Microbial Nutrition and Growth Microbial Ecology Microbiology for Health Sciences

Factors that May Affect Growth Oxygen Temperature pH Osmotic Pressure

Oxygen Requirements Aerobe: uses oxygen for metabolism Facultative Anaerobe: does not require oxygen for metabolism Microaerophile: requires small amounts of oxygen for metabolism Strict Anaerobe: require element other than oxygen for metabolism, N, S, CO2

Several General Categories of Oxygen Requirements Aerobe: can use gaseous oxygen in its metabolism and possesses the enzymes needed to process toxic oxygen products Obligate aerobe: cannot grow without oxygen Facultative anaerobe: an aerobe that does not require oxygen for its metabolism and is capable of growth in the absence of it Microaerophile: does not grow at normal atmospheric concentrations of oxygen but requires a small amount of it in metabolism Anaerobe: lacks the metabolic enzyme systems for using oxygen in respiration Strict, or obligate, anaerobes: also lack the enzymes for processing toxic oxygen and cannot tolerate any free oxygen in the immediate environment and will die if exposed to it. Aerotolerant anaerobes: do not utilize oxygen but can survive and grow to a limited extent in its presence

Carbon Dioxide All microbes require some carbon dioxide in their metabolism Capnophiles grow best at a higher CO2 tension than is normally present in the atmosphere

Physical Requirements for Growth Temperature: Psychrophiles: cold-loving microbes (-10 to 20 degrees C) Mesophiles: moderate- temperature-loving microbes (10 to 50 degrees C) Thermophiles: heat-loving microbes (40 to 73 degrees C) Refrigeration severely retards the growth of most pathogenic bacteria

Psychrophile A microorganism that has an optimum temperature below 15°C and is capable of growth at 0°C. True psychrophiles are obligate with respect to cold and cannot grow above 20°C. Psychrotrophs or facultative psychrophiles- grow slowly in cold but have an optimum temperature above 20°C.

Mesophile An organism that grows at intermediate temperatures Optimum growth temperature of most: 20°C to 40°C Temperate, subtropical, and tropical regions Most human pathogens have optima between 30°C and 40°C

Temperature and Growth Requirements [INSERT FIGURE 6.4]

Thermophile A microbe that grows optimally at temperatures greater than 45°C Vary in heat requirements General range of growth of 45°C to 80°C Hyperthermophiles- grow between 80°C and 120°C

Environmental Factors that Influence Microbes Temperature Adaptations Microbial cells cannot control their temperature, so they assume the ambient temperature of their natural habitat The range of temperatures for the growth of a given microbial species can be expressed as three cardinal temperatures: Minimum temperature: the lowest temperature that permits a microbe’s continued growth and metabolism Maximum temperature: The highest temperature at which growth and metabolism can proceed Optimum temperature: A small range, intermediate between the minimum and maximum, which promotes the fast rate of growth and metabolism Some microbes have a narrow cardinal range while others have a broad one Another way to express temperature adaptation- to describe whether an organism grows optimally in a cold, moderate, or hot temperature range

pH Requirements pH: low pH = acidic, high pH = basic pH range 0-14 Normal growth range for bacteria is pH 6 to 8 (most bacteria can’t grow in orange juice) Acidophiles (minority) can grow in pH 4 Molds and yeasts grow in pH 4-6

Effects of pH Majority of organisms live or grow in habitats between pH 6 and 8 Obligate acidophiles Euglena mutabilis- alga that grows between 0 and 1.0 pH Thermoplasma- archae that lives in hot coal piles at a pH of 1 to 2, and would lyse if exposed to pH 7

Osmotic Pressure Most microbes live either under hypotonic or isotonic conditions Osmophiles- live in habitats with a high solute concentration Halophiles- prefer high concentrations of salt Obligate halophiles- grow optimally in solutions of 25% NaCl but require at least 9% NaCl for growth

Osmotic Pressure Osmotic pressure: addition of salts to foods results in shrinkage of cell due to loss of water Plasma membrane pulls away from cell wall which results in inhibition of cell growth Ex. In food prep = salted fish, honey, sweetened and condensed milk (addition of high amounts of salt or sugar) Halophiles: org. that can grow in high salt concentrations

Ecological Associations Among Microorganisms Most microbes live in shared habitats Interactions can have beneficial, harmful, or no particular effects on the organisms involved They can be obligatory or nonobligatory to the members They often involve nutritional interactions

Symbiosis A general term used to denote a situation in which two organisms live together in a close partnership

Symbiosis A general term used to denote a situation in which two organisms live together in a close partnership Mutualism: when organisms live in an obligatory but mutually beneficial relationship

Symbiosis A general term used to denote a situation in which two organisms live together in a close partnership Commensalism: the member called the commensal receives benefits, while its coinhabitant is neither harmed nor benefited Satellitism: when one member provides nutritional or protective factors needed by the other

Symbiosis A general term used to denote a situation in which two organisms live together in a close partnership Parasitism: a relationship in which the host organism provides the parasitic microbe with nutrients and a habitat

Nonsymbiotic Relationship Organisms are free-living and relationships are not required for survival Synergism: an interrelationship between two or more free-living organisms that benefits them but is not necessary for their survival Antagonism: an association between free-living species that arrises when members of a community compete One microbe secretes chemical substances into the surrounding environment that inhibit or destroy another microbe in the same habitat

Interrelationships Between microbes and Humans Normal microbiotia: microbes that normally live on the skin, in the alimentary tract, and in other sites in humans Can be commensal, parasitic, and synergistic relationships