182 ANTIBACTERIAL SUBSTANCES FOR TREATMENT OF INFECTIONS 



Fig. 28. — The assay of penicillin by the plate method. 

 With falling concentrations of penicillin in the cups, the zone of inhibition of Staph, aureus 



is progressively less. 

 (From a photograph kindly supplied by Dr. N. G. Heatley.) 



or by the absence of some characteristic growth-product of the test organism, for example, 

 the absence of hsemolysin of Str. pyogenes detectable in slide-cells (Wright, Colebrook 

 and Storer 1923) or in culture media (Rake and Jones 1943, Wilson 1943) containing 

 red blood cells ; or the absence of acid production in a medium containing an indicator 

 and a carbohydrate fermented by the test bacterium (Fleming 1942). Strains of species 

 other than Staph, aureus are suitable for assay. Rammelkamp (1942), for example, 

 employed a highly sensitive strain of Sir. pyogenes for the measurement of low concen- 

 trations of penicillin in small quantities of fluid. (See also Lochhead and Timonin 1943.) 



Laboratory Use of Penicillin. The selective action of penicillin in vitro has 

 been fruitfully employed in devising selective media for isolating resistant organisms 

 from mixtures. Following Fleming's original observations on this aspect of 

 penicillin (Fleming 1929) we may note its use for isolation of H. influenzce and 

 other htemophilic bacteria (Fleming and Maclean 1930, Fleming 1932, Schoenbach 

 and Seidman 1942) ; H. pertussis (Maclean 1937, Buxbaum and Fiegoli 1943, 

 Cruickshank 1944) and the acne bacillus (Craddock 1942). 



Mode of Action and Constitution. Little is known of the mode of action of 

 penicillin on bacteria. It is bacteriostatic in low concentrations, and appears 

 to be bactericidal in high concentrations. In low concentrations its action is 

 relatively slow (Fleming 1929, Heilman and Herrell 1942). In completely inhibitory 

 concentrations, bacterial respiration may continue for some hours, and some 

 bacteria survive for 24 hours or longer (Abraham et al. 1941). The rate of killing 

 depends to some extent on the bacterium employed, and for any one strain is 

 relatively constant until some 99 per cent, of the bacteria are dead, after which 

 the numbers of survivors may remain constant, or even increase slightly. Anti- 

 bacterial activity is marked at 37° C, slight at 18° C, absent at 4° C. The rate 

 increases with diminishing inoculum, and with increasing concentrations of peni- 

 cillin, though only to a certain point, beyond which the rate does not increase. 

 Within the limits of experimental error, no utilization of penicillin is detectable 

 (Hobby, Meyer and Chaffee 19426). Though a bacterial strain may be trained to 



