RESISTANCE 861 



they are differentiated into an opaque centre and a translucent periphery ; their 

 edge may be entire, but is more often woolly, erose, or presents that curious reticular 

 filamentous appearance of a cigarette thrown into water. There is a general 

 correspondence between the form of surface and deep colonies. Thus round, 

 entire-edged and raised surface colonies usually correspond to opaque and lenticular 

 colonies in deep agar ; irregular or coarsely rhizoidal to opaque and lumpy ; 

 delicately rhizoidal to fluffy ; and spreading colonies to deep colonies like a snow- 

 flake. Again, surface colonies with central papillae usually correspond to deep 

 colonies with a marked central opacity. 



Blood Agar Plates. — On these, not only is the colonial form characteristic, 

 but the degree and type of haemolysis afford a useful differentiating feature 

 between the members of the group. Haemolysis is well marked after 3-days' 

 incubation at 37° C. ; if the plates are then stored in a dark cupboard at room 

 temperature it often continues to increase. With a thick seeding the whole plate 

 may be completely decolorized. 



Many organisms give haemolysis of the a-prime type after 3 days' incubation 

 (see Chapter 24) ; after a further 3 days this passes into the fully developed 

 /^-variety. In some cases, it is possible to specify the haemolytic factors concerned. 

 For instance, the relatively wide zone of haemolysis produced by toxigenic strains 

 of CI. welchii on routine horse blood agar is usually due to the 0-toxin (p. 866) ; 

 if the action of 0-toxin is suppressed by 0-antitoxin, a narrower ill-defined zone 

 of partial haemolysis is revealed, due to a-toxin. The action of the a-lysin is greatly 

 enhanced by calcium ions. Some strains of CI. ivelchii produce only the a-type 

 of haemolysis (Evans 1945). 



Cooked Meat Medium. — Most of the members grow well in this medium. All 

 render the fluid turbid to some extent, and most produce gas. The proteolytic 

 members turn the meat black and may obviously digest it ; the saccharolytic 

 members do not digest the meat, and frequently turn it pink. Varying reactions 

 are recorded in this medium, depending on the strain used, the batch of medium, 

 and the length of incubation. Both in this medium and in other media the pro- 

 teolytic members form characteristic foul and pervasive odours, while in cultures 

 of the saccharolytic members there is no odour or, if there is, it is not foul. 



Coagulated Serum and Coagulated Egg.— These media are used for testing 

 the proteolytic powers. None of the saccharolytic organisms is able to liquefy them. 



Gelatin. — At 23° C. most members grow poorly. In stab culture CI. tetani 

 gives a characteristic fir-tree growth, followed later by liquefaction. At 37° C. 

 growth is improved, and is generally accompanied by permanent liquefaction. 



Resistance. — In the sporing stage all the members present a marked but 

 variable resistance to heat, drying, and disinfectants. Thus the spores of CI. 

 hotulinum withstand boiling for 3 or 4 hours, and even at 105° C. are not killed 

 completely in less than 100 minutes. CI. oedematiens is a little less resistant than 

 CI. hotulinum (Hoyt, Chancy and Cavell 1938). On the other hand, spores of 

 CI. welchii are said to be destroyed by boiling in less than 5 minutes (Headlee 

 1931). CI. sporogenes can survive exposure for 8 days to a 5 per cent, phenol 

 solution. In dried earth or dust CI. tetani may live for years. Stock cultures 

 of most members in cooked meat medium remain viable for months ; some, 

 such as CI. fallax and CI. cochlearium, are more delicate and require transferring 

 frequently. 



