DIPHTHERIA BACILLUS. 337 
But though the morphology of the diphtheria bacillus 
is more regular under some circumstances than others, 
its chief morphological characteristic is its irregularity 
of form and size. 
Biology. The Klebs-Loffler bacillus is non-motile 
and non-liquefying. It is aérobic. It grows most 
readily in the presence of oxygen, but also without 
it; it is thus facultative anaérobic. It does not form 
spores. Its thermal death-point with ten minutes’ 
exposure is about 58° C., and with longer exposure a 
lower temperature; it is more easily destroyed by dis- 
infectants than many other bacteria. In the dry state 
and exposed to diffuse light diphtheria bacilli usually 
die in a few days or may live for weeks or months; 
when in the dark, or protected by a film of mucus or 
albumin, they may live for even longer periods. Thus 
I found scrapings from a dry bit of membrane to 
contain vigorous and virulent living bacilli for a 
period of four months after removal from the throat, 
and if the membrane had not been at that time com- 
pletely used, living bacilli could probably have been 
obtained for a2 much longer period; in culture media 
when kept at the blood heat they usually die after a 
few weeks, but under certain conditions, as when sealed 
' in tubes and protected from heat and light, they retain 
their virulence for years. The bacillus is not sensitive 
to cold, for I found it to retain its virulence after ex- 
posure for two hours to several hundred degrees below 
zero. It begins to develop, but grows slowly, at a 
temperature of 20° C., or even less. It grows more 
rapidly as the temperature rises, and attains its maximum 
development at 37° C.. It may grow at a temperature 
as high as 41° C. and retain its virulence for months, 
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