- Volume 166, Issue 11, 2020

Expansins, cerato-platanins and swollenins (which we will henceforth refer to as expansin-related proteins) are a group of microbial proteins involved in microbe-plant interactions. Although they share very low sequence similarity, some of their composing domains are near-identical at the structural level. Expansin-related proteins have their target in the plant cell wall, in which they act through a non-enzymatic, but still uncharacterized, mechanism. In most cases, mutagenesis of expansin-related genes affects plant colonization or plant pathogenesis of different bacterial and fungal species, and thus, in many cases they are considered virulence factors. Additionally, plant treatment with expansin-related proteins activate several plant defenses resulting in the priming and protection towards subsequent pathogen encounters. Plant-defence responses induced by these proteins are reminiscent of pattern-triggered immunity or hypersensitive response in some cases. Plant immunity to expansin-related proteins could be caused by the following: (i) protein detection by specific host-cell receptors, (ii) alterations to the cell-wall-barrier properties sensed by the host, (iii) displacement of cell-wall polysaccharides detected by the host. Expansin-related proteins may also target polysaccharides on the wall of the microbes that produced them under certain physiological instances. Here, we review biochemical, evolutionary and biological aspects of these relatively understudied proteins and different immune responses they induce in plant hosts.

A formylglycine-generating enzyme (FGE)–sulfatase-based whole-cell biosensor was genetically improved into a single-copy system by integrating the Sinorhizobium meliloti transcriptional activator ChpR and the chpA promoter–FGE–sulfatase fusion into the Escherichia coli chromosome. The sensitivity was further enhanced through a random mutagenesis of the chpR. The new integrated biosensor offered both a lower detection limit [5 nM chlorpyrifos (CPF)] and fluorescence background. The ready-to-use kit was developed using silica gel for on-field detection. The biosensor kit was stable for 20 days when stored at 4 °C. Moreover, a 1-(1-naphthylmethyl)-piperazine (NMP) efflux pump inhibitor can improve the sensitivity by 57 %.

Vibrio cholerae, the aetiological agent of cholera, possesses multiple iron acquisition systems, including those for the transport of siderophores. How these systems benefit V. cholerae in low-iron, polymicrobial communities in environmental settings or during infection remains poorly understood. Here, we demonstrate that in iron-limiting conditions, co-culture of V. cholerae with a number of individual siderophore-producing microbes significantly promoted V. cholerae growth in vitro. We further show that in the host environment with low iron, V. cholerae colonizes better in adult mice in the presence of the siderophore-producing commensal Escherichia coli . Taken together, our results suggest that in aquatic reservoirs or during infection, V. cholerae may overcome environmental and host iron restriction by hijacking siderophores from other microbes.

Homologous recombination plays key roles in fundamental processes such as recovery from DNA damage and in bacterial horizontal gene transfer, yet there are still open questions about the distribution of recognized components of recombination machinery among bacteria and archaea. RecBCD helicase-nuclease plays a central role in recombination among Gammaproteobacteria like Escherichia coli ; while bacteria in other phyla, like the Firmicute Bacillus subtilis , use the related AddAB complex. The activity of at least some of these complexes is controlled by short DNA sequences called crossover hotspot instigator (Chi) sites. When RecBCD or AddAB complexes encounter an autologous Chi site during unwinding, they introduce a nick such that ssDNA with a free end is available to invade another duplex. If homologous DNA is present, RecA-dependent homologous recombination is promoted; if not (or if no autologous Chi site is present) the RecBCD/AddAB complex eventually degrades the DNA. We examined the distribution of recBCD and addAB genes among bacteria, and sought ways to distinguish them unambiguously. We examined bacterial species among 33 phyla, finding some unexpected distribution patterns. RecBCD and addAB are less conserved than recA, with the orthologous recB and addA genes more conserved than the recC or addB genes. We were able to classify RecB vs. AddA and RecC vs. AddB in some bacteria where this had not previously been done. We used logo analysis to identify sequence segments that are conserved, but differ between the RecBC and AddAB proteins, to help future differentiation between members of these two families.

Sphingomyelinases produced by the pathogenic members of the genus Leptospira are implicated in the haemorrhagic manifestations seen in the severe form of leptospirosis. With multiple sphingomyelinase genes present in the genome of pathogenic Leptospira , much remains to be understood about these molecules. They include factors regulating their expression, post-translational modifications, and release of the biologically active forms of these molecules. In this study, serovar Pomona was chosen as it is reported to express high levels of sphingomyelinase that explained the haemolytic activity seen in experimental animals infected with this pathogen. Here, we demonstrate the cytotoxicity of a 42 kDa sphingomyelinase secreted by Leptospira interrogans serovar Pomona strain Pomona upon infecting Vero cells. This sphingomyelinase detected using specific anti-sphingomyelinase antibodies, exhibited haemolytic and sphingomyelinase activities that caused host-cell damage evident from the confocal images and scanning electron micrographs. The implications of these findings and the detection of a 42 kDa sphingomyelinase in the urine of human patients with leptospirosis in our earlier study is discussed with an emphasis on the potential of these sphingomyelinases as candidate markers for the early diagnosis of leptospirosis.

Antibiotic resistance in Pseudomonas aeruginosa is a serious concern in healthcare systems. Among the determinants of antibiotic resistance in P. aeruginosa , efflux pumps belonging to the resistance–nodulation–division (RND) family confer resistance to a broad range of antibacterial compounds. The MexXY efflux system is widely overexpressed in P. aeruginosa isolates from cystic fibrosis (CF) patients. MexXY can form functional complexes with two different outer membrane factors (OMFs), OprA and OprM. In this study, using state-of-the-art genetic tools, the substrate specificities of MexXY–OprA and MexXY–OprM complexes were determined. Our results show, for the first time, that the substrate profile of the MexXY system from P. aeruginosa PA7 can vary depending on which OM factor (OprM or OprA) it complexes with. While both MexXY–OprA and MexXY–OprM complexes are capable of effluxing aminoglycosides, the bi-anionic β-lactam molecules carbenicillin and sulbenicillin were found to only be the substrate of MexXY–OprA. Our study therefore shows that by partnering with different OMF proteins MexY can expand its substrate profile.