Microorganisms are capable of surviving and reproducing in various environments. Many of these environments produce stresses such as nutrient limitation, acid/base, thermal stress, oxidation, desiccation and exposure to bile salts and antimicrobial peptides which affect the reaction kinetics and the performance of the organism under these conditions. For example, bacteria living in the alimentary canal must be able to sense and respond according to changes in pH and bile salts that they encounter in their environment. To survive, microorganisms have evolved both physiological and genetic mechanisms to tolerate extreme conditions. The production of specific key sets of proteins, maintaining pH homeostasis plus other passive and active mechanisms act as survival strategies.
Key words: stress responses, survival strategies, unfavorable environmental conditions
Microorganisms used in research laboratories and various industries such as food and fermentation technologies are supplied with plenty of nutrients and grown at optimal temperature, pH, oxygen levels and solute concentrations. However in the various natural ecosystems, conditions that allow maximum growth are few and most organisms live in a constant state of stress. Moreover, the extent of the change in the various environmental parameters will determine whether the organism continues to grow, is killed, or survives in a state of dormancy. In order to survive potentially lethal changes, organisms must be able to percept every particular stimuli and induce the appropriate response against the vast array of stresses. Responses to these conditions not only have an impact on growth and survival but also influence the degree of virulence of certain pathogens and their ability to resist antimicrobial drugs.
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The stringent response is used by microorganisms to adapt to lack of amino acids and fatty acids. It controls operons regulating the transcription of rRNA and tRNA. By decreasing the rate of rNA synthesis, it reduces the rate of protein synthesis and stops certain energy draining functions for example cell replication and cell cycle. The stringent response is expressed by the stringent factor RelA and spoT in response to lack of amino acid in the environment.
Some bacteria can also make use of alternative sources of energy by making use of the catabolite repression mechanism during deficiency of certain nutrients especially carbon source in the environment. For example the lac operon in E.coli allows the bacteria to use lactose once glucose is depleted. This allows the organisms to make use of available nutrient sources in the environment whenever their primary sources of energy is completely depleted or lacking in the surrounding environment.
The Pho system is a mechanism which is activated during the presence of very low levels of phosphates available in the surrounding environment of the microorganisms. This mechanism like catabolite repression also causes the microorganisms to make use of other sources of phosphates other than inorganic phosphates from the environment. For example instead of using inorganic phosphates the microorganisms can use organic sources of phosphates. This system also causes an increased production of alkaline phosphatases which is responsible for dephosphorylating phosphate groups so that they can be absorbed into the cells.
During starvation some bacteria have the capability to produce endospores which is a differentiation to a reproducing form. Endospore formation is usually repressed in the presence of glucose and other growth substances. Spore formation in certain bacteria is regulated by sigma factors during the lack of essential elements for growth and reproduction. Endospore formation is a survival strategy developed by certain bacteria that enable them to be dormant and survive in nutrient deficient environments for longer period of time without reproducing until optimum conditions for growth and reproduction returns back.
Weak lipophilic acids, which are present in many fruits and vegetables, are used to preserve low pH foods such as fruit juices, wines and salad dressings. This is done by maintaining their microbial stability. Spoilage of food is usually caused by yeasts, molds and lactic acid bacteria. Some acids act as fungistatic agents whereas some inhibit bacterial growth. Weak acid preservatives affect the cell's ability to maintain pH homeostasis, disrupting substrate transport and inhibiting metabolic pathways. This results in a hurdle for microbes to grow. However, despite the high level of preservative used, osmophilic yeasts such as Zygosaccharamyces rouxii can still grow to cause spoilage of the food. Also, microorganisms can develop resistance to strong doses of the weak acid if they were previously exposed to mild concentrations. In the presence of weak acid preservatives, bacteria can survive but unable to grow. Inactivating or interfering with the cell membrane, cell wall, metabolic enzymes, protein synthesis system or genetic material can cause growth inhibition. Weak acid preservatives may also affect the cell yield, ATP levels and the cells' ability to maintain pH. This results in the disruption of substrate transport and oxidative phosphorylation. However, fungi have developed the H+ translocating ATPase of the plasma membrane to counteract the effect of weak acids and maintain pH. Resistance mechanisms are more difficult in Gram-negative bacteria than in Gram-positive bacteria. The tolerance is determined by the structure and chemical composition of the outer layers of the bacterial cell.
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The electron transport chain depends on the catalytic spin pairing of triplet oxygen to generate energy. During this process, toxic compounds of oxygen can be formed which damage DNA, protein and lipid components of the cell. Superoxide can interact with other chemical reactions producing more highly reactive oxygen derivatives such as hydrogen peroxide and hydroxyl radicals. Peroxynitrite anion can also be formed which is highly reactive to proteins such as methionine, cysteine, tyrosine and tryptophan. Enzyme inactivation, growth deficiencies and DNA damage can take result.
However, aerobic microorganisms are protected from those toxic compounds by the enzyme superoxide dismutase (SOD) and catalase. Cytoplasmic SOD protects DNA and proteins from oxidation whereas periplasmic SOD protects the periplasmic and membrane constituents from exogenous superoxide. Anaerobes protect themselves from the toxic compounds by using NADH oxidase which catalyses the direct four electron reduction of oxygen to water. The superoxide reductase system has the advantage of eliminating superoxide without the formation of molecular oxygen.
Thermal stress and envelope (extracytoplasmic) stress:
Heat Shock Proteins are required for thermo-tolerance. Studies on Salmonella enterica shows that a molecular switch act as a built in molecular thermometer to monitor thermal changes and the production of HSPs. Upon a temperature upshift, intramolecular hydrogen bonds preventing translation of rpoH mRNA which encodes σH are broken and the secondary structure of rpoH mRNA opens, allowing ribosomal binding and translation. σH binds to RNA polymerase and directs the transcription of more than 30 HSPs which function as molecular chaperones, proteases and misfolded proteins. A negative control feedback loop control the over expression of HSPs. At low temperature, DnaK- DnaJ chaperone complex binds to σH, however upon heat stress, DnaK- DnaJ chaperone complex binds to the misfolded proteins allowing more RNA polymerase and σH binding, therefore increased HSPs expression. While the σH regulon controls the accumulation of misfiled proteins in the cytoplasmic membrane, the σE controls multiple envelope and extracytoplasmic accumulation of misfolded proteins. The σE regulon include periplasmic chaperones, peptidyl-prolyl isomerases, proteases and their factors associated with extracytoplasmic functions and biosynthesis.
A decrease in temperature would alter the fluidity of the cell membrane resulting in leakage of the cell components. However, most cells remodel their membrane lipid composition to ensure membrane function such as solute transport and enzyme activity is maintained. Major homeoviscous adaptations involve increased in fatty acid saturation and shortening of the average fatty acid length which disturb the interactions and packing between adjoining chains resulting in an increase in membrane fluidity. Moreover, repression of the heat shock proteins and induction of cold shock proteins (Csp) have been observed in E.coli. After a reduction from 37 oC to 10 oC, CspA which is a transcriptional regulator recognizes gene promoters and induces the production of more cold shock proteins. Another cold shock protein Hcs66 act as a molecular chaperone which ensures refolding of proteins and the conformation of proteins is maintained.
Antimicrobial Peptide (AP) stress:
Soil bacteria(e.g. Paenibacillus polymyxa) produce antimicrobial peptides ( polymyxins and bacteriocins/lantibiotics) in order to kill competing microbes for nutrient.
Different types of APs have been observed to have negative impact on a wide range of microbes. APs produced by eukaryotic organisms are cationic and non-cationic APs. The cationic AP (alpha and beta)includes defensins, cathelicidins and thrombocidins and the non-cationic AP exhibit much lower antimicrobial .
The AP resistance mediated by LPS modification includes the addition of 4-aminoarabinose(Ara4N) and phosphoenthanolamine(pEtN) to one or both phosphate groups in the lipid A moiety thus reducing the overall negative charge at the outer membrane.
pEtN addition to the first heptose phosphate residue in the core of the polysaccharide is mediated by the gene product cptA. Also, the dephosphorylation of the second core heptose phosphate by the pmrG gene product helps in AP resistance.
Another mechanism of LPS modification is the alteration of the acylation character of lipid A moiety. The addition of palmitate to the second position of the N-linked 3-hydroxymyristate on the proximal glucosamine of lipid A is catalyzed by the product of the PhoP-regulated pagP gene.
The production of a surface-associated protease that degrades AP before it can interact with the outer membrane and the pgtE gene encodes a surface protease that exhibits some specificity for alpha-helical APs such as C18G and human cathelicidin.
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Bile is composed of proteins, ions, pigments, cholesterol and various salts. Enteric bacteria are able to resist the high concentrations of bile present in the gastrointestinal tract as they have developed several mechanisms that allow for protection and continued proliferation. Bile salts interact mainly with the bacterial cell membranes. If the membrane is harmed by the bile salts, then the toxic effects could be lead to the DNA, causing great damage in the form of reactive oxygen species. This will eventually result to a halt in replication following cell death. The resistance of the bacteria is not only overcoming damage to the membrane or the DNA, but rather is a result of a combination of defense and repair mechanisms. They possess efflux pumps to remove the bile salts from the cell, thus preventing harmful damage to the membrane.
Multi dug resistance:
The rise of multidrug -resistant bacteria is of high concern as it is the major cause of failure in the treatment of infectious diseases. There are several mechanisms, which have evolved in bacteria conferring them with antibiotic resistance. These mechanisms can chemically alter the antibiotic or convert it inactive through physical removal from the cell or lastly modify target site so that the antibiotic does not recognize it. An existing cellular enzyme can also be modified to react with the antibiotic in such a way so that it does not affect the microorganism.
Antibiotic resistance in bacteria may be an inherent trait of the organism that makes it naturally resistant. For example, the cell wall of the bacteria is covered with an outer membrane that establishes a permeability barrier against the antibiotic. However, it may be acquired by means of mutation in its own DNA or acquisition of resistance-conferring DNA from another source. There is the vertical gene transfer and the horizontal gene transfer.
The vertical gene transfer is a process driven by the principles of natural selection. A spontaneous mutation in the bacterial chromosome gives resistance to a member of the bacterial population and in the presence of the antibiotic, the non-mutants are killed whereas the resistant mutant is allowed to grow and flourish.
On the other hand, horizontal gene transfer is a process whereby plasmids can be transferred between individual bacteria of the same species or even between different species. There are three mechanisms present. Conjugation takes place when there is direct cell-to-cell contact between two bacteria and the transfer takes place. Transformation is a process where parts of the DNA are taken up by the bacteria from the external environment. Finally, transduction occurs when bacteriophages transfer DNA between two closely related bacteria.
For many microbial cells, hypertonic or hyperosmotic conditions result in water loss from the cytoplasm, causing the cell to shrink(plasmolysis).
Hypotonic or hypoosmotic conditions result in an influx of water into the cytoplasm, which causes the cell to swell (plasmoptysis) and can also burst in a process referred to as osmotic lysis.
The function of osmoregulatory mechanisms or osmotic stress responses is to maintain turgor within limits, allowing for maintenance of cell viability.
Movement of water occurs by diffusion and, in a much more rapid process, through water-selective channels called aquaporins. The AqpZ channel of E. coli has been shown to mediate rapid and large water fluxes in both directions in response to sudden osmotic upshifts or downshifts, although its role in the cell is not essential. Turgor is maintained by regulating the total osmotic solute pool in the cytoplasm and the relative level of solutes in the periplasm (in Gram-negative bacteria) immediately outside the cytoplasmic membrane. In low-osmolality media, cytosolic osmolality is largely due to ionic solutes (e.g., K+ ions); in high-osmolality media it largely involves neutral solutes (e.g., trehalose).
As the osmolality of the surrounding environment increases, turgor pressure drops and growth slows . Macromolecular biosynthesis is inhibited and respiration rates decrease. The most rapid response to this osmotic upshock is an increase in K+ ion influx through three uptake systems in E. coli : Trk, Kdp, and Kup.
The Trk and Kdp systems are the major systems for K+ uptake under these conditions, since they can achieve sufficiently high rates of uptake. The Trk System is composed of three components: TrkA (peripheral membrane protein), TrkE(membrane associated), and either TrkH (in E. coli ; membrane-spanning protein) or TrkG (E. coli and other bacteria; membrane-spanning protein). The Trk system binds NAD(H) via TrkA and may regulate K+ ion uptake.
The Kdp system is also a three-component system composed of KdpA (membrane-spanning protein),KdpB (integral membrane protein), and KdpC (peripheral membrane protein).
KdpB is a P-type ATPase and likely provides the energy to drive K+ ion influx through this
The Kup is a single, large membrane-spanning protein possessing a significant cytoplasmic tail domain. In addition to influx, K+ ion accumulation results from plasmolysis and the closing of stretch-sensitive K+ ion efflux channels.
Concurrent with the increase in K+ ion influx, as a result of high external osmolality, is a decrease in intracellular putrescine levels due to increased excretion.
The major anionic compound involved in osmoregulation is glutamate.
Glutamate is synthesized and accumulates quickly following osmotic upshock and is dependent on K+ ion uptake. In E. coli and other enteric bacteria, glutamate is synthesized by two enzymes: glutamate dehydrogenase (GDH) and glutamate synthase (GS).
Many microbes also accumulate the disaccharide trehalose as a compatible solute.
Membrane-derived oligosaccharides(MDOs)are found in the periplasm of Gram-negative bacteria. MDOs are substituted with sn-1-phosphoglycerol and phosphoethanolamine derived from the membrane phospholipids and also with O-succinyl ester residues. Synthesis of these compounds is induced by growth in conditions of low osmolality.
It is referred as water loss due to drying or water stress and affects greatly the survival of microbial cells on inanimate surfaces and environmental habitats such as soil.
Extracellular defenses such as bacterial glycocalyces (composed of exopolysaccharides and associated proteins) have an important role in protecting the bacteria by forming a gel-like extracellular matrix that holds a significant amount of bound water. Thus this water is lost slowly to the evaporative and matric forces.
Colloidal surface structures help in slowing the drying process. The regulatory protein CsgD in S.Typhimurium controls the biosynthesis extracellular cellulose and the thin aggregate fimbriae(curli) which are the major factors in desiccation resistance.
Also, the O-antigen polysaccharide chain of LPS protects the S.Typhimurium from complete desiccation.
During drying, the disaccharide trehalose acts as a compatible solute and aids in maintaining the structure and function of proteins and membrane lipids. The trehalose may also replace water under conditions of extreme desiccation therefore preventing denaturation of proteins and hence stabilizing membrane phospholipids.
Iron stress in many bacteria is regulated by the ferric uptake regulator (Fur) which is the global Fe homeostasis regulator and ryhB (sRNA). During Fe starvation, Fur is inactivated and ryhB becomes expressed. The expression of ryhB reduces the use of Fe by non-essential proteins thus making Fe available for essential Fe-using proteins such as ribonucleotide reductase involved in DNA synthesis and repair. RyhB is also involved in the repression of Fur mRNA translation which prevents the Fur repression of Fe metabolising genes (for example for Fe uptake) in low Fe conditions.
RyhB can also interact with mRNA shiA, which encodes for shikimate transporter increasing the uptake of shikimate from the surrounding environment. Shikimate is essential for the biogenesis of siderophores which are important in the acquisition of more Fe from the external environment due to their very high affinity for Fe3+.Enterochellin is an example of one of the main siderophore synthesised by enterobacteriaceae family for example in E.coli, Shigella, Salmonella and Klebsiella.
Once Fe is repleted in the external environment, there is a decrease in the production of siderophores to prevent excessive uptake of Fe which may be harmful to the cell. The further uptake of Fe is usually repressed by the formation of the Fur- Fe2+ complex.
At least 3 different ryhB-like systems have been identified in other bacteria:
Pseudomonas aeruginosa; 2 tandem sRNAs regulated by Fur is used to repress mRNAs encoding Fe-using proteins during Fe starvation
Neisseria meningitidis; the sRNA, NrrF, is involved in the rapid degradation of sdhCDAB mRNA during Fe starvation
Bacillus subtilis; the Fur regulated sRNA, FsrA, represses several Fe-using proteins for example succinate dehydrogenase
Microorganisms have developed several mechanisms in order to adapt themselves to unfavorable environmental conditions. The mechanisms are usually stress-specific and they are generated for survival of the microorganism during high levels of the stress. The microorganisms have sometimes evolved stress mechanism induced by one stress factor only but which allows the microorganisms to survive many other different stresses at the same time. These stress mechanisms allow the microorganisms to anticipate and prepare for possible adverse environmental conditions in the future and thus increasing their chances of survival. These stress mechanisms are controlled by a number of protein regulators and systems and allow the microorganism to persist either in a host or non-host environment for a much longer period of time. Although much research have been already been done in the subject in order to study the various stress mechanisms that have been developed to maintain survival of many microorganisms, there still remains much to be covered in the field inviting future incentives for research into the field.