RESUME OF PERTINENT LITERATURE ON CERTAIN RELATIONSHIPS 

 BETWEEN EXTRACELLULAR BACTERIA AND INSECTS AED TICKS. 



There are approximately 250 known species of "bacteria 

 which have "been found associated, in one way or another, 

 with insects and ticks. This does not include the intra- 

 cellular forms such as the rickettsiae and intracellular 

 "symbiotes. " A large number of the "bacteria associated 

 with these arthropods have never "been completely described 

 nor correctly classified. In general, however, the "bac- 

 teria found in insects are not characteristically differ- 

 ent from most "bacteria. Limited studies have shown that 

 the "bacteria isolated from insects consist roughly of ko 

 to 50 per cent gram- negative short rods, 15 to 25 per cent 

 gram-positive spore-forming rods, 15 to 25 per cent gram- 

 positive cocci, 10 to 12 per cent gram-positive short rods, 

 and lower percentages of spirilla, "coccohacilli," and • 

 other forms. Bacteria isolated from ticks appear to "be 

 predominantly cocci (50 per cent or more), with smaller 

 numbers of non-spore -forming and spore -forming rods. 

 Percentages such as the above will prohahly have to "be 

 modified as more complete studies on the flora of insects 

 are made. 



External "bacterial flora of insects . The "bacteria 

 found on the external surface of insects are, for the 

 most part, adventitious. This is especially the case with 

 such insects as the housefly and cockroach which frequent 

 areas of filth and decomposing organic matter. Such in- 

 sects acquire a great and ever-changing variety of bacteria, 

 many of which are of public health importance. 



Due to peculiarities in "body structure, such as the 

 housefly with its "bristles and sticky pads, many insects 

 carry enormous numbers of microorganisms. Even in these 

 cases many more "bacteria are usually found inside than on 

 the outside of the fly. For example, Yao, Yuan and Huie 

 (1929) found an average of 3,683,000 "bacteria (externally) 

 per fly from the slum. district of Peiping, China, and 

 1,9^1,000 per fly from the cleanest district. They found 

 the inside of the flies to harbor from eight to ten times 

 as many "bacteria as the outside. Similarly, Torrey (1912) 

 found the "bacteria in the intestine of the housefly to he 

 8.6 times as numerous as those occurring on the external 

 surface of the insect. Other insects, whose external "body 

 structure is not so complex, carry relatively few "bacteria 

 of any kind on their "body surfaces. It is interesting to • 

 note that "bees, whose "body structures are so well adapted 



