Ear

Intro to the Ear

The Human Ear: The human ear is comprised of 3 main parts: outer, middle, and inner. The outer ear consists of the pinna, which is the visible part of the ear, and the auditory ear canal to the tympanic membrane(ear drum). The membrane separates the outer ear from the middle ear which is an air-filled space that is connected to the nasopharynx via the eustachian tube. The middle ear consists of the malleus, incus, and stapes. These three tiny bones carry sound from the ear drum to the inner ear. The inner ear (aka the bony labyrinth) is comprised of the membraneous labyrinth which encloses both cochlea and the vestibular apparatus.(1)

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Physical Conditions of the Normal Healthy Ear

The conditions of the pinna are similar to that of the skin. Closer to the ear canal the skin develops a thin lining of ear wax. Ear wax is composed mostly of dead skin cells and keratin with a small mixture of cerumen, sweat, and oil. Cerumen is secreted from the ceruminous glands located in the first third outer part of the ear canal and is thought to be composed mainly of cholesterol, squalene, wax esters, ceramides, and triglycerides. The cerumen also has antibacterial/fungal properties which can be attributed to its slight acidic pH of 6.1 and the presence of lysozyme. In normal individuals, the ear wax is continuously pushed out of the ear canal by the slow migration of the top layer of skin cells from the tympanic membrane towards the outer ear. The ear wax traps any foreign particles/organisms on its way out. The ear canal is a dark, moist, close to body temperature(~37C), and abundant with nutrients from dead skin cells.(2)(3) If not for the cerumen, the ear canal would be a very hospital place for microbes.

The middle ear, or tympanic cavity, is an air filled space that is connected to the outer ear via the tympanic membrane and the inner ear via the fenestra vestibuli. A mucous membrane lines the middle ear that is continuous with the nasal passageways and nasopharynx via the eustachian tube. This continuity allows the person to equalize the air pressure inside the cavity with that of the outside environment. The mucus serves to trap any particles and microbes that are inside and slowly move it towards the throat to be swallowed or coughed out. This migration of mucus is important to prevent microbes from colonizing as the tympanic cavity would otherwise present a very favorable environment for microbes. It is abundant in nutrients, moist, dark, protected from the external environment, and at body temperature. (4)

The inner ear consists of two labyrinth structures. The bony labyrinth and the membraneous labyrinth. The bony labyrinth is filled with a fluid called perilymph and encloses the membraneous labyrinth. The membraneous labyrinth comprises the vestibular apparatus and the cochlea. It is filled with a seperate fluid that does not mix with the perilymph called endolymph. The both fluids are derived from blood plasma implying a high concentration of nutrients and a similar pH (~7.4). A characteristic of perilymph is that its concentration of sodium is high (about 150 milliequivalents per litre) and its concentration of potassium is low (about 5 milliequivalents per litre), similar to other extracellular fluids. Conversely endolymph has a low concentration of sodium (about 15 milliequivalents per litre) and a high concentration of potassium (about 140 milliequivalents per litre).(5) Since the inner ear is completely membrane enclosed, it should be virtually imperveous to bacterial colonization under normal circumstances.

Microbes in the Healthy Ear

Because it is exposed to the outside environment, despite the best efforts of the ceruminous glands, the normal healthy outer ear still houses a variety of microbes. The some of the most common bacteria are Staphylococcus epidermis, Turicellaotitidis, Alloiococousotitis, Pseudomonas aeruginosa, Corynebacterium, Staphylococcus aureus, and Streptococcus saprophyticum. The most common fungal microbe known to reside in the ear is Candida albicans. (7)(8)

Microbes that are known to inhabit the middle ear are Streptococci, Haemophilus pneumoniae, Moraxella catarrhalis, and less commonly mycobacterium.

The healthy inner ear should be virtually microbe free.

Interactions Between microbes

In the middle ear, research shows that alpha hemolytic streptococci from healthy children inhibit pneumococci and H. influenzae, two pathogens that cause Otitis media. (9)

Normal Changes to the Environment of the Ear

Do any of the physical conditions change? How do those changes affect the microbes?

In regards to the ear, the environment of the outer ear is subject to the most variation as it is exposed to the outside environment. One of the biggest effects on the environment of the ear would be those that remove the ear wax. This can occur through excessive showering or swimming as water/soap can wash away the thin protective lining of ear wax. Loss of the cerumen encourages the growth of microbes that are normally kept under control by its antimicrobial properties. Use of cotton swaps or hair pins is also common practice as a means to remove ear wax. In addition to the removal of the ear wax, they can also scratch the surface of the skin which can allow microbes to get in to the unprotected tissue. Use of cotton swabs also can push the ear wax further into the canal, which if allowed to build up can become impacted and damage the inner ear canal and the tympanic membrane. These too provide opportunities for microbes to infect the underlying tissue.

The infectous opportunity of the ear also grows smaller as the person grows older. Young children with immature immune systems are more susceptible to infection by microbes than are adults with developed immune systems.

Blockage of the Eustachian tube can also increase the likelihood of ear infections. This occurs when the mucus in the tube gets too thick or becomes blocked as can occur during a cold or allergies. Blockage of the tube prevents a healthy flow of mucus out of the middle ear.

Infections of the Ear

When do microbes infect the ear, what conditions? Why do those conditions allow the microbes do infect the ear? How do other infections cause a change in the conditions of the ear? How do those change in conditions allow other microbes to infect the ear?

Resources

1)

2)

3)^Bortz JT, Wertz PW, Downing DT. "Composition of cerumen lipids." J Am Acad Dermatol. 1990 Nov;23(5 Pt 1):845-9.

4)^James E. Crouch. "Functional Human Anatomy" Lea and Febiger 1978

5)^Encyclopedia Britannica. "Anatomy of the Human Ear" http://www.britannica.com/EBchecked/topic/175622/ear/65043/Endolymph-and-perilymph#tab=active~checked%2Citems~checked&title=human%20ear%20%3A%3A%20Endolymph%20and%20perilymph%20--%20Britannica%20Online%20Encyclopedia

7)^Stroman DW, Roland PS, Dohar J, Burt W. "Microbiology of normal external auditory canal." Laryngoscope. 2001 Nov;111(11 Pt 1):2054-9

8)^Campos A, Arias A, Betancor L, Rodríguez C, Hernández AM, López Aguado D, Sierra A. "Study of common aerobic flora of human cerumen." J Laryngol Otol. 1998 Jul;112(7):613-6

9)^Tano K, Grahn-Håkansson E, Holm SE, Hellström S. "Inhibition of OM pathogens by alpha-hemolytic streptococci from healthy children, children with SOM and children with rAOM." Int J Pediatr Otorhinolaryngol. 2000 Dec 22;56(3):185-90.

http://www.meddean.luc.edu/depts/otolaryn/patient_ed/pdf/ENT%20INNER%20EAR%20FLUID%20IMBALANCE.pdf

http://jama.ama-assn.org/cgi/content/abstract/291/8/981 ( Outbreak of Pseudomonas aeruginosa Infections Caused by Piercing of Upper Ear Cartilage)

a. Ronna Staley, MD, James J. Fitzgibbon, MD, Catherine Anderson, and LSM, MSN. "Auricular Infections Caused by High Ear Piercing in Adolescents." PEDIATRICS. Vol. 99 No. 4 April 1997, pp. 610-611