Lori L. Burrows

Weapons of mass retraction

Twitching motility is a unique form of bacterial propulsion on solid surfaces associated with cycles of extension, tethering and retraction of type IV pili (T4P). Although investigations over the last two decades in a number of species have identified the majority of the genes involved in this process, we are still learning how these pili are assembled and the mechanics by which bacteria use T4P to drag themselves from one place to another. Among the puzzles that remain to be solved is the mechanism by which hydrolysis of ATP is coupled to pilus assembly and disassembly, and how the cell envelope structure is modified to accommodate the passage of the pilus through the periplasm. Unravelling of these and other enigmas in the T4P system will not only teach us more about these important colonization and virulence factors, but also help us to understand related processes such as type II secretion, which relies on a set of proteins homologous to those in the T4P system, and bacterial conjugation, involving retractable pili belonging to the F‐like subgroup of the type IV secretion family. This review focuses on recent discoveries relating to the assembly and function of T4P in generation of twitching motility.

Basis for Selectivity of Cationic Antimicrobial Peptides for Bacterial Versus Mammalian Membranes

Novel cationic antimicrobial peptides typified by structures such as KKKKKKAAXAAWAAXAA-NH2, where X = Phe/Trp, and several of their analogues display high activity against a variety of bacteria but exhibit no hemolytic activity even at high dose levels in mammalian erythrocytes. To elucidate their mechanism of action and source of selectivity for bacterial membranes, phospholipid mixtures mimicking the compositions of natural bacterial membranes (containing anionic lipids) and mammalian membranes (containing zwitterionic lipids + cholesterol) were challenged with the peptides. We found that peptides readily inserted into bacterial lipid mixtures, although no insertion was detected in model “mammalian” membranes. The depth of peptide insertion into model bacterial membranes was estimated by Trp fluorescence quenching using doxyl groups variably positioned along the phospholipid acyl chains. Peptide antimicrobial activity generally increased with increasing depth of peptide insertion. The overall results, in conjunction with molecular modeling, support an initial electrostatic interaction step in which bacterial membranes attract and bind peptide dimers onto the bacterial surface, followed by the “sinking” of the hydrophobic core segment to a peptide sequence-dependent depth of ∼2.5–8 Å into the membrane, largely parallel to the membrane surface. Antimicrobial activity was likely enhanced by the fact that the peptide sequences contain AXXXA sequence motifs, which promote their dimerization, and possibly higher oligomerization, as assessed by SDS-polyacrylamide gel analysis and fluorescence resonance energy transfer experiments. The high selectivity of these peptides for nonmammalian membranes, combined with their activity toward a wide spectrum of Gram-negative and Gram-positive bacteria and yeast, while retaining water solubility, represent significant advantages of this class of peptides.

Biofilm Formation by Hyperpiliated Mutants of Pseudomonas aeruginosa

Under static growth conditions, hyperpiliated, nontwitching pilT and pilU mutants of Pseudomonas aeruginosa formed dense biofilms, showing that adhesion, not twitching motility, is necessary for biofilm initiation. Under flow conditions, the pilT mutant formed mushroom-like structures larger than those of the wild type but the pilU mutant was defective in biofilm formation. Therefore, twitching motility affects the development of biofilm structure, possibly through modulation of detachment.

Disparate Subcellular Localization Patterns of Pseudomonas aeruginosa Type IV Pilus ATPases Involved in Twitching Motility

The opportunistic pathogen Pseudomonas aeruginosa expresses polar type IV pili (TFP), which are responsible for adhesion to various materials and twitching motility on surfaces. Twitching occurs by alternate extension and retraction of TFP, which arise from assembly and disassembly of pilin subunits at the base of the pilus. The ATPase PilB promotes pilin assembly, while the ATPase PilT or PilU or both promote pilin dissociation. Fluorescent fusions to two of the three ATPases (PilT and PilU) were functional, as shown by complementation of the corresponding mutants. PilB and PilT fusions localized to both poles, while PilU fusions localized only to the piliated pole. To identify the portion of the ATPases required for localization, sequential C-terminal deletions of PilT and PilU were generated. The conserved His and Walker B boxes were dispensable for polar localization but were required for twitching motility, showing that localization and function could be uncoupled. Truncated fusions that retained polar localization maintained their distinctive distribution patterns. To dissect the cellular factors involved in establishing polarity, fusion protein localization was monitored with a panel of TFP mutants. The localization of yellow fluorescent protein (YFP)-PilT and YFP-PilU was independent of the subunit PilA, other TFP ATPases, and TFP-associated proteins previously shown to be associated with the membrane or exhibiting polar localization. In contrast, YFP-PilB exhibited diffuse cytoplasmic localization in a pilC mutant, suggesting that PilC is required for polar localization of PilB. Finally, localization studies performed with fluorescent ATPase chimeras of PilT and PilU demonstrated that information responsible for the characteristic localization patterns of the ATPases likely resides in their N termini.

Functional role of conserved residues in the characteristic secretion NTPase motifs of the Pseudomonas aeruginosa type IV pilus motor proteins PilB, PilT and PilU.

Type IV pili are retractable protein fibres used by many bacterial pathogens for adherence, twitching motility, biofilm development and host colonization. In Pseudomonas aeruginosa, PilB and PilT are bipolar proteins belonging to the secretion NTPase superfamily, and power pilus extension and retraction, respectively, while the unipolar PilT paralogue PilU supports pilus retraction in an unknown manner. Assay of purified 6xHis-tagged PilB, PilT and PilU from P. aeruginosa showed that all three proteins have ATPase activities in vitro. Conserved residues in the Walker A (WA), Walker B (WB), Asp Box and His Box motifs characteristic of secretion NTPases were mutated, and complementation of twitching motility was tested. Mutation of conserved WA or WB residues in any of the three ATPases abrogated twitching motility, and for the WA mutant of PilT caused loss of polar localization. The requirement for three invariant acidic residues in the Asp Box motif, and for two invariant His residues in the His Box motif varied, with PilB being the least tolerant of changes. In all three proteins, the third acidic residue in the Asp Box and the second His of the His Box were crucial for function; mutation of these residues caused loss of PilT ATPase activity in vitro. Modelling of the effects of these mutations on the crystal structures of Aquifex aeolicus PilT and Vibrio cholerae EpsE (a PilB homologue) showed that the critical Asp Box and His Box residues contribute to a catalytic pocket that surrounds the ligand. These results provide experimental evidence differentiating widely conserved Asp and His Box residues that are essential for function from those whose roles are modulated by specific local environments.

Common β-lactamases inhibit bacterial biofilm formation

β‐Lactamases, which evolved from bacterial penicillin‐binding proteins (PBPs) involved in peptidoglycan (PG) synthesis, confer resistance to β‐lactam antibiotics. While investigating the genetic basis of biofilm development by Pseudomonas aeruginosa, we noted that plasmid vectors encoding the common β‐lactamase marker TEM‐1 caused defects in twitching motility (mediated by type IV pili), adherence and biofilm formation without affecting growth rates. Similarly, strains of Escherichia coli carrying TEM‐1‐encoding vectors grew normally but showed reduced adherence and biofilm formation, showing this effect was not species‐specific. Introduction of otherwise identical plasmid vectors carrying tetracycline or gentamicin resistance markers had no effect on biofilm formation or twitching motility. The effect is restricted to class A and D enzymes, because expression of the class D Oxa‐3 β‐lactamase, but not class B or C β‐lactamases, impaired biofilm formation by E. coli and P. aeruginosa. Site‐directed mutagenesis of the catalytic Ser of TEM‐1, but not Oxa‐3, abolished the biofilm defect, while disruption of either TEM‐1 or Oxa‐3 expression restored wild‐type levels of biofilm formation. We hypothesized that the A and D classes of β‐lactamases, which are related to low molecular weight (LMW) PBPs, may sequester or alter the PG substrates of such enzymes and interfere with normal cell wall turnover. In support of this hypothesis, deletion of the E. coli LMW PBPs 4, 5 and 7 or combinations thereof, resulted in cumulative defects in biofilm formation, similar to those seen in β‐lactamase‐expressing transformants. Our results imply that horizontal acquisition of β‐lactamase resistance enzymes can have a phenotypic cost to bacteria by reducing their ability to form biofilms. β‐Lactamases likely affect PG remodelling, manifesting as perturbation of structures involved in bacterial adhesion that are required to initiate biofilm formation.

Use of In-Biofilm Expression Technology To Identify Genes Involved in Pseudomonas aeruginosa Biofilm Development

Mature Pseudomonas aeruginosa biofilms form complex three-dimensional architecture and are tolerant of antibiotics and other antimicrobial compounds. In this work, an in vivo expression technology system, originally designed to study virulence-associated genes in complex mammalian environments, was used to identify genes up-regulated in P. aeruginosa grown to a mature (5-day) biofilm. Five unique cloned promoters unable to promote in vitro growth in the absence of purines after recovery from the biofilm environment were identified. The open reading frames downstream of the cloned promoter regions were identified, and knockout mutants were generated. Insertional mutation of PA5065, a homologue of Escherichia coli ubiB, was lethal, while inactivation of PA0240 (a porin homologue), PA3710 (a putative alcohol dehydrogenase), and PA3782 (a homologue of the Streptomyces griseus developmental regulator adpA) had no effect on planktonic growth but caused defects in biofilm formation in static and flowing systems. In competition experiments, mutants demonstrated reduced fitness compared with the parent strain, comprising less than 0.0001% of total biofilm cells after 5 days. Therefore, using in-biofilm expression technology, we have identified novel genes that do not affect planktonic growth but are important for biofilm formation, development, and fitness.

Glycosylation of Pseudomonas aeruginosa Strain Pa5196 Type IV Pilins with Mycobacterium-Like α-1,5-Linked d-Araf Oligosaccharides

Pseudomonas aeruginosa is a gram-negative bacterium that uses polar type IV pili for adherence to various materials and for rapid colonization of surfaces via twitching motility. Within the P. aeruginosa species, five distinct alleles encoding variants of the structural subunit PilA varying in amino acid sequence, length, and presence of posttranslational modifications have been identified. In this work, a combination of mass spectrometry and nuclear magnetic resonance spectroscopy was used to identify a novel glycan modification on the pilins of the group IV strain Pa5196. Group IV pilins continued to be modified in a lipopolysaccharide (wbpM) mutant of Pa5196, showing that, unlike group I strains, the pilins of group IV are not modified with the O-antigen unit of the background strain. Instead, the pilin glycan was determined to be an unusual homo-oligomer of alpha-1,5-linked d-arabinofuranose (d-Araf). This sugar is uncommon in prokaryotes, occurring mainly in the cell wall arabinogalactan and lipoarabinomannan (LAM) polymers of mycobacteria, including Mycobacterium tuberculosis and Mycobacterium leprae. Antibodies raised against M. tuberculosis LAM specifically identified the glycosylated pilins from Pa5196, confirming that the glycan is antigenically, as well as chemically, identical to those of Mycobacterium. P. aeruginosa Pa5196, a rapidly growing strain of low virulence that expresses large amounts of glycosylated type IV pilins on its surface, represents a genetically tractable model system for elucidation of alternate pathways for biosynthesis of d-Araf and its polymerization into mycobacterium-like alpha-1,5-linked oligosaccharides.

Modification of Pseudomonas aeruginosa Pa5196 Type IV Pilins at Multiple Sites with d-Araf by a Novel GT-C Family Arabinosyltransferase, TfpW

Pseudomonas aeruginosa Pa5196 produces type IV pilins modified with unusual alpha1,5-linked d-arabinofuranose (alpha1,5-D-Araf) glycans, identical to those in the lipoarabinomannan and arabinogalactan cell wall polymers from Mycobacterium spp. In this work, we identify a second strain of P. aeruginosa, PA7, capable of expressing arabinosylated pilins and use a combination of site-directed mutagenesis, electrospray ionization mass spectrometry (MS), and electron transfer dissociation MS to identify the exact sites and extent of pilin modification in strain Pa5196. Unlike previously characterized type IV pilins that are glycosylated at a single position, those from strain Pa5196 were modified at multiple sites, with modifications of alphabeta-loop residues Thr64 and Thr66 being important for normal pilus assembly. Trisaccharides of alpha1,5-D-Araf were the principal modifications at Thr64 and Thr66, with additional mono- and disaccharides identified on Ser residues within the antiparallel beta sheet region of the pilin. TfpW was hypothesized to encode the pilin glycosyltransferase based on its genetic linkage to the pilin, weak similarity to membrane-bound GT-C family glycosyltransferases (which include the Mycobacterium arabinosyltransferases EmbA/B/C), and the presence of characteristic motifs. Loss of TfpW or mutation of key residues within the signature GT-C glycosyltransferase motif completely abrogated pilin glycosylation, confirming its involvement in this process. A Pa5196 pilA mutant complemented with other Pseudomonas pilins containing potential sites of modification expressed nonglycosylated pilins, showing that TfpWs pilin substrate specificity is restricted. TfpW is the prototype of a new type IV pilin posttranslational modification system and the first reported gram-negative member of the GT-C glycosyltransferase family.

Functional Conservation of the Polysaccharide Biosynthetic Protein WbpM and Its Homologues in Pseudomonas aeruginosa and Other Medically Significant Bacteria

ABSTRACT WbpM is a highly conserved protein involved in synthesis of the O antigens of Pseudomonas aeruginosa. Homologues of this protein have been identified in a large number of bacteria, and they can be divided into two subfamilies: subfamily 1, including WbpM, contains large proteins (∼600 amino acids), while subfamily 2, typified by HP0840 (FlaA1) of Helicobacter pylori, contains smaller proteins (∼350 amino acids) homologous to the C termini of proteins in subfamily 1. Analysis of knockout mutants ofwbpM in P. aeruginosa serotypes O3, O10, O15, and O17 showed that although all 20 serotypes of P. aeruginosa possess wbpM, it is not universally required for O-antigen biosynthesis. Homologous genes fromBordetella pertussis (wlbL),Staphylococcus aureus (cap8D), and H. pylori (flaA1) complemented a P. aeruginosa O5 wbpM mutant to various degrees. These conserved proteins may represent interesting targets for the design of inhibitors of bacterial exopolysaccharide biosynthesis.