HOW PARASITIC AND PATHOGENIC BACTERIA REACH MAN 97 
1. Skin and Adnexa.^— The intact skin is a natural barrier which 
protects the underlying tissues of the body from bacterial invasion. 
Its free exposure to the environment suggests that a great variety of 
organisms find lodgment upon it from time to time; a majority of 
these organisms are harmless, and probably transient saprophytes 
which come and go irregularly. The moisture and excretions, how- 
ever, appear to favor the limited development of a few types of bac- 
teria, mainly those of the coccal group, which occur with sufficient 
regularity to be regarded provisionally as habitually parasitic bacteria. 
Of these, the pyogenic cocci are usually the most numerous; they 
exist as "opportunists" on the surface of the skin or penetrate into 
hair follicles and the ducts of the cutaneous glands, ordinarily, how- 
ever, without becoming invasive so long as the continuity of the skin 
is maintained. Abrasions and cuts furnish a portal of entry to the 
subcutaneous tissues, in which these parasitic bacteria frequently 
set up inflammatory reactions. Friction may actually force them 
through 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 
particularly 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 insignificant 
an abrasion as an insect bite may furnish the necessary atrium for 
infection. The umbilicus of the new-born 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 neo- 
natorum" which has been so common in the tropics in the past. Con- 
tused 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 
' A very excellent and complete discussion of the bacteriology of the skin and mucus 
surfaces is by William H. Welch: Surgical Bacteriology, in Dennis's System of Surgery. 
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