Near-zero growth kinetics of Pseudomonas putida deduced from proteomic analysis.

Intensive microbial growth typically observed in laboratory rarely occurs in nature. Because of severe nutrient deficiency, natural populations exhibit near-zero growth (NZG). There is a long-standing controversy about sustained NZG, specifically whether there is a minimum growth rate below which cells die or whether cells enter a non-growing maintenance state. Using chemostat with cell retention (CCR) of Pseudomonas putida, we resolve this controversy and show that under NZG conditions, bacteria differentiate into growing and VBNC (viable but not non-culturable) forms, the latter preserving measurable catabolic activity. The proliferating cells attained a steady state, their slow growth balanced by VBNC production. Proteomic analysis revealed upregulated (transporters, stress response, self-degrading enzymes and extracellular polymers) and downregulated (ribosomal, chemotactic and primary biosynthetic enzymes) proteins in the CCR versus batch culture. Based on these profiles, we identified intracellular processes associated with NZG and generated a mathematical model that simulated the observations. We conclude that NZG requires controlled partial self-digestion and deep reconfiguration of the metabolic machinery that results in the biosynthesis of new products and development of broad stress resistance. CCR allows efficient on-line control of NZG including VBNC production. A well-nuanced understanding of NZG is important to understand microbial processes in situ and for optimal design of environmental technologies.

A screen for and validation of prodrug antimicrobials.

The rise of resistant pathogens and chronic infections tolerant to antibiotics presents an unmet need for novel antimicrobial compounds. Identifying broad-spectrum leads is challenging due to the effective penetration barrier of Gram-negative bacteria, formed by an outer membrane restricting amphipathic compounds, and multidrug resistance (MDR) pumps. In chronic infections, pathogens are shielded from the immune system by biofilms or host cells, and dormant persisters tolerant to antibiotics are responsible for recalcitrance to chemotherapy with conventional antibiotics. We reasoned that the dual need for broad-spectrum and sterilizing compounds could be met by developing prodrugs that are activated by bacterium-specific enzymes and that these generally reactive compounds could kill persisters and accumulate over time due to irreversible binding to targets. We report the development of a screen for prodrugs, based on identifying compounds that nonspecifically inhibit reduction of the viability dye alamarBlue, and then eliminate generally toxic compounds by testing for cytotoxicity. A large pilot of 55,000 compounds against Escherichia coli produced 20 hits, 3 of which were further examined. One compound, ADC111, is an analog of a known nitrofuran prodrug nitrofurantoin, and its activity depends on the presence of activating enzymes nitroreductases. ADC112 is an analog of another known antimicrobial tilbroquinol with unknown mechanism of action, and ADC113 does not belong to an approved class. All three compounds had a good spectrum and showed good to excellent activity against persister cells in biofilm and stationary cultures. These results suggest that screening for overlooked prodrugs may present a viable platform for antimicrobial discovery.

Pseudomonas aeruginosa biofilms in disease.

Pseudomonas aeruginosa is a ubiquitous organism that is the focus of intense research because of its prominent role in disease. Due to its relatively large genome and flexible metabolic capabilities, this organism exploits numerous environmental niches. It is an opportunistic pathogen that sets upon the human host when the normal immune defenses are disabled. Its deadliness is most apparent in cystic fibrosis patients, but it also is a major problem in burn wounds, chronic wounds, chronic obstructive pulmonary disorder, surface growth on implanted biomaterials, and within hospital surface and water supplies, where it poses a host of threats to vulnerable patients (Peleg and Hooper, N Engl J Med 362:1804-1813, 2010; Breathnach et al., J Hosp Infect 82:19-24, 2012). Once established in the patient, P. aeruginosa can be especially difficult to treat. The genome encodes a host of resistance genes, including multidrug efflux pumps (Poole, J Mol Microbiol Biotechnol 3:255-264, 2001) and enzymes conferring resistance to beta-lactam and aminoglycoside antibotics (Vahdani et al., Annal Burns Fire Disast 25:78-81, 2012), making therapy against this gram-negative pathogen particularly challenging due to the lack of novel antimicrobial therapeutics (Lewis, Nature 485: 439-440, 2012). This challenge is compounded by the ability of P. aeruginosa to grow in a biofilm, which may enhance its ability to cause infections by protecting bacteria from host defenses and chemotherapy. Here, we review recent studies of P. aeruginosa biofilms with a focus on how this unique mode of growth contributes to its ability to cause recalcitrant infections.

Emergence of Pseudomonas aeruginosa strains producing high levels of persister cells in patients with cystic fibrosis.

The majority of cystic fibrosis (CF) patients succumb to a chronic infection of the airway with Pseudomonas aeruginosa. Paradoxically, pathogenic strains are often susceptible to antibiotics, but the infection cannot be eradicated with antimicrobial therapy. We find that in a majority of patients with airway infections, late isolates of P. aeruginosa produce increased levels of drug-tolerant persister cells. The genomes of a clonal pair of early/late isolates from a single patient have been previously sequenced, and the late isolate (obtained at age 96 months) showed a 100-fold increase in persister levels. The 96-month isolate carries a large number of mutations, including a mutation in mutS that confers a hypermutator phenotype. There is also a mutation in the mexZ repressor controlling the expression of the MexXY-OprM multidrug pump, which results in a moderate increase in the ofloxacin, carbenicillin, and tobramycin MICs. Knocking out the mexXY locus restored the resistance to that of the parent strain but did not affect the high levels of persisters formed by the 96-month isolate. This suggests that the late isolate is a high-persister (hip) mutant. Increased persister formation was observed in exponential phase, stationary phase, and biofilm populations of the 96-month isolate. Analysis of late isolates from 14 additional patients indicated that 10 of them are hip mutants. Most of these hip mutants did not have higher drug resistance. Increased persister formation appears to be their sole mechanism for surviving chemotherapy. Taken together, these findings suggest a link between persisters and recalcitrance of CF infection and identify an overlooked culprit-high-persister mutants producing elevated levels of drug-tolerant cells. Persisters may play a similarly critical role in the recalcitrance of other chronic infections.

Discovery of new peptides from old prohormones: insights for energy balance and beyond.

A complex network of peptide hormones secreted from the gut, adipose tissue, the brain, and other tissues regulate energy balance. Intensive investigation has uncovered the role of many of the major players in energy balance. However, many of these peptide hormones are derived from precursor proteins whose in vivo processing have not been fully studied. In this review we highlight the importance of fully understanding the processing of prohormones and highlight a particular case of why this is important with the recent discovery of the peptide obestatin.

Disruption of disulfide bond formation alters the trafficking of prothyrotropin releasing hormone (proTRH)-derived peptides.

Rat prothyrotropin releasing hormone (proTRH) is processed in the regulated secretory pathway (RSP) of neuroendocrine cells yielding five TRH peptides and several non-TRH peptides. It is not understood how these peptides are targeted to the RSP. We show here that a disulfide bond in the carboxy-terminus of proTRH plays an important role in the trafficking of this prohormone. Recombinant proTRH was observed to migrate faster on a native gel when treated with dithiothreitol (DTT) suggesting the presence of a disulfide bond. In vitro disulfide bond formation was prevented either by DTT treatment or by mutating cysteines 213 and 219 to glycines. In both cases the peptides derived from these mutants exhibited increased constitutive release and processing defects when expressed in AtT20 cells, a neuroendocrine cell line used in our prior studies on proTRH processing. Immunocytochemistry revealed that wild-type proTRH and mutant proTRH localized in a punctate pattern typical of proteins sorted to the regulated secretory pathway. These data suggest that the proposed disulfide bond of proTRH is involved in sorting of proTRH-derived peptides and in their retention within maturing secretory granules. This is the first evidence of structural motifs being important for the sorting of proTRH.

Killing by bactericidal antibiotics does not depend on reactive oxygen species.

Bactericidal antibiotics kill by modulating their respective targets. This traditional view has been challenged by studies that propose an alternative, unified mechanism of killing, whereby toxic reactive oxygen species (ROS) are produced in the presence of antibiotics. We found no correlation between an individual cell's probability of survival in the presence of antibiotic and its level of ROS. An ROS quencher, thiourea, protected cells from antibiotics present at low concentrations, but the effect was observed under anaerobic conditions as well. There was essentially no difference in survival of bacteria treated with various antibiotics under aerobic or anaerobic conditions. This suggests that ROS do not play a role in killing of bacterial pathogens by antibiotics.

Persister eradication: lessons from the world of natural products.

Persisters are specialized survivor cells that protect bacterial populations from killing by antibiotics. Persisters are dormant phenotypic variants of regular cells rather than mutants. Bactericidal antibiotics kill by corrupting their targets into producing toxic products; tolerance to antibiotics follows when targets are inactive. Transcriptome analysis of isolated persisters points to toxin/antitoxin modules as a principle component of persister formation. Mechanisms of persister formation are redundant, making it difficult to eradicate these cells. In Escherichia coli, toxins RelE and MazF cause dormancy by degrading mRNA; HipA inhibits translation by phosphorylating Ef-Tu; and TisB forms an anion channel in the membrane, leading to a decrease in pmf and ATP levels. Prolonged treatment of chronic infections with antibiotics selects for hip mutants that produce more persister cells. Eradication of tolerant persisters is a serious challenge. Some of the existing antibiotics are capable of killing persisters, pointing to ways of developing therapeutics to treat chronic infections. Mitomycin is a prodrug which is converted into a reactive compound forming adducts with DNA upon entering the cell. Prolonged treatment with aminoglycosides that cause mistranslation leading to misfolded peptides can sterilize a stationary culture of Pseudomonas aeruginosa, a pathogen responsible for chronic, highly tolerant infections of cystic fibrosis patients. Finally, one of the best bactericidal agents is rifampin, an inhibitor of RNA polymerase, and we suggest that it "kills" by preventing persister resuscitation.

Prohormone-convertase 1 processing enhances post-Golgi sorting of prothyrotropin-releasing hormone-derived peptides.

Rat prothyrotropin-releasing hormone (pro-TRH) is endoproteolyzed within the regulated secretory pathway of neuroendocrine cells yielding five TRH peptides and seven to nine other unique peptides. Endoproteolysis is performed by two prohormone convertases, PC1 and PC2. Proteolysis of pro-TRH begins in the trans-Golgi network and forms two intermediates that are then differentially processed as they exit the Golgi and are packaged into immature secretory granules. We hypothesized that this initial endoproteolysis may be necessary for downstream sorting of pro-TRH-derived peptides as it occurs before Golgi exit and thus entry into the regulated secretory pathway. We now report that when pro-TRH is transiently expressed in GH4C1 cells, a neuroendocrine cell line lacking PC1, under pulse-chase conditions release is constitutive and composed of more immature processing intermediates. This is also observed by radioimmunoassay under steady-state conditions. When a mutant form of pro-TRH, which has the dibasic sites of initial processing mutated to glycines, is expressed in AtT20 cells, a neuroendocrine cell line endogenously expressing PC1, both steady-state and pulse-chase experiments revealed that peptides derived from this mutant precursor are secreted in a constitutive fashion. A constitutively secreted form of PC1 does not target pro-TRH peptides to the constitutive secretory pathway but results in sorting to the regulated secretory pathway. These results indicated that initial processing action of PC1 on pro-TRH in the trans-Golgi network, and not a cargo-receptor relationship, is important for the downstream sorting events that result in storage of pro-TRH-derived peptides in mature secretory granules.