Freezing Technology

1.4.1 Native bacterial flora of fishes

The flesh and body fluids of live healthy fish are generally free from bacteria, i.e. sterile. But, even when the fish is alive, they harbour bacteria, mainly on three sites of their body (i.e.) the slime on the skin surface the gill tissue and the intestine. Bacteria, which are naturally present on fishes, are called the native bacterial flora of fish. The population and nature of such flora depend on the waters from where the fish were caught, i.e. whether seawater, brackish water or fresh water. Seawater has more dissolved salts, i.e. a higher salinity than both brackish and fresh water.

Salinity range of water bodies

Water	Salinity range
Sea water	30-36 ppt
Brackish water	10-28 ppt
Fresh water	0.2-1.0 ppt

Depending on the salinity and pollution range of waters, the bacterial flora of fishes varies from water to water. Generally the bacterial population of the fishes from tropical water is in the following ranges .

Bacterial population of marine fishes from tropical waters

Sample	Oil sardine (Sardinella longiceps)	Indian mackerel (Rastrelliger kanagurta)
Skin with slime/cm2	103-107	104-106
Gills/g	105-108	104-109
Intestine with contents/g	105-109	105-108

Bacterial population of brackish water fishes

Sample	Pearl spot  (Etroplus suratensis )	Milkfish (Chanos chanos)
Skin with slime/cm2	103-104	103-105
Gills/g	104-108	105-108
Intestine with contents/g	105-108	106-108

Bacterial population of fresh water fishes

Sample	Rohu (Labeo rohita)	Mrigal (Cirrhinus mrigala)
Skin with slime/cm2	104-105	104-105
Gills/g	104-106	105-107
Intestine with contents/g	105-107	104-106

Generally, the bacterial populations on the skin surface are the least, while bacterial counts in the intestine are the highest. Bacterial counts in the gill tissue are more or less between the bacterial counts of skin surface and the intestine. Bacterial populations exhibit seasonal variations. During warmer months, the counts used to be higher while during colder seasons, the bacterial counts are lower. Qualitatively, the composition of bacterial flora of the fishes from the three waters differ considerably. Majority of the bacterial flora of marine fishes are gram negative, non-spore forming (asporogenous) rods or cocci. Bacterial flora of fresh water fishes is predominantly gram positive in nature. The microflora of the brackish water fishes are evenly composed of both gram +ves and gram –ves.

From these examples, it can be seen that in the case of marine fishes, not only there is a larger proportion of gram-negative bacteria, but also, there is a large number of bacterial types. But for fresh water fishes, the number of bacterial genera is quite limited.

a. Microbial flora on the skin surface of marine fish:

Gram negative: Vibrio , Pseudomonas , Moraxella , Acinetobacter , Flavobacteria , Aeromonas , Photobacteria.

Gram positive: Arthrobacter , Micrococcus , Bacillus.

b. Bacterial flora on the skin surface of brackish water fish:

Gram negative: Pseudomonas , Alcaligenes , Flavobacter , Moraxella , Vibrio , Acinetobacter , Coliforms.

Gram positive: Micrococcus , Bacillus , Arthrobacter , Lactobacillus , Streptococci.

c. Bacterial flora on the skin surface of freshwater fish :

Gram negative: Acinetobacter , Pseudomonas , Coliforms.

Grampositive: Micrococcus , Bacillus.