the latter causing serious infectious diseases. O antigen variation is current standard in defining strains for taxonomy and epidemiology, providing the basis for many serotyping schemes for Gram-negative bacteria. This review covers the diversity in E. coli O antigen structures and gene clusters, and the genetic basis for the structural diversity. Of the 187 formally defined O antigens, six (O31, O47, O67, O72, O94 and O122) have since been removed and three (O34, O89 and O144) strains do not produce any O antigen. Therefore, structures are presented for 176 of the 181 E. coli O antigens, some of which include subgroups. Most (93%) of these O antigens are synthesized via the Wzx/Wzy pathway, 11 via the ABC transporter pathway, with O20, O57 and O60 still uncharacterized due to failure to find their O antigen gene clusters. Biosynthetic pathways are given for 38 of the 49 sugars found in E. coli O antigens, and several pairs or groups of the E. coli antigens that have related structures show close relationships of the O antigen gene clusters within clades, thereby highlighting the genetic basis of the evolution of diversity.

Molecular serogrouping ofEscherichia coli

O-antigens present on the surface ofEscherichia coliprovide antigenic specificity for the strain and are the main components for O-serogroup designation. Serotyping using O-group-specific antisera for the identification ofE. coliO-serogroups has been traditionally the gold-standard for distinguishingE. colistrains. Knowledge of the O-group is important for determining pathogenic lineage, classifyingE. colifor epidemiological studies, for determining virulence, and for tracing outbreaks of diseases and sources of infection. However, serotyping has limitations, as the antisera generated against each specific O-group may cross-react, many strains are non-typeable, and others can autoagglutinate or be rough (lacking an O-antigen). Currently, the nucleotide sequences are available for most of the 187 designatedE. coliO-groups. Public health and other laboratories are considering whole genome sequencing to develop genotypic methods to determine O-groups. These procedures require instrumentation and analysis that may not be accessible and may be cost-prohibitive at this time. In this review, we have identified unique gene sequences within the O-antigen gene clusters and have targeted these genes for identification of O-groups using the polymerase chain reaction. This information can be used to distinguish O-groups by developing other platforms forE. colidiagnostics in the future.

Structures and gene clusters of the O-antigens of Escherichia albertii O3, O4, O6, and O7

The O-specific polysaccharides (OPSs) called O-antigens were obtained by mild acid degradation of the lipopolysaccharides of Escherichia albertii serotypes O3, O4, O6, and O7 and studied by sugar analysis along with 1D and 2D ¹H and ¹³C NMR spectroscopy. The following structure was established for the OPS of E. albertii O4, which, to our knowledge, is unique among known bacterial polysaccharide structures: →2)-α-l-Rhap-(1 → 2)-α-l-Fucp-(1 → 2)-β-d-Galp-(1 → 3)-α-d-GalpNAc-(1 → 3)-β-d-GlcpNAc-(1→ The OPS structure of the strain of E. albertii O7 studied was identical to that of strain LMG 20973 (= Albert 10457), whose structure has been reported earlier (R. Eserstam et al. Eur. J. Biochem. 269 (2002) 3289-3295). E. albertii O3 and O6 shared the OPS structures with Escherichia coli O181 and O3, respectively, except for the lack of O-acetylation in E. albertii O3, which is present in E. coli O181. The gene clusters driving the O-antigen biosynthesis of the E. albertii strains were sequenced, the genes were annotated by comparison with sequences in the available databases, and the predicted functions of the encoded proteins were found to be consistent with the OPS structures established. In accordance with the relatedness of the OPS structures, the O-antigen gene clusters of E. albertii O3 and O6 contain the same genes and have the same organization as those of E. coli O181 and O3, the entire gene clusters being 83% and 98% identical, respectively.

Structure of O-Antigen and Hybrid Biosynthetic Locus in Burkholderia cenocepacia Clonal Variants Recovered from a Cystic Fibrosis Patient

Burkholderia cenocepacia is an opportunistic pathogen associated with chronic lung infections and increased risk of death in patients with cystic fibrosis (CF). In this work, we investigated the lipopolysaccharide (LPS) of clinical variants of B. cenocepacia that were collected from a CF patient over a period of 3.5 years, from the onset of infection until death by necrotizing pneumonia (cepacia syndrome). We report the chemical structure of the LPS molecule of various sequential isolates and the identification of a novel hybrid O-antigen (OAg) biosynthetic cluster. The OAg repeating unit of the LPS from IST439, the initial isolate, is a [→2)-β-D-Ribf-(1→4)-α-D-GalpNAc-(1→] disaccharide, which was not previously described in B. cenocepacia. The IST439 OAg biosynthetic gene cluster contains 7 of 23 genes that are closely homologous to genes found in B. multivorans, another member of the Burkholderia cepacia complex. None of the subsequent isolates expressed OAg. Genomic sequencing of these isolates enabled the identification of mutations within the OAg cluster, but none of these mutations could be associated with the loss of OAg. This study provides support to the notion that OAg LPS modifications are an important factor in the adaptation of B. cenocepacia to chronic infection and that the heterogeneity of OAgs relates to variation within the OAg gene cluster, indicating that the gene cluster might have been assembled through multiple horizontal transmission events.

Lipopolysaccharide modification in Gram-negative bacteria during chronic infection

The Gram-negative bacterial lipopolysaccharide (LPS) is a major component of the outer membrane that plays a key role in host-pathogen interactions with the innate immune system. During infection, bacteria are exposed to a host environment that is typically dominated by inflammatory cells and soluble factors, including antibiotics, which provide cues about regulation of gene expression. Bacterial adaptive changes including modulation of LPS synthesis and structure are a conserved theme in infections, irrespective of the type or bacteria or the site of infection. In general, these changes result in immune system evasion, persisting inflammation and increased antimicrobial resistance. Here, we review the modifications of LPS structure and biosynthetic pathways that occur upon adaptation of model opportunistic pathogens (Pseudomonas aeruginosa, Burkholderia cepacia complex bacteria, Helicobacter pylori and Salmonella enterica) to chronic infection in respiratory and gastrointestinal sites. We also discuss the molecular mechanisms of these variations and their role in the host-pathogen interaction.

Structural diversity in Salmonella O antigens and its genetic basis

This review covers the structures and genetics of the 46 O antigens of Salmonella, a major pathogen of humans and domestic animals. The variation in structures underpins the serological specificity of the 46 recognized serogroups. The O antigen is important for the full function and virulence of many bacteria, and the considerable diversity of O antigens can confer selective advantage. Salmonella O antigens can be divided into two major groups: those which have N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) and those which have galactose (Gal) as the first sugar in the O unit. In recent years, we have determined 21 chemical structures and sequenced 28 gene clusters for GlcNAc-/GalNAc-initiated O antigens, thus completing the structure and DNA sequence data for the 46 Salmonella O antigens. The structures and gene clusters of the GlcNAc-/GalNAc-initiated O antigens were found to be highly diverse, and 24 of them were found to be identical or closely related to Escherichia coli O antigens. Sequence comparisons indicate that all or most of the shared gene clusters were probably present in the common ancestor, although alternative explanations are also possible. In contrast, the better-known eight Gal-initiated O antigens are closely related both in structures and gene cluster sequences.

Development of a DNA microarray for molecular identification of all 46 Salmonella O serogroups

Salmonella is a major cause of food-borne disease in many countries. Serotype determination of Salmonella is important for disease assessment, infection control, and epidemiological surveillance. In this study, a microarray system that targets the O antigen-specific genes was developed for simultaneously detecting and identifying all 46 Salmonella O serogroups. Of these, 40 serogroups can be confidently identified, and the remaining 6, in three pairs (serogroups O67 and B, E1 and E4, and A and D1), need to be further distinguished from each other using PCR methods or conventional serotyping methods. The microarray was shown to be highly specific when evaluated against 293 Salmonella strains, 186 Shigella strains, representative Escherichia coli strains, and 10 strains of other bacterial species. The assay correctly identified 288 (98%) of the Salmonella strains. The detection sensitivity was determined to be 50 ng genomic DNA per sample. By testing simulated samples in a tomato background, 2 to 8 CFU per gram inoculated could be detected after enrichment. This newly developed microarray assay is the first molecular protocol that can be used for the comprehensive detection and identification of all 46 Salmonella O serogroups. Compared to the traditional serogrouping method, the microarray provides a reliable, high-throughput, and sensitive approach that can be used for rapid identification of multiple Salmonella O serogroups simultaneously.

Association of nucleotide polymorphisms within the O-antigen gene cluster of Escherichia coli O26, O45, O103, O111, O121, and O145 with serogroups and genetic subtypes

Shiga toxin-producing Escherichia coli (STEC) strains are important food-borne pathogens capable of causing hemolytic-uremic syndrome. STEC O157:H7 strains cause the majority of severe disease in the United States; however, there is a growing concern for the amount and severity of illness attributable to non-O157 STEC. Recently, the Food Safety and Inspection Service (FSIS) published the intent to regulate the presence of STEC belonging to serogroups O26, O45, O103, O111, O121, and O145 in nonintact beef products. To ensure the effective control of these bacteria, sensitive and specific tests for their detection will be needed. In this study, we identified single nucleotide polymorphisms (SNPs) in the O-antigen gene cluster that could be used to detect STEC strains of the above-described serogroups. Using comparative DNA sequence analysis, we identified 22 potentially informative SNPs among 164 STEC and non-STEC strains of the above-described serogroups and designed matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) assays to test the STEC allele frequencies in an independent panel of bacterial strains. We found at least one SNP that was specific to each serogroup and also differentiated between STEC and non-STEC strains. Differences in the DNA sequence of the O-antigen gene cluster corresponded well with differences in the virulence gene profiles and provided evidence of different lineages for STEC and non-STEC strains. The SNPs discovered in this study can be used to develop tests that will not only accurately identify O26, O45, O103, O111, O121, and O145 strains but also predict whether strains detected in the above-described serogroups contain Shiga toxin-encoding genes.

Detection of O antigens in Escherichia coli

Lipopolysaccharide on the surface of Escherichia coli constitutes the O antigens which are important virulence factors that are targets of both the innate and adaptive immune systems and play a major role in host-pathogen interactions. O antigens are responsible for antigenic specificity of the strain and determine the O serogroup. The designation of O serogroups is important for classifying E. coli strains, for epidemiological studies, in tracing the source of outbreaks of gastrointestinal or other illness, and for linking the source to the infection. For conventional serogroup identification, serotyping by agglutination reactions against antisera developed for each of the O serogroups has been used. In the last decade, many O-antigen gene clusters that encode for the enzymes responsible for the synthesis of the variable oligosaccharide region on the surface of the bacteria have been sequenced and characterized. Unique gene sequences within the O-antigen gene clusters have been targeted for identification and detection of many O groups using the polymerase chain reaction and microarrays. This review summarizes current knowledge on the DNA sequences of the O-antigen gene clusters, genetic-based methods for O-group determination and detection of pathogenic E. coli based on O-antigen and virulence gene detection, and provides perspectives on future developments in the field.

Structure elucidation of the O-Antigen of Salmonella enterica O51 and its structural and genetic relation to the O-Antigen of Escherichia coli O23

The O-polysaccharide (O-antigen) of Salmonella enterica O51 was isolated by mild acid degradation of the lipopolysaccharide and its structure was established using sugar analysis and NMR spectroscopy. The O-antigen of Escherichia coli O23, whose structure was elucidated earlier, possesses a similar structure and differs only in the presence of an additional lateral α-D-Glcp residue at position 6 of the GlcNAc residue in the main chain. Sequencing of the O-antigen gene clusters of S. enterica O51 and E. coli O23 revealed the same genes with a high-level similarity. By comparison with opened gene databases, all genes expected for the synthesis of the common structure of the two O-antigens were assigned functions. It is suggested that the gene clusters of both bacteria originated from a common ancestor, whereas the O-antigen modification in E. coli O23, which, most probably, is induced by prophage genes outside the gene cluster, could be introduced after the species divergence.

Immunochemical studies of Salmonella Dakar and Salmonella Telaviv O-antigens (serogroup O:28)

Salmonella Dakar and Salmonella Telaviv bacteria belong to serogroup O:28, which represents 107 serovars and possesses only the epitope O28. Salmonella Telaviv has the subfactors O28(1) and O28(2) , whereas S. Dakar has O28(1) and O28(3) . So far, only limited serological and immunological information for this serogroup is available in the literature. Knowledge of the structures of their O-polysaccharides and the immunochemical investigations performed in this work allowed to reveal the nature of subfactor O28(1) as attributed to the presence of 3-linked (or 3,4-disubstituted) α-d-GalpNAc in the main chains of S. Dakar and S. Telaviv O-polysaccharides. An explanation for the cross-reactions between Salmonella enterica O28 O-antigens and other Salmonella O-polysaccharides and their structural similarity to Escherichia coli O-serogroups is also given.

Molecular characterization of Cronobacter lipopolysaccharide O-antigen gene clusters and development of serotype-specific PCR assays

Cronobacter (formerly Enterobacter sakazakii) is a recently defined genus consisting of six species, C. sakazakii, C. malonaticus, C. dublinensis, C. muytjensii, C. turicensis, and Cronobacter genomospecies 1. In this study, MboII restriction fragment length polymorphism (RFLP) patterns of O-antigen gene clusters, located between galF and gnd, were used to identify serotypes in Cronobacter spp. Seven O-antigen RFLP clusters were generated, including three C. sakazakii clusters, previously identified as serotypes O1, O2, and O3. The O-antigen regions of six strains with unique RFLP patterns, including two C. sakazakii strains, two C. malonaticus strains, one C. turicensis strain, and one C. muytjensii strain, revealed three O-antigen gene clusters shared among Cronobacter species. PCR assays were developed, targeting the wzx O-antigen polymerase gene, and used to screen 231 Cronobacter strains to determine the frequency of these newly identified serotypes.