Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule.

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin-orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Efficiency of charge transfer in changing the dissociation dynamics of OD+ transients formed after the photo-fragmentation of D2O.

We present an investigation of the relaxation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV. We focus on the very rare D+ + O+ + D reaction channel in which the sequential fragmentation mechanisms were found to dominate the dynamics. Aided by theory, the state-selective formation and breakup of the transient OD+(a1Δ, b1Σ+) is traced, and the most likely dissociation path-OD+: a1Δ or b1Σ+ → A 3Π â†’ X 3Σ- → B 3Σ--involving a combination of spin-orbit and non-adiabatic charge transfer transitions is determined. The multi-step transition probability of this complex transition sequence in the intermediate fragment ion is directly evaluated as a function of the energy of the transient OD+ above its lowest dissociation limit from the measured ratio of the D+ + O+ + D and competing D+ + D+ + O sequential fragmentation channels, which are measured simultaneously. Our coupled-channel time-dependent dynamics calculations reproduce the general trends of these multi-state relative transition rates toward the three-body fragmentation channels.

Hypochlorous acid-generating electrochemical scaffold eliminates Candida albicans biofilms.

AIMS: Wound infections involving Candida albicans can be challenging to treat because of the fungus' ability to penetrate wound tissue and form biofilms. The goal of this study was to assess the activity of a hypochlorous acid (HOCl)-generating electrochemical scaffold (e-scaffold) against C. albicans biofilms in vitro and on porcine dermal explants (ex vivo). METHODS AND RESULTS: C. albicans biofilms were grown either on acrylic-bottom six-well plates (in vitro) or on skin tissue excised from porcine ears (ex vivo), and the polarized e-scaffold was used to generate a continuous supply of low concentration HOCl near biofilm surfaces. C. albicans biofilms grown in vitro were reduced to undetectable amounts within 24 h of e-scaffold exposure, unlike control biofilms (5·28 ± 0·034 log10  (CFU cm- 2 ); P < 0·0001). C. albicans biofilms grown on porcine dermal explants were also reduced to undetectable amounts in 24 h, unlike control explant biofilms (4·29 ± 0·057 log10 (CFU cm- 2 ); P < 0·0001). There was a decrease in the number of viable mammalian cells (35·6 ± 6·4%) in uninfected porcine dermal explants exposed to continuous HOCl-generating e-scaffolds for 24 h compared to explants exposed to nonpolarized e-scaffolds (not generating HOCl) (P < 0·05). CONCLUSIONS: Our HOCl-generating e-scaffold is a potential antifungal-free strategy to treat C. albicans biofilms in chronic wounds. SIGNIFICANCE AND IMPACT OF THE STUDY: Wound infections caused by C. albicans are difficult to treat due to presence of biofilms in wound beds. Our HOCl producing e-scaffold provides a promising novel approach to treat wound infections caused by C. albicans.

Microcin MccPDI reduces the prevalence of susceptible Escherichia coli in neonatal calves.

AIMS: Microcin MccPDI-producing Escherichia coli have a fitness advantage in dairy calves. For this project, we determined whether MccPDI is responsible for the in vivo fitness advantage, which is a necessary condition before MccPDI strains can be considered viable candidates for inhibiting pathogenic serovars of E. coli. METHODS AND RESULTS: Neonatal calves were coinoculated with either MccPDI-producing E. coli or MccPDI-knockout mutants in conjunction with a susceptible strain. After 6 days, the MccPDI-producing E. coli-25 strain clearly dominated the E. coli-186 susceptible strain in the inoculated calves (P = 0·003). MccPDI-producing E. coli composed a higher log percentage of the total population of lactose-fermenting bacteria in the faeces (5·51 log CFU per 8·03 log CFU) compared with the knockout strain (2·6 log CFU per 8·23 log CFU) (P = 0·01), and it was more consistently recovered from the lower gastrointestinal tract at the time of necropsy (P = 0·01). CONCLUSION: Our findings support the hypothesis that MccPDI is functional in vivo and it is most likely responsible for a fitness advantage in vivo. SIGNIFICANCE AND IMPACT OF THE STUDY: MccPDI-producing E. coli strongly inhibit pathogenic E. coli strains in vitro. We show herein that MccPDI functions in vivo, and thus, these strains may be candidate probiotics against pathogenic strains of E. coli.

Informal sale of antibiotics in Guatemalan convenience stores before and after implementation of federal antibiotic dispensing legislation.

BACKGROUND: Convenience stores in Guatemala provide essential consumer goods in communities, but many dispense antibiotics illegally. Federal legislation, passed in August of 2019, requires prescriptions for antibiotic purchase at pharmacies but it is unclear if this legislation is enforced or if it has any impact on unlawful sales of antibiotics. METHODS: To determine if antibiotic availability changed in convenience stores, we carried out a repeated measures study collecting antibiotic availability data before and after implementation of the dispensing regulation. RESULTS: There was no statistical difference in the proportion of convenience stores that sold antibiotics before and after antibiotic regulations [66.6% (295/443) and 66.7% (323/484), respectively, P>0.96], nor in the number of stores selling amoxicillin [55.5% (246/443) and 52.3% (253/484), respectively, P>0.96], but fewer stores (20%) sold tetracycline capsules after regulation was passed (P<0.05). For stores visited both before and after passage of legislation (n=157), 15% stopped selling antibiotics while 25% started selling antibiotics. Antibiotics from convenience stores were reportedly sold for use in people and animals. CONCLUSIONS: Antibiotics remain widely available in convenience stores consistent with no significant change in the informal sector after implementation of prescription requirements for pharmacies. Importantly, effects from regulatory change could have been masked by potential changes in antibiotic use during the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic.

Factors associated with hand hygiene adherence among healthcare workers in Guatemala during the COVID-19 pandemic.

BACKGROUND: Healthcare-associated infections are prevalent in low- and middle-income countries and may be reduced through proper hand hygiene (HH) adherence during patient care. AIM: We produced and distributed alcohol-based hand rub (ABHR) to 19 public primary- and secondary-level healthcare facilities in Quetzaltenango, Guatemala, and carried out HH observations to assess healthcare workers' (HCWs) HH adherence, and to identify factors associated with this practice. HH adherence was defined as washing hands with soap and water or using ABHR. METHODS: Observations were conducted before (2021, baseline) and after (2022, follow-up) ABHR distribution to evaluate the evolution of HH practices over time. Bivariate comparisons and mixed-effects logistic regression models were used to explore associations between HH adherence and the following independent variables: healthcare facility level, type of contact performed, timing of HH performance, occupational category of HCW and materials present (e.g., water, soap, ABHR). FINDINGS: We observed 243 and 300 patient interactions among 67 and 82 HCWs at each time point, respectively. HH adherence was low for both observation periods (40% at baseline and 35% at follow-up). HCWs were more likely to adhere to HH during invasive contacts, after patient contact, and if the HCW was a physician. CONCLUSION: HH adherence varied by scenario, which underscores the importance of addressing multiple determinants of behaviour change to improve adherence. This requires interventions implemented with a multi-modal approach that includes both increasing access to HH materials and infrastructure, as well as HH education and training, monitoring and feedback, reminders, and promoting a HH safety culture.

Validation of diagnostic assays to screen broodstock for Flavobacterium psychrophilum infections.

It is hypothesized that the frequency of bacterial coldwater disease outbreaks can be reduced through the detection of the aetiologic agent, Flavobacterium psychrophilum, in broodstock followed by culling of eggs from heavily infected broodstock. Before a culling programme can be instituted, however, it is necessary to determine the sensitivity and specificity of existing assays for the detection of F. psychrophilum. In this study, tissue and ovarian fluid samples were collected from 224 fish at five hatcheries and screened using an enzyme-linked immunosorbent assay (ELISA), a membrane-filtration fluorescent antibody test (MF-FAT), bacteriological culture and nested PCR. Latent class analysis was used to estimate sensitivity and specificity of kidney culture, kidney ELISA, nested PCR and MF-FAT. Analytical sensitivity of the ELISA varied but was greatest when bacteria were cultured under iron-limiting conditions. Diagnostic sensitivity estimates ranged from 0.02 (kidney culture) to 0.97 (kidney ELISA). Specificity estimates ranged from 0.02 (MF-FAT) to 0.98 (kidney ELISA). In a separate challenge experiment, the ELISA confirmed the presence of F. psychrophilum in sub-clinically infected fish. Results from this study demonstrate that the ELISA is an appropriate tool to screen broodstock and provides an indication of infection severity, which is crucial for implementation of a screening/culling programme.

Comparative proteomic analysis of virulent and rifampicin-attenuated Flavobacterium psychrophilum.

Flavobacterium psychrophilum is the aetiologic agent of bacterial coldwater disease and rainbow trout fry syndrome. In this study, we compared a wild-type strain (CSF 259-93) with a rifampicin-resistant strain and virulence-attenuated strain of F. psychrophilum (CSF 259-93B.17). The attenuated strain harboured a mutation in the rpoB gene consistent with resistance to rifampicin. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry demonstrated an altered proteome with eight proteins characteristic for the parent strain and six that were unique to the attenuated strain. Immunoblotting with a diagnostic monoclonal antibody (FL-43) identified a putative antigen (FP1493) that was subsequently cloned, expressed as a recombinant protein and confirmed as recognized by FL-43. 2D-PAGE, immunoblotting with rainbow trout, Oncorhynchus mykiss (Walbaum), convalescent antisera and mass spectrometry of bacterial whole-cell lysates revealed several uniquely expressed immunoreactive proteins including FP1493. An FP1493 recombinant subunit vaccine was tested, but did not provide protection against challenge with the CSF259-93 strain. While the exact mechanism responsible for altered protein synthesis and attenuation of CSF 259-93B.17 is still unknown, the differentially expressed immunoreactive proteins are a valuable resource to develop subunit vaccines and to identify proteins that are potentially involved in disease.

Quantitative PCR demonstrates a positive correlation between a Rickettsia-like organism and severity of strawberry disease lesions in rainbow trout, Oncorhynchus mykiss (Walbaum).

Strawberry disease (SD) is an inflammatory skin disorder in rainbow trout, Oncorhynchus mykiss (Walbaum). The aetiology of SD is unknown although the 16S rDNA sequence of a Rickettsia-like organism (RLO) has been associated with SD lesions using a nested PCR assay. In this study, we developed a Taqman quantitative PCR assay (qPCR) that targeted the RLO 16S rDNA sequence to examine the distribution of RLO relative to lesion status. We compared 18 lesion samples from 13 fish representing high or low lesion severity as judged by gross examination. QPCR results showed that there was a higher number of RLO sequences in high severity lesions (mean of 12,068 copies) compared with fewer copies of RLO sequence in low severity lesions (mean of 3287 copies, P = 0.012). Grossly normal skin samples (n = 13) from SD-affected fish were all negative by qPCR except two samples (121 and 139 copies). The qPCR assay described herein is a useful tool to investigate the role of RLO in SD in the absence of a culture system for RLO. Our results demonstrate a positive correlation between copy number and lesion severity consistent with the hypothesis that the RLO is the aetiologic agent of SD.

Identification of immunogenic proteins within distinct molecular mass fractions of Flavobacterium psychrophilum.

Flavobacterium psychrophilum is the aetiological agent of bacterial coldwater disease (CWD), and this pathogen has large economic impacts on salmonid aquaculture worldwide. Previously, it was demonstrated that high levels of protection against F. psychrophilum challenge were conferred to rainbow trout, Oncorhynchus mykiss (Walbaum), by immunization with distinct molecular mass fractions of the bacterium, and specific antibodies were correlated with protection. In this study, an immunoproteomic analysis of F. psychrophilum was performed using two-dimensional polyacrylamide gel electrophoresis and Western blotting with serum from fish immunized with high- and mid-molecular mass fractions of the bacterium. Mass spectrometry was used to determine the protein identity, and 15 immunogenic proteins were positively identified following Mascot searches of the F. psychrophilum genome. Based on known function and immunogenicity of homologous proteins in other bacterial pathogens, antibodies specific for several of the identified proteins may be important for protective immunity from CWD. These include outer membrane protein OmpA (P60), trigger factor, ClpB, elongation factor G, gliding motility protein GldN and a conserved hypothetical protein. This work increases the understanding of the protective humoral immune response of rainbow trout against these distinct molecular mass fractions of F. psychrophilum and provides new potential targets for recombinant protein vaccine development.

Improved identification of epidemiologically related strains of Salmonella enterica by use of a fusion algorithm based on pulsed-field gel electrophoresis and multiple-locus variable-number tandem-repeat analysis.

Pulsed-field gel electrophoresis (PFGE) and multiple-locus variable-number tandem-repeat analysis (MLVA) are used to assess genetic similarity between bacterial strains. There are cases, however, when neither of these methods quantifies genetic variation at a level of resolution that is well suited for studying the molecular epidemiology of bacterial pathogens. To improve estimates based on these methods, we propose a fusion algorithm that combines the information obtained from both PFGE and MLVA assays to assess epidemiological relationships. This involves generating distance matrices for PFGE data (Dice coefficients) and MLVA data (single-step stepwise-mutation model) and modifying the relative distances using the two different data types. We applied the algorithm to a set of Salmonella enterica serovar Typhimurium isolates collected from a wide range of sampling dates, locations, and host species. All three classification methods (PFGE only, MLVA only, and fusion) produced a similar pattern of clustering relative to groupings of common phage types, with the fusion results being slightly better. We then examined a group of serovar Newport isolates collected over a limited geographic and temporal scale and showed that the fusion of PFGE and MLVA data produced the best discrimination of isolates relative to a collection site (farm). Our analysis shows that the fusion of PFGE and MLVA data provides an improved ability to discriminate epidemiologically related isolates but provides only minor improvement in the discrimination of less related isolates.

Association of red-mark syndrome with a Rickettsia-like organism and its connection with strawberry disease in the USA.

Red-mark syndrome (RMS), a disease seen mostly in rainbow trout, Oncorhynchus mykiss, is of unknown aetiology. The research presented here indicates the presence of an intracellular bacterium in RMS-affected fish. A positive reaction was observed using immunohistochemistry (IHC) with skin lesions, liver, kidney and spleen of affected fish sampled from several locations within the United Kingdom using two different polyclonal antisera raised against Piscirickettsia salmonis. The same reaction was also seen with a number of different anti-P. salmonis monoclonal antibodies (MAbs). A disease with similar clinical signs to RMS, referred to as strawberry disease (SD), has been reported in the USA. A Rickettsia-like organism (RLO) has recently been associated with SD based on analysis of 16S rDNA sequences. Using the same panel of anti-P. salmonis antibodies used to screen the RMS samples, similar staining was obtained in tissue of SD-affected fish by IHC. A polymerase chain reaction (PCR) using RLO-specific primers was also performed on RMS-affected fish from the United Kingdom, and the samples were positive for the RLO 16S rRNA sequence. These findings suggest that the same aetiological agent may be responsible for RMS in the United Kingdom and SD in the USA.

Proteomic analysis of Flavobacterium psychrophilum cultured in vivo and in iron-limited media.

Flavobacterium psychrophilum is the etiologic agent of bacterial coldwater disease, but the pathogenic mechanisms of this important fish pathogen are not fully understood. Identifying bacterial genes of F. psychrophilum differentially expressed in vivo may lead to a better understanding of pathogenesis and provide targets for vaccine development. Therefore, the present study used a proteomic approach to identify and quantify proteins of F. psychrophilum following growth in vivo and under iron-limited growth conditions. As determined by 2D polyacrylamide gel electrophoresis (2D-PAGE), numerous proteins exhibited different spot intensities following culture of the bacterium in vivo, and of these, 20 were selected and identified by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) analysis and Mascot searches of the F. psychrophilum genome. Eighteen proteins exhibited increased spot intensities in vivo, and these included: several chaperone and stress proteins, gliding motility protein GldN, outer membrane protein OmpH, 2 probable outer membrane proteins (OmpA family), probable aminopeptidase precursor, probable lipoprotein precursor, 3-oxoacyl-[acyl-carrier-protein]-reductase, and several proteins with unknown function. Two proteins exhibited decreased spot intensities in vivo and were identified as ferritin FtnA and outer membrane protein OmpA (P60). Culture of F. psychrophilum in iron-limited media resulted in similar protein spot intensity changes for 6 of the 20 proteins identified following growth in vivo. Results from the present study suggest a role of upregulated proteins in the pathogenesis of F. psychrophilum and these may represent potential vaccine candidate antigens.

Genetic diversity of Flavobacterium psychrophilum recovered from commercially raised rainbow trout, Oncorhynchus mykiss (Walbaum), and spawning coho salmon, O. kisutch (Walbaum).

Flavobacterium psychrophilum is the aetiological agent of rainbow trout fry syndrome and bacterial cold water disease. This study examined the genetic diversity of F. psychrophilum isolates retrieved from multiple epizootics at rainbow trout, Oncorhynchus mykiss, rearing facilities and from spawning coho salmon, O. kisutch. A total of 139 isolates were confirmed as F. psychrophilum by PCR assay and were further typed using pulsed-field gel electrophoresis (PFGE). Multiple epizootics at three proximally located rainbow trout rearing facilities were numerically dominated by three PFGE profiles, which accounted for 76% of all trout isolates. In coho salmon, 19 PFGE profiles were differentiated by PFGE and four numerically dominant PFGE profiles represented 56% of all coho salmon isolates. PFGE analysis also indicated that the average similarity of macrorestriction patterns of F. psychrophilum isolates was greater in rainbow trout than in coho salmon (88% vs. 70%). Furthermore, it was not unusual to isolate multiple PFGE profiles from a single coho salmon sample whereas only two PFGE profiles were shared between two sample dates separated by 1 month. It is clear that the domestic rainbow trout aquaculture facilities studied here were primarily affected by a complex of genetically related strains whereas spawning coho salmon supported a much more genetically diverse collection of F. psychrophilum.

Assessment and promotion of judicious antibiotic use on dairy farms in Washington State.

The aims of this study were to describe antibiotic use and biosecurity practices among Washington State dairy producers and to evaluate the effectiveness of a collaborative approach to promoting judicious antibiotic use on dairy farms. In collaboration with a statewide industry group, Washington State dairy producers participated in a written, self-administered survey in 2003. They were then provided several educational interventions followed by a second written survey in 2005. Sixty-five percent (381) of dairy producers completed the 2003 survey. The most commonly cited drugs used for disease treatment were penicillin, ceftiofur, and oxytetracycline. Participants also indicated significant preventive uses with 28% using medicated milk replacer. Most producers appeared to consider intramammary infusion at dry-off to be a treatment rather than a preventative practice. Twenty-three percent of initial respondents indicated at least one extra-label use of antibiotics, yet only half routinely consulted with a veterinarian when doing so. Most agreed that using written protocols for disease treatment could reduce errors, but less than one-third had protocols. After the educational intervention there was a tendency toward reduced use of antibiotic medicated milk replacer: 51% of producers who originally reported using medicated milk replacer discontinued this practice, whereas 12% of producers began using medicated milk replacer between the 2003 and 2005 surveys. Most reported that the resources and educational materials were useful. Areas where additional work is needed include reducing the use of medicated milk replacer, increasing veterinary involvement in antibiotic use decisions, implementing treatment protocols, enhancing biosecurity, and ensuring optimal cow and calf immunity.

A sandwich enzyme-linked immunoabsorbent assay for measurement of picogram quantities of murine granulocyte colony-stimulating factor.

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation and differentiation of hematopoietic progenitor cells of the neutrophil lineage. Measurement of murine G-CSF levels will allow examination of its role in host defense using murine models. Therefore, we developed a sensitive sandwich enzyme-linked immunoabsorbent assay (ELISA) for murine G-CSF. A polyclonal antibody to recombinant murine G-CSF was produced in rabbits and isolated using a protein A column. This purified native IgG served as the capture antibody and a portion of the IgG was biotinylated to serve as the developing antibody. Specificity was verified by lack of reactivity to GM-CSF, IL-6, IL-3, prolactin, and growth hormone. The lower limit of sensitivity routinely extended to 16 pg/ml in multiple ELISAs. Intra-assay coefficient of variation (CV) ranged from 3.4 to 21.5% across the detection limits of the assay, with the greatest variance occurring near the standard curve maximum. Interassay CV ranged from 11.5 to 23.3%. The ability of the ELISA to detect G-CSF in different sample preparations was examined in RPMI 1640 with 10% FCS, Hanks balanced salt solution, PBS/Tween-20/2% FCS, and the dilution media for ELISA (10% BLOTTO/PBS/0.05% Tween-20). Average recovery in these media ranged from 98 to 107%. Heparin anti-coagulated normal mouse plasma had a suppressive effect on the ELISA that varied between individual mice. Recovery was also determined from liver, spleen, and lung homogenate suspensions at dilutions of 1:5, 1:10, and 1:20 in dilution buffer. Recovery from liver was optimal at the 1:10 and 1:20 dilutions at 105%, with that of the 1:5 dilution at 135%. Recovery from spleen ranged from 94 to 96%. Lung homogenate displayed enhanced recovery (139% or greater) across all dilutions. The ability of the assay to detect G-CSF was explored by measurement of G-CSF levels in peritoneal lavage following polymicrobial intra-abdominal infection. Peak levels of G-CSF production occurred at 16 h after cecal ligation and puncture surgery with 18- and 21-guage needles (75.7 ng/ml and 111.4 ng/ml, respectively) as compared to the sham animals (0.61 ng/ml). The assay was found to be specific, sensitive, and accurate for measurement of murine G-CSF in a variety of sample types.

Fabrication of DNA microarrays using unmodified oligonucleotide probes.

Microarrays printed on glass slides are often constructed by covalently linking oligonucleotide probes to a derivatized surface. These procedures typically require relatively expensive amine- or thiol-modified oligonucleotide probes that add considerable expense to larger arrays. We describe a system by which unmodified oligonucleotide probes are bound to either nonderivatized or epoxy-silane-derivatized glass slides. Biotinylated PCR products are heat denatured, hybridized to the arrays, and detected using an enzymatic amplification system. Unmodified probes appear to detach from the slide surface at high pH (> 10.0), suggesting that hydrogen bonding plays a significant role in probe attachment. Regardless of surface preparation, high temperature (up to 65 degrees C) and low ionic strength (deionized water) do not disturb probe attachment; hence, the fabrication method described here is suitable for a wide range of hybridization stringencies and conditions. We illustrate kinetics of room temperature hybridizations for probes attached to nonderivatized slides, and we demonstrate that unmodified probes produce hybridization signals equal to amine-modified, covalently bound probes. Our method provides a cost-effective alternative to conventional attachment strategies that is particularly suitable for genotyping PCR products with nucleic acid microarrays.

A QSAR evaluation of Ah receptor binding of halogenated aromatic xenobiotics.

Because of their widespread occurrence and substantial biological activity, halogenated aromatic hydrocarbons such as polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs) comprise one of the more important classes of contaminants in the environment. Some chemicals in this class cause adverse biological effects after binding to an intracellular cytosolic protein called the aryl hydrocarbon receptor (AhR). Toxic responses such as thymic atrophy, weight loss, immunotoxicity, and acute lethality, as well as induction of cytochrome P4501A1, have been correlated with the relative affinity of PCBs, PCDFs, and PCDDs for the AhR. Therefore, an important step in predicting the effects of these chemicals is the estimation of their binding to the receptor. To date, however, the use of quantitative structure activity relationship (QSAR) models to estimate binding affinity across multiple chemical classes has shown only modest success possibly due, in part, to a focus on minimum energy chemical structures as the active molecules. In this study, we evaluated the use of structural conformations other than those of minimum energy for the purpose of developing a model for AhR binding affinity that encompasses more of the halogenated aromatic chemicals known to interact with the receptor. Resultant QSAR models were robust, showing good utility across multiple classes of halogenated aromatic compounds.