What happens in vivo to bacterial pathogens?

Chromosome replication as a measure of bacterial growth rate during Escherichia coli infection in the mouse peritonitis model

The efficacy of most antibiotics is dependent on active bacterial growth, yet little is known about the growth dynamics during infection. Therefore, means to measure in-host bacterial growth rate is of importance. Here, we use chromosome replication as readout for in situ bacterial growth rate during infection; obtained from a single biological specimen. We have applied two independent methods: quantitative PCR (qPCR) and fluorescence microscopy, to quantify the level of chromosome replication present during Escherichia coli propagation in the mouse peritonitis model. We find that the methods complement each other and allow for quantification of growth rate, both on a population average and on a single-cell level. We demonstrate the presence of heterogeneous growth rates within bacterial populations propagating during infection. Also, no growth cessation was observed during the apparent stationary phase in vivo, and, by comparison of growth dynamics at different anatomical sites, we demonstrate that E. coli is unlikely to grow independently intravascularly. These findings provide novel insight into bacterial growth during host infection, and underscore the importance of pinpointing the primary site of infection in septicaemia of unknown origin and ensuring antibiotic availability at this site.

Microbial Endocrinology: An Ongoing Personal Journey

The development of microbial endocrinology is covered from a decidedly personal perspective. Specific focus is given to the role of microbial endocrinology in the evolutionary symbiosis between man and microbe as it relates to both health and disease. Since the first edition of this book series 5 years ago, the role of microbial endocrinology in the microbiota-gut-brain axis is additionally discussed. Future avenues of research are suggested.

Bacterial pathogen genomics and vaccines

Infectious diseases remain a major cause of deaths and disabilities in the world, the majority of which are caused by bacteria. Although immunisation is the most cost effective and efficient means to control microbial diseases, vaccines are not yet available to prevent many major bacterial infections. Examples include dysentery (shigellosis), gonorrhoea, trachoma, gastric ulcers and cancer (Helicobacter pylori). Improved vaccines are needed to combat some diseases for which current vaccines are inadequate. Tuberculosis, for example, remains rampant throughout most countries in the world and represents a global emergency heightened by the pandemic of HIV. The availability of complete genome sequences has dramatically changed the opportunities for developing novel and improved vaccines and facilitated the efficiency and rapidity of their development. Complete genomic databases provide an inclusive catalogue of all potential candidate vaccines for any bacterial pathogen. In conjunction with adjunct technologies, including bioinformatics, random mutagenesis, microarrays, and proteomics, a systematic and comprehensive approach to identifying vaccine discovery can be undertaken. Genomics must be used in conjunction with population biology to ensure that the vaccine can target all pathogenic strains of a species. A proof in principle of the utility of genomics is provided by the recent exploitation of the complete genome sequence of Neisseria meningitidis group B.

Human macrophage gamma interferon decreases gene expression but not replication of Mycobacterium tuberculosis: analysis of the host-pathogen reciprocal influence on transcription in a comparison of strains H37Rv and CMT97

Mycobacterium tuberculosis is an intracellular pathogen that readily survives and replicates in human macrophages (MPhi). Host cells have developed different mycobactericidal mechanisms, including the production of inflammatory cytokines. The aim of this study was to compare the MPhi response, in terms of cytokine gene expression, to infection with the M. tuberculosis laboratory strain H37Rv and the clinical M. tuberculosis isolate CMT97. Both strains induce the production of interleukin-12 (IL-12) and IL-16 at comparable levels. However, the clinical isolate induces a significantly higher and more prolonged MPhi activation, as shown by reverse transcription-PCR analysis of IL-1beta, IL-6, IL-10, transforming growth factor beta, tumor necrosis factor alpha, and gamma interferon (IFN-gamma) transcripts. Interestingly, when IFN-gamma transcription is high, the number of M. tuberculosis genes expressed decreases and vice versa, whereas no mycobactericidal effect was observed in terms of bacterial growth. Expression of 11 genes was also studied in the two M. tuberculosis strains by infecting resting or activated MPhi and compared to bacterial intracellular survival. In both cases, a peculiar inverse correlation between expression of these genes and multiplication was observed. The number and type of genes expressed by the two strains differed significantly.

Assessment of bacterial pathogenesis by analysis of gene expression in the host

A number of techniques have been developed to assess the expression of microbial virulence genes within the host (in vivo). These studies have shown that bacteria employ a wide variety of mechanisms to coordinately regulate the expression of these genes during infection. Two tenets have emerged from these studies: bacterial adaptation responses are critical to growth within the host, and interactions between microorganisms and the microenvironments of their hosts cannot be revealed from in vitro studies alone. Results that support these tenets include (i) the prevalent class of in vivo expressed genes are involved in adaptation to environmental stresses, (ii) pathogens recovered from host tissues (versus laboratory growth) are often more resistant to host killing mechanisms, and (iii) virulence gene expression can differ in the animal compared to laboratory media. Thus, pathogenicity comprises the unique ability to adapt to the varied host milieus encountered as the infection proceeds.

Host factors that influence the behaviour of bacterial pathogens in vivo

Interest is increasing in how bacteria behave and produce virulence determinants within the infected host. There are three aspects of this process; observations on the bacteria themselves, recognition of host factors that affect them and investigation of metabolic interactions between the two. The first aspect is relatively easy to investigate and attracts much interest. The second and third are difficult to work on and hence understudied. The review aims to stimulate interest in them by indicating methods of investigation and describing some successful studies. After discussing host factors that determine growth in vivo consideration is given to factors that influence the production of the determinants of mucosal colonization, penetration, interference with host defence and damage to the host. The final section deals with the influence of host-derived cytidine 5'-monophospho-N-acetyl neuraminic acid and lactate on the pathogenicity of gonococci, meningococci and Haemophilus influenzae.

Challenge of investigating biologically relevant functions of virulence factors in bacterial pathogens

Recent innovations have increased enormously the opportunities for investigating the molecular basis of bacterial pathogenicity, including the availability of whole-genome sequences, techniques for identifying key virulence genes, and the use of microarrays and proteomics. These methods should provide powerful tools for analysing the patterns of gene expression and function required for investigating host-microbe interactions in vivo. But, the challenge is exacting. Pathogenicity is a complex phenotype and the reductionist approach does not adequately address the eclectic and variable outcomes of host-microbe interactions, including evolutionary dynamics and ecological factors. There are difficulties in distinguishing bacterial 'virulence' factors from the many determinants that are permissive for pathogenicity, for example those promoting general fitness. A further practical problem for some of the major bacterial pathogens is that there are no satisfactory animal models or experimental assays that adequately reflect the infection under investigation. In this review, we give a personal perspective on the challenge of characterizing how bacterial pathogens behave in vivo and discuss some of the methods that might be most relevant for understanding the molecular basis of the diseases for which they are responsible. Despite the powerful genomic, molecular, cellular and structural technologies available to us, we are still struggling to come to grips with the question of 'What is a pathogen?'

Broad-host-range shuttle vectors for screening of regulated promoter activity in viridans group streptococci: isolation of a pH-regulated promoter

Viridans group streptococci are major constituents of the normal human oral flora and are also identified as the predominant pathogenic bacteria in native valve infective endocarditis. Little information is available regarding the regulation of gene expression in viridans group streptococci, either in response to changes in the oral environment or during development of endocarditis. We therefore constructed a set of broad-host-range vectors for the isolation of promoters from viridans group streptococci that are activated by specific environmental stimuli in vitro or in vivo. A genomic library of Streptococcus gordonii strain CH1 was constructed in one of the new vectors, and this library was introduced into a homologous bacterium by using an optimized electroporation protocol for viridans group streptococci. Because viridans group streptococci entering the bloodstream from the oral cavity encounter an increase in pH, we selected promoters upregulated by this specific stimulus. One of the selected promoter sequences showed homology to the promoter region of the hydA gene from Clostridium acetobutylicum, the expression of which is known to be regulated by the environmental pH. The isolation of this pH-regulated promoter shows that S. gordonii can sense an increase in the environmental pH, which serves as a signal for bacterial gene activation. Furthermore, this demonstrates the usefulness of these new selection vectors in research on adaptive gene expression of viridans group streptococci and possibly also of other gram-positive bacteria.

In vivo adaptation of attenuated Salmonella typhimurium results in increased growth upon exposure to norepinephrine

Two studies were conducted to determine whether attenuated strains of Salmonella typhimurium, currently being investigated as possible vectors for mucosal vaccines, are able to respond to norepinephrine (NE). Bacteria were tested for NE responsiveness before and for 1 week after passage through juvenile rhesus monkeys. NE significantly increased the growth of the attenuated bacteria after being shed from the animal, but not before animal infection. Follow-up in vitro tests were performed by passaging the bacteria in Lauria-Bertani (LB) broth with or without selective antibiotic for the attenuation insert and supplementing with NE. NE increased the growth of bacteria passaged in LB broth with no selective antibiotic, but not in bacteria passaged in LB broth with selective antibiotic. These results show that the attenuated bacteria assumed to be safe for use as a vaccine are able to respond to environmental stimuli, such as NE, and change their characteristics. The results suggest that there may be problems with the stability of attenuated bacteria used as vectors for mucosal vaccines.

What happens to bacterial pathogens in vivo?

Most of our current knowledge about the molecular determinants of bacterial pathogenicity comes from studies with cultures in vitro. However, interest is increasing in bacterial behaviour in the complex and ever-changing environment of the infected host. New methods are revealing how bacteria behave in their hosts, providing many surprises and indicating how much of the subject remains unexplored.