OTHER BIOCHEMICAL AND PHYSIOLOGICAL ACTIVITIES OF BACTERIA 73 



To take a few illustrative examples among pathogenic and potentially pathogenic 

 species, Bact. coli grows best at 37° C, but wiU grow at any temperature within the range 

 of 15°-45° C. Among the Gram-negative cocci there are characteristic differences in 

 reaction to temperature ; thus N. gonorrhceoe, N. meningitidis and N. catarrhalis all show 

 optima at about 37° C. ; but the range of growth of N. catarrhalis extends from approxi- 

 mately 18° to 42° C, while N. gonorrlioece and N. meningitidis show a very restricted 

 range of about 30° to 38° C. The different types of tubercle bacilli show temperature 

 optima in conformity with the body-temperatures of the host-species that they infect ; 

 thus the human and bovine types of tubercle bacillus grow best at 37° C. and fail to grow 

 below 30° C. ; the avian type grows best at 40° C, and again fails to grow below 30° C, 

 while the cold-blooded type grows freely at 22° C. 



As examples of non-pathogenic species that depart from the temperature optima 

 given above we may note (see Buchanan and Fulmer 1928-30) that bacteria have been 

 isolated from fish, brine and similar sources that grow well at 0° C. ; while from a variety 

 of natural sources (soil, excreta, silos and especially hot springs) thermophilic species 

 have been isolated that have optima at 55° C. or over, and are able to multiply at a tem- 

 perature of 75° C. These thermophiles are of considerable economic importance, since 

 they are a source of difficulty when it is desired to sterilize any material at a relatively 

 low temperature (see Chapter 5). 



There appears to be a definite relationship between the thermolability of enzyme 

 systems in a given organism, and the maximum temperature at which growth occurs. 

 In a study of eighteen species, including thermophilic organisms, Edwards and Rettger 

 (1937) demonstrated close agreement between the minimum temperatures at which the 

 three thermolabUe respiratory enzymes — catalase, succinic dehydrogenase, and cytochrome 

 oxidase — were destroyed and the maximum growth temperatures. The temperature at 

 which the relatively thermostable peroxidase was destroyed was not related to growth 

 temperatures in this way. 



The relationship also appears to hold for spores, for Lamanna (1942), working with 

 seventy-two strains of the spore-bearing bacillus species, found an association between the 

 maximum temperature at which growth took place and the length of time the spore 

 resisted heating at 95° C. 



In this connection, we may note the effects of heat on bacteria, prior to their growth 

 in a given medium. If the death of a bacterium depends on the inactivation of certain 

 enzymic systems, or the destruction of some essential nutrient, we might expect that 

 sub-lethal degrees of heating would destroy some but not all of the essential metabolic 

 functions of the bacterium. For instance, yeast cells, increasingly exposed to heat, first 

 lose their ability to reproduce, and then their fermentative ability (Rahn and Barnes 

 1932). The count of viable bacteria from cultures subjected to sub-lethal doses of 

 heat or ultra-violet light may be greatly increased if the test media are enriched with 

 blood or extra carbohydrates (Fay 1934, Curran and Evans 1937, Nelson 1943). Davis 

 (1940) records a phenomenon he refers to as " pseudo-death " among spores of CI. welchii. 

 Only a small number of originally viable spores resisted a certain exposure to heat 

 sufficiently to produce colonies when subsequently seeded on to a standard agar medium. 

 In some cases the area of agar seeded with heated spore suspension was free of colonies 

 after 5 days' incubation. When a fresh culture of CI. welchii was grown on the agar 

 adjacent to the original inoculum, a number of the heated spores germinated. Pre- 

 sumably these " pseudo-dead " spores were without some heat-labile substance essential 

 for germination, which was supplied by the growing CI. welchii. 



Other Biochemical and Physiological Activities of Bacteria. 



We have endeavoured to present a broad picture of bacterial metabolism, 

 particularly as it affects the heterotrophic organisms that constitute the greater 

 part of the bacteria pathogenic for man and the higher animals. Many other 

 activities of bacteria — -the production of pigments and of various toxic substances, 



