1. CWSR (Cell Wall Stress-sensing Regulatory) Systems in Gram Negative Bacteria

Juan A. Ayala, Felipe Cava and Miguel A. de Pedro

The cell envelope is the major line of defence against environmental threats. It is an essential but vulnerable structure that shapes the cell and counteracts the turgor pressure. It provides a sensory interface, a molecular sieve and a structural support, mediating information flow, transport and assembly of supramolecular structures. Therefore, maintenance of cell envelope integrity in the presence of deleterious conditions is crucial for survival. Several envelope stress responses, including two components regulatory systems (TCRS), of Escherichia coli are involved in the maintenance, adaptation and protection of the bacterial cell wall in response to a variety of stresses. Recent studies indicate that these stress responses exist in many Gram negative pathogens. Particular emphasis has been made on the identified TCRS and their activating signals. Another aspect of stress response is the generation of morphological modifications. Most bacteria alter shape when growth conditions change and upon symbiotic or parasitic processes. Any modification in cell shape is connected with cell wall metabolism and requires specific regulatory mechanisms. Recent advances support the existence of complex mechanisms mediating morphological responses to stress involving inter and intra-specific signalling.

2. Stress Responses in Streptococcus

Jacqueline Abranches and Josá A. Lemos

The genus Streptococcus is comprised of a diverse group of organisms, which includes food-associated, commensal and pathogenic species. The importance of this genus to the food industry and the capacity of certain species to infect animals and humans make streptococci one of the best-studied Gram-positive bacteria. In this chapter, we will describe the stress responses of the four major pathogenic streptococcal species: Streptococcus mutans, the etiologic agent of dental caries, S. pyogenes (commonly known as Group A Streptococcus or GAS), responsible for a variety of suppurative disesases as well as life-threatening invasive infections and post-infection sequelae, S. agalactiae (Group B Streptococcus or GBS), a major bacterial pathogen associated with neonatal infections, and S. pneumoniae, the leading causative agent of bacterial pneumoniae. In the following pages, the description of the stress response mechanisms for each individual species is presented in the context of the environmental stress condition. In addition to highlighting the cross-species conservation of certain stress reponses, this organization will allow the reader to follow the progresses obtained in each species, and, at the same time, identify areas that have been poorly explored.

3. Oxidative and Nitrosative Stress Responses in Pathogenic Neisseria

Isabel Delany and Kate L. Seib

Mechanisms to sense, avoid and scavenge oxidants as well as repair damaged biomolecules are important survival and virulence factors of the obligate human pathogens Neisseria meningitidis and Neisseria gonorrhoeae. These bacteria are routinely exposed to several forms of oxidative and nitrosative stress during colonisation and interaction with the host, of which superoxide, hydrogen peroxide and nitric oxide are some of the key oxidants that result in damage to the bacteria. However, the pathogenic Neisseria express an array of defense mechanisms to combat oxidative and nitrosative stress, such as catalase, superoxide dismutase, nitric oxide reductase, as well as thiol-based defenses and proteins involved in metal homeostasis and repair of damage to DNA and proteins. The expression of these defenses is tightly regulated by a series of transcription factors containing redox-sensitive active sites, including OxyR, Fur, PerR/Zur, FNR, MseR, LexA NsrR, NmlR, which sense and maintain the redox homeostasis of the cell.

4. Stress Response in Listeria monocytogenes

Ewa Wałecka and Jacek Bania

L. monocytogenes is a food-borne pathogen widespread in the environment. The majority of human listeriosis is associated with consumption of contaminated food. It has the ability to invade many types of nonphagocytic cells and spread from cell to cell, crossing important barriers in host organism. Despite intensified surveillance in food manufacturing serious cases of listeriosis are still reported. Before L. monocytogenes causes disease it has to endure adverse conditions encountered in food during its processing and storage, such as supraoptimal temperatures, low pH, high osmolarity, presence of oxidants. In the human intestinal tract L. monocytogenes must overcome another set of challenges as the low pH of the stomach, volatile fatty acids, low oxygen levels, osmotic stress, nutrient variability, bile stress and natural flora in the intestine. To survive in hostile environment bacteria adjust their metabolism which involves expression of stress response genes. Consequently, bacteria synthesize proteins that repair damages, maintain the cell stability, eliminate the stress factor, and restore homeostasis. The stress response not only affects L. monocytogenes resistance to subsequent doses of stress factors, but can also alter the pathogen's virulence.

5. Mechanisms Involved in Low-temperature Adaptation in Bacillus cereus

Julien Brillard and Véronique Broussolle

Among the soil bacteria of the spore former genus Bacillus, the human pathogens mostly belong to the B. cereus group. This species is divided in seven phylogenetic groups, with particular traits in virulence, and particular growth temperature ranges, where each of these seven phylogenetic groups corresponds to a specific "thermotype", showing clear differences in ability to grow at low or high temperatures. After a temperature downshift, changes that occur in the bacterial cell include a decrease of the membrane fluidity, a stabilisation of secondary structures of nucleic acids which consequently causes a decreased efficiency in transcription and translation, a misfolding of some proteins, etc. The bacterial cell response involves various mechanisms which, among the Bacillus genus, have been mostly studied in Bacillus subtilis. This chapter focuses on current research about B. cereus low-temperature adaptation, compared to what is well described in B. subtilis.

6. Stress Responses in Salmonella

Suzanne Humphrey, Tom J. Humphrey and Mark A. Jepson

Salmonella enterica are the causative agents of a spectrum of diseases, including enteric fever and self-limiting gastroenteritis and remain significant foodborne pathogens throughout both the developed and developing worlds. The ability to actively invade and reside within gut epithelia and macrophages is an important process in the establishment of Salmonella infection, generating localised inflammatory responses and facilitating systemic spread of the pathogen within the host. Many environments, including food matrices, the external environment and conditions within the host, present a range of stressful challenges that Salmonella must overcome in order to survive and establish infection. Salmonella utilise a diverse range of stress response strategies, including expression of alternative RNA polymerase sigma factors, uptake of compatible solutes, increased expression of genes encoding uptake or efflux pumps, and production of proteins with roles in protecting and repairing stress-induced damage, in order to facilitate their survival in suboptimal and stressful growth environments. Additionally, the ability of Salmonella to undergo morphological changes during stress exposure and rapidly recover from stress conditions commonly encountered within food matrices represents a pertinent issue for food processing and public health.

7. Stress Response in the Pathogenic Yersinia Species

N. Kaye Horstman and Andrew J. Darwin

Pathogenic Yersinia species have long been studied as important causes of human disease and as model organisms to understand widely conserved mechanisms of bacterial virulence. Like all bacteria, these pathogens must respond to a variety of potentially damaging conditions to ensure their survival. This chapter begins by introducing the pathogenic Yersinia and the aspects of their lifestyles that are likely to require successful response to stress. The emphasis is primarily on conditions relevant to pathogenesis. Then, some genome-wide transcription and gene function studies that have identified or implicated stress response mechanisms are summarized. Next, more focused analyses of response to increased and decreased temperature, encounter with macrophages, and macrophage-like conditions are covered in more detail. Finally, the so-called extracytoplasmic stress responses (ESRs) that are activated by changes to the cell envelope will be described. Several of these ESRs have been directly associated with the infectious process in Yersinia. Inactivation of one, the phage-shock-protein (Psp) system, completely attenuates Y. enterocolitica. As a result, the Psp system has become the most extensively studied Yersinia stress response. Therefore, the final section specifically describes the regulation and function of this critical stress response system.

8. Adaptations to Environmental Changes: Stress Response Mechanisms Among Vibrio Species

W. Brian Whitaker and E. Fidelma Boyd

Members of the genus Vibrio are Gram-negative ubiquitous marine bacteria. They can be isolated directly from the water column but are perhaps most known for their association with eukaryotic organisms. In their association with eukaryotic hosts, be it pathogenic or symbiotic, these bacteria must respond to a variety of stress conditions present within the host environment. Often times, these stress response systems are vitally important for the vibrios to successfully establish in the host. Here, we will discuss the systems used by the three main human pathogens of the genus, V. cholerae, V. parahaemolyticus, and V. vulnificus as well as briefly discussing the stress response systems of V. fischeri, V. splendidus, and V. anguillarum, all of which form close associations with marine organisms.

9. Stress Responses in Mycobacterium

Richard W. Stokes

There are many species of mycobacteria, some of which are pathogens of man. Mycobacterium tuberculosis, the etiological agent of tuberculosis, is a major pathogen of man with about one third of the world's population being infected. It resides within host macrophages where it can survive in a dormant state for the lifetime of the host with about 10% of all infections resulting in disease. This environment results in the bacteria being exposed to numerous stresses including nutrient deprivation, reduced oxygen availability, exposure to pH changes and exposure to the antimicrobial activities of the host's cell-mediated immune response. The bacterium responds with its own defense mechanisms that include the increased expression of stress proteins (also called heat shock proteins). This review describes the regulation and function of the major stress proteins within mycobacteria such as the GroEL, GroES and DnaK homologues along with hspX (alpha-crystallin) and others. The multiple copies of cpn60 (GroEL homologue) that are found within mycobacteria are discussed along with their putative roles as chaperonins but also as "moonlighting" proteins with roles in immunomodulation and receptor/ligand interactions that facilitate the pathogenesis of M. tuberculosis.

10. Stress Response in Mycoplasmas

Melissa L. Madsen and F. Chris Minion

Because mycoplasmas lack cell walls and have a limited genome capacity, there has been interest in their regulation of gene expression in these unique organisms. Their restriction to the host environment only adds to the intrigue. That environment, however, is not constant, changing with the host adaption to colonization and disease by the mycoplasma. This review focuses on the types of stresses the mycoplasma might encounter in vivo including heat shock, oxidative stress, osmolarity shifts, hormone exposure, and iron deprivation. Biofilm studies are included because of their use by other pathogens as a defense measure to resist killing in the host. The field of mycoplasmology is still in its infancy, particularly in regards to gene regulatory mechanisms. There have even been suggestions that mycoplasmas may not have the capacity to respond to changing environmental conditions. The studies reported here, however, show unequivocally that mycoplasmas do respond to their environment by altering transcription rates. How that is accomplished is still unknown except in one instance, heat shock. In summary, like all bacteria, mycoplasmas respond to their environment. That response may be limited, but it appears essential to their survival.

11. Stress Responses in Yeast

Eulàlia de Nadal and Francesc Posas

Adaptation to environmental stress requires changes in many aspects of cellular physiology essential for cell survival, such as gene expression, translation, metabolism, morphogenesis or cell cycle progression. Accordingly, the ability of eukaryotic cells to survive and thrive within adverse environments depends on rapid and robust stress responses. Stress-activated protein kinases (SAPKs) pathways are key elements on intracellular stress-signalling networks to respond and adapt to extracellular changes. In this review, we describe the different mechanisms used by model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, as well as the pathogenic fungus Candida albicans, to sense and transduce stress signals to SAPKs in response to osmo, heat and oxidative stresses. Moreover, other signalling pathways related to stress are discussed. Although much remains to be learned, studies from yeast have served to understand how stress signalling molecules adjust precise and efficient adaptation strategies to maximize cell survival in response to extracellular stimuli.

12. Stress Response in the Human Malaria Parasite Plasmodium falciparum

Sylke Müller and Christian Doerig

The life cycle of malaria parasites comprises a complex succession of developmental stages occurring in two different hosts, the human patient and the mosquito vector. In both hosts, the parasite encounters hostile environments and must deal with stresses such as immune responses, sharp temperature shifts and exposure to drugs; partly because of large-scale haemoglobin degradation in the infected erythrocyte and resulting haeme release, oxidative stress is another challenge that the parasite must face. In contrast to other eukaryotes where stress response is largely mediated through a well-defined and robust transcriptional response, it appears that malaria parasites opted for a different strategy. In line with the largely fixed transcriptional programme that characterises the progression of the organisms through their life cycle stages, the transcriptional response to several stresses (such as drug treatments) consists primarily of low-amplitude, genome-wide changes of transcript abundance. However, recent findings suggest that specific transcriptome adaptations, that affect selected aspects of the parasites' physiology, also occur. Overall, the absence in the parasite's kinome of classical stress response mediators such as SAPKs/JNKs, together with the relative scarcity in transcription factors, suggest a low level of flexibility of the parasite in implementing classical eukaryotic stress response pathways. Post-transcriptional mechanisms are expected to play crucial roles in stress response in Plasmodium as exemplified by the demonstrated involvement of an eIF2alpha kinases in response to starvation stress.

13. Toxoplasma gondii: Without Stress There Is No Life

Maria J. Figueras, Sergio O. Angel, Verónica M. Cóceres and Maria L. Alomar

Toxoplasma gondii is an important pathogen of human and domestic animals. It has a complex life cycle which includes the transition from one host to another, being only exposed to the environment during one stage, as highly resistant oocysts. Interestingly, in the intermediate host (non-feline mammalians and birds) the parasite presents an asexual cycle with two stages that can interconvert without its passage in the definite host (felines). The asexual cycle is very important in the establishment of the infection and on its pathogenesis and it could be driven by different kind of stressors. Therefore, the response to environmental and host stresses is essential to their viability and successful progression through their life cycle. The heat shock proteins are key molecules not only in the resistance to different stressors, but they are also involved in the optimal differentiation as well as in other biological processes in T. gondii. This chapter summarizes the findings on different aspects of T. gondii stress responses and the implication of these processes in the biology and pathogenesis of this parasite.

14. The Stressful Life of Pathogenic Leishmania Species

Jose M. Requena

Leishmania parasites are unicellular protozoa descending from one of the oldest eukaryotic lineages. During its digenetic life cycle, Leishmania alternates between the alimentary tract of the sandfly vector as an extracellular promastigote and the acidic phagolysosomes of macrophage cells as an intracellular amastigote. Parasites must cope with varied and heterogeneous environments: changes in temperature, in pH, in nutrient and oxygen concentrations. Also, they must face the immune defences, such as complement factors, free radicals and other antimicrobial effectors. The focus of this chapter will be on our current knowledge of the different stress responses in Leishmania, ranging from description of the prototypical heat shock response to more specific responses found in this parasite. A comprehensive view on the implications of the stress response in parasite survival, in cytodifferentiation and in apoptotic processes will be presented. Future studies, which should be directed mainly to the uncovering of the stress sensors, signal transduction pathways and regulatory mechanisms leading to the induction of the appropriate stress response will be also discussed.

15. The Stress Response of Trypanosoma cruzi

Turán P. Ürményi, Deivid C. Rodrigues, Rosane Silva and Edson Rondinelli

Trypanosoma cruzi, the causal agent of Chagas' disease, is a flagellated protozoan parasite with a complex life cycle that involves infecting an insect and a mammalian host. Several environmental stresses occur during its life cycle, such as heat, reactive oxygen species, and osmolarity changes, and the parasite has evolved a variety of stress responses to cope with these challenges. The stress responses range from synthesis of several proteins and small molecules to modulation of the activity of organelles, and they are essential for the parasite's viability and survival in both hosts. Here we review the components and operation of T. cruzi's stress response with emphasis on its relevance to the parasite's biology and to Chagas' disease transmission, pathogenesis and treatment.

16. Stress Response in the Infective Stage of Trypanosoma brucei

Marcelo A. Comini, Andrea Medeiros and Bruno Manta

African trypanosomes (Trypanosoma brucei sp.) are unicellular eukaryotic organisms that undergo a complex life cycle shuttling between an invertebrate (vector) and a mammalian host. The parasites have evolved sophisticated and efficient mechanisms to cope with, and adapt to, different environmental conditions. Distinct physical (temperature, pH, osmotic pressure) and biological (endo- and exo-biotic molecules, antibodies, proteases, etc) stimuli acting individually or in a concerted manner induce an adaptive response in the parasite. Depending on the nature and extent of the stress, the cellular response can be transient or long-term and associated with minor or major morphological and metabolic changes. In this chapter we compile the most significant molecular and biological aspects related to the mechanisms and components of the stress response of T. brucei to adapt and survive in the bloodstream of mammals.

17. Stress Response in Entamoeba histolytica

Alfonso Olivos-García, Emma Saavedra, Erika Rubí Luis-García, Mario Nequiz and Ruy Pérez-Tamayo

Several species belonging to the genus Entamoeba can colonize the mouth or the human gut; only Entamoeba histolytica, however, is pathogenic to the host, causing the disease amebiasis. This illness is responsible for one hundred thousand human deaths per year worldwide, affecting mainly underdeveloped countries. Throughout its entire life cycle, or invasion of human tissues, the parasite is constantly subjected to stress conditions. Under in vitro culture, this microaerophilic parasite can tolerate up to 5% oxygen concentrations; however, during tissue invasion the parasite has to cope with the higher oxygen content found in well perfused tissues (4-14%) and with reactive oxygen and nitrogen species (ROS and NOS, respectively) derived from both host and parasite. In almost all living cells, a low-dose, tightly regulated generation of ROS and NOS mediates several physiological functions such as growth, differentiation and metabolism; an excess of ROS and NOS, however, damages DNA, proteins and lipids, leading to cell death. In this chapter we review the latest findings regarding the physiological and pathological molecular functions of oxidative and nitrosative stresses in E. histolytica and discuss whether the molecules involved in the antioxidant system of the parasite can be appropriate drug targets for the treatment of amebiasis.