A Borrelia burgdorferi Surface-Exposed Transmembrane Protein Lacking Detectable Immune Responses Supports Pathogen Persistence and Constitutes a Vaccine Target.

Borrelia burgdorferi harbors a limited set of transmembrane surface proteins, most of which constitute key targets of humoral immune responses. Here we show that BB0405, a conserved membrane-spanning protein of unknown function, fails to evoke detectable antibody responses despite its extracellular exposure. bb0405 is a member of an operon and ubiquitously expressed throughout the rodent-tick infection cycle. The gene product serves an essential function in vivo, as bb0405-deletion mutants are unable to transmit from ticks and establish infection in mammalian hosts. Despite the lack of BB0405-specific immunoglobulin M or immunoglobulin G antibodies during natural infection, mice immunized with a recombinant version of the protein elicited high-titer and remarkably long-lasting antibody responses, conferring significant host protection against tick-borne infection. Taken together, these studies highlight the essential role of an apparently immune-invisible borrelial transmembrane protein in facilitating infection and its usefulness as a target of protective host immunity blocking the transmission of B. burgdorferi.

Instantaneous within-patient diversity of Pseudomonas aeruginosa quorum-sensing populations from cystic fibrosis lung infections.

In the opportunistic pathogen Pseudomonas aeruginosa, acyl-homoserine lactone (acyl-HSL) quorum sensing (QS) regulates biofilm formation and expression of many extracellular virulence factors. Curiously, QS-deficient variants, often carrying mutations in the central QS regulator LasR, are frequently isolated from infections, particularly from cystic fibrosis (CF) lung infections. Very little is known about the proportion and diversity of these QS variants in individual infections. Such information is desirable to better understand the selective forces that drive the evolution of QS phenotypes, including social cheating and innate (nonsocial) benefits. To obtain insight into the instantaneous within-patient diversity of QS, we assayed a panel of 135 concurrent P. aeruginosa isolates from eight different adult CF patients (9 to 20 isolates per patient) for various QS-controlled phenotypes. Most patients contained complex mixtures of QS-proficient and -deficient isolates. Among all patients, deficiency in individual phenotypes ranged from 0 to about 90%. Acyl-HSL, sequencing, and complementation analyses of variants with global loss-of-function phenotypes revealed dependency upon the central QS circuitry genes lasR, lasI, and rhlI. Deficient and proficient isolates were clonally related, implying evolution from a common ancestor in vivo. Our results show that the diversity of QS types is high within and among patients, suggesting diverse selection pressures in the CF lung. A single selective mechanism, be it of a social or nonsocial nature, is unlikely to account for such heterogeneity. The observed diversity also shows that conclusions about the properties of P. aeruginosa QS populations in individual CF infections cannot be drawn from the characterization of one or a few selected isolates.

Cross-Species Interferon Signaling Boosts Microbicidal Activity within the Tick Vector.

Evolution of hematophagy in blood-sucking parasites likely involves communication with their hosts. We find that Ixodes ticks are responsive to IFNγ acquired in a blood meal from mice infected with the Lyme disease-causing bacteria Borrelia burgdorferi, leading to induction of antimicrobial responses. Ixodes ticks parasitizing B. burgdorferi-infected mice upregulated an I. scapularis Rho-like GTPase (IGTPase). IGTPase knockdown enhanced B. burgdorferi levels in post-fed ticks, suggesting this protein controls spirochete survival. Notably, IGTPase was only induced during pathogen acquisition from mice and not upon transmission to naive hosts. Microinjection of ticks with IFNγ induced IGTPase, and ticks parasitizing IFNγ knockout mice, failed to upregulate IGTPase. Additionally, ticks lacking the transcription factor STAT, which signals downstream of IFNγ, did not induce IGTPase. IGTPase expression induced antimicrobial peptides, including Dae2, previously shown to inhibit B. burgdorferi. These results identify an interspecies signaling cascade allowing ticks to detect invading bacteria and mount microbicidal responses.

Cooperation and cheating in Pseudomonas aeruginosa: the roles of the las, rhl and pqs quorum-sensing systems.

Pseudomonas aeruginosa coordinates the transcription of hundreds of genes, including many virulence genes, through three hierarchically arranged quorum-sensing (QS) systems, namely las, rhl and pqs. Each system consists of genes involved in autoinducer synthesis, lasI, rhlI and pqsABCDH, as well as cognate-regulatory genes, lasR, rhlR and pqsR. In this study, we analyzed the social behavior of signal-blind (ΔlasR, ΔrhlR, ΔpqsR) and signal-negative (ΔlasI, ΔrhlI, ΔpqsA) mutants from each QS system. As each system controls extracellular common goods but differs in the extent of regulatory control, we hypothesized that all signal-blind mutants can behave as cheaters that vary in their ability to invade a QS-proficient population. We found that lasR and pqsR, but not rhlR, mutants evolve from a wild-type ancestor in vitro under conditions that favor QS. Accordingly, defined lasR and pqsR mutants enriched in wild-type co-culture, whereas rhlR and all signal-negative mutants did not. Both lasR and pqsR mutants enriched with negative frequency dependence, suggesting social interactions with the wild type, although the pqsR mutant also grew well on its own. Taken together, the lasR mutant behaved as a typical cheater, as reported previously. However, the pqsR and rhlR mutants exhibited more complex behaviors, which can be sufficiently explained by positive and negative pleiotropic effects through differential regulation of pqs gene expression in the interconnected QS network. The evolutionary approach adopted here may account for the prevalence of naturally occurring QS mutants.