Factors Influencing Microbial Growth

NUTRIENTS

• Microbes acquire nutrients from environment for growth and metabolism with other elements generally required for survival.
• Macro-nutrients - required in high amounts for cell components and metabolism - carb and protein synthesis.
• Micro-nutrients - required in lesser amounts for enzyme function and protein structure maintenance (Zn, Mg, Ni)
• Microbes either: Heterotrophs or Autotrophs
  • Heterotrophs - must obtain carbon in organic form (carbs, proteins, lipids, nucleic acids)- some simply absorbed (monosac, amino acids) but larger molecules need digestion than absorption.
  • Autotrophs - use CO2 as carbon source and convert to organic compunds - not dependent on other living things - form base of food chain.

• Metabolism - biochemical assimilation (in anabolic pathways) & dissimilation (in catabolic pathways) of nutrients by a cell. 
  • Anabolic pathways - include reductive process leading to production of new cellular material.
  • Catabolic pathways - oxidative processes which remove electrons from substrates or intermediates used to generate energy.

(These processes use NADP or NAD as co-factors which can be recycled to generate energy as ATP - processes are also mainly regulated by environmental factors.)

• Major source of energy in eukaryotes is glucose - glycolysis
  • Conversion of glucose to pyruvate - production of ATP + generation of NADH for biosynthetic pathways.
  • In presence of oxygen - respiration - pyruvate enters mitochondrial matrix where its oxidatively decarboxylated to acetyl co-enzyme A by pyruvate dehydrogenase.
  • Links to citric acid cycle where acetyl CoA is oxidised = 2Co2 + NADH + FADH2
  • In absence of oxygen, pyruvate is decarboxylated to acetaldehyde (fermentation) - reduced to ethanol + 2CO2 - NAD recycled to maintain redox balance.

• Water acitivity (Aw) - amount of water available to a cell for growth.
• Pure water Aw = 1
• Water containing many solutes (salts, sugars) has lower Aw like 0.6
• Bacteria get nutrients from watery environment - sensitive to osmotic pressure
• All bacterial/plant/animal cells regulate water movement across membranes
• Hypotonic solution - cell more concentrated - water moves in - prvent cell from bursting (lyse)
  • Microbes - rigid cell walls - maintain shape + accumulate glycerol + similar compounds to counteract effect of this water pressure. 
• Hypertonic solution - solution more concentrated than cell - water moves out cell into surroundings - cell faces threat of dehydration.
  • If too much water lost - plasma membrane pulled away from cell wall (plasmolysis) & cell dies. 
  • To counteract dehydration - many bacterial species surround themselves with capsule/layer of mucilage, like glycan. 

• Osmotolerant organisms - grow over wide range of water activies - like microbes that cause sweet food spoilage.
• Fungi can grow at lower Aw than bacteria - cause spoilage of dry foods (bread)
• Osmophiles - microbes adapted to grow under high osmotic presure
  • Halophiles - prefer high concentration of NaCl
  • Archaea - more resistant to extreme conditions than bacteria eg: haloarchaea - thrive in Dead Sea.
  • These orgs adapted - plasma membrane + cell walls stabilze high sodium ion concentrations - accumulate K ions in cytoplasm to balance ionic strength outsid cell - cell walls lack peptidoglycan allowing more cell wall flexibility. 

PH

• Each microbial species grows within defined pH range - max growth at optimum pH.
• Most bacteria = neutrophiles - preferring growth at pH 7.
• Some acidophiles - growing optimally at pH 1 - 5.5
• Some alkalophiles - growing optimally at pH 8.5 - 11.5
• Pseudophiles - like fungi - prefer slightly acidic conditions - pH 4 - 6.

• Microbes have preferred temp ranges
  • most bacteria & fungi prefer more moderate temperatures - upper limit below 70 degrees celcius. 
  • Archaea - survive extreme temps - some very cold(just above 0) - others very hot (80-100+)
  • Psychrophiles (cold loving microbes) - grow optimally at 15 (range 0 - 20)
  • Psychrotrophs - prefer warmer conditions (20-30) - but can survive wide range (0-35) - capable of growing on refrigerated foods.
  • Mesophiles - most human pathogens - prefer range 20-45.
  • Thermophiles - optimum = 55-65 (range 45-100) - volcanoes, hot springs.
  • Hyperthermophiles - 80+ - upper temp limitsof life. 

• Oxygen levels in environ. can vary - determines which microbes grow in diff. niches. 
• Obligate anaerobes - find oxygen toxic and cannot grow in its presence.
  • Have to be cultured in special anaerobic cabinets in presence of N2 + Co2 gas.
• All other microbes require and/or tolerate atmospheric oxygen (205 O2) 
  • Obligate aerobes - completely dependent on O2 
  • Facultative anaerobes - Dont require O2 but grow better in its presence.
  • Aerotolerant - not influenced by O2
  • Microaerophiles - require 2-10% O2 
• Moulds + fungi = aerobic
• Yeasts - facultative anaerobes
• Bacteria found in most groupings.