Micro-organisms in Fish

Micro-organisms are living creatures which are microscopic in nature. They are relatively simple, often single cell in structure.

            Bacteria:

They are prokaryotes (their genetic material is not bound by a membrane).

Native bacterial flora of fishes: - Bacteria which are naturally present in fish are called native bacteria flora.

The population and nature of such flora depends on water from where the fish are caught. The flesh of live healthy fish are generally free from bacteria i.e. sterile but even when the  fish are alive they harbor bacteria mainly of three sites of their body i.e. slime of the skin surface, gill tissue and intestine generally the bacterial population on the skin surface are the least while bacterial count in the intestine are the highest. Bacterial counts in the gill tissue are more or less between the bacterial counts of skin surface and intestine. Also bacterial population exhibit seasonal variations during warmer months the counts are higher while during colder season bacterial counts are lower. The composition of bacterial flora of the fishes from the three waters, (fresh water, brackish water, sea water) differs considerably. Majority of the bacterial flora of marine fishes are gram negative, non-spore forming, rods are cocci. Bacterial flora of fresh water fishes are predominantly gram positive in nature where as the micro flora of brackish water fishes evenly composed of both gram positive and gram negative bacteria.

Examples of Gram negative bacteria

1.      Vibrio

2.      Pseudomonas

3.      Moroxella

4.      Acinetobacter

5.      Flavobacteria

6.      Aermonas

7.      Photobacterium

8.      Coliforms.

Examples of Gram positive bacteria.

1.      Arthrobacter

2.       Micrococcus

3.       Bacillus.

              Intrinsic and extrinsic parameters affecting the growth of bacteria

Intrinsic parameters

Parameters which are naturally present in fish called intrinsic parameters. It includes nutrients, growth factors and inhibitors, water activity, pH, oxidation reduction potential (redox potential).

A.     Nutrients :-

                        They are very essential for growth of living cells, including bacteria. These nutrients include carbohydrate, proteins, minerals and vitamins. Water is not considered as a nutrient but it is essential medium for biochemical reaction necessary for synthesis of energy.  Fish is one of the highly nutritious commodities.  The fish muscle contains around 15-23% protein.  Bacteria attack the protein and break it down to peptides and amino acids

Proteins

                        In general meat is rich in protein, lipids, minerals and vitamins but poor in carbohydrate.  Foods of plant sources are rich in carbohydrates, but can be poor sources of proteins, minerals and some vitamins.  Some foods such as milk have major all five nutrient group in sufficient amount for microbial growth. The major proteinaceous compounds are simple protein, conjugated proteins, non protein nitrogenous compounds(amino acid, urea, ammonia, creatine, trimethyl amine).

                        Amino acid inside microbial cell are metabolize via different pathways to synthesize cellular component, energy and many different types of byproducts. Most of these products can be undesirable (Ammonia and H₂s production causes spoilage of food while toxin causes health hazards).

                        Production of specific metabolic products is used for the laboratory identification of microbial isolates from foods, such as Production of indole from tryptophan by e.coli.

Lipids

                        Lipids in fish include compound that can be extracted by organic solvents. Some of which are free fatty acids, glycerides, phospholipids etc. lipids in fish vary depending on the species in fatty fish, i.e., oil sardine is higher lipids are general less preferred substrate for microbial synthesis of energy. However, certain bacterial group as Pseudomonas, Achromobacter and alkaligens can produce enzyme associated with oxidation of unsaturated fatty acids.

Mineral and vitamins in food

                        Microbes need several minerals in small amount such as phosphorus, calcium, magnesium, iron, sulphur, manganese and potassium. Microbes also posses B group water soluble vitamins and fat soluble vitamins in small amount in growth.

B.      Growth factors and inhibitors

Food can also have some factors that can either stimulate; growth is adversely affecting the growth of microbes. The exact nature of growth factors is not known but they are naturally present in some foods, e.g. of growth factor is in curd that stimulate growth of some lactobacillus species these factors can be added two row materials during food bio  processing  or  in to media to isolate some bacteria from foods. Foods also contain many chemicals that can adversely affect microbial growth. Some of the natural antimicrobial agents are lysozyme in egg, agglutinin in milk and raganol in cloves.

C.      Water activity (aw)  

Water activity is a major of the availability of water for biological function and relates to water present in free form. The water activity of fresh fish is in the range of 0.95 -0.99. The free water in the fish is necessary for enzymatic and microbial activity. It is also necessary to transport nutrient and remove waste material, synthesis cellular material and take part in other biochemical’s reactions. In general the minimum water activity values for the growth of microbial growth are as follows.

Gram –ve bacteria – 0.93.

Gram +ve bacteria – 0.90

But S. aureus can also grow at water activity of 0.85 and halophilic bacteria at 0.75. Fungi can grow at water activity range of 0.82- 0.85.

D.     P^H

PH indicate the H⁺ ion concentration in a system and is expressed as [log^-] of H ion or proton concentration. The P^H of fish and fish product has a profound effect on the growth and viability of microbial cell. Each species of pathogenic bacteria has an optimum p^H for the growth. In general mould and yeasts can able to grow at lower P^H than that of bacteria. Gram –ve bacteria are more sensitive to low PH than gram +ve.

Group	PH  Range
Moulds	1.5 to 9.0
Yeast	2.0 to 2.5
Gram +ve bacteria	4.0 to8.5
Gram –ve bacteria	4.5 to 9.0

                                                           

                                               

                                                           

                                   

E.      Redox potential

Redox or oxidation reduction potential measures the potential different in a system generated by a coupled reaction in which one substance is oxidized and 2^nd substance reduced simultaneously. Oxidation means loss of one electron and reduction means gain of one electron. The electrons donor that reduces an oxidize substance is called as reducing agent. Similarly the electron acceptor is called as an oxidizing agent. The redox potential designated as “Eh” is measured electrical unit of mill volt. The redox potential of fish is influenced by its chemical composition, specific processing treatment and storage condition. Fish stored in air will have higher reduced potential than stored under vaccum or modified gases.

Extrinsic parameters   

It includes the environmental condition in which the fish is stored. Following are the main extrinsic parameters.

1.      Temperature

2.      Gaseous composition

3.      Salt concentration

4.      Water quality

1.      Temperature

Microbial growth is accomplished through enzymatic reaction. It is well known that with every 10⁰ c rise in temp, the catalytic rate of an enzyme get double. Similarly the enzymatic reaction role is reduced to half by decreasing the temp of 10⁰c. Foods are exposed to different temperature from the time of production until conversion. Each bacterial spp. has a range of temp in which it has optimum growth depending on their temp difference bacteria are broadly classify in to three groups.

1.      Psychrophilic

2.      Mesophilic

3.      Thermophilic

   Psychrophilic bacteria.

They are called cold loving bacteria. They grow usually between the ranges of 0 to 20⁰C. The optimum is 15⁰C. This groups includes most of the bacteria causing spoilage to refrigerated or ice foods.

E.g. Pseudomonas, Alteromonas, Morax.

Mesophilic bacteria

They grow within temperature range of 20-45⁰C with an optimum temp of 37⁰C. Most of the pathogen belongs to this group.  

For example Salmonella, Vibrio, Streptococcus etc.    

Thermophilic bacteria

Bacteria which grow best at higher temperature come under this group. Their growth temperature range is 45-65⁰C with optimum temperature of 55⁰C. Certain bacteria causing spoilage of canned food belongs to this group.

 For example Bacillus coagulants and B. stearothermophilus

Gaseous environment

 Depending on requirement of oxygen,

1.      Aerobic Bacteria: Bacteria requiring presence of oxygen called Aerobic or Aerobes.

Example = S. aureus.

2.      Anaerobic bacteria: Bacteria which can only grow in absence of oxygen called anaerobic bacteria.

Example= C. botulinum

3.      Facultative: Bacteria growing in presence or absence of oxygen.

Example= Listiria

4.      Microaerohilic bacteria. Bacteria under this group can grow only in the presence of very little oxygen (3 to 15%).

Example= lactobacillus, streptococci

Salt concentration:-

Depending on requirement of salt concentration for bacterial growth, following major groups have been classify.

a.      Halophilic bacteria: - salt loving bacteria are called as halophilic bacteria.

Example= Vibrio.

b.      Halophilic bacteria. Bacteria which can survive even in small salt concentration.

Example= Salmonella, E.coli etc.

c.       Halotolerent bacteria. Bacteria that can survive higher salt concentration (> 10%).

Example= Bacillus micrococcus, Staphylococcus.