What are the factors that affect growth of bacteria?

 

Food microbiology

 

 

Factors affecting the growth of bacteria in food

Following are some factors affecting the growth of the bacteria

·        Moisture

·        Temperature

·        PH value

·        Osmotic pressure

·        Food composition

·        Oxygen availability

·        Time duration

·        Hydrostatic pressure

·        Radiations

·        Nutrients availability

·        Mechanical stress

·        Light

·        Salt concentration and water activity

 

 

 Moisture

                    The growth of bacteria is influenced by the availability of moisture content. High the moisture in any food can lead to an increasing growth of bacteria. For example in soggy bread certain bacteria and mould start to grow at high moisture content.  The growth of bacteria can be inhibited by drying to lower the moisture content. Therefore certain meat products are dried to lower their moisture in order to prevent the attack of bacteria that otherwise can cause various food borne diseases.

Temperature

                  Temperature is very important factor that affect the growth of bacteria because most of metabolic reactions of bacteria is catalyzed by enzymes which denatures at high temperature, Moreover the lipid membranes also effected by temperature. If temperature is too low the membranes may become non flexible, sensitive to break and most of metabolic activities stopped down. But on the other hand if temperature is too high the cell structure may damage and all the cell organelles my diminished and cell presents fluidic appearance. The cell may lose DNA.

There are three categories of bacteria depending upon their existence in temperature ranges.

1. Psychrophilic

                The groups of bacteria that grow at about -20℃ to 15℃ are called psychrophilic bacteria. There are great chances of having their growth during freezing the food items. But they are heat resisting microbes thus proper cooking may kill such bacteria. They mostly incorporate in foods during improper thawing after freezing food items like meat.

Examples

 

·       Methanogenium

·       Chlymydomonas

·       Flavobacterium

·       Serratia

·       Bacillus

·       Clostridium

 

2. Mesophilic

            The types of bacteria that grow at 20℃ to 45℃ are called Mesophilic bacteria. It includes all the pathogens as human body has optimum 37℃ temperature.

Proper cooking of food may kill mesophylls. Thus it is recommended that meat should be cooked properly to prevent the diseases.

     Examples

·        Listeria monocytogens

·        Pseudomonas maltophilia

·        Staphylococcus aureus

·        Staphylococcus pyrogens

·        Salmonella

 

3. Thermophilic

         The types of bacteria that grow best at 45℃ to above 100℃ are called Thermophilic bacteria. Most of them show resistence to cooking and canning thus causing food borne diseases or food intoxication. For such bacteria ultra-heat treatments are required to inhibit their growth. But there are less chances of getting disease from such bacteria because their metabolic activities are slowed down at body temperature.

                Examples

·       Syanidium caldarium

·       Thermococcus litoralis

·       Calothrix

               PH value                   

PH affects the growth of bacteria. It affects the plasma membrane of cell and inhibits activity of enzymes and transport proteins in the cell.

There are three classes of bacteria depending upon PH value for their existence

1.  Acidophilic

         Those bacteria that grow at PH value ranges between 0-5 are called acidophilic. They grow in acidic foods, when they have other optimum conditions like in soft drinks, lemonades, and some fruits like oranges, lemon, strawberry etc. Their growth can be inhibited by applying alkaline conditions.

Examples

·        Thiobacillus acidophilus

·        Crenarchaeota

2.  Alkalophilic

          Those bacteria that grow at temperature ranges of 7.5 to 14, these bacteria can affect meat quality and cause food spoilage.

Examples

·       Thermococcus alkaliphilus

·       Halorhodospira

·       Thiohalospira

 

3.  Neutrophils

       Those bacteria that grows best at temperature ranges of 6.5 to 7.5. Most bacteria prefer neutral PH because acidic conditions inhibit their metabolic activities.    

These bacteria can cause diseases to humans.

  Examples

·       Escherichia coli

·       Staphylococcus

·       Salmonella

         Osmotic pressure

                             Osmotic pressure is the pressure created by the movement of solute in a solvent through a permeable membrane. Osmotic pressure affects the growth of bacteria. High osmotic pressure causes the water moves out of the cell, as a result shrinkage of cell (plasmolysis)occurs and bacteria cannot promote its metabolic activities and hence do not deteriorate the food.  Osmotic pressure is considerably affected by concentration of solute and temperature at a membrane. High concentration of solute and temperature causes high osmotic pressure, because solvent with high solute concentration causes difficulty in the passage through permeable membrane and when there is high temperature there will be more possibility of passage of solvent through membrane and at low temperature molecules get closer to each other and possess difficulty in their passage and osmotic pressure will definitely be increased.

              Food composition                      

                         Food composition correlates the bacterial growth in food. Food with high content of protein is more likely to spoilage. Moreover certain essential nutrients in food like nitrogen, phosphorus, iron, manganese etc. also play an effective role in the growth of bacteria. Food with high content of water is major factor that contribute in the growth of microbes. Therefore soggy bread and meat products deteriorate easily.

 

Oxygen availability

                    Microbes grow best in the presence of oxygen. The regulation of oxygen in food system can be exploited in selective control of bacteria for food preservation and in the production of certain food items, For example by forming the anaerobic conditions in canned foods by facilitating the vacuum system. Anaerobic atmosphere prevents growth of all microbes that require oxygen therefore processing associated with such preservation technique is usually aimed at destroying all pathogenic bacteria.

There are four classes of bacteria on the basis of oxygen.

Obligate aerobes

                              Those bacteria that grow in the presence of plenty of oxygen are called obligate aerobes. They produce fats and carbohydrates as a source of energy by utilizing oxygen for respiration.

Facultative aerobes

                              Those bacteria that grow best if sufficient oxygen is available, but can also grow anaerobically or under reduced oxygen tension.

Obligate anaerobes

                             Those bacteria that grow in the absence of oxygen are called anaerobes. Their metabolic activities will be inhibited when they come into contact of oxygen. They obtain their energy by glycolysis.

Facultative anaerobes

                            Facultative bacteria can grow best in the absence of oxygen but can also grow in the presence of oxygen. These bacteria have ability to respire and ferment substances thus can survive both in the presence and absence of oxygen.

 

        Time duration

                         Bacterial growth is greatly influenced by time duration. Bacterial count becomes double after every 20 minutes. So as long as food is come into contact with bacteria more spoilage of food occurs. For example if milk is placed for a long time at room temperature the bacterial growth can be incredibly increased and milk starts to form clots by separating the whey protein and water. As a result the milk will be no longer suitable for consumption. Time, temperature relation is very essential. If food items are stored at low temperature then its shelf life can be increased or if food items are heated for some time then their microbial growth can also be inhibited.

      Hydrostatic pressure

                             Hydrostatic pressure affects greatly bacterial growth. High hydrostatic pressure affect the DNA and RNA synthesis thus causes difficulty in replication. It also affect the nucleus function and proteins are also susceptible to proteins, restricted the amino acid formulation. The most important organelle which is affected by high pressure is ribosomes. Ribosome restriction causes the difficulty in the growth of microbes.

Bacteria can be classified into two classes based on hydrostatic pressure

1.     Barotolerant

                   Those bacteria that are not affected by high temperature are called Barotolerant.                                                                                                                                                                                                                                                                        

.      Barophiles

                   Those bacteria that grow at moderately high temperature

       Radiation

                Microorganisms are sensitive to radiations. Radiations of high energy level destroy the cell structure while radiation of low energy causes mutation in its genes and provide difficulty in replication and ultimately its growth and sometimes causes death of microbes. X-rays and gamma rays are most notably radiations that affect microbial growth. However certain bacteria show resistance to ionizing radiations most probably by endospore of bacterium. Visible light is essential for optimal growth because it’s a source of energy for photosynthesis.

     Nutrients availability

                            For optimal growth of microorganisms optimum amount of nutrients are required. Both micronutrients and macronutrients are essential. Macronutrients include all the proteins, carbohydrates and fats while micronutrient includes zinc, chloride, manganese, iodine, vitamins and other trace metals. Macronutrients help maintain cell structure and metabolism while micronutrients help enzyme function and maintain protein structure.

     Mechanical stress

                             The flow rate of fluid causes mechanical stress to bacteria. Some bacteria remain attached to surfaces in the form of colonies. When mechanical                                       stress applied to bacteria they disrupt their colonies, and wash away secreted compounds, affecting biofilm density. Mechanical stress also affects the growth of bacteria. It can stop the bacteria to spread in new locations.                     

Light

        Light is necessary for growth and survival of photosynthetic bacteria like cyanobacteria. Light of optimum range is necessary. Ultraviolet light causes destruction in the DNA and affecting growth of bacteria while Infrared light causes temperature to increase and disturb the colonies of bacteria.

Salt concentration and water activity        

                                                  High salt concentration can cause dehydration due to osmosis in bacteria and water activity is lowered as a result bacterial metabolism is disturbed and in most cases bacteria can be killed. Most bacteria do not survive at higher water activity more than 0.91.