134 



THE ACTINOMYCETES, Vol. I 



range. Bacteria pathogenic to man and to 

 warm-blooded animals develop within a 

 much narrower range of temperature than 

 do saprophytic bacteria. M. tuberculosis, for 

 example, has its minimum at about 30° C, 

 its maximum at 42° C, thus giving 12° C as 

 a range of growth. (3n the other hand, many 

 saprophytes have a range of growth of al- 

 most 40° C. 



According to their temperature relations, 

 microbes are usually divided into three 

 groups : 



1. Psychrophilic forms, with a minimum 

 at 0°, an optimum at 15 to 20°, and a maxi- 

 mum at 30° C. 



2. Mesophilic types, with a minimum at 

 9 to 30° C, an optimum at 28 to 38°, and a 

 maximum at 43 to 50° C. 



3. Thermophilic organisms, with a mini- 

 mum at 40 to 49°, an optimum at 50 to 65°, 

 and a maximum at 60 to 75° C. 



The water-inhabiting organisms, particu- 

 larly the marine types, and most lumines- 

 cent bacteria comprise the first group. The 

 pathogenic forms and most of the sapro- 

 phytes belong to the second group. The 

 third group includes certain bacteria which 

 develop in hot springs and in soils of warm 

 climates; many bacteria and actinomycetes 

 are found in high-temperature composts. 

 This classification is purely arbitrary, since 

 there are no sharp lines of demarcation be- 

 tween these groups. The above limits of 

 temperature are not constant, and depend 

 on the composition of the medium, concen- 

 tration of nutrients, and other environmen- 

 tal conditions. 



When the temperature is reduced below 

 the minimum or raised above the maximum, 

 growth of the organism stops. An injurious 

 effect follows only a considerable change in 

 temperature. This effect is more marked on 

 the upward scale than on the downward 

 scale. Microbes are much more resistant to 

 temperatures below the minimum than to 

 those above the maximum. 



Lowest possible temperatures are usually 

 not sufficient to kill microbes. Even tem- 

 peratures of li([uid air w^ere not sufficient to 

 kill staphylococci and M. tuberculosis. How- 

 ever, some bacteria are rapidly killed at 0°. 

 Repeated freezing and thawing have a more 

 deleterious effect than freezing alone; dif- 

 ferent organisms vary in this respect. Bac- 

 teria and spores in a dry state are preserved 

 by liquid air. The mycelium and spores of 

 certain fungi were not killed even by — 110°, 

 whereas the mycelium of other fungi was 

 destroyed readily at low temperatures, the 

 spores being more resistant. 



Most bacterial cells, except the thermo- 

 philic forms, are killed at a temperature of 

 56° C for 1 hour; at 60° C, 10 minutes is 

 sufficient, whereas at 80° C only 1 minute 

 will kill these bacteria. The temperature and 

 time necessary to kill bacterial cells vary 

 with the kind of bacteria and with the 

 composition of the medium; a longer period 

 is required for bacteria in organic media 

 than for suspensions of bacteria in water or 

 in salt solution. 



The optimmn temperature for growth of 

 most of the actinomycetes usually falls be- 

 tween 23 and 37° C. Certain actinomycetes 

 are able to grow at temperatures lower than 

 20° C, whereas some prefer temperatures of 

 20° to 23° C. The more common forms are 

 readily destroyed at the higher tempera- 

 tures, the resistance of the spores being only 

 slightly greater than that of the mycelium. 

 When a culture is kept for 10 minutes at 

 70° C, not only the mycelium but also the 

 spores lose their viability. 



The actinomycetes are able to withstand 

 much higher degrees of dry heat than of 

 moist heat. Knori (1933) found that ex- 

 posure of soil to dry heat at 180 to 200° C 

 may not be sufficient to destroy all actinomy- 

 cetes. Some actinomycetes can be adapted 

 to grow at higher temperatures. Lieske, for 

 example, was able, after a few transfers, to 

 grow his culture Nocardia 74 at 48° C, 



