The document discusses various types of stress responses in bacteria: - Aerobic to anaerobic transition is regulated by the FNR protein in E. coli in response to oxygen levels. Facultative bacteria like E. coli can grow with either aerobic or anaerobic metabolism. - Thermal stress induces heat shock proteins that help bacteria cope with heat damage to proteins. - Nutrient starvation stress triggers expression of starvation response proteins that help bacteria survive long-term starvation and resist other stresses. - Osmoregulation maintains salt and water balance across bacterial cell membranes in response to external osmotic conditions. - The stringent response is triggered by amino acid starvation and other stresses and regulates gene expression through

1. Aerobic to Anaerobic Transition
Pragya Tyagi
Microbiology

2. • The FNR (fumarate nitrate reduction) protein plays a central role in the
global oxygen response of a variety of bacteria
• In Escherichia coli, FNR is the master transcriptional regulator of the transition
between aerobic and anaerobic growth
• Regulation of FNR is achieved by cycling the molecule between three states in
a process dependent on oxygen.
• Facultative microorganisms, such as E. coli and S. enterica are
capable of modifying their metabolism to accommodate growth under
either aerobic or anaerobic conditions. .
• The transition between aerobic and anaerobic metabolism is
accompained by alterations in the rate, route and efficiency of
pathways of electron flow.
• Under anaerobic conditions without alternate electron acceptors,
pyruvate is converted to formate, acetate, or ethanol, CO2 and H2
gas (mixed acid fermentation). However, the choices and energy yield
become more plentiful when alternate electron acceptors are
available.

3. • E. coli, even under aerobic conditions, synthesizes two distinct cytochrome
oxidases — cytochrome o (cyo operon) and cytochrome d (cyd operon)—
produced under high O2 and low O2 conditions, respectively. Under anaerobic
conditions, at least five more terminal oxidoreductases can be produced.

4. OXIDATIVE STRESS-------------------------
Oxidative stress, defined as a disturbance in the balance between the production of reactive oxygen species
(free radicals) and antioxidant defenses, is discussed in relation to its possible role in the production of tissue
damage in diabetes mellitus.
Free Radicals----
A free radical is an oxygen containing molecule that has one or more unpaired electrons, making it highly
reactive with other molecules. Oxygen by-products are relatively unreactive but some of these can undergo
metabolism within the biological system to give rise to these highly reactive oxidants. Not all reactive
oxygen species are harmful to the body. Some of them are useful in killing invading pathogens or microbes
Antioxidants ------------------------------
Every cell that utilizes enzymes and oxygen to perform functions is exposed to oxygen free radical reactions
that have the potential to cause serious damage to the cell. Antioxidants are molecules present in cells that
prevent these reactions by donating an electron to the free radicals without becoming destabilized
themselves. An imbalance between oxidants and antioxidants is the underlying basis of oxidative stress.

5. Damaged Caused by Oxidative Stress---------------------
Oxidative stress leads to many pathophysiological conditions in the body. Some of
these include neurodegenerative diseases such as Parkinson’s disease and
Alzheimer’s disease, gene mutations and cancers, chronic fatigue syndrome, fragile
X syndrome, heart and blood vessel disorders, atherosclerosis, heart failure, heart
attack and inflammatory diseases

6. pHSTRESSAND
ACID
TOLERANCE
 Acid stress can be described as the combined biological
effect of low pH and weak (organic) acids present in the
environment.
 Weak acids include volatile fatty acids (VFAs) like
butyrate, propionate and acetate produced as a result
of fermentation. Weak acids in their uncharged,
protonated forms can diffuse across the cell membrane
and dissociate inside the cell, lowering internal pH
(pHi) in the process.
 The lower the external pH (pHo), the more
undissociated weak acid will be available (based upon
pKa values) to cross the membrane and affect pHi.

7. • The combined biological effect of H+ ions (i.e. pH ) and weak acid concentrations.
• Microorganisms live in fluctuating pH conditions.
• Permeability of membrane to protons is very low, however extremely low external
pH (pH0) will cause H+ to leak across the membrane and acidify the internal pH (pHi).
• Decrease in pHi causes lethal effect on biochemical reaction and
macromolecular structures.
• Even more moderate acidic environment can be lethal if combined with weak acids .
• Bacteria has evolved strategies to minimize acid or alkaline damage.

8. THERMALSTRESS
ANDHEATSHOCK
RESPONSE
 A large amount of studies has been carried out in
which stress to cells was provided in the form of heat
shock, i.e., exposing the cells to elevated temperatures
(40- 42°C, in some cases up to 60°C as well)
 It is a known fact that prokaryotes possess only a
single copy of a heat shock gene.
 However, in general, eukaryotes possess at least two
copies of heat shock genes. Of the two, one is under
heat shock regulation and the other is under
constitutive control.

9. • The stress induced in the body of microbes due to high temperature is called Thermal
Stress.
• The heat shock represent a ubiquitous protective and homeostatic cellular response to
cope with heat induced damage in proteins.
• There are two types of heat shock response---
Heat shock response against Cytoplasmic thermal stress
Heat shock response against periplasmic thermal stress.

10. NUTRIENT STRESS
ANDSTARVATION
STRESS
 Nutrient starvation and other environmental stresses
are routine occurrences for most bacteria.
 microbes are most frequently found in a state of
nutrient starvation or stress-induced slow growth or no
growth.

11. Starvation-Stress Response---------------
• When E. coli, Salmonella, and many other non-differentiating microbes
are starved for an essential nutrient such as a carbon-energy (C)
source, they respond by inducing the expression of up to 50 or so new
proteins or preexisting proteins
• The genetic and physiologic reprogramming that occurs is the
starvation-stress response (SSR). The function of the SSR is to allow for
the long-term starvation survival of the bacteria and to provide a
general cross-resistance to a variety of other environmental stresses
including extremes in temperature, pH, and osmolarity as well as
exposure to reactive oxygen and nitrogen species and antimicrobial
peptides/proteins.
• . Typically, stationary-phase cells populate cultures that have stopped
growing following exponential growth in rich or non-limiting media, in
contrast to starved cells, which populate cultures that have ceased
growing in response to exhaustion of one or more defined nutrients
• The SSR refers specifically to the response of starved cells.

12. r Gram-negative bacteria, starved cells are morphologically and
physiologically very different from log-phase cells. The initial
response to carbon-energy source limitation is to try and avoid the
stress by increasing expression or expressing new uptake or
scavenging systems to be able to utilize any nutrients that may
become available

13. Osmoregulation
 Osmoregulation is the process of maintaining salt and
water balance (osmotic balance) across membranes
within the body
 The fluids inside and surrounding cells are composed of
water, electrolytes, and nonelectrolytes.

14. • An electrolyte is a compound that dissociates into ions when
dissolved in water.
• . A nonelectrolyte, in contrast, does not dissociate into ions in
water.
• The body does not exist in isolation. There is a constant input of
water and electrolytes into the system. Excess water, electrolytes,
and wastes are transported to the kidneys and excreted, helping
to maintain osmotic balance
• . Insufficient fluid intake results in fluid conservation by the
kidneys.
• Biological systems constantly interact and exchange water and
nutrients with the environment by way of consumption of food
and water and through excretion in the form of sweat, urine, and
feces.
• Without a mechanism to regulate osmotic pressure, or when a
disease damages this mechanism, there is a tendency to
accumulate toxic waste and water, which can have dire
consequences.

15. Stringent response:---------------
• The stringent response is a stress response that occurs
in bacteria in reactions to amino acids starvation or
other stress conditions.
• In other bacteria stringent response is mediated by a
variety of proteins. Some only have synthetic
,hydrolytic or both activities.
• The stringent response also called stringent control is a
stress response that occurs in bacteria and plant
chloroplast in reaction to amino acids starvation ,fatty
acids limitation, iron limitation, heat shock ,and other
stress conditions.

16. Thank You.