ZoBell — 172 — Marine Microbiology 



Reciprocal relations of bacteria and animals : — There are several 

 ways in which bacteria and animals are beneficial to each other either 

 directly or indirectly. The bacterial production of plant nutrients, 

 thereby providing for the growth of plants which animals may consume as 

 food, is regarded as an indirect benefit. The part that bacteria play in 

 aiding animals to digest their food is a direct benefit. 



The digestive tracts of most animals contain numerous bacteria which 

 as a group are very versatile biochemically, being capable of attacking a 

 wide range of materials. Collectively, these bacteria elaborate a formid- 

 able equipment of enzymes which attack many of the substances swal- 

 lowed by animals for which the consumer itself has no enzyme. The 

 power to break down pectins, hemicelluloses, cellulose, lignin, chitin, and 

 other organic complexes is much more widespread among enteric bacteria 

 than it is among animals which ingest these organic complexes. There are 

 few data on the extent to which commensal bacteria may aid animals in 

 the digestion of food, but with many animal species such bacteria may be 

 virtually indispensable. Certain ship worms and wood borers, which are 

 discussed elsewhere in this volume, are believed to depend upon com- 

 mensal bacteria which help to digest cellulose and lignin. 



The function of bacteria in the pre-digestion or partial digestion of 

 food is not confined to enteric forms. Grazing, filter-feeding, and mud- 

 eating animals assimilate much finely particulate food which has been 

 broken down by bacterial activity. 



Large numbers of bacteria, along with partially decomposed organic 

 complexes, are ingested and digested by animals. Other bacteria find con- 

 ditions in the intestinal tracts or on the integuments of the animals con- 

 ducive to their growth and multiplication. Evidence is forthcoming from 

 analyses which reveals that the intestinal contents of many marine ani- 

 mals contain millions of bacteria per ml. Likewise the slimy integuments 

 of aquatic animals are invested with an extensive bacterial population. 

 From his observations on marine fish, Sanborn (1932) concluded that a 

 more or less definite bacterial flora occurs on the surface of fresh fish. 



The integuments or cell walls of virtually all aquatic animals both 

 large and small appear to be veritable bacterial gardens. Illustrative 

 examples are the large bacterial populations associated with copepod 

 tows, the difficulty experienced in attempting to obtain bacteria-free 

 cultures of protozoa, and the bacterial content of fish slime. 



Predominating on the surface of halibut freshly taken at depths of 

 30 to 60 fathoms in the Atlantic Ocean by Sanborn (1932) were Achromo- 

 bacter pellucidum and Rhodococcus agilis. These, together with Micro- 

 coccus varians, M. citreus, M. candidus, Flavohacterium turcosum, and Fl. 

 fucatmn, were observed by Sanborn on Atlantic halibut and by Harrison 

 (1929) on Pacific halibut. Sanborn (1932) also noted the presence of 

 Micrococcus nitrificans, M. halophilus, Achromohacter geniculatum, A. am- 

 biguum, and Flavohacterium annulatum on halibut. Unlike Harrison's 

 findings on Pacific halibut, Sanborn noted on Atlantic halibut the pres- 

 ence of Pseudomonasfluorescens, which was very actively proteolytic. San- 

 born believed that Ps. fiuorescens, along with two other equally active 

 proteolytic marine species, Achromohacter geniculatum and Flavohacterium 

 ucatum, have a direct economic bearing upon the keeping quality of fish. 

 The formation of slimy or viscous growth, which is a common property of 

 bacteria found on the integument of marine fish, is particularly pro- 

 nounced with Ps. fiuorescens, Achromohacter pellucidum, and A. viscidum. 



