Bacterial Responses to Environmental Stress

Bacteria face frequent changes in temperature, acidity, osmolarity, nutrient supply, and oxidative conditions, and they survive by mounting coordinated stress responses that reprogramme gene expression and physiology. These responses range from specific systems for a single stressor, such as osmotic or heat stress, to a broad general stress response that protects the cell against many adverse conditions at once.

Definition

A bacterial stress response is a regulated change in gene expression and physiology by which a bacterium senses an adverse environmental condition and adjusts to survive it, ranging from stressor-specific systems to broad, multi-stressor protective programmes.

Scope

This topic covers the main bacterial responses to environmental stress: the general (stationary-phase) stress response, osmoregulation, heat- and acid-stress responses, the oxidative stress response, the nutrient-limitation stringent response, and the phenomenon of stress-tolerant persister cells. It is a reference topic in microbial physiology and provides no clinical guidance.

Core questions

• How do bacteria sense and respond to changes in their environment?
• What is the general stress response and what does it protect against?
• How do bacteria cope with osmotic, oxidative, and nutrient stress?
• Why do some stressed bacterial cells become dormant persisters?

Key concepts

• General stress response and the sigma factor RpoS
• Osmoregulation and compatible solutes
• Heat-shock and acid-stress responses
• Oxidative stress response
• Stringent response and (p)ppGpp
• Persister cells and dormancy

Mechanisms

Bacteria detect stress through sensors that alter the activity of regulators controlling protective genes. In many bacteria a general stress response, governed by the alternative sigma factor RpoS, switches on a broad protective programme as cells enter stationary phase or encounter adverse conditions (Battesti et al., 2011). Specific responses include accumulating compatible solutes to counter osmotic stress (Csonka, 1991), inducing chaperones during heat shock, and detoxifying reactive oxygen species during oxidative stress. Nutrient limitation triggers the stringent response through (p)ppGpp, which slows growth and redirects resources (Potrykus & Cashel, 2008). A small fraction of cells can enter a dormant, stress-tolerant persister state that survives conditions lethal to growing cells (Lewis, 2010).

Clinical relevance

Stress responses help bacteria survive hostile conditions, including those encountered during infection, and dormant persister cells are associated with the difficulty of eradicating some chronic and recurrent infections. Acid and oxidative stress responses are relevant to how bacteria withstand host defences. This topic describes these survival mechanisms for understanding and does not provide diagnostic or treatment recommendations.

History

Research from the 1980s onward revealed that bacteria possess dedicated regulatory systems for coping with specific stresses such as heat, acid, and high osmolarity (Csonka, 1991), and identified a broad general stress response coordinated by the sigma factor RpoS (Battesti et al., 2011). The concept of dormant persister cells, first noted in the mid-twentieth century, was revived as a distinct stress-survival phenomenon in modern microbiology (Lewis, 2010), linking stress physiology to the persistence of infection.

Key figures

• Susan Gottesman
• Laszlo Csonka
• Kim Lewis

Seminal works

• battesti-2011
• csonka-1991
• lewis-2010

Frequently asked questions

What is the bacterial general stress response?

It is a broad protective programme, in many bacteria controlled by the alternative sigma factor RpoS, that switches on numerous genes to defend the cell against a range of adverse conditions, typically as cells stop growing or encounter stress.

What are persister cells?

Persister cells are a small, dormant subpopulation of bacteria that tolerate stresses, including conditions that kill actively growing cells, without being genetically resistant; they are associated with the difficulty of clearing some chronic infections.