In vivo genetic analysis of bacterial virulence

Signature-tagged and directed mutagenesis identify PABA synthetase as essential for Aspergillus fumigatus pathogenicity

Signature-tagged mutagenesis (STM) is a method that has been used to screen for genes required for in vivo survival of pathogenic bacteria, but has not been used to investigate a eukaryotic pathogen in an animal model of disease. We have adapted STM to identify genes required for in vivo growth of the opportunistic fungal pathogen Aspergillus fumigatus. Using a mouse model of invasive pulmonary aspergillosis, we have isolated several mutant strains with defects in their ability to replicate in vivo. One strain unable to cause lethal infection was further characterized and found to have an insertion into the promoter of a gene (pabaA) encoding para-aminobenzoic acid synthetase, an enzyme catalyzing a late step in the biosynthesis of folate. The complete inability of this strain, and other pabaA- strains constructed in this study by targeted gene deletion, to cause lethal infection in mice confirms the importance of the folate synthesis pathway for in vivo survival of this pathogen. The successful application of STM to A. fumigatus demonstrates that in vivo genetic analysis of eukaryotic pathogens is feasible and could result in the identification of potential targets, such as para-aminobenzoic acid synthetase, for novel antifungal therapies.

Measurement of bacterial gene expression in vivo

The complexities of bacterial gene expression during mammalian infection cannot be addressed by in vitro experiments. We know that the infected host represents a complex and dynamic environment, which is modified during the infection process, presenting a variety of stimuli to which the pathogen must respond if it is to be successful. This response involves hundreds of ivi (in vivo-induced) genes which have recently been identified in animal and cell culture models using a variety of technologies including in vivo expression technology, differential fluorescence induction, subtractive hybridization and differential display. Proteomic analysis is beginning to be used to identify IVI proteins, and has benefited from the availability of genome sequences for increasing numbers of bacterial pathogens. The patterns of bacterial gene expression during infection remain to be investigated. Are ivi genes expressed in an organ-specific or cell-type-specific fashion? New approaches are required to answer these questions. The uses of the immunologically based in vivo antigen technology system, in situ PCR and DNA microarray analysis are considered. This review considers existing methods for examining bacterial gene expression in vivo, and describes emerging approaches that should further our understanding in the future.

Identification of Brucella suis genes affecting intracellular survival in an in vitro human macrophage infection model by signature-tagged transposon mutagenesis

Bacteria of the genus Brucella are facultative intracellular pathogens which have developed the capacity to survive and multiply in professional and nonprofessional phagocytes. The genetic basis of this aspect of Brucella virulence is still poorly understood. To identify new virulence factors, we have adapted signature-tagged transposon mutagenesis, which has been used essentially in animal models, to an in vitro human macrophage infection model. A library of 1,152 Brucella suis 1330 tagged mini-Tn5 Km2 mutants, in 12 pools, was screened for intracellular survival and multiplication in vitamin D(3)-differentiated THP1 cells. Eighteen mutants were identified, and their attenuation was confirmed in THP1 macrophages and HeLa cells. For each avirulent mutant, a genomic fragment containing the transposon was cloned. The genomic DNA sequence flanking the transposon allowed us to assign functions to all of the inactivated genes. Transposon integration had occurred in 14 different genes, some of which were known virulence genes involved in intracellular survival or biosynthesis of smooth lipopolysaccharide (the virB operon and manB), thus validating the model. Other genes identified encoded factors involved in the regulation of gene expression and enzymes involved in biosynthetic or metabolic pathways. Possible roles in the virulence of Brucella for the different factors identified are discussed.

Reactive nitrogen intermediates and the pathogenesis of Salmonella and mycobacteria

Over the past decade, reactive nitrogen intermediates joined reactive oxygen intermediates as a biochemically parallel and functionally non-redundant pathway for mammalian host resistance to many microbial pathogens. The past year has brought a new appreciation that these two pathways are partially redundant, such that each can compensate in part for the absence of the other. In combination, their importance to defense of the murine host is greater than previously appreciated. In addition to direct microbicidal actions, reactive nitrogen intermediates have immunoregulatory effects relevant to the control of infection. Genes have been characterized in Mycobacterium tuberculosis and Salmonella typhimurium that may regulate the ability of pathogens to resist reactive nitrogen and oxygen intermediates produced by activated macrophages.

Using DNA microarrays to study host-microbe interactions

Complete genomic sequences of microbial pathogens and hosts offer sophisticated new strategies for studying host-pathogen interactions. DNA microarrays exploit primary sequence data to measure transcript levels and detect sequence polymorphisms, for every gene, simultaneously. The design and construction of a DNA microarray for any given microbial genome are straightforward. By monitoring microbial gene expression, one can predict the functions of uncharacterized genes, probe the physiologic adaptations made under various environmental conditions, identify virulence-associated genes, and test the effects of drugs. Similarly, by using host gene microarrays, one can explore host response at the level of gene expression and provide a molecular description of the events that follow infection. Host profiling might also identify gene expression signatures unique for each pathogen, thus providing a novel tool for diagnosis, prognosis, and clinical management of infectious disease.