40 LEECHES 



Graetz and x\utrum (1935) failed to find any proteolytic enzymes 

 in extracts of Hirudo gut wall. They suggested that digestion 

 depends on bacterial decomposition, but this is difficult to reconcile 

 with the observation that the corpuscles remain unchanged for 

 many months. A careful bacteriological examination was made by 

 Busing (1951) who found that only one kind of bacterium was 

 present, Pseudomonas hirudinis. Its properties were investigated 

 by Busing et al. (1953). They were first concerned to show that 

 it was capable of digesting blood protein, transforming it into 

 soluble nitrogenous comipounds. They therefore took two samples 

 of sterile defibrinated sheep's blood and inoculated one with 

 P. hirudinis. In subsamples taken at intervals the protein was 

 separated by coagulation and the nitrogen content of the residue 

 was determined. In the sterile blood the figure remained approxi- 

 mately constant at 25-28 mg% but in the inoculated samples the 

 figure rose to over 200 mg^() in 65 days (Fig. 23). This indicates 

 a progressive breakdown of protein during the whole period and 

 shows that the bacterium is capable of digesting blood at the slow 

 rate normally found in leeches. To complete the picture Busing 

 et al.y showed that P. hirudinis can also break down fats. 



When an antibiotic, Chloromycetin, was injected into the crops 

 of leeches, further digestion was inhibited. When P. hirudinis was 

 added to a culture of Staphylococcus aureus in sheep's blood the 

 Staphylococcus was eliminated, suggesting that the Pseudomonas 

 has the ability to prevent the development of other bacteria. This 

 would account for the freedom from putrefaction of the blood in 

 the crop. Presumably the pseudomonads are well mixed with the 

 meal of blood when it comes into the crop, eliminate any organ- 

 isms which might cause putrefaction and slowly attack the red 

 blood cells one by one. The products of digestion are absorbed 

 by the leech as they are released and if the contents of the crop 

 are examined some time after ingestion only the corpuscles which 

 have not been attacked are seen. 



It is Hkely that the problem of ensuring that pseudomonads are 

 present in the crop of every leech is met by arranging that during 

 the process of cocoon formation, when the leech is forming the 

 plugs of the cocoon with its mouth, it passes some of the pseudo- 

 monads into the nutritive fluid surrounding the eggs, for it is 



