Large-scale preparation of bacterial cell membranes by tangential flow filtration

Development of ELISA-based diagnostic methods for the detection of haemorrhagic septicaemia in animals

Haemorrhagic septicaemia (HS) is an acute infection of cattle and buffaloes caused by the B:2 serotype of Pasteurella multocida. This disease is highly endemic in South Asia. In some peracute cases, there is 100% mortality in infected animals within a few hours of infection. Therefore, timely diagnosis of infection may contribute to its treatment and control to minimize economic losses. The current work reported the development of ELISA-based assays for the detection of anti-P. multocida antibodies and pathogen i.e. P. multocida. Owing to high immunogenicity, membrane proteins (MPs) extracted from local isolates of P. multocida serotype B:2 (PM1, PM2, and PM3) were employed as a potential diagnostic antigen for the development of indirect ELISA (i-ELISA) to detect HS antibodies in animals. MPs extracted from PM1, PM2 and PM3 isolates showed very low heterogeneity; hence MPs from the PM3 isolate were selected for the development of i-ELISA. The concentration of MPs (as coating antigen) of 3.13 μg/well and test sera dilution 1:100 was found to be optimal to perform i-ELISA. The developed method was validated through the detection of anti-P. multocida antibodies in sera of mice, immunized with MPs and formalin killed cells from the three local isolates (PM1, PM2 and PM3) of P. multocida. The significantly higher antibody titer in immunized mice was determined compared to unimmunized mice with the cut off value of 0.139. To detect P. multocida directly from the blood of infected animals, whole cell-based ELISA (cb-ELISA) assay was developed. A better detection signal was observed in the assay where bacterial cells were directly adsorbed on plate wells as compared to poly L-lysine (PLL) assisted attachment at a cell concentration of 10⁶ CFU and 10⁷ CFU respectively. The developed assays can be scaled up and potentially be used for the rapid detection of HS antibodies to gauge the immune status of the animal as well as vaccination efficacy and pathogen detection.

Cryo-EM Structure and Molecular Dynamics Analysis of the Fluoroquinolone Resistant Mutant of the AcrB Transporter from Salmonella

Salmonella is an important genus of Gram-negative pathogens, treatment of which has become problematic due to increases in antimicrobial resistance. This is partly attributable to the overexpression of tripartite efflux pumps, particularly the constitutively expressed AcrAB-TolC. Despite its clinical importance, the structure of the Salmonella AcrB transporter remained unknown to-date, with much of our structural understanding coming from the Escherichia coli orthologue. Here, by taking advantage of the styrene maleic acid (SMA) technology to isolate membrane proteins with closely associated lipids, we report the very first experimental structure of Salmonella AcrB transporter. Furthermore, this novel structure provides additional insight into mechanisms of drug efflux as it bears the mutation (G288D), originating from a clinical isolate of Salmonella Typhimurium presenting an increased resistance to fluoroquinolones. Experimental data are complemented by state-of-the-art molecular dynamics (MD) simulations on both the wild type and G288D variant of Salmonella AcrB. Together, these reveal several important differences with respect to the E. coli protein, providing insights into the role of the G288D mutation in increasing drug efflux and extending our understanding of the mechanisms underlying antibiotic resistance.

A urea channel from Bacillus cereus reveals a novel hexameric structure

Urea is exploited as a nitrogen source by bacteria, and its breakdown products, ammonia and bicarbonate, are employed to counteract stomach acidity in pathogens such as Helicobacter pylori. Uptake in the latter is mediated by UreI, a UAC (urea amide channel) family member. In the present paper, we describe the structure and function of UACBc, a homologue from Bacillus cereus. The purified channel was found to be permeable not only to urea, but also to other small amides. CD and IR spectroscopy revealed a structure comprising mainly α-helices, oriented approximately perpendicular to the membrane. Consistent with this finding, site-directed fluorescent labelling indicated the presence of seven TM (transmembrane) helices, with a cytoplasmic C-terminus. In detergent, UACBc exists largely as a hexamer, as demonstrated by both cross-linking and size-exclusion chromatography. A 9 Å (1 Å=0.1 nm) resolution projection map obtained by cryo-electron microscopy of two-dimensional crystals shows that the six protomers are arranged in a planar hexameric ring. Each exhibits six density features attributable to TM helices, surrounding a putative central channel, while an additional helix is peripherally located. Bioinformatic analyses allowed individual TM regions to be tentatively assigned to the density features, with the resultant model enabling identification of residues likely to contribute to channel function.