Factors affecting microbial growth

Factors affecting microbial growth

1. pH

2. Moisture

3. Nutrient Content

4. Oxygen

5. Light

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

• 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

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

• 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

• Effects of low water levels – longer lag, slower growth

– impaired transport

– loss of membrane fluidity

Nutrient content

• Required by all organisms: water, carbon, nitrogen, minerals

• Organic growth factors needed to varying degrees: – Gram + > Gram - > yeasts

> moulds

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

• Obligate aerobes, e.g. Mycobacterium, will grow only in presence of free oxygen

Oxygen

• Facultative anaerobes, e.g. Saccharomyces (yeast) will grow in the absence of oxygen, but more slowly than if oxygen were present.

Oxygen

• Obligate anaerobes, e.g. Clostridium will grow only in absence of oxygen which is toxic to them

Light

• Essential for photoautotrophs.

• As well as its intensity its wavelength may be significant.