Biology subjects

Scribner, M. R.

Publications and source records attributed to Scribner, M. R..

5 recordsLinked to original sources

bioRxiv2025

Filamentous cheater phages drive bacterial and phage populations to lower fitness

Many bacteria carry phage genome(s) in their chromosome, which intertwines the fitness of the bacterium and the phage. Most Pseudomonas aeruginosa strains carry filamentous phages called Pf that establish chronic infections and do not require host lysis to spread. However, spontaneous mutations in the Pf repressor gene (pf5r) can allow extreme phage production that slows bacterial growth and increases cell death, violating an apparent detente between bacterium and phage. We observed this paradoxical outcome in an evolution experiment with P. aeruginosa in media simulating nutrients from the cystic fibrosis airway. Bacteria containing pf5r mutant phage grow to a lower density but directly outcompete their ancestor and convert them into pf5r mutants via phage superinfection. Reduced fitness therefore spreads throughout the bacterial population, driven by weaponized Pf. Yet high intracellular phage replication facilitates another evolutionary conflict: "cheater miniphages" lacking capsid genes and the superinfection exclusion gene (pfsE) invade populations of full-length phages within cells. Although bacteria containing both full-length phages and miniphages are most immune to superinfection by limiting the Pf receptor, this hybrid vigor is extremely unstable, as a classic Tragedy of the Commons scenario ensues that causes complete prophage loss. The entire cycle - from phage hyperactivation to miniphage invasion to prophage loss - can occur within 24h, showcasing rapid coevolution between bacteria and their filamentous phages. This study demonstrates that P. aeruginosa, and potentially many other bacterial species that carry filamentous prophages, risk being exploited by these phages in a runaway process that reduces fitness of both host and virus. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=168 SRC="FIGDIR/small/646652v4_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1f3d9c9org.highwire.dtl.DTLVardef@5f5591org.highwire.dtl.DTLVardef@eac39borg.highwire.dtl.DTLVardef@d00f3a_HPS_FORMAT_FIGEXP M_FIG C_FIG

microbiology↗

bioRxiv2021

The nutritional environment is sufficient to select coexisting biofilm and quorum-sensing mutants of Pseudomonas aeruginosa

The evolution of bacterial populations during infections can be influenced by various factors including available nutrients, the immune system, and competing microbes, rendering it difficult to identify the specific forces that select on evolved traits. The genomes of Pseudomonas aeruginosa isolated from the airway of patients with cystic fibrosis (CF), for example, have revealed commonly mutated genes, but which phenotypes led to their prevalence is often uncertain. Here, we focus on effects of nutritional components of the CF airway on genetic adaptations by P. aeruginosa grown in either well-mixed (planktonic) or biofilm-associated conditions. After only 80 generations of experimental evolution in a simple medium with glucose, lactate, and amino acids, all planktonic populations diversified into lineages with mutated genes common to CF infections: morA, encoding a regulator of biofilm formation, or lasR, encoding a quorum sensing regulator that modulates the expression of virulence factors. Although mutated quorum sensing is often thought to be selected in vivo due to altered virulence phenotypes or social cheating, isolates with lasR mutations demonstrated increased fitness when grown alone and outcompeted the ancestral PA14 strain. Nonsynonymous SNPs in morA increased fitness in a nutrient concentration-dependent manner during planktonic growth and surprisingly also increased biofilm production. Populations propagated in biofilm conditions also acquired mutations in loci associated with chronic infections, including lasR and cyclic-di-GMP regulators roeA and wspF. These findings demonstrate that nutrient conditions and biofilm selection are alone sufficient to select mutants with problematic clinical phenotypes including increased biofilm and altered quorum sensing. ImportancePseudomonas aeruginosa produces dangerous chronic infections that are known for their rapid diversification and recalcitrance to treatment. We performed evolution experiments to identify adaptations selected by two specific aspects of the CF respiratory environment: nutrient levels and surface attachment. Propagation of P. aeruginosa in nutrients present within the CF airway was alone sufficient to drive diversification into subpopulations with identical mutations in regulators of biofilm and quorum sensing to those arising during infection. Thus, the adaptation of opportunistic pathogens to nutrients found in the host may select mutants with phenotypes that complicate treatment and clearance of infection.

microbiology↗

bioRxiv2020

Quantitative mapping of mRNA 3' ends in Pseudomonas aeruginosa reveals putative riboregulators and a pervasive role for transcription termination in response to azithromycin

Pseudomonas aeruginosa produces serious chronic infections in hospitalized patients and immunocompromised individuals, including patients with cystic fibrosis. The molecular mechanisms by which P. aeruginosa responds to antibiotics and other stresses to promote persistent infections may provide new avenues for therapeutic intervention. Azithromycin (AZM), an antibiotic frequently used in cystic fibrosis treatment, is thought to improve clinical outcomes through a number of mechanisms including impaired biofilm growth and quorum sensing (QS). The mechanisms underlying the transcriptional response to AZM remain unclear. Here, we interrogated the P. aeruginosa transcriptional response to AZM using a fast, cost-effective genome-wide approach to quantitate RNA 3 ends (3pMap). We also identified hundreds of P. aeruginosa genes with high incidence of premature 3 end formation indicative of riboregulation in their transcript leaders using 3pMap. AZM treatment of planktonic and biofilm cultures alters the expression of hundreds of genes, including those involved in QS, biofilm formation, and virulence. Strikingly, most genes downregulated by AZM in biofilms had increased levels of intragenic 3 ends indicating premature transcription termination, transcriptional pausing, or accumulation of stable intermediates resulting from the action of nucleases. Reciprocally, AZM reduced premature intragenic 3 end termini in many upregulated genes. Most notably, reduced termination accompanied robust induction of obgE, a GTPase involved in persister formation in P. aeruginosa. Our results support a model in which AZM-induced changes in 3 end formation alter the expression of central regulators which in turn impairs the expression of QS, biofilm formation and stress response genes, while upregulating genes associated with persistence.

microbiology↗

bioRxiv2019

Parallel evolution of tobramycin resistance across species and environments

An important problem in evolution is identifying the genetic basis of how different species adapt to similar environments. Understanding how various bacterial pathogens evolve in response to antimicrobial treatment is a pressing example of this problem, where discovery of molecular parallelism could lead to clinically useful predictions. Evolution experiments with pathogens in environments containing antibiotics combined with periodic whole population genome sequencing can be used to characterize the evolutionary dynamics of the pathways to antimicrobial resistance. We separately propagated two clinically relevant Gram-negative pathogens, Pseudomonas aeruginosa and Acinetobacter baumannii, in increasing concentrations of tobramycin in two different environments each: planktonic and biofilm. Independent of the pathogen, populations adapted to tobramycin selection by parallel evolution of mutations in fusA1, encoding elongation factor G, and ptsP, encoding phosphoenolpyruvate phosphotransferase. As neither gene is a direct target of this aminoglycoside, both are relatively novel and underreported causes of resistance. Additionally, both species acquired antibiotic-associated mutations that were more prevalent in the biofilm lifestyle than planktonic, in electron transport chain components in A. baumannii and LPS biosynthesis enzymes in P. aeruginosa populations. Using existing databases, we discovered both fusA1 and ptsP mutations to be prevalent in antibiotic resistant clinical isolates. Additionally, we report site-specific parallelism of fusA1 mutations that extend across several bacterial phyla. This study suggests that strong selective pressures such as antibiotic treatment may result in high levels of predictability in molecular targets of evolution despite differences between organisms genetic background and environment.

microbiology↗

bioRxiv2019

Biofilm-dependent evolutionary pathways to antibiotic resistance

Bacterial populations vary in their stress tolerance and population structure depending upon whether growth occurs in well-mixed or structured environments. We hypothesized that evolution in biofilms would generate greater genetic diversity than well-mixed environments and lead to different pathways of antibiotic resistance. We used experimental evolution and whole genome sequencing to test how the biofilm lifestyle influenced the rate, genetic mechanisms, and pleiotropic effects of resistance to ciprofloxacin in Acinetobacter baumannii populations. Both evolutionary dynamics and the identities of mutations differed between lifestyle. Planktonic populations experienced selective sweeps of mutations including the primary topoisomerase drug targets, whereas biofilm-adapted populations acquired mutations in regulators of efflux pumps. An overall trade-off between fitness and resistance level emerged, wherein biofilm-adapted clones were less resistant than planktonic but more fit in the absence of drug. However, biofilm populations developed collateral sensitivity to cephalosporins, demonstrating the clinical relevance of lifestyle on the evolution of resistance.

microbiology↗