Factors affecting microbial growth : Factors affecting microbial growth Factors affecting microbial growth : Factors affecting microbial growth pH
Temperature pH : pH Most spoilage bacteria grow best near neutral pH
Pathogenic bacteria even more narrow in tolerance range of near neutral
Yeast and moulds have much greater tolerance to acidic (lower) pH pH : pH The optimum pH range is usually quite narrow so that small changes in the pH can have large effects on the growth rate of the organism pH : pH Effects of acids on organisms
energy required to maintain cell's internal pH
enzyme activity affected
proteins, DNA, other molecules denatured
longer lag, less rapid growth Moisture : Moisture Microbes must have a supply of of water available
Bacteria most restricted, then yeasts, then moulds
Organisms tolerant of low water levels:
halophilic bacteria osmophilic yeasts
xerophilic molds Moisture : Moisture Effects of low water levels
longer lag, slower growth
loss of membrane fluidity Nutrient content : Nutrient content Required by all organisms: water, carbon, nitrogen, minerals
Organic growth factors needed to varying degrees:
Gram + > Gram - > yeasts > moulds Nutrient content : Nutrient content How easily are energy sources metabolized?
sugar > alcohol > amino acids > complex molecules
How easily are nitrogen sources metabolized?
amino acids > proteins
B vitamins required by many bacteria Oxygen : Oxygen Obligate aerobes, e.g. Mycobacterium, will grow only in presence of free oxygen Oxygen : Oxygen Facultative anaerobes, e.g. Saccharomyces (yeast) will grow in the absence of oxygen, but more slowly than if oxygen were present. Oxygen : Oxygen Obligate anaerobes, e.g. Clostridium will grow only in absence of oxygen which is toxic to them Light : Light Essential for photoautotrophs.
As well as its intensity its wavelength may be significant. Temperature : Temperature Psychrophiles:
Grow at -10 to 20oC (in aquatic and soil environment)
Can contaminate and grow in food and dairy products when refrigerated.e.g. Vibrio
marinus Mesophiles : Mesophiles Grow at 30-37oC (most human pathogens are mesophiles). e.g. Corynbacterium dephtherius, Nesseria gonorrhoea Thermophiles : Thermophiles Grow at 45 – 70oC (not pathogenic to human). e.g. Thurmus aquaticus.