98 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



the intact skin. The plague bacillus and certain types of staphylococci 

 are said to pass through the skin occasionally in this manner. 



Streptococci and staphylococci are the more common habitual 

 bacterial parasites found on the skin. Staphylococcus epidermidis 

 albus (Welch), a variant of Staphylococcus pyogenes albus, is a particu- 

 larly common factor in the causation of the troublesome, but relatively 

 benign stitch abscesses which frequently develop where sutures are 

 introduced through the skin. 



The damaged skin is the usual portal of entry for spore-forming 

 bacteria as well as the cocci mentioned above. Spores of the bacilli 

 of tetanus, anthrax, symptomatic anthrax, malignant edema and the 

 "gas bacillus," (B. aerogenes capsulatus, Welch) may pass to the 

 underlying tissues through abrasions of the skin and cause either 

 localized infections or widely distributed lesions. Even so insignifi- 

 cant an abrasion as an insect bite may furnish the necessary atrium 

 for infection. The umbilicus of the newborn furnishes a portal of 

 entry for certain bacteria; particularly severe is the infection of the 

 stump of the umbilicus with B. tetani, causing that very fatal "tetanus 

 neonatorurn" which has been so common in the tropics in the past. 

 Contused wounds and compound fractures are particularly dangerous; 

 the inflamed tissues furnish anaerobic conditions particularly favoring 

 the growth of anaerobic bacteria, as the tetanus and gas bacilli. 

 Clean-cut wounds are usually less liable to infection with anaerobic 

 bacteria. The free flow of blood with its bactericidal properties 

 washes out many bacteria, inhibits the growth of residual microbes, 

 and by virtue of the clot which soon seals the wound prevents the 

 entrance of other organisms. 



The sebaceous secretions, particularly of the axilla and external 

 genitalia, are good culture-media for certain acid-fast bacteria, par- 

 ticularly B. smegmatis. The cerumen of the external ear is frequently 

 infected with Microcbccus cereus flavus, and the puncture of the 

 tympanic membrane may lead to direct infection of the middle ear 

 from the outside, with this or other organisms. Infection of the 

 middle ear may also take place directly through the Eustachian tube. 

 The blood and lymph may also deposit bacteria in the middle ear. 



The conjunctiva, by virtue of its very exposed position, must 

 receive bacteria upon it very frequently. Its polished surface and 

 the mechanical cleansing by the flow of tears (which do not possess 

 germicidal properties) usually suffice to remove adventitious bacteria 

 and to prevent bacterial development under ordinary conditions. The 



