Swarming populations of Salmonella represent a unique physiological state coupled to multiple mechanisms of antibiotic resistance

Understanding Antimicrobial Resistance Using Genome-Scale Metabolic Modeling

The urgent necessity to fight antimicrobial resistance is universally recognized. In the search of new targets and strategies to face this global challenge, a promising approach resides in the study of the cellular response to antimicrobial exposure and on the impact of global cellular reprogramming on antimicrobial drugs' efficacy. The metabolic state of microbial cells has been shown to undergo several antimicrobial-induced modifications and, at the same time, to be a good predictor of the outcome of an antimicrobial treatment. Metabolism is a promising reservoir of potential drug targets/adjuvants that has not been fully exploited to date. One of the main problems in unraveling the metabolic response of cells to the environment resides in the complexity of such metabolic networks. To solve this problem, modeling approaches have been developed, and they are progressively gaining in popularity due to the huge availability of genomic information and the ease at which a genome sequence can be converted into models to run basic phenotype predictions. Here, we review the use of computational modeling to study the relationship between microbial metabolism and antimicrobials and the recent advances in the application of genome-scale metabolic modeling to the study of microbial responses to antimicrobial exposure.

Dead cells release a 'necrosignal' that activates antibiotic survival pathways in bacterial swarms

Swarming is a form of collective bacterial motion enabled by flagella on the surface of semi-solid media. Swarming populations exhibit non-genetic or adaptive resistance to antibiotics, despite sustaining considerable cell death. Here, we show that antibiotic-induced death of a sub-population benefits the swarm by enhancing adaptive resistance in the surviving cells. Killed cells release a resistance-enhancing factor that we identify as AcrA, a periplasmic component of RND efflux pumps. The released AcrA interacts on the surface of live cells with an outer membrane component of the efflux pump, TolC, stimulating drug efflux and inducing expression of other efflux pumps. This phenomenon, which we call 'necrosignaling', exists in other Gram-negative and Gram-positive bacteria and displays species-specificity. Given that adaptive resistance is a known incubator for evolving genetic resistance, our findings might be clinically relevant to the rise of multidrug resistance.

Agaric acid reduces Salmonella biofilm formation by inhibiting flagellar motility

Salmonella biofilms are a common cause of contaminations in the food or feed industry. In a screening for novel compounds to combat biofilm-associated foodborne outbreaks, we identified agaric acid as a Salmonella Typhimurium biofilm inhibitor that does not affect planktonic growth. Importantly, the remaining biofilm cells after preventive treatment with agaric acid were significantly more sensitive to the common disinfectant hydrogen peroxide. Screening of a GFP-promoter fusion library of biofilm related genes revealed that agaric acid downregulates the transcription of genes responsible for flagellar motility. Concurrently, swimming motility was completely abrogated in the presence of agaric acid, indicating that biofilm inhibition occurs via interference with the motility phenotype. Moreover, agaric acid also reduced biofilm formation of Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli. Agaric acid thus shows potential as an anti-virulence compound that inhibits both motility and biofilm formation.

ptsI gene in the phosphotransfer system is a potential target for developing a live attenuated Salmonella vaccine

Salmonella enterica serovar Typhimurium causes invasive non-typhoidal Salmonella diseases in animals and humans, resulting in a high mortality rate and huge economic losses globally. As the prevalence of antibiotic-resistant Salmonella has been increasing, vaccination is thought to be the most effective and economical strategy to manage salmonellosis. The present study aimed to investigate whether dysfunction in the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS), which is critical for carbon uptake and survival in macrophages, may be adequate to generate Salmonella-attenuated vaccine strains. A Salmonella strain (KST0555) was generated by deleting the ptsI gene from the PTS and it was revealed that this auxotrophic mutant was unable to efficiently utilize predominant carbon sources during infection (glucose and glycerol), reduced its invasion and replication capacity in macrophages, and significantly (P=0.0065) lowered its virulence in the setting of a mouse colitis model, along with a substantially decreased intestinal colonization and invasiveness compared with its parent strain. The reverse transcription-quantitative PCR results demonstrated that the virulence genes in Salmonella pathogenicity island-1 (SPI-1) and -2 (SPI-2) and the motility of KST0555 were all downregulated compared with its parent strain. Finally, it was revealed that when mice were immunized orally with live KST0555, Salmonella-specific humoral and cellular immune responses were effectively elicited, providing protection against Salmonella infection. Thus, the present promising data provides a strong rationale for the advancement of KST0555 as a live Salmonella vaccine candidate and ptsI as a potential target for developing a live attenuated bacterial vaccine strain.

Dynamic motility selection drives population segregation in a bacterial swarm

Population expansion in space, or range expansion, is widespread in nature and in clinical settings. Space competition among heterogeneous subpopulations during range expansion is essential to population ecology, and it may involve the interplay of multiple factors, primarily growth and motility of individuals. Structured microbial communities provide model systems to study space competition during range expansion. Here we use bacterial swarms to investigate how single-cell motility contributes to space competition among heterogeneous bacterial populations during range expansion. Our results revealed that motility heterogeneity can promote the spatial segregation of subpopulations via a dynamic motility selection process. The dynamic motility selection is enabled by speed-dependent persistence time bias of single-cell motion, which presumably arises from physical interaction between cells in a densely packed swarm. We further showed that the dynamic motility selection may contribute to collective drug tolerance of swarming colonies by segregating subpopulations with transient drug tolerance to the colony edge. Our results illustrate that motility heterogeneity, or "motility fitness," can play a greater role than growth rate fitness in determining the short-term spatial structure of expanding populations.

Phenolics with Bactericidal Activity Alter Motility and Biofilm Formation in Enterotoxigenic, Enteropathogenic, and Enterohemorrhagic Escherichia coli

Most Escherichia coli strains are innocuous to human beings; however, some strains can cause diarrhea and are grouped into pathotypes. Since current trends promote the use of natural-origin compounds to control bacteria, in this study, the effects of the phenolic compounds (PCs) tannic acid (TA), gallic acid (GA), methyl gallate (MG), and epigallocatechin gallate (EG) on the growth, swarming motility, biofilm formation, and expression of selected virulence genes of three E. coli pathotypes (enteropathogenic Escherichia coli [EPEC], enterohemorrhagic Escherichia coli [EHEC], and enterotoxigenic Escherichia coli [ETEC]) were evaluated. Minimum bactericidal concentrations (MBCs) were determined by using microtiter plates, and the effects of sublethal PC concentrations on swarming motility were evaluated on Luria-Bertani agar. Biofilm formation was assessed in microtiter plates via crystal violet staining, and the expression levels of genes involved in biofilm formation (flhC, fliA, fliC, and csgA) and swarming motility (csgD and cyaA) were evaluated via quantitative PCR. All PC were bactericidal with minimal bactericidal concentrations ranging from 0.07 to 2.1 mg/mL. At concentrations lower than the MBC, PCs decreased swarming motility (14.8-100%). GA reduced biofilm formation in all of the tested strains; however, TA, MG, and EG induced biofilm formation in some strains at specific concentrations. TA induced the overexpression of csgA, csgD, and cyaA, whereas the other PCs did not have any effects or reduced their expression levels. The PCs tested in this study showed potential to control E. coli strains belonging to the EHEC, ETEC, and EPEC pathotypes by affecting their growth, swarming motility, and virulence gene expression; however, proper concentrations must be used to avoid the induction of undesirable virulence factor genes.

Detection and characterization of multidrug-resistant Salmonella enterica serovar Infantis as an emerging threat in poultry farms of Faisalabad, Pakistan

AIMS

The aim of this study was the molecular identification of Salmonella enterica serovar Infantis (S. Infantis) isolated from poultry samples and their antimicrobial resistance and virulence profiling.

METHODS AND RESULTS

A total of 149 isolates, belonging to genus Salmonella, originally isolated from 340 suspected poultry post mortem specimens reported by us earlier were preliminary identified as Salmonella by biochemical methods and confirmed by polymerase chain reaction targeting genus-specific gene invA. Targeting serovar-specific gene fragment (fljB) resulted in confirmation of 54 isolates as S. Infantis which were further confirmed by sequencing of 16S RNA and fljB genes. Swimming and swarming motilities were detected in 98·1 and 11·1% isolates respectively. Phenotypic disc diffusion assay against 23 antimicrobial agents showed the highest resistance against pefloxacin (PEF) (94·4%), chloramphenicol (83·3%) and imipenem (77·7%) while 5·3% isolates showed extended-spectrum β-lactamase production. Fifty-nine genes reported for antimicrobial resistance and 12 for conferring virulence were targeted. The most prevalent resistance gene for aminoglycosides was aadA (42·3%), for quinolone resistance determining region parE (62·5%), for penicillin's Int1 (62·9%), for chloramphenicol cat3 (66·1%) and for beta-lactams bla_{TEM -1} (44·4%). Among efflux pump coding genes, armA showed highest (74·2%) prevalence and for virulence, a high prevalence of SopE (89·2%) showed the zoonotic potential of the isolates. The activity of efflux pumps was detected through Ethidium Bromide-agar method.

CONCLUSIONS

Poultry could act as reservoirs of multidrug resistance Salmonella.

SIGNIFICANCE AND IMPACT OF THE STUDY

We firstly report the prevalence and molecular characterization of virulence/drug resistance in S. Infantis from this region and the results may contribute to designing precisely targeted therapy. This study has also highlighted the possible emergence of S. Infantis with zoonotic potential.

Proteomic Analysis of Resistance of Gram-Negative Bacteria to Chlorhexidine and Impacts on Susceptibility to Colistin, Antimicrobial Peptides, and Ceragenins

Use of chlorhexidine in clinical settings has led to concerns that repeated exposure of bacteria to sub-lethal doses of chlorhexidine might result in chlorhexidine resistance and cross resistance with other cationic antimicrobials including colistin, endogenous antimicrobial peptides (AMPs) and their mimics, ceragenins. We have previously shown that colistin-resistant Gram-negative bacteria remain susceptible to AMPs and ceragenins. Here, we investigated the potential for cross resistance between chlorhexidine, colistin, AMPs and ceragenins by serial exposure of standard strains of Gram-negative bacteria to chlorhexidine to generate resistant populations of organisms. Furthermore, we performed a proteomics study on the chlorhexidine-resistant strains and compared them to the wild-type strains to find the pathways by which bacteria develop resistance to chlorhexidine. Serial exposure of Gram-negative bacteria to chlorhexidine resulted in four- to eight-fold increases in minimum inhibitory concentrations (MICs). Chlorhexidine-resistant organisms showed decreased susceptibility to colistin (8- to 32-fold increases in MICs) despite not being exposed to colistin. In contrast, chlorhexidine-resistant organisms had the same MICs as the original strains when tested with representative AMPs (LL-37 and magainin I) and ceragenins (CSA-44 and CSA-131). These results imply that there may be a connection between the emergence of highly colistin-resistant Gram-negative pathogens and the prevalence of chlorhexidine usage. Yet, use of chlorhexidine may not impact innate immune defenses (e.g., AMPs) and their mimics (e.g., ceragenins). Here, we also show that chlorhexidine resistance is associated with upregulation of proteins involved in the assembly of LPS for outer membrane biogenesis and virulence factors in Pseudomonas aeruginosa. Additionally, resistance to chlorhexidine resulted in elevated expression levels of proteins associated with chaperones, efflux pumps, flagella and cell metabolism. This study provides a comprehensive overview of the evolutionary proteomic changes in P. aeruginosa following exposure to chlorhexidine and colistin. These results have important clinical implications considering the continuous application of chlorhexidine in hospitals that could influence the emergence of colistin-resistant strains.

Switching and Torque Generation in Swarming E. coli

Escherichia coli swarm on semi-solid surfaces with the aid of flagella. It has been hypothesized that swarmer cells overcome the increased viscous drag near surfaces by developing higher flagellar thrust and by promoting surface wetness with the aid of a flagellar switch. The switch enables reversals between clockwise (CW) and counterclockwise (CCW) directions of rotation of the flagellar motor. Here, we measured the behavior of flagellar motors in swarmer cells. Results indicated that although the torque was similar to that in planktonic cells, the tendency to rotate CCW was higher in swarmer cells. This suggested that swarmers likely have a smaller pool of phosphorylated CheY. Results further indicated that the upregulation of the flagellin gene was not critical for flagellar thrust or swarming. Consistent with earlier reports, moisture added to the swarm surface restored swarming in a CCW-only mutant, but not in a FliG mutant that rotated motors CW-only (FliG^CW). Fluorescence assays revealed that FliG^CW cells grown on agar surfaces carried fewer flagella than planktonic FliG^CW cells. The surface-dependent reduction in flagella correlated with a reduction in the number of putative flagellar preassemblies. These results hint toward a possibility that the conformational dynamics of switch proteins play a role in the proper assembly of flagellar complexes and flagellar export, thereby aiding bacterial swarming.

The Transient Multidrug Resistance Phenotype of Salmonella enterica Swarming Cells Is Abolished by Sub-inhibitory Concentrations of Antimicrobial Compounds

Swarming motility is the rapid and coordinated multicellular migration of bacteria across a moist surface. During swarming, bacterial cells exhibit increased resistance to multiple antibiotics, a phenomenon described as adaptive or transient resistance. In this study, we demonstrate that sub-inhibitory concentrations of cefotaxime, ciprofloxacin, trimethoprim, or chloramphenicol, but not that of amikacin, colistin, kanamycin or tetracycline, impair Salmonella enterica swarming. Chloramphenicol-treated S. enterica cells exhibited a clear decrease in their flagellar content, while treatment with other antibiotics that reduced swarming (cefotaxime, ciprofloxacin, and trimethoprim) inhibited polar chemoreceptor array assembly. Moreover, the increased resistance phenotype acquired by swarming cells was abolished by the presence of these antimicrobials. The same occurred in cells treated with these antimicrobial agents in combination with others that had no effect on swarming motility. Our results reveal the potential of inhibiting swarming ability to enhance the therapeutic effectiveness of antimicrobial agents.

Molecular Interaction and Cellular Location of RecA and CheW Proteins in Salmonella enterica during SOS Response and Their Implication in Swarming

In addition to its role in DNA damage repair and recombination, the RecA protein, through its interaction with CheW, is involved in swarming motility, a form of flagella-dependent movement across surfaces. In order to better understand how SOS response modulates swarming, in this work the location of RecA and CheW proteins within the swarming cells has been studied by using super-resolution microscopy. Further, and after in silico docking studies, the specific RecA and CheW regions associated with the RecA-CheW interaction have also been confirmed by site-directed mutagenesis and immunoprecipitation techniques. Our results point out that the CheW distribution changes, from the cell poles to foci distributed in a helical pattern along the cell axis when SOS response is activated or RecA protein is overexpressed. In this situation, the CheW presents the same subcellular location as that of RecA, pointing out that the previously described RecA storage structures may be modulators of swarming motility. Data reported herein not only confirmed that the RecA-CheW pair is essential for swarming motility but it is directly involved in the CheW distribution change associated to SOS response activation. A model explaining not only the mechanism by which DNA damage modulates swarming but also how both the lack and the excess of RecA protein impair this motility is proposed.

Curcumin Reduces the Motility of Salmonella enterica Serovar Typhimurium by Binding to the Flagella, Thereby Leading to Flagellar Fragility and Shedding

One of the important virulence properties of the pathogen is its ability to travel to a favorable environment, cross the viscous mucus barrier (intestinal barrier for enteric pathogens), and reach the epithelia to initiate pathogenesis with the help of an appendage, like flagella. Nonetheless, flagella can act as an "Achilles heel," revealing the pathogen's presence to the host through the stimulation of innate and adaptive immune responses. We assessed whether curcumin, a dietary polyphenol, could alter the motility of Salmonella, a foodborne pathogen. It reduced the motility of Salmonella enterica serovar Typhimurium by shortening the length of the flagellar filament (from ∼8 μm to ∼5 μm) and decreasing its density (4 or 5 flagella/bacterium instead of 8 or 9 flagella/bacterium). Upon curcumin treatment, the percentage of flagellated bacteria declined from ∼84% to 59%. However, no change was detected in the expression of the flagellin gene and protein. A fluorescence binding assay demonstrated binding of curcumin to the flagellar filament. This might make the filament fragile, breaking it into smaller fragments. Computational analysis predicted the binding of curcumin, its analogues, and its degraded products to a flagellin molecule at an interface between domains D1 and D2. Site-directed mutagenesis and a fluorescence binding assay confirmed the binding of curcumin to flagellin at residues ASN120, ASP123, ASN163, SER164, ASN173, and GLN175.

IMPORTANCE

This work, to our knowledge the first report of its kind, examines how curcumin targets flagellar density and affects the pathogenesis of bacteria. We found that curcumin does not affect any of the flagellar synthesis genes. Instead, it binds to the flagellum and makes it fragile. It increases the torsional stress on the flagellar filament that then breaks, leaving fewer flagella around the bacteria. Flagella, which are crucial ligands for Toll-like receptor 5, are some of the most important appendages of Salmonella Curcumin is an important component of turmeric, which is a major spice used in Asian cooking. The loss of flagella can, in turn, change the pathogenesis of bacteria, making them more robust and fit in the host.

SOS System Induction Inhibits the Assembly of Chemoreceptor Signaling Clusters in Salmonella enterica

Swarming, a flagellar-driven multicellular form of motility, is associated with bacterial virulence and increased antibiotic resistance. In this work we demonstrate that activation of the SOS response reversibly inhibits swarming motility by preventing the assembly of chemoreceptor-signaling polar arrays. We also show that an increase in the concentration of the RecA protein, generated by SOS system activation, rather than another function of this genetic network impairs chemoreceptor polar cluster formation. Our data provide evidence that the molecular balance between RecA and CheW proteins is crucial to allow polar cluster formation in Salmonella enterica cells. Thus, activation of the SOS response by the presence of a DNA-injuring compound increases the RecA concentration, thereby disturbing the equilibrium between RecA and CheW and resulting in the cessation of swarming. Nevertheless, when the DNA-damage decreases and the SOS response is no longer activated, basal RecA levels and thus polar cluster assembly are reestablished. These results clearly show that bacterial populations moving over surfaces make use of specific mechanisms to avoid contact with DNA-damaging compounds.

Nontoxic colloidal particles impede antibiotic resistance of swarming bacteria by disrupting collective motion and speed

A monolayer of swarming B. subtilis on semisolid agar is shown to display enhanced resistance against antibacterial drugs due to their collective behavior and motility. The dynamics of swarming motion, visualized in real time using time-lapse microscopy, prevents the bacteria from prolonged exposure to lethal drug concentrations. The elevated drug resistance is significantly reduced when the collective motion of bacteria is judiciously disrupted using nontoxic polystyrene colloidal particles immobilized on the agar surface. The colloidal particles block and hinder the motion of the cells, and force large swarming rafts to break up into smaller packs in order to maneuver across narrow spaces between densely packed particles. In this manner, cohesive rafts rapidly lose their collectivity, speed, and group dynamics, and the cells become vulnerable to the drugs. The antibiotic resistance capability of swarming B. subtilis is experimentally observed to be negatively correlated with the number density of colloidal particles on the engineered surface. This relationship is further tested using an improved self-propelled particle model that takes into account interparticle alignment and hard-core repulsion. This work has pertinent implications on the design of optimal methods to treat drug resistant bacteria commonly found in swarming colonies.

Shelter in a Swarm

Flagella propel bacteria during both swimming and swarming, dispersing them widely. However, while swimming bacteria use chemotaxis to find nutrients and avoid toxic environments, swarming bacteria appear to suppress chemotaxis and to use the dynamics of their collective motion to continuously expand and acquire new territory, barrel through lethal chemicals in their path, carry along bacterial and fungal cargo that assists in exploration of new niches, and engage in group warfare for niche dominance. Here, we focus on two aspects of swarming, which, if understood, hold the promise of revealing new insights into microbial signaling and behavior, with ramifications beyond bacterial swarming. These are as follows: how bacteria sense they are on a surface and turn on programs that promote movement and how they override scarcity and adversity as dense packs.

Bacterial swarms recruit cargo bacteria to pave the way in toxic environments

Swarming bacteria are challenged by the need to invade hostile environments. Swarms of the flagellated bacterium Paenibacillus vortex can collectively transport other microorganisms. Here we show that P. vortex can invade toxic environments by carrying antibiotic-degrading bacteria; this transport is mediated by a specialized, phenotypic subpopulation utilizing a process not dependent on cargo motility. Swarms of beta-lactam antibiotic (BLA)-sensitive P. vortex used beta-lactamase-producing, resistant, cargo bacteria to detoxify BLAs in their path. In the presence of BLAs, both transporter and cargo bacteria gained from this temporary cooperation; there was a positive correlation between BLA resistance and dispersal. P. vortex transported only the most beneficial antibiotic-resistant cargo (including environmental and clinical isolates) in a sustained way. P. vortex displayed a bet-hedging strategy that promoted the colonization of nontoxic niches by P. vortex alone; when detoxifying cargo bacteria were not needed, they were lost. This work has relevance for the dispersal of antibiotic-resistant microorganisms and for strategies for asymmetric cooperation with agricultural and medical implications.

Antibiotic resistance is a major health threat. We show a novel mechanism for the local spread of antibiotic resistance. This involves interactions between different bacteria: one species provides an enzyme that detoxifies the antibiotic (a sessile cargo bacterium carrying a resistance gene), while the other (Paenibacillus vortex) moves itself and transports the cargo. P. vortex used a bet-hedging strategy, colonizing new environments alone when the cargo added no benefit, but cooperating when the cargo was needed. This work is of interest in an evolutionary context and sheds light on fundamental questions, such as how environmental antibiotic resistance may lead to clinical resistance and also microbial social organization, as well as the costs, benefits, and risks of dispersal in the environment.

Comparative analysis of Salmonella susceptibility and tolerance to the biocide chlorhexidine identifies a complex cellular defense network

Chlorhexidine is one of the most widely used biocides in health and agricultural settings as well as in the modern food industry. It is a cationic biocide of the biguanide class. Details of its mechanism of action are largely unknown. The frequent use of chlorhexidine has been questioned recently, amidst concerns that an overuse of this compound may select for bacteria displaying an altered susceptibility to antimicrobials, including clinically important anti-bacterial agents. We generated a Salmonella enterica serovar Typhimurium isolate (ST24(CHX)) that exhibited a high-level tolerant phenotype to chlorhexidine, following several rounds of in vitro selection, using sub-lethal concentrations of the biocide. This mutant showed altered suceptibility to a panel of clinically important antimicrobial compounds. Here we describe a genomic, transcriptomic, proteomic, and phenotypic analysis of the chlorhexidine tolerant S. Typhimurium compared with its isogenic sensitive progenitor. Results from this study describe a chlorhexidine defense network that functions in both the reference chlorhexidine sensitive isolate and the tolerant mutant. The defense network involved multiple cell targets including those associated with the synthesis and modification of the cell wall, the SOS response, virulence, and a shift in cellular metabolism toward anoxic pathways, some of which were regulated by CreB and Fur. In addition, results indicated that chlorhexidine tolerance was associated with more extensive modifications of the same cellular processes involved in this proposed network, as well as a divergent defense response involving the up-regulation of additional targets such as the flagellar apparatus and an altered cellular phosphate metabolism. These data show that sub-lethal concentrations of chlorhexidine induce distinct changes in exposed Salmonella, and our findings provide insights into the mechanisms of action and tolerance to this biocidal agent.

Sub-inhibitory fosmidomycin exposures elicits oxidative stress in Salmonella enterica serovar Typhimurium LT2

Fosmidomycin is a time-dependent nanomolar inhibitor of methylerythritol phosphate (MEP) synthase, which is the enzyme that catalyzes the first committed step in the MEP pathway to isoprenoids. Importantly, fosmidomycin is one of only a few MEP pathway-specific inhibitors that exhibits antimicrobial activity. Most inhibitors identified to date only exhibit activity against isolated pathway enzymes. The MEP pathway is the sole route to isoprenoids in many bacteria, yet has no human homologs. The development of inhibitors of this pathway holds promise as novel antimicrobial agents. Similarly, analyses of the bacterial response toward MEP pathway inhibitors provides valuable information toward the understanding of how emergent resistance may ultimately develop to this class of antibiotics. We have examined the transcriptional response of Salmonella enterica serovar typhimurium LT2 to sub-inhibitory concentrations of fosmidomycin via cDNA microarray and RT-PCR. Within the regulated genes identified by microarray were a number of genes encoding enzymes associated with the mediation of reactive oxygen species (ROS). Regulation of a panel of genes implicated in the response of cells to oxidative stress (including genes for catalases, superoxide dismutases, and alkylhydrogen peroxide reductases) was investigated and mild upregulation in some members was observed as a function of fosmidomycin exposure over time. The extent of regulation of these genes was similar to that observed for comparable exposures to kanamycin, but differed significantly from tetracycline. Furthermore, S. typhimurium exposed to sub-inhibitory concentrations of fosmidomycin displayed an increased sensitivity to exogenous H2O2 relative to either untreated controls or kanamycin-treated cells. Our results suggest that endogenous oxidative stress is one consequence of exposures to fosmidomycin, likely through the temporal depletion of intracellular isoprenoids themselves, rather than other mechanisms that have been proposed to facilitate ROS accumulation in bacteria (e.g. cell death processes or the ability of the antibiotic to redox cycle).

Characterization of the Burkholderia thailandensis SOS response by using whole-transcriptome shotgun sequencing

The bacterial SOS response is a well-characterized regulatory network encoded by most prokaryotic bacterial species and is involved in DNA repair. In addition to nucleic acid repair, the SOS response is involved in pathogenicity, stress-induced mutagenesis, and the emergence and dissemination of antibiotic resistance. Using high-throughput sequencing technology (SOLiD RNA-Seq), we analyzed the Burkholderia thailandensis global SOS response to the fluoroquinolone antibiotic, ciprofloxacin (CIP), and the DNA-damaging chemical, mitomycin C (MMC). We demonstrate that a B. thailandensis recA mutant (RU0643) is ∼4-fold more sensitive to CIP in contrast to the parental strain B. thailandensis DW503. Our RNA-Seq results show that CIP and MMC treatment (P < 0.01) resulted in the differential expression of 344 genes in B. thailandensis and 210 genes in RU0643. Several genes associated with the SOS response were induced and include lexA, uvrA, dnaE, dinB, recX, and recA. At the genome-wide level, we found an overall decrease in gene expression, especially for genes involved in amino acid and carbohydrate transport and metabolism, following both CIP and MMC exposure. Interestingly, we observed the upregulation of several genes involved in bacterial motility and enhanced transcription of a B. thailandensis genomic island encoding a Siphoviridae bacteriophage designated E264. Using B. thailandensis plaque assays and PCR with B. mallei ATCC 23344 as the host, we demonstrate that CIP and MMC exposure in B. thailandensis DW503 induces the transcription and translation of viable bacteriophage in a RecA-dependent manner. This is the first report of the SOS response in Burkholderia spp. to DNA-damaging agents. We have identified both common and unique adaptive responses of B. thailandensis to chemical stress and DNA damage.

Extracellular DNA-induced antimicrobial peptide resistance in Salmonella enterica serovar Typhimurium

Background

The Salmonella enterica serovar Typhimurium PhoPQ two component system (TCS) is activated by low Mg²⁺ levels, low pH and by antimicrobial peptides (AP). Under Mg²⁺ limitation, the PhoPQ system induces pmrD expression, which post-translationally activates the PmrAB TCS. PhoPQ and PmrAB control many genes required for intracellular survival and pathogenesis. These include the polymyxin resistance (pmr) operon, which is required for aminoarabinose modification of LPS and protecting the outer membrane from antimicrobial peptide disruption and killing. Extracellular DNA is a ubiquitous polymer in the matrix of biofilms and accumulates in some infection sites. Extracellular DNA chelates cations and thus activates the Pseudomonas aeruginosa PhoPQ/PmrAB systems, leading to expression of the orthologous arn (pmr) operon.

Results

Here we show that extracellular DNA induces expression of the S. Typhimurium pmr antimicrobial peptide resistance operon in a PhoPQ and PmrAB-dependent manner. Induction of the pmr genes by DNA was blocked when present with excess Mg²⁺. Exogenous DNA led to increased resistance of planktonic cultures to aminoglycosides, antimicrobial peptides (AP) and ciprofloxacin, but only AP resistance was PhoPQ/PmrAB-dependent. Extracellular DNA was shown to be a matrix component of S. Typhimurium biofilms cultivated in flow chambers and on glass surfaces. A pmrH-gfp fusion was highly expressed in flow chamber biofilms cultivated in medium with repressing levels of 10 mM Mg²⁺ and co-localized with eDNA. Expression of pmrH-lux was monitored in plastic peg biofilms and shown to require PhoPQ and PmrAB. Biofilms had higher levels of pmrH expression compared to planktonic cultures. We propose that DNA accumulation in biofilms contributes to the increased pmrH-lux expression in biofilms.

Conclusions

The Salmonella PhoPQ/PmrAB systems and antimicrobial peptide resistance are activated by the cation chelating properties of extracellular DNA. DNA-induced AP resistance may allow immune evasion and increased survival of S. Typhimurium biofilms formed during extracellular growth stages of an infection or outside the host.

Concentration-dependent activity of antibiotics in natural environments

Bacterial responses to antibiotics are concentration-dependent. At high concentrations, antibiotics exhibit antimicrobial activities on susceptible cells, while subinhibitory concentrations induce diverse biological responses in bacteria. At non-lethal concentrations, bacteria may sense antibiotics as extracellular chemicals to trigger different cellular responses, which may include an altered antibiotic resistance/tolerance profile. In natural settings, microbes are typically in polymicrobial communities and antibiotic-mediated interactions between species may play a significant role in bacterial community structure and function. However, these aspects have not yet fully been explored at the community level. Here we discuss the different types of interactions mediated by antibiotics and non-antibiotic metabolites as a function of their concentrations and speculate on how these may amplify the overall antibiotic resistance/tolerance and the spread of antibiotic resistance determinants in a context of polymicrobial community.

Influence of Pseudomonas aeruginosa pvdQ gene on altering antibiotic susceptibility under swarming conditions

In Pseudomonas aeruginosa PAO1, the pvdQ gene has been shown to have at least two functions. It encodes the acylase enzyme and hydrolyzes 3-oxo-C12-HSL, the key signaling molecule of quorum sensing system. In addition, pvdQ is involved in swarming motility. It is required and up-regulated during swarming motility, which is triggered by high cell densities. As high density bacterial populations also display elevated antibiotics resistance, studies have demonstrated swarm-cell differentiation in P. aeruginosa promotes increased resistance to various antibiotics. PvdQ acts as a signal during swarm-cell differentiation, and thus may play a role in P. aeruginosa antibiotic resistance. The aim of this study was to examine whether pvdQ was involved in modifying antibiotic susceptibility during swarming conditions and to investigate the mechanism by which this occurred. We constructed the PAO1pMEpvdQ strain, which overproduces PvdQ. PAO1pMEpvdQ promotes swarming motility, while PAO1ΔpvdQ abolishes swarming motility. In addition, both PAO1 and PAO1pMEpvdQ acquired resistance to ceftazidime, ciprofloxacin, meropenem, polymyxin B, and gentamicin, though PAO1pMEpvdQ exhibited a twofold to eightfold increase in antibiotic resistance compared to PAO1. These results indicate that pvdQ plays an important role in elevating antibiotic resistance via swarm-cell differentiation and possibly other mechanisms as well. We analyzed outer membrane permeability. Our data also suggest that pvdQ decreases P. aeruginosa outer membrane permeability, thereby elevating antibiotic resistance under swarming conditions. Our results suggest new approaches for reducing P. aeruginosa resistance.

Expression of the DisA amino acid decarboxylase from Proteus mirabilis inhibits motility and class 2 flagellar gene expression in Escherichia coli

In Proteus mirabilis, a putative phenylalanine decarboxylase (DisA) acts in a regulatory pathway to inhibit class 2 flagellar gene expression and motility. In this study, we demonstrate that DisA expression in Escherichia coli blocked motility and resulted in a 50-fold decrease in the expression of class 2 (fliA) and class 3 (fliC) flagellar genes. However, the expression of flhDC encoding the class 1 activator of the flagellar cascade was unchanged by DisA expression at both the transcriptional and translational levels. Phenethylamine, a decarboxylation product derived from phenylalanine, was able to mimic DisA overexpression and decrease both motility and class 2/3 flagellar gene expression. In addition, both DisA overexpression and phenethylamine strongly inhibited biofilm formation in E. coli. DisA overexpression and exogenous phenethylamine could also reduce motility in other enteric bacteria, but had no effect on motility in non-enteric Gram-negative bacteria. It is hypothesized that phenethylamine or a closely related compound formed by the DisA decarboxylation reaction inhibits the formation or activity of the FlhD(4)C(2) complex required for activation of class 2 genes.

Cysteine catabolism and cysteine desulfhydrase (CdsH/STM0458) in Salmonella enterica serovar typhimurium

Cysteine is potentially toxic and can affect diverse functions such as oxidative stress, antibiotic resistance, and swarming motility. The contribution of cysteine catabolism in modulating responses to cysteine has not been examined, in part because the genes have not been identified and mutants lacking these genes have not been isolated or characterized. We identified the gene for a previously described cysteine desulfhydrase, which we designated cdsH (formerly STM0458). We also identified a divergently transcribed gene that regulates cdsH expression, which we designated cutR (formerly ybaO, or STM0459). CdsH appears to be the major cysteine-degrading and sulfide-producing enzyme aerobically but not anaerobically. Mutants with deletions of cdsH and ybaO exhibited increased sensitivity to cysteine toxicity and altered swarming motility but unaltered cysteine-enhanced antibiotic resistance and survival in macrophages.

A third mode of surface-associated growth: immobilization of Salmonella enterica serovar Typhimurium modulates the RpoS-directed transcriptional programme

Although the growth of bacteria has been studied for more than a century, it is only in recent decades that surface-associated growth has received attention. In addition to the well-characterized biofilm and swarming lifestyles, bacteria can also develop as micro-colonies supported by structured environments in both food products and the GI tract. This immobilized mode of growth has not been widely studied. To develop our understanding of the effects of immobilization upon a food-borne bacterial pathogen, we used the IFR Gel Cassette model. The transcriptional programme and metabolomic profile of Salmonella enterica serovar Typhimurium ST4/74 were compared during planktonic and immobilized growth, and a number of immobilization-specific characteristics were identified. Immobilized S.Typhimurium did not express motility and chemotaxis genes, and electron microscopy revealed the absence of flagella. The expression of RpoS-dependent genes and the level of RpoS protein were increased in immobilized bacteria, compared with planktonic growth. Immobilized growth prevented the induction of SPI1, SPI4 and SPI5 gene expression, likely mediated by the FliZ transcriptional regulator. Using an epithelial cell-based assay, we showed that immobilized S.Typhimurium was significantly less invasive than planktonic bacteria, and we suggest that S.Typhimurium grown in immobilized environments are less virulent than planktonic bacteria. Our findings identify immobilization as a third type of surface-associated growth that is distinct from the biofilm and swarming lifestyles of Salmonella.

Influence of Pseudomonas aeruginosa pvdQ gene on altering antibiotic susceptibility under swarming conditions

In Pseudomonas aeruginosa PAO1, the pvdQ gene has been shown to have at least two functions. It encodes the acylase enzyme and hydrolyzes 3-oxo-C12-HSL, the key signaling molecule of quorum sensing system. In addition, pvdQ is involved in swarming motility. It is required for up-regulated during swarming motility, which is triggered by high cell densities. As high-density bacterial populations also display elevated antibiotic resistance, studies have demonstrated that swarm-cell differentiation in P. aeruginosa promotes increased resistance to various antibiotics. PvdQ acts as a signal during swarm-cell differentiation, and thus may play a role in P. aeruginosa antibiotic resistance. The aim of this study is to examine whether pvdQ was involved in modifying antibiotic susceptibility during swarming conditions, and to investigate the mechanism by which this occurred. We constructed the PAO1pMEpvdQ strain, which overproduced PvdQ. PAO1pMEpvdQ promotes swarming motility, while PAO1ΔpvdQ abolishes swarming motility. In addition, both PAO1 and PAO1pMEpvdQ acquired resistance to ceftazidime, ciprofloxacin, meropenem, polymyxin B, and gentamicin, though PAO1pMEpvdQ exhibited a two to eightfold increase in antibiotic resistance compared to PAO1. These results indicate that pvdQ plays an important role in elevating antibiotic resistance via swarm-cell differentiation and possibly other mechanisms as well. We analyzed outer membrane permeability. Our data also suggest that pvdQ decreases P. aeruginosa outer membrane permeability, thereby elevating antibiotic resistance under swarming conditions. Our results suggest new approaches for reducing P. aeruginosa resistance.

Metabolic regulation of antibiotic resistance

It is generally assumed that antibiotics and resistance determinants are the task forces of a biological warfare in which each resistance determinant counteracts the activity of a specific antibiotic. According to this view, antibiotic resistance might be considered as a specific response to an injury, not necessarily linked to bacterial metabolism, except for the burden that the acquisition of resistance might impose on the bacteria (fitness costs). Nevertheless, it is known that changes in bacterial metabolism, such as those associated with dormancy or biofilm formation, modulate bacterial susceptibility to antibiotics (phenotypic resistance), indicating that there exists a linkage between bacterial metabolism and antibiotic resistance. The analyses of the intrinsic resistomes of bacterial pathogens also demonstrate that the building up of intrinsic resistance requires the concerted action of many elements, several of which play a relevant role in the bacterial metabolism. In this article, we will review the current knowledge on the linkage between bacterial metabolism and antibiotic resistance and will discuss the role of global metabolic regulators such as Crc in bacterial susceptibility to antibiotics. Given that growing into the human host requires a metabolic adaptation, we will discuss whether this adaptation might trigger resistance even in the absence of selective pressure by antibiotics.

Global gene expression profile for swarming Bacillus cereus bacteria

Bacillus cereus can use swarming to move over and colonize solid surfaces in different environments. This kind of motility is a collective behavior accompanied by the production of long and hyperflagellate swarm cells. In this study, the genome-wide transcriptional response of B. cereus ATCC 14579 during swarming was analyzed. Swarming was shown to trigger the differential expression (>2-fold change) of 118 genes. Downregulated genes included those required for basic cellular metabolism. In accordance with the hyperflagellate phenotype of the swarm cell, genes encoding flagellin were overexpressed. Some genes associated with K(+) transport, phBC6A51 phage genes, and the binding component of the enterotoxin hemolysin BL (HBL) were also induced. Quantitative reverse transcription-PCR (qRT-PCR) experiments indicated an almost 2-fold upregulation of the entire hbl operon during swarming. Finally, BC1435 and BC1436, orthologs of liaI-liaH that are known to be involved in the resistance of Bacillus subtilis to daptomycin, were upregulated under swarming conditions. Accordingly, phenotypic assays showed reduced susceptibility of swarming B. cereus cells to daptomycin, and P(spac)-induced hyper-expression of these genes in liquid medium highlighted the role of BC1435 and BC1436 in the response of B. cereus to daptomycin.

Creeping baselines and adaptive resistance to antibiotics

The introduction of antimicrobial drugs in medicine gave hope for a future in which all infectious diseases could be controlled. Decades later it appears certain this will not be the case, because antibiotic resistance is growing relentlessly. Bacteria possess an extraordinary ability to adapt to environmental challenges like antimicrobials by both genetic and phenotypic means, which contributes to their evolutionary success. It is becoming increasingly appreciated that adaptation is a major mechanism behind the acquisition and evolution of antibiotic resistance. Adaptive resistance is a specific class of non-mutational resistance that is characterized by its transient nature. It occurs in response to certain environmental conditions or due to epigenetic phenomena like persistence. We propose that this type of resistance could be the key to understanding the failure of some antibiotic therapy programs, although adaptive resistance mechanisms are still somewhat unexplored. Similarly, hard wiring of some of the changes involved in adaptive resistance might explain the phenomenon of "baseline creep" whereby the average minimal inhibitory concentration (MIC) of a given medically important bacterial species increases steadily but inexorably over time, making the likelihood of breakthrough resistance greater. This review summarizes the available information on adaptive resistance.

Features of Bacillus cereus swarm cells

When propagated on solid surfaces, Bacillus cereus can produce differentiated swarm cells under a wide range of growth conditions. This behavioural versatility is ecologically relevant, since it allows this bacterium to adapt swarming to environmental changes. Swarming by B. cereus is medically important: swarm cells are more virulent and particularly prone to invade host tissues. Characterisation of swarming-deficient mutants highlights that flagellar genes as well as genes governing different metabolic pathways are involved in swarm-cell differentiation. In this review, the environmental and genetic requirements for swarming and the role played by swarm cells in the virulence this pathogen exerts will be outlined.

Cysteine biosynthesis, oxidative stress and antibiotic resistance in Salmonella typhimurium

The efficacy of antibiotics varies under different growth conditions due to the induction of specific or more general defense pathways, but the mechanisms are not completely understood. Actively swarming Salmonella show elevated resistance to many types of antibiotics. Previously, we had shown that cysteine biosynthesis was important for the induced antibiotic resistance phenotype of swarm cells. Here we examine the connection of cysteine to oxidative stress and demonstrate that the antioxidant properties of cysteine or cysteine-derived metabolites contribute to the antibiotic resistance in both vegetative and swarm cell populations. We observed that cys auxotrophs were oxidatively stressed, and in wild-type cells expression of the cys regulon was induced during periods of oxidative stress. In swarm cells, we found a 6-fold increase in reduced glutathione compared to swim cells and a corresponding increased resistance to oxidants. Wild-type and cys auxotrophs exhibited the same sensitivities to gentamicin, polymyxin and ciprofloxacin when grown anaerobically, suggesting that induced oxidative stress defense was contributing to elevated antibiotic resistance in swarm cells aerobically. Induction of the CysB regulon by addition of exogenous inducer resulted in elevated antibiotic resistance independently of swarming.

Characterization of swarming motility in Rhizobium leguminosarum bv. viciae

We have characterized swarming motility in Rhizobium leguminosarum strains 3841 and VF39SM. Swarming was dependent on growth on energy-rich media, and both agar concentration and incubation temperature were critical parameters for surface migration. A cell density-dependent lag period was observed before swarming motility was initiated. Surface migration began 3-5 days after inoculation and a full swarming phenotype was observed 3 weeks after inoculation. The swarming front was preceded by a clear extracellular matrix, from which we failed to detect surfactants. The edge of the swarming front formed by VF39SM was characterized by hyperflagellated cells arranged in rafts, whereas the cells at the point of inoculation were indistinguishable from vegetative cells. Swarmer cells formed by 3841, in contrast, showed a minor increase in flagellation, with each swarmer cell exhibiting an average of three flagellar filaments, compared with an average of two flagella per vegetative cell. Reflective of their hyperflagellation, the VF39SM swarmer cells demonstrated an increased expression of flagellar genes. VF39SM swarmed better than 3841 under all the conditions tested, and the additional flagellation in VF39SM swarm cells may contribute to this difference. Metabolism of the supplemented carbon source appeared to be necessary for surface migration as strains incapable of utilizing the carbon source failed to swarm. We also observed that swarmer cells have increased resistance to several antibiotics.

Cell density and mobility protect swarming bacteria against antibiotics

Swarming bacteria move in multicellular groups and exhibit adaptive resistance to multiple antibiotics. Analysis of this phenomenon has revealed the protective power of high cell densities to withstand exposure to otherwise lethal antibiotic concentrations. We find that high densities promote bacterial survival, even in a nonswarming state, but that the ability to move, as well as the speed of movement, confers an added advantage, making swarming an effective strategy for prevailing against antimicrobials. We find no evidence of induced resistance pathways or quorum-sensing mechanisms controlling this group resistance, which occurs at a cost to cells directly exposed to the antibiotic. This work has relevance to the adaptive antibiotic resistance of bacterial biofilms.

Induction of Salmonella pathogenicity island 1 under different growth conditions can affect Salmonella-host cell interactions in vitro

Salmonella invade non-phagocytic cells by inducing massive actin rearrangements, resulting in membrane ruffle formation and phagocytosis of the bacteria. This process is mediated by a cohort of effector proteins translocated into the host cell by type III secretion system 1, which is encoded by genes in the Salmonella pathogenicity island (SPI) 1 regulon. This network is precisely regulated and must be induced outside of host cells. In vitro invasive Salmonella are prepared by growth in synthetic media although the details vary. Here, we show that culture conditions affect the frequency, and therefore invasion efficiency, of SPI1-induced bacteria and also can affect the ability of Salmonella to adapt to its intracellular niche following invasion. Aerobically grown late-exponential-phase bacteria were more invasive and this was associated with a greater frequency of SPI1-induced, motile bacteria, as revealed by single-cell analysis of gene expression. Culture conditions also affected the ability of Salmonella to adapt to the intracellular environment, since they caused marked differences in intracellular replication. These findings show that induction of SPI1 under different pre-invasion growth conditions can affect the ability of Salmonella to interact with eukaryotic host cells.

Effect of the O-antigen length of lipopolysaccharide on the functions of Type III secretion systems in Salmonella enterica

The virulence of Salmonella enterica critically depends on the functions of two type III secretion systems (T3SS), with the Salmonella pathogenicity island 1 (SPI1)-encoded T3SS required for host cell invasion and the SPI2-T3SS enabling Salmonella to proliferate within host cells. A further T3SS is required for the assembly of the flagella. Most serovars of Salmonella also possess a lipopolysaccharide with a complex O-antigen (OAg) structure. The number of OAg units attached to the core polysaccharide varies between 16 and more than 100 repeats, with a trimodal distribution. This work investigated the correlation of the OAg length with the functions of the SPI1-T3SS and the SPI2-T3SS. We observed that the number of repeats of OAg units had no effect on bacterial motility. The interaction of Salmonella with epithelial cells was altered if the OAg structure was changed by mutations in regulators of OAg. Strains defective in synthesis of very long or long and very long OAg species showed increased translocation of a SPI1-T3SS effector protein and increased invasion. Invasion of a strain entirely lacking OAg was increased, but this mutant strain also showed increased adhesion. In contrast, translocation of a SPI2-T3SS effector protein and intracellular replication were not affected by modification of the OAg length. Mutant strains lacking the entire OAg or long and very long OAg were highly susceptible to complement killing. These observations indicate that the architecture of the outer membrane of Salmonella is balanced to permit sufficient T3SS function but also to confer optimal protection against antimicrobial defense mechanisms.

Brominated furanones inhibit biofilm formation by Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium is a main cause of bacterial food-borne diseases. As Salmonella can form biofilms in which it is better protected against antimicrobial agents on a wide diversity of surfaces, it is of interest to explore ways to inhibit biofilm formation. Brominated furanones, originally extracted from the marine alga Delisea pulchra, are known to interfere with biofilm formation in several pathogens. In this study, we have synthesized a small focused library of brominated furanones and tested their activity against S. enterica serovar Typhimurium biofilm formation. We show that several furanones inhibit Salmonella biofilm formation at non-growth-inhibiting concentrations. The most interesting compounds are (Z)-4-bromo-5-(bromomethylene)-3-alkyl-2(5H)-furanones with chain lengths of two to six carbon atoms. A microarray study was performed to analyze the gene expression profiles of Salmonella in the presence of (Z)-4-bromo-5-(bromomethylene)-3-ethyl-2(5H)-furanone. The induced genes include genes that are involved in metabolism, stress response, and drug sensitivity. Most of the repressed genes are involved in metabolism, the type III secretion system, and flagellar biosynthesis. Follow-up experiments confirmed that this furanone interferes with the synthesis of flagella by Salmonella. No evidence was found that furanones act on the currently known quorum-sensing systems in Salmonella. Interestingly, pretreatment with furanones rendered Salmonella biofilms more susceptible to antibiotic treatment. Conclusively, this work demonstrates that particular brominated furanones have potential in the prevention of biofilm formation by Salmonella serovar Typhimurium.

Biofilm induced tolerance towards antimicrobial peptides

Increased tolerance to antimicrobial agents is thought to be an important feature of microbes growing in biofilms. We address the question of how biofilm organization affects antibiotic susceptibility. We established Escherichia coli biofilms with differential structural organization due to the presence of IncF plasmids expressing altered forms of the transfer pili in two different biofilm model systems. The mature biofilms were subsequently treated with two antibiotics with different molecular targets, the peptide antibiotic colistin and the fluoroquinolone ciprofloxacin. The dynamics of microbial killing were monitored by viable count determination, and confocal laser microscopy. Strains forming structurally organized biofilms show an increased bacterial survival when challenged with colistin, compared to strains forming unstructured biofilms. The increased survival is due to genetically regulated tolerant subpopulation formation and not caused by a general biofilm property. No significant difference in survival was detected when the strains were challenged with ciprofloxacin. Our data show that biofilm formation confers increased colistin tolerance to cells within the biofilm structure, but the protection is conditional being dependent on the structural organization of the biofilm, and the induction of specific tolerance mechanisms.

Effect of physiological heterogeneity of E. coli population on antibiotic susceptivity test

According to the instantaneous growth rate (dN/dt) of E. coli CVCC249 growing in batch culture, the entire growth progress was distinguished into four phases: accelerating growth phase, constant growth phase, decelerating growth phase and declining phase, in each of which obvious variation in physiological and biochemical properties was detected, including total DNA, total protein, and MTT-dehydrogenase activity, etc., that led to difference in their antibiotic susceptivity. Antibiotic susceptivity of the population sampled from each phase was tested by Concentration-killing Curve (CKC) approach following the formula N=N (0)/{1+exp[r.(x-BC (50))]}, showing as normal distribution at the individual cell level for an internal population, in which the median bactericidal concentration BC (50) represents the mean level of susceptivity, while the bactericidal span BC (1-99)=(2lnN (0))/r indicates the variation degree of the antibiotic susceptivity. Furthermore, tested by CKC approach, the antibiotic susceptivity of E. coli CVCC249 population in each physiological phase to gentamicin or enoxacin was various: susceptivity of the population in the constant growth phase and declining phase all increased compared with that in the accelerating growth phase for gentamicin but declined for enoxacin. The primary investigations revealed that the physiological phase should be taken into account in the context of antibiotic susceptivity and research into antimicrobial mechanism. However there are few reports concerned with this study. Further research using different kinds of antibiotics with synchronized continuous culture of different bacterial strains is required.

Isolation of transposon mutants and characterization of genes involved in biofilm formation by Pseudomonas fluorescens TC222

Biofilm formation mutants are often found to have defective or altered motility. The motility phenotype was exploited to identify Pseudomonas fluorescens biofilm formation mutants. Fourteen motility mutants were obtained from P. fluorescens isolate TC222 and eight stable mutants were studied further. The eight transposon insertion mutants showed altered ability to form biofilm compared with the parent. Five Tn5-inserted genes from these mutants were cloned and sequenced. Genetic analysis showed that two insertions were located in genes affecting multiple cell surface characteristics, including lipopolysaccharide (rfbD) and polar flagella (fliR). Three genes encoding for a putative Mig-14 family protein (epsB), a probable bacteriophage signal peptide protein (bspA) and a soluble pyridine nucleotide transhydrogenase (pyrA) were reported for the first time to be involved in biofilm formation. Complementation experiments of rfbD and epsB genes proved that biofilm formation of the corresponding mutants could be restored. Further semi-quantitative reverse transcription-PCR analysis showed that both rfbD and epsB can express their transcripts much higher in the complemented strains than that in wild-type strains. The transcripts of both genes in their mutants could not be detected.

Proteome analysis of virulence factor regulated by autoinducer-2-like activity in Escherichia coli O157:H7

Many pathogenic bacteria, including Escherichia coli O157:H7, can control gene expression in a cell density-dependent manner by producing small signaling molecules (autoinducers) in a process known as quorum sensing. In this study, the effects of the autoinducer-2-like activity on the expression of proteins, including virulence factors, in E. coli O157:H7 were characterized by proteomic analysis. Compared with the control, E. coli O157:H7 strains in the presence of autoinducer-2-like activity exhibited elevated virulence by more rapidly forming cell aggregates on epithelial cells and rapidly killing the nematode Caenorhabditis elegans, the surrogate host. Two-dimensional gel electrophoresis revealed 18 proteins that were upregulated by autoinducer-2-like activity and 4 proteins that were down-regulated. These proteins were further characterized by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry and are involved in the metabolic process, adaptation and protection, cell motility, secretion, envelope biogenesis, and protein translation. These results indicate that the newly identified proteins are associated with the control of virulence in E. coli O157:H7 and that these proteins can be potential targets for the development of antibiotics and other antimicrobial agents.

Coordinated regulation of two independent cell-cell signaling systems and swarmer differentiation in Salmonella enterica serovar Typhimurium

Almost all members of the genus Salmonella differentiate and migrate on semisolid surfaces in a coordinated population behavior known as swarming. Important virulence determinants are coupled to swarmer differentiation in several other pathogenic organisms, collectively suggesting that conditions that trigger swarming in the laboratory may fortuitously promote the cells to enter a robust physiological state relevant to the host environment. Here, we present evidence that expression of two independent cell-cell signaling systems are also coupled to swarmer differentiation in S. enterica serovar Typhimurium. Expression of both pfs and sdiA genes was up-regulated in the actively migrating swarmers compared to their vegetative counterparts propagated in broth or spread plated on the surface of swim, swarm, and solid media. Accordingly, swarmers produced elevated levels of a universally recognized signaling molecule, autoinducer-2, and exhibited increased sensitivity to N-acyl homoserine lactones (AHLs), signaling molecules that Salmonella does not produce. Expression of the rck operon was concomitantly up-regulated in the swarmers in an SdiA-dependent manner only in the presence of exogenous AHLs. In addition to the previously reported adaptive antibiotic resistance phenotype and global shift in metabolism, this work presents another component of the physiological changes that are specifically associated with swarmer differentiation in serovar Typhimurium and not simply due to growth on a surface.

Prevalence of surface swarming behavior in Salmonella

Swarming behavior among 167 Salmonella sp. isolates, representing all eight groups, was assessed. Only eight strains failed to swarm under standard conditions. Four of the defective strains swarmed on alternate carbon sources, and four harbored general defects in motility or lipopolysaccharide. Thus, swarming may represent an evolutionarily conserved behavior in Salmonella spp.

Azithromycin inhibits the formation of flagellar filaments without suppressing flagellin synthesis in Salmonella enterica serovar typhimurium

The present study shows that a sub-MIC of the macrolide antibiotic azithromycin (AZM) diminishes the virulence function of Salmonella enterica serovar Typhimurium. We first constructed an AZM-resistant strain (MS248) by introducing ermBC, an erythromycin ribosome methylase gene, into serovar Typhimurium. The MIC of AZM for MS248 exceeded 100 microg/ml. Second, we managed to determine the efficacy with which a sub-MIC of AZM reduced the virulence of MS248 in vitro. On the one hand, AZM (10 microg/ml) in the culture medium was unable to inhibit the total protein synthesis, growth rate, or survival within macrophages of MS248. On the other hand, AZM (10 microg/ml) reduced MS248's swarming and swimming motilities in addition to its invasive activity in Henle-407 cells. Electron micrographs revealed no flagellar filaments on the surface of MS248 after overnight growth in L broth supplemented with AZM (10 microg/ml). However, immunoblotting analysis showed that flagellin (FliC) was fully synthesized within the bacterial cells in the presence of AZM (10 microg/ml). In contrast, the same concentration of AZM reduced the export of FliC to the culture medium. These results indicate that a sub-MIC of AZM was able to affect the formation of flagellar filaments, specifically by reducing the amount of flagellin exported from bacterial cells, but it was not involved in suppressing the synthesis of flagellin. Unfortunately, AZM treatment was ineffective against murine salmonellosis caused by MS248.

Metabolic differentiation in actively swarming Salmonella

Most current paradigms of microbial metabolism have been derived from studying cells grown under a variety of nutrient compositions in aqueous environments. With recent advances in genomics and experimental techniques, alternative forms of bacterial growth are increasingly being explored. When propagated on nutrient-rich semi-solid media, several species of bacteria undergo a morphological differentiation into swarmers that are capable of migrating on surfaces. Recent studies indicate that swarmer differentiation represents much more than a motility phenotype, as several clinically important attributes are also co-regulated. We demonstrate that migrating swarmer cells of Salmonella are metabolically differentiated compared to the vegetative swimmer cells grown in the same nutrient environment. Furthermore, once the cells have differentiated, the swarmers remain in this physiological state under conditions that do not promote the initial differentiation. The bacterium's capacity to override some of the classic paradigms of metabolic regulation established in aqueous environments represents a unique physiological response by the pathogen that may be advantageous in polymicrobial environments such as the host.