Bacterial infection and immunity in lower vertebrates and invertebrates

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Abstract
A review is given of some of the literature concerned with bacterial infections and immunity in invertebrates and cold-blooded vertebrates. Among the former, insects have been most fully studied, although a certain amount is known of the immunological reactions of marine invertebrates. Diseases of insects are mainly generalized, bacteraemic conditions, and the great majority are caused by Gramnegative cocco-bacilli. Diseases of cold-blooded vertebrates also are usually generalized, but bacteria from a very wide variety of groups have been isolated from them.It is probable that this resemblance in the infections of these widely separated groups of animals is due to the fact that, unlike mammals and birds, with whose reactions we are more familiar, the regulation of their temperature and the constitution of their body fluids is not exact. This toleration of wide variations of their own physical and chemical constitution must reduce their sensitivity to the changes produced by infection, and hence decrease the likelihood of a local reaction, designed to confine the invading organism to the immediate region of its point of entry, i.e. a local inflammation.The production of humoral antibodies appears to be almost universal in the animal kingdom, although greatly affected by changes in temperature.The effect of temperature upon the balance between host and parasite is also discussed. This question bears upon the problem of occasional pathogenesis by saprophytes. Even among mammalian pathogens the borderline between parasite and saprophyte is an indistinct one, especially in such cases asProteusandPseudomonas pyocyanea, and even those species which are usually regarded as exclusively parasitic may readily be constrained to adopt a saprophytic existence on artificial culture. Where cold-blooded animals are concerned the boundary is even more difficult to draw, and it is possible that under suitable conditions, bacteria which are normally saprophytes may be capable of causing infection. This point will be impossible of proof until more sensitive means of definition of bacterial species are discovered.

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