Variation in within-host replication kinetics among virus genotypes provides evidence of specialist and generalist infection strategies across three salmonid host species

Theory of the evolution of pathogen specialization suggests that a specialist pathogen gains high fitness in one host, but this comes with fitness loss in other hosts. By contrast, a generalist pathogen does not achieve high fitness in any host, but gains ecological fitness by exploiting different hosts, and has higher fitness than specialists in nonspecialized hosts. As a result, specialist pathogens are predicted to have greater variation in fitness across hosts, and generalists would have lower fitness variation across hosts. We test these hypotheses by measuring pathogen replicative fitness as within-host viral loads from the onset of infection to the beginning of virus clearance, using the rhabdovirus infectious hematopoietic necrosis virus (IHNV) in salmonid fish. Based on field prevalence and virulence studies, the IHNV subgroups UP, MD, and L are specialists, causing infection and mortality in sockeye salmon, steelhead, and Chinook salmon juveniles, respectively. The UC subgroup evolved naturally from a UP ancestor and is a generalist infecting all three host species but without causing severe disease. We show that the specialist subgroups had the highest peak and mean viral loads in the hosts in which they are specialized, and they had low viral loads in nonspecialized hosts, resulting in large variation in viral load across hosts. Viral kinetics show that the mechanisms of specialization involve the ability to both maximize early virus replication and avoid clearance at later times, with different mechanisms of specialization evident in different host-virus combinations. Additional nuances in the data included different fitness levels for nonspecialist interactions, reflecting different trade-offs for specialist viruses in other hosts. The generalist UC subgroup reached intermediate viral loads in all hosts and showed the smallest variation in fitness across hosts. The evolution of the UC generalist from an ancestral UP sockeye specialist was associated with fitness increases in steelhead and Chinook salmon, but only slight decreases in fitness in sockeye salmon, consistent with low- or no-cost generalism. Our results support major elements of the specialist-generalist theory, providing evidence of a specialist-generalist continuum in a vertebrate pathogen. These results also quantify within-host replicative fitness trade-offs resulting from the natural evolution of specialist and generalist virus lineages in multi-host ecosystems.

Risk assessment strategies for early detection and prediction of infectious disease outbreaks associated with climate change

A new generation of surveillance strategies is being developed to help detect emerging infections and to identify the increased risks of infectious disease outbreaks that are expected to occur with climate change. These surveillance strategies include event-based surveillance (EBS) systems and risk modelling. The EBS systems use open-source internet data, such as media reports, official reports, and social media (such as Twitter) to detect evidence of an emerging threat, and can be used in conjunction with conventional surveillance systems to enhance early warning of public health threats. More recently, EBS systems include artificial intelligence applications such machine learning and natural language processing to increase the speed, capacity and accuracy of filtering, classifying and analysing health-related internet data. Risk modelling uses statistical and mathematical methods to assess the severity of disease emergence and spread given factors about the host (e.g. number of reported cases), pathogen (e.g. pathogenicity) and environment (e.g. climate suitability for reservoir populations). The types of data in these models are expanding to include health-related information from open-source internet data and information on mobility patterns of humans and goods. This information is helping to identify susceptible populations and predict the pathways from which infections might spread into new areas and new countries. As a powerful addition to traditional surveillance strategies that identify what has already happened, it is anticipated that EBS systems and risk modelling will increasingly be used to inform public health actions to prevent, detect and mitigate the climate change increases in infectious diseases.

Pediatric biopsy of a single native kidney

Technological improvements have reduced the frequency of complications in children receiving a percutaneous renal biopsy. No study has systematically compared the safety of open and percutaneous kidney biopsy. Yet many nephrologists consider a single native kidney an absolute contraindication to percutaneous biopsy. We have established an international registry of single native kidney biopsies in children and we now report our early results. Eight biopsies are included. Seven patients had percutaneous biopsies and one an open biopsy. None of the patients had major complications, and adequate tissue was obtained from all. Our limited experience indicates that the presence of a single native kidney is not an absolute indication for an open approach. We encourage our colleagues to report to the international registry in order to further document the safety of percutaneous biopsy of the single native kidney in children.

Broadly protective vaccine for Staphylococcus aureus based on an in vivo-expressed antigen

Vaccines based on preferential expression of bacterial antigens during human infection have not been described. Staphylococcus aureus synthesized poly-N-succinyl beta-1-6 glucosamine (PNSG) as a surface polysaccharide during human and animal infection, but few strains expressed PNSG in vitro. All S. aureus strains examined carried genes for PNSG synthesis. Immunization protected mice against kidney infections and death from strains that produced little PNSG in vitro. Nonimmune infected animals made antibody to PNSG, but serial in vitro cultures of kidney isolates yielded mostly cells that did not produce PNSG. PNSG is a candidate for use in a vaccine to protect against S. aureus infection.

The ica locus of Staphylococcus epidermidis encodes production of the capsular polysaccharide/adhesin

Clinical isolates of coagulase-negative staphylococci often elaborate a biofilm involved in adherence to medical devices and resistance to host defenses. The biofilm contains the capsular polysaccharide/adhesin (PS/A), which mediates cell adherence to biomaterials, and another antigen, termed polysaccharide intercellular adhesin (PIA), which is thought to mediate bacterial accumulation into cellular aggregates. PIA is a polymer of beta-1, 6-linked N-acetyl glucosamine residues with a molecular mass of <30, 000 kDa. We found that recombinant Staphylococcus carnosus and Staphylococcus aureus carrying a plasmid with genes of the ica locus, which was reported to encode the biosynthetic proteins for production of PIA, were also able to synthesize PS/A. PS/A and a chemically and immunologically identical polysaccharide isolated from S. carnosus carrying the ica genes on plasmid pCN27 were found to be high-molecular-mass (>250,000 kDa), acid-stable polymers of beta-1,6-linked glucosamine substituted on the amino group primarily with succinate, although some preparations also contained acetate. Moreover, all recombinant staphylococcal strains with the ica genes had the biologic properties previously attributed to PS/A. ica-positive strains readily formed an in vitro biofilm on plastic, adhered 3- to 10-fold more to catheters during a 30-min assay compared with control strains carrying only the cloning vector, adsorbed out antibodies to PS/A from immune serum, and elaborated a capsule visualized by immunoelectron microscopy with antisera to PS/A. These properties were also seen with PS/A-producing strains of Staphylococcus epidermidis, but not with transposon mutants lacking PS/A. An antiserum raised to PIA contained high-titer antibody to PS/A that was readily adsorbed out by PS/A-positive strains of S. epidermidis and recombinant strains of staphylococci carrying the ica genes. We conclude that the ica locus encodes production of PS/A and that the properties of S. epidermidis associated with initial bacterial adherence, biofilm formation, and intercellular adhesion can be correlated with elaboration of PS/A.

Influence of growth rate and nutrient limitation on susceptibility of Burkholderia cepacia to ciprofloxacin and tobramycin

The influence of growth rate and oxygen availability on ciprofloxacin and tobramycin sensitivity and cell surface hydrophobicity in Burkholderia cepacia was assessed for cells grown in a chemostat under iron-limitation. Whereas susceptibility to both antibiotics decreased with increasing growth rate and oxygen repletion yet increased under oxygen depletion, hydrophobicity decreased with increasing growth rate and oxygen repletion but was relatively unaffected under oxygen depletion. These results emphasize the modulating effect of the growth environment on antibiotic susceptibility and the need to perform antimicrobial susceptibility tests under conditions which closely mimic, in vitro, bacterial growth in vivo.

Influence of Pseudomonas aeruginosa exoproducts on virulence factor production in Burkholderia cepacia: evidence of interspecies communication

The effect of concentrated cell-free extracellular material from stationary-phase cultures of Burkholderia cepacia 10661 and Pseudomonas aeruginosa PAO1 on virulence factor production in B. cepacia was assessed. While increasing concentrations of the B. cepacia exoproduct caused a slight increase in siderophore, lipase, and protease production in the producing organism, a significant in productivity was observed for all three virulence factors with the addition of the PAO1 exoproduct. Moreover, the addition of the exoproduct from a strain of P. aeruginosa producing reduced amounts of autoinducer caused only a slightly greater response than that of the control. Both B. cepacia 10661 and P. aeruginosa PAO1, along with two matched clinical isolates of both organisms obtained from a cystic fibrotic patient, were shown to produce variable amounts of three different types of autoinducer. The potential for interspecies signalling in microbial pathogenicity is discussed.

Effects of growth rate and nutrient limitation on virulence factor production in Burkholderia cepacia

The influence of growth rate and oxygen availability on siderophore, protease, and lipase production in Burkholderia cepacia was assessed for cells grown in a chemostat under iron limitation. Whereas siderophore and protease production increased with growth rate and oxygen yet decreased under oxygen depletion, lipase production demonstrated the opposite trend.

Effect of sub-MIC antibiotics on the cell surface and extracellular virulence determinants of Pseudomonas cepacia

The effects of sub-MICs of ciprofloxacin and tobramycin on the cell surface characteristics and extracellular virulence factors of Pseudomonas cepacia were evaluated. Cells were grown in batch culture under iron-deficient and iron-replete conditions. At sub-MIC levels that did not affect bacterial growth cell surface hydrophobicity decreased under both iron-replete and iron-depleted conditions with ciprofloxacin, but increased with tobramycin under iron-sufficient conditions. Exopolysaccharide synthesis, lipase production and siderophore production were all significantly increased by the presence of ciprofloxacin under both growth conditions. Outer membrane protein and lipopolysaccharide profiles were not affected by exposure to the two antibiotics.

Alteration of glucose transport and diauxic growth in 5-thio-D-glucose-resistant mutants of Azotobacter vinelandii

Spontaneous mutants of Azotobacter vinelandii defective for glucose utilization were selected as resistant to 5-thio-D-glucose. Mutant strains AM2, AM38, and AM39 exhibited longer generation times than the wild type when grown on glucose. Mutant strain AM2 also exhibited an altered Km and Vmax for glucose uptake. During acetate-glucose diauxie, glucose utilization in the 5-thio-D-glucose-resistant mutants was subject to severe inhibition by acetate. These mutants did not exhibit the normal glucose phase of diauxie. Transport studies during diauxie indicated that glucose uptake was not induced in mutant strain AM2. However, increasing the glucose concentration from 25 to 200 mM relieved the severe acetate inhibition, and under these conditions the mutant strain AM2 exhibited normal diauxie. Revertants of mutant strain AM2 exhibited normal glucose and diauxie growth. The results are discussed in terms of a model for acetate regulation of glucose utilization in A. vinelandii.

Isolation and characterization of ack and pta mutations in Azotobacter vinelandii affecting acetate-glucose diauxie

Azotobacter vinelandii mutants defective for acetate utilization that were resistant to fluoroacetate (FA) were isolated. FA-resistant mutant AM6 failed to transport [14C]acetate and lacked enzymatic activity for both acetate kinase and phosphotransacetylase. Growth of wild-type A. vinelandii was sensitive to 10 mM glycine; however, all FA-resistant strains were resistant to glycine toxicity. Isolated mutants that were spontaneously resistant to glycine were also resistant to FA and lacked both acetate kinase and phosphotransacetylase activity. The glycine-resistant mutant AM3, unlike mutant AM6, was capable of growth on acetate. The mutant strain AM6 was unable to growth under acetate-glucose diauxie conditions. Glucose utilization in this mutant, unlike that in wild-type A. vinelandii, was permanently arrested in the presence of acetate. Revertants of strain AM6 were selected on plates with acetate or acetate-glucose. Two classes of revertants were isolated. Class I revertant mutants AM31 and AM35 were positive for both acetate kinase and phosphotransacetylase activities. These revertants were also sensitive to both FA and glycine. Class II revertant strains AM32 and AM34 still lacked acetate kinase and phophotransacetylase activity. Both of these revertants remained resistant to FA and glycine.