- Volume 168, Issue 2, 2022

The polymyxin and lipopeptide classes of antibiotics are membrane-targeting drugs of last resort used to treat infections caused by multi-drug-resistant pathogens. Despite similar structures, these two antibiotic classes have distinct modes of action and clinical uses. The polymyxins target lipopolysaccharide in the membranes of most Gram-negative species and are often used to treat infections caused by carbapenem-resistant species such as   Escherichia coli  ,   Acinetobacter baumannii   and   Pseudomonas aeruginosa  . By contrast, the lipopeptide daptomycin requires membrane phosphatidylglycerol for activity and is only used to treat infections caused by drug-resistant Gram-positive bacteria such as methicillin-resistant   Staphylococcus aureus   and vancomycin-resistant enterococci. However, despite having distinct targets, both antibiotic classes cause membrane disruption, are potently bactericidal in vitro and share similarities in resistance mechanisms. Furthermore, there are concerns about the efficacy of these antibiotics, and there is increasing interest in using both polymyxins and daptomycin in combination therapies to improve patient outcomes. In this review article, we will explore what is known about these distinct but structurally similar classes of antibiotics, discuss recent advances in the field and highlight remaining gaps in our knowledge.

  Geobacter metallireducens   has served as the initial model for a substantial number of newly recognized microbial physiologies that play an important role in biogeochemical cycling of carbon, metals and nutrients. The strategies used by   G. metallireducens   for microbial interaction with minerals, contaminants, other microbes and electrodes have led to new technologies for bioremediation, bioenergy conversion and the sustainable production of ‘green’ electronics.

Transferable linezolid resistance due to optrA, poxtA, cfr and cfr-like genes is increasingly detected in enterococci associated with animals and humans globally. We aimed to characterize the genetic environment of optrA in linezolid-resistant Enterococcus faecalis isolates from Scotland. Six linezolid-resistant E. faecalis isolated from urogenital samples were confirmed to carry the optrA gene by PCR. Short read (Illumina) sequencing showed the isolates were genetically distinct (>13900 core SNPs) and belonged to different MLST sequence types. Plasmid contents were examined using hybrid assembly of short and long read (Oxford Nanopore MinION) sequencing technologies. The optrA gene was located on distinct plasmids in each isolate, suggesting that transfer of a single plasmid did not contribute to optrA dissemination in this collection. pTM6294-2, BX5936-1 and pWE0438-1 were similar to optrA-positive plasmids from China and Japan, while the remaining three plasmids had limited similarity to other published examples. We identified the novel Tn6993 transposon in pWE0254-1 carrying linezolid (optrA), macrolide (ermB) and spectinomycin [ANT(9)-Ia] resistance genes. OptrA amino acid sequences differed by 0–20 residues. We report multiple variants of optrA on distinct plasmids in diverse strains of E. faecalis . It is important to identify the selection pressures driving the emergence and maintenance of resistance against linezolid to retain the clinical utility of this antibiotic.

The ubiquitous unicellular eukaryote, Acanthamoeba, is known to play a role in the survival and dissemination of Campylobacter jejuni. C. jejuni is the leading cause of bacterial foodborne gastroenteritis world-wide and is a major public health problem. The ability of C. jejuni to interact and potentially invade epithelial cells is thought to be key for disease development in humans. We examined C. jejuni grown under standard laboratory conditions, 11168HCBA with that harvested from within Acanthamoeba castellanii (11168HAC/CBA) or Acanthamoeba polyphaga (11168HAP/CBA), and compared their ability to invade different cell lines. C. jejuni harvested from within amoebae had a ~3.7-fold increase in invasiveness into T84 human epithelial cells and a striking ~11-fold increase for re-entry into A. castellanii cells. We also investigated the invasiveness and survivability of six diverse representative C. jejuni strains within Acanthamoeba spp., our results confirm that invasion and survivability is likely host-cell-dependent. Our survival assay data led us to conclude that Acanthamoeba spp. are a transient host for C. jejuni and that survival within amoebae pre-adapts C. jejuni and enhances subsequent cell invasion. This study provides new insight into C. jejuni interactions with amoebae and its increased invasiveness potential in mammalian hosts.

Virulent non-tuberculous Mycobacteria (NTMs) successfully reside and multiply within the phagosomes of phagocytic cells such as monocytes and macrophages. Macrophages play a very important role in the innate clearance of intracellular pathogens including NTMs. Attenuated Mycobacterium avium subsp. hominissuis 100 enters macrophages but is incapable of escaping these cells via canonical mycobacteria escape mechanisms. Alternatively, virulent Mycobacterium avium subsp. hominissuis 104 and Mycobacterium abscessus subsp. abscessus are able to modify macrophages to suit their growth, survival and ultimately escape from macrophages, while non-virulent Mycobacterium smegmatis is readily killed by macrophages. In this study we focused on early infection of macrophages with NTMs to determine the phenotypic response of macrophages, M1 or M2 differentiation, and phosphorylation alterations that can affect cellular response to invading bacteria. Our findings indicate that infection of the macrophage with MAH 100 and M. smegmatis favours the development of M1 macrophage, a pro-inflammatory phenotype associated with the killing of intracellular pathogens, while infection of the macrophage with MAH 104 and M. abscessus favoured the development of M2 macrophage, an anti-inflammatory phenotype associated with the healing process. Interference with the host post-translational mechanisms, such as protein phosphorylation, is a key strategy used by many intracellular bacterial pathogens to modulate macrophage phenotype and subvert macrophage function. By comparing protein phosphorylation patterns of infected macrophages, we observed that uptake of both MAH 100 and M. smegmatis resulted in MARCKS-related protein phosphorylation, which has been associated with macrophage activation. In contrast, in macrophages infected with MAH 104 and M. abscessus , methionine adenosyltransferase IIβ, an enzyme that catalyses the biosynthesis of S-adenosylmethionine, a methyl donor for DNA methylation. Inhibition of DNA methylation with 5-aza-2 deoxycytidine, significantly impaired the survival of MAH 104 in macrophages. Our findings suggest that the virulent MAH 104 and M. abscessus enhance its survival in the macrophage possibly through interference with the epigenome responses.

Vibrio cholerae O1 and O139 isolates deploy cholera toxin (CT) and toxin-coregulated pilus (TCP) to cause the diarrhoeal disease cholera. The ctxAB and tcpA genes encoding CT and TCP are part of two acquired genetic elements, the CTX phage and Vibrio pathogenicity island-1 (VPI-1), respectively. ToxR and ToxT proteins are the key regulators of virulence genes of V. cholerae O1 and O139. V. cholerae isolates belonging to serogroups other than O1/O139, called non-O1/non-O139, are usually devoid of virulence-related elements and are non-pathogenic. Here, we have analysed the available whole genome sequence of an environmental toxigenic V. cholerae non-O1/non-O139 strain, VCE232, carrying the CTX phage and VPI-1. Extensive bioinformatics and phylogenetic analyses indicated high similarity of the VCE232 genome sequence with the genome of V. cholerae O1 strains, including organization of the VPI-1 locus, ctxAB, tcpA and toxT genes, and promoters. We established that the VCE232 strain produces an optimal amount of CT at 30 °C under AKI conditions. To investigate the role of ToxT and ToxR in the regulation of virulence factors, we constructed ΔtoxT, ΔtoxR and ΔtoxTΔtoxR deletion mutants of VCE232. Extensive genetic analyses of these mutants indicated that the toxT and toxR genes of VCE232 are crucial for CT and TCP production. However, unlike O1 isolates, the presence of either toxT or toxR gene is sufficient for optimal CT production in VCE232. In addition, the VCE232 ΔtoxR mutant showed differential regulation of the major outer membrane proteins, OmpT and OmpU. This is the first attempt to explore the regulation of expression of major virulence genes and regulators in an environmental toxigenic V. cholerae non-O1/non-O139 strain.