xii SYNOPSIS 



CHAPTER VI 



VARIATIONS IN VIRULENCE 



Bacteria pathogenic and non-pathogenic. Pathogenic character due to 

 two factors — parasitism — nature of activity in tissues. Most bacteria cannot 

 invade tissues — activities of some invaders harmless — actual invasion not 

 essential. Effects of bacterial invasion due to (a) their metabolism, (6) their 

 disintegi-ation, (c) their mechanical action, {d) response of living tissues. 

 Viability — pathogenesis — virulence. 



Vaeiations IN VIRULENCE. 1. At different stagcs of epidemic— possibly 

 explained by unequal resistance met with. 2. Sporadic cases of infectious 

 disease imply weakened virulence. 3. Endemic diseases become less viru- 

 lent — ^possibly explained by acquired immunity. 4. Epidemics vary in 

 severity with date and locality. 5. Intensity of infection by same specific 

 organism varies. 6. Virulence altered by abnormal conditions of cultiva- 

 tion, {a) temperature — possibly protective influence of fever — disproved, 

 (6) presence of antiseptics — carbolic acid, potassium bichromate, iodine tri- 

 chloride, saliva, (c) oxygen, {d) sunlight, (e) reaction of medium. 7. Virulence 

 altered by prolonged cultivation outside the body. Results due to several 

 factors, (a) chemical composition of media— blood media — pathological exu- 

 dations — urine, (6) physical character of artificial media, (c) response of 

 tissues, (<^) purity of culture. 8. Virulence increased by growth in patho- 

 logical secretions. 9. Symbiosis — affects viability — also affects virulence. 

 10. Virulence altered by "passage" — passage alternating with culture more 

 effective. 11. Simultaneous inoculation with another organism intensifies 

 results — even when symbiosis of same organism outside the body ineffective. 

 Simultaneous subcutaneous and sub-peritoneal inoculations with different 

 organisms also effectiva Exalted virulence is towards species used for 

 passage — not necessarily towards othei"s. 



The significance of variation in virulence. Evolution of bacteria 

 — virulence is latest property acquired and first to be lost. Its re-acquire- 

 ment an example of the survival of the fittest —"fittest" not necessarily most 

 robust, but most capable of defence. Firidence results from adaptation 

 and is not due to increased robustness, (a) increased virulence to one species 

 of animal does not apply to another, (&) most virulent not always most robust 

 — contrary true of pneumococcus, (c) analogy suggests adaptation, e.ff. 

 increased resistance -to antiseptics, (d) increased virulence accompanied by 

 other changes obviously adaptive, e.ff. growth at body temperature. Difii- 

 cidties in accepting natural selection as developing virulence, (a) Intra- 

 cellular toxins only set free after death of organism — may nevertheless be 

 of advantage to strain — their effect perhaps purely physiological and not 

 the result of adaptation. (6) Why are common infective diseases not of 

 deadly virulence ? — death of host involves death of organism, (c) Vinilence 

 established by single "passage" — virulence possibly results fi-om sudden 

 change in metabolism, (d) Toxic saprophyte assists non-toxic as well as 



