Bacterial Temperature Tolerance

Temperature

• Psychrotrophs can grow at 0°C and have an optimum temperature of 20-30°C, found in soil, water, milk, and dairy products, e.g. Listeria monocytogenes.
• Mesophiles are most bacteria associated with human infection, growing optimally between 9 and 40°C.
• Thermophiles grow at 55°C or higher, found in compost, hay stacks, hot water lines, and hot springs, with more heat-stable enzymes and protein synthesis systems.
• Hyperthermophiles can grow at 90°C or 100°C, found in hot areas of the sea floor.

Bacteria and Oxygen

• Aerobes are capable of growth in full oxygen tension (21% in the atmosphere), using oxygen as a terminal receptor for electron transport chain in aerobic respiration.
• Anaerobic organisms lack a respiratory system that can use oxygen, with obligate anaerobes being killed by oxygen, and facultative anaerobes able to decompose toxic oxygen products.
• Microaerophiles are anaerobes that can tolerate oxygen and grow in its presence, but do not use it.
• Strict anaerobes are killed by oxygen but can be found in otherwise aerobic environments, e.g. the mouth.

Bacterial Culture Media

• Understanding growth requirements helps in designing culture media for a given genera and enables selectivity.
• Knowledge of metabolic pathways helps in designing identification systems/media.

Environmental Factors Affecting Bacterial Growth

• Source of nitrogen, carbon, essential ions, and nutrients.
• Temperature.
• Water, salt levels.
• pH.
• Atmosphere (O2 levels).

Essential Elements

• Carbon, oxygen, nitrogen, hydrogen, sulphur, phosphorous, potassium, magnesium, calcium, iron, sodium, and chloride.
• Bacteria can synthesise molecules called siderophores to sequester iron from host iron stores.

Salt (Osmolarity)

• Halophiles have adapted to higher salt levels (seawater has 3% salt), requiring salt levels of 2.8M to saturation of 6.2M.
• Halophiles modify structure of proteins and membranes, and extreme halophiles accumulate levels of potassium to remain hypertonic (4–7M).
• Halotolerant bacteria can tolerate some reduction in salt levels.

Water

• 80% or more of the cell mass is water, which is essential for growth.
• Free available water (A) is necessary for nutrients and waste products to enter and leave the cell.

Effect of pH

• Most environments have a pH of between 5 and 8-9, and most organisms grow within these parameters.
• Acidophiles grow optimally between pH 0 and 5.5.
• Neutrophiles grow between pH 5.5 and 8.
• Alkalophiles grow between pH 8.5-11, with extreme alkalophiles at pH 10 or above.
• Most bacteria and protozoa are neutrophiles, while fungi require slightly acid conditions (pH 4-6), and algae favour acidity.