DETERIORATIVE CHANGES 351 



shellfish, but bacterial populations of the same order of magnitude can 

 be anticipated for these animals, too. The tissues and internal organs 

 of healthy fish and shellfish appear to be sterile. 



In general, the bacteria found on living fish are predominately psy- 

 chrophilic (growing significantly at 32°F), sea water loving (though not 

 necessarily absolutely halophilic), aerobic, and attack proteinaceous ma- 

 terials more actively than carbohydrates. 



Representatives of most of the genera of heterotrophic true bacteria 

 reportedly have been isolated from living fish. In many cases, however, 

 it is probable that the species described represent casual contaminants 

 or transient strains not normally present on the fish. Genera repeatedly 

 found to be present on fish to a greater or lesser extent include Pseudo- 

 monas, Achromobacter, Vibrio, Flavobacterium, Corynebacterium, Alcalige- 

 nes, Photobacterium, Alicrococcus, Mycoplana, Proteus, Bacillus and (in 

 the intestine) Clostridium. Most recent reports, however, indicate an 

 overwhelming preponderance of gram negative rod-shaped organisms of 

 the genera Pseudomonas, Achromobacter, and Vibrio in fish and shellfish 

 floras. Flavobacterium, Corynebacterium, and Micrococcus also seem to 

 occur consistently but at much lower levels of incidence. Fish and (par- 

 ticularly) shellfish living in water polluted from terrigenous sources may 

 carry significant numbers of bacteria normally associated with land ani- 

 mals, though even under these conditions such bacteria never occupy a 

 preponderant position in the total associated bacterial flora. However, 

 the occurrence of such types is important from the sanitary and public 

 health standpoint. 



Bacteriological Changes Post Mortem. Quantitative. During the life- 

 time of the fish or shellfish there is no effective penetration of the tissues 

 by the bacteria present on the body surfaces and in the intestine, and a 

 balanced situation seems to exist whereby the numbers of these bacteria 

 remain at a rather stable level. On the death of the fish (or other animal) 

 the humoral defenses against bacterial invasion cease to operate and the 

 mechanical barriers such as skin and membranes gradually lose their im- 

 permeability. As a result of these and other changes post mortem, the 

 balance between bacteria and host animal is upset, and qualitative and 

 quantitative changes in these bacterial populations soon become evident. 



Very little is known concerning the nature and extent of these changes 

 when the dead fish remains in its natural environment, but considerable 

 information is available on the changes which take place in dead fish 

 stored in ice. Technologically, these changes in the bacterial flora are of 

 enormous importance since, together with associated endogeneous bio- 

 chemical and perhaps physical changes in the fish tissue, they lead to the 

 process of spoilage and ultimate decay and dissolution of the cadaver. 



