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multiply enormously; when conditions become unfavourable such as exhaustion
of food supply, or change of temperature, or the presence of some
harmful chemical or virus, there is equally enormous destruction in the colony.
But new bacteria are constantly being produced slightly different from their
parents and these in turn give rise to whole new strains which can withstand
the changed circumstances and continue to survive. We can thus produce
strains of organisms which are resistant to penicillin and the other new antibiotic
drugs which have been produced in recent years.
In some diseases the infecting agent is transmitted direct from person to
person. Thus, in measles, the virus is transmitted in droplets from an infectious
person, generally a child, to another; the virus appears to breed true, and once
the infective stage is over the virus is entirely eliminated from the body. On
the other hand, many diseases which were at one time thought to be due to a
single organism are now known to be caused by many varying strains in a single
species. Thus scarlet fever is caused by many different strains of streptococci;
in food poisoning there have been recognised over 150 different strains of
salmonellae; recently it has been found that poliomyelitis is caused by at
least three and probably four strains of virus. The problem becomes more
complicated when the relationship between the infecting parasite and the
human patient is an indirect one. Thus we have diseases like yellow fever and
plague in which the organism has its natural existence in some wild animal, is
then transferred to an insect such as a flea or louse, and then in turn to man;
or as in malaria where the organism is transferred from person to person by a
mosquito carrier. But whatever the nature of the strain of the species of infective
organism, or the particular species and strain of intermediate animal or insect
carrier, or the uncertainties encountered in transmission to a host, the organism
will most surely have adapted itself to a role which has enabled it to survive
throughout the countless ages since the dawn of evolution.
Food Sources.
It is important from the point of view of the organism that the host should
live and be capable of passing on the infection; for if all infected hosts were to
die, the species of organism would be extinguished. The meningococcus, the
organism which causes meningitis, lives as a parasite on the mucus membrane
of the nose and throat. Under normal circumstances it goes on feeding on the
debris, mucus and food particles it finds there, reproducing itself quite happily.
When some change takes place either in the virulence of the organism, or in
the resistance of the patient, the organism can invade the lining membrane
of the brain and cause the disease meningitis. If the patient dies it is a tragedy
for the organism (as well as for the patient's relatives) because the chain of
survival of the organism ceases. Even if the patient recovers the germs reproduced
inside the skull will never give rise to a continuing line of descent.
The quantity and quality of the food supply of all living creatures is of
paramount importance. Micro-organisms are composed of the same constituents
as the cells of the body of any animal, that is of proteins, carbo-hydrates and
various salts. Some organisms can manufacture these constituents for themselves
out of very simple substances; however, when the organisms are grown
on a medium which contains the constituents ready made, the new generations
of organisms gradually lose the power of synthesising the complicated constituents
in their own bodies, and come to rely on finding them ready made in
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