Fundamentals of Microbiology

2.4.8 Physical requirements of growth

Temperature :

Microorganisms grow best at warm ambient temperatures. Some bacteria are capable of growth at extreme temperatures unlike eukaryotic organisms.

Microorganism is divided into three primary groups, based on their preferred range of growth temperature.

                    Type                              Growth range           Optimum

1. Psychrophilic (cold loving): -             5⁰C to 20⁰C             15⁰C
2. Mesophilic (moderate-temp loving): 15⁰C to 40⁰C          35-37⁰C
3. Termophilic (heat loving) :              40⁰C – 70⁰            55 – 60⁰C

 

Each bacterial species grows at particular minimum, optimum and maximum temperatures (Cardinal temperature). Optimal growth temperature is usually near the top of the range, above that temperature, growth drops off rapidly. Temperature influences growth in two ways:

1) as temperature increases enzymes and chemical reactions proceed rapidly; but beyond a range, irreversible changes occur such as protein denaturation and collapse of membranes

2) as temperature decreases the transport processes slow down such that growth can not occur due to membrane gelling.

There is another group of bacteria, other than psychrophiles that can grow at 0⁰C and has higher optimal temperature usually between 20⁰C and 30⁰C. These are called psychrotrophs and are important in food spoilage.

The optimal temperature for human pathogenic bacteria is about 37⁰C.

Thermophilic bacteria are important in organic compost, where temperature can rise rapidly to 50⁰ or 60⁰C. Some microbes, members of archaebacteria have optimal temperature of 80⁰C or higher. These are called hyperthermophiles or extreme thermophiles. These live in hot springs.

pH

Most bacteria grow best in a narrow range of pH near neutrality (6.5 – 7.5) and are called neutrophiles. Very few bacteria grow at an acidic pH below 4.0. Therefore, food preservation employs lower pH. Some bacteria called acidophiles are tolerant of acidity. One type of chemoautotrophic bacteria, found in drainage waters of coal mines oxidizes sulphur to sulphuric acid and can survive at a pH of 1 (Thiobacillus sp). Molds and yeast grow over a wide range of pH, and their optimal range is about pH 5-6. Bacteria that grow over a pH range of 8.5 to 11.5 are called alkalinophiles, for eg. Some of the Bacillus spp - live in soda lakes (high carbonate solutions). When bacteria are cultured in laboratory certain chemicals called buffers are included in the medium. Peptones and amino acids can act as buffers. Phosphate salts exhibit buffering effect and are non toxic and provide phosphorous, an essential nutrient.

Osmotic pressure:

Microbes obtain their nutrients in solution from the surrounding water. Higher osmotic pressures remove water from a cell. When a cell is kept in a hypertonic (higher solute concentration than in the cell) solution, the cellular water passes out through the plasma membrane causing shrinkage of the membrane and growth is inhibited. The phenomenon is called plasmolysis.

Therefore, in food preservation, addition of salts helps to prevent microbial growth. A low molecular weight compound such as NaCl has a greater effect than higher molecular weight compound such as sucrose. Some bacteria, extreme halophiles have good adaptation at high salt concentrations. There are facultative halophiles (halotolerant) that can grow normally up to 2% salt. A few facultative halophiles can tolerate 15% salt for eg. Staphylococcus aureus. Some microbes are designated as Osmophiles (those that can grow in sugar solutions) and some as Xerophiles (those that grow in low moisture conditions)

The term ‘water activity’ is generally used to explain this function the amount of water available to microorganism can be reduced by interaction with solute molecules. Water activity of a solution is 1/100 the relative humidity of the solution. Water activity is the ratio of the vapour pressure of air in equilibrium with a substance or a solution (P soln) to that of pure water (P water) at the same temperature.

                                           P soln.

Water activity (aw) = ----------------------------

                                            P water

Water activity is inversely related to osmotic pressure. If a solution has high osmotic pressure it’s aw is low. (A_w of pure water is 1.0; Seawater – 0.98; Fruit jams – 0.80; Salted fish - 0.75). Most microorganism grows well at a_w of 0.98 Osmotolerant organisms like Staphylococcus can grow up to aw 0.8. Some yeasts and molds grow up to a_w of 0.6.

Oxygen requirement / tolerance of microbes