The outer layer (the epidermis), which comes into direct contact with the environment, is essentially contiguous and made up of several layers of dead or dying cells. There is constant shedding of these cells, and this shedding keeps pathogens from successfully attaching to the skin. The production of perspiration has a flushing action that constantly moves pathogens off the surface of the skin. The surface of the skin usually has a pH of 5.0 to 6.0, which is too acidic for many microorganisms, but the skin is home to a selection of microbes (bacteria and yeasts) that are part of the normal human microbiota. The number and type of microbes found in a particular area of the skin depends on the local environment, but they all act as additional barriers to opportunistic pathogens. Sebum, which is produced by the sebaceous glands, contains free fatty acids and lipids that are broken down by the normal microbiota into more free fatty acids. These fatty acids are naturally antibacterial and inhibit the growth of such pathogens as group A streptococci.
Chlamydia trachomatis, which causes non-gonococcal urethritis, can infect the eyes of newborn infants as they pass through the birth canal. This infection causes large amounts of pus to form in the eyes, and without treatment it will lead to ulceration and scarring of the cornea. The eyes of Alice’s baby will be treated with erythromycin to prevent the onset of this infection. A similar infection can be caused by Neisseria gonorrhoeae, which is sexually transmitted and often occurs together with a Chlamydial infection. The eyes of newborns used to be treated with an application of silver nitrate, but this has no effect on Chlamydia, so antibiotics are used instead.