Bacterial pathogenesis as an imperfect symbiosis

Els patogens bacterians adaptats als humans com ara Salmonella enterica serovar Typhi, Helicobacter pylori o Mycobacterium tuberculosis causen infeccions agudes i també infeccions latents i asimptomàtiques. Durant una infecció latent, el patogen atenua la seva virulència i adopta un estil de vida que redueix l'impacte de la infecció sobre el benestar de l'hoste. Estratègies evolutives d'aquest tipus poden dirigir alguns patògens bacterians vers el comensalisme.Human-adapted bacterial pathogens such as Salmonella enterica serovar Typhi, Helicobacter pylori, and Mycobacterium tuberculosis cause acute infections and also latent, asymptomatic infections. During latent infection the pathogen undergoes self-attenuation of virulence, a lifestyle that reduces the impact of infection on host fitness. Evolutionary strategies of this kind may drive certain bacterial pathogens towards commensalism

Programmed heterogeneity: Epigenetic mechanisms in bacteria

Contrary to the traditional view that bacterial populations are clonal, single-cell analysis reveals that phenotypic heterogeneity is common in bacteria. Formation of distinct bacterial lineages appears to be frequent during adaptation to harsh environments, including the colonization of animals by bacterial pathogens. Formation of bacterial subpopulations is often controlled by epigenetic mechanisms that generate inheritable phenotypic diversity without altering the DNA sequence. Such mechanisms are diverse, ranging from relatively simple feedback loops to complex self-perpetuating DNA methylation patterns

Roles of DNA adenine methylation in host-pathogen interactions: mismatch repair, transcriptional regulation, and more

The Dam methylase of gamma-proteobacteria and the CcrM methylase of alpha-proteobacteria catalyze an identical reaction (methylation of adenosine moieties using S-adenosyl-methionine as methyl donor) at similar DNA targets (GATC and GANTC, respectively). Dam and CcrM are of independent evolutionary origin. Each may have evolved from an ancestral restriction-modification system that lost its restriction component, leaving an “orphan” methylase devoted solely to epigenetic genome modification. Formation of 6-methyladenine lowers the thermodynamic stability of DNA and changes DNA curvature. As a consequence, the methylation state of specific adenosine moieties can affect DNA-protein interactions. Well known examples include binding of the replication initiation complex to the methylated oriC, recognition of hemimethylated GATCs in newly replicated DNA by the MutHLS mismatch repair complex, and discrimination of methylation states in promoters and regulatory DNA motifs by RNA polymerase and transcription factors. In recent years, Dam and CcrM have been shown to play roles in host-pathogen interactions. These roles are diverse and only partially understood. Especially intriguing is the evidence that Dam methylation regulates virulence genes in E. coli, Salmonella, and Yersinia at the postranscriptional level

Host-pathogen interactions in typhoid fever: the model is the message

Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Fund PCIN-2015-131 (Infect-ERA) and BIO2016-75235-

Single cell analysis of bistable expression of pathogenicity island 1 and the flagellar regulon in Salmonella enterica

Bistable expression of the Salmonella enterica pathogenicity island 1 (SPI-1) and the flagellar network (Flag) has been described previously. In this study, simultaneous monitoring of OFF and ON states in SPI-1 and in the flagellar regulon reveals independent switching, with concomitant formation of four subpopulations: SPI-1OFF FlagOFF, SPI-1OFF FlagON, SPI-1ON FlagOFF, and SPI-1ON FlagON. Invasion assays upon cell sorting show that none of the four subpopulations is highly invasive, thus raising the possibility that FlagOFF cells might contribute to optimal invasion as previously proposed for SPI-1OFF cells. Time lapse microscopy observation indicates that expression of the flagellar regulon contributes to the growth impairment previously described in SPI-1ON cells. As a consequence, growth resumption in SPI-1ON FlagON cells requires switching to both SPI-1OFF and FlagOFF states.Ministerio de Economía, Industria y Competitividad. Gobierno de España-BIO2016-75235-PFondo Europeo de Desarrollo Regional. Unión EuropeaVI Plan Propio de Investigación y Transferencia. Universidad de Sevilla. Españ

Adaptation of Salmonella enterica to bile: essential role of AcrAB-mediated efflux

Adaptation to bile is the ability to endure the lethal effects of bile salts after growth on sublethal concentrations. Surveys of adaptation to bile in Salmonella enterica ser. Tyhimurium reveal that active efflux is essential for adaptation while other bacterial functions involved in bile resistance are not. Among S. enterica mutants lacking one or more efflux systems, only strains lacking AcrAB are unable to adapt, thus revealing an essential role for AcrAB. Transcription of the acrAB operon is upregulated in the presence of a sublethal concentration of sodium deoxycholate (DOC) while other efflux loci are either weakly upregulated or irresponsive. Upregulation of acrAB transcription is strong during exponential growth, and weak in stationary cultures. Single cell analysis of ethidium bromide accumulation indicates that DOC-induced AcrAB-mediated efflux occurs in both exponential and stationary cultures. Upregulation of acrAB expression may thus be crucial at early stages of adaptation, while sustained AcrAB activity may be sufficient to confer bile resistance in nondividing cells.Ministerio de Economía y Competitividad (MINECO) de España y el Fondo Regional Europeo (FEDER) BIO2013–44220-R y BIO2016–75235-PConsejería de Innovación, Ciencia y Empresa, Junta de Andalucía. CVI-587

Contribution of SPI-1 bistability to Salmonella enterica cooperative virulence: insights from single cell analysis

Salmonella enterica pathogenicity island 1 (SPI-1) is a gene cluster that encodes a type III secretion system and effectors involved in epithelial cell invasion. SPI-1 undergoes bistable expression, with concomitant formation of SPI-1ON and SPI-1OFF lineages. This study describes single cell analysis of SP1-1 bistability and epithelial cell invasion, and reports the unsuspected observation that optimal invasion of epithelial cells requires the presence of both SPI-1ON and SPI-1OFF subpopulations. The contribution of SPI-1OFF cells to optimal invasion may rely on their ability to invade epithelial cells if a SPI-1ON subpopulation is present. In fact, Salmonella SPI-1 mutants are also able to invade epithelial cells in the presence of SPI-1ON Salmonellae, a phenomenon described in the 1990’s by Galán and co-workers. Invasion by SPI-1OFF cells does not seem to involve a diffusible factor. A small number of SPI-1ON cells is sufficient to endow the bacterial population with invasion capacity, a feature that may permit host colonization regardless of the bottlenecks encountered by Salmonella populations inside animals.España, MINECO BIO2016-75235-

Interactions between Bacteria and Bile Salts in the Gastrointestinal and Hepatobiliary Tracts

Bile salts and bacteria have intricate relationships. The composition of the intestinal pool of bile salts is shaped by bacterial metabolism. In turn, bile salts play a role in intestinal homeostasis by controlling the size and the composition of the intestinal microbiota. As a consequence, alteration of the microbiome–bile salt homeostasis can play a role in hepatic and gastrointestinal pathological conditions. Intestinal bacteria use bile salts as environmental signals and in certain cases as nutrients and electron acceptors. However, bile salts are antibacterial compounds that disrupt bacterial membranes, denature proteins, chelate iron and calcium, cause oxidative damage to DNA, and control the expression of eukaryotic genes involved in host defense and immunity. Bacterial species adapted to the mammalian gut are able to endure the antibacterial activities of bile salts by multiple physiological adjustments that include remodeling of the cell envelope and activation of efflux systems and stress responses. Resistance to bile salts permits that certain bile-resistant pathogens can colonize the hepatobiliary tract, and an outstanding example is the chronic infection of the gall bladder by Salmonella enterica. A better understanding of the interactions between bacteria and bile salts may inspire novel therapeutic strategies for gastrointestinal and hepatobiliary diseases that involve microbiome alteration, as well as novel schemes against bacterial infectionsEspaña, MINECO BIO2013-44220-

Contribution of phenotypic heterogeneity to adaptive antibiotic resistance

Antibiotic-resistant isolates of Salmonella enterica were selected on plates containing lethal concentrations of rifampicin, kanamycin, and nalidixic acid. The stability of the resistance phenotype was scored after nonselective growth. Rifampicin-resistant (Rifr) isolates were stable, suggesting that they had arisen by mutation. Mutations in the rpoB gene were detected indeed in Rifr mutants. In contrast, a fraction of kanamycin-resistant (Kmr) and nalidixic acid-resistant (Nal r) isolates showed reduced resistance after nonselective growth, suggesting that mechanisms other than mutation had contributed to bacterial survival upon lethal selection. Single-cell analysis revealed heterogeneity in expression of the porin gene ompC, and subpopulation separation provided evidence that reduced ompC expression confers adaptive resistance to kanamycin. In the case of Nalr isolates, mutations in the gyrA gene were present in most nalidixic acid-resistant isolates. However, the efflux pump inhibitor Phe-Arg-β-naphtylamide (PAβN) reduced the level of resistance in Nalr mutants, indicating that active efflux contributes to the overall level of nalidixic acid resistance. Heterogeneous efflux pump activity was detected in single cells and colonies, and a correlation between high efflux and increased resistance to nalidixic acid was found. These observations suggest that fluctuations in the expression and the activity of critical functions of the bacterial cell, alone or combined with mutations, can contribute to adaptive resistance to antibiotics.Ministerio de Economía e Innovación BIO2010-15023, CSD 2008-00013Junta de Andalucía. Consejería de Innovación, Ciencia y Empresa. CVI-587