Difference between revisions of "Microbiota of the Human Ear"

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[[Image:Ear_compartments.jpg|thumb|300px|right| Fig. 2 The Three Compartments of the Human Ear. http://riversideonline.com/health_reference/Ear-Nose-Throat/EI00027.cfm]]
<br>Three main compartments subdivide the human ear: the outer, middle and inner ear.  Figure 1 shows the three different sections of the human ear.<br> Figure 2 shows more details, and various structures in each section of the ear.
<br>Three main compartments subdivide the human ear: the outer, middle and inner ear.  Figure 1 shows the three different sections of the human ear.  Figure 2 shows more details, and various structures in each section of the ear.<br>
[[Image:The-human-ear.gif|thumb|300px|right| Fig. 2 More Detailed Anatomy of the Human Ear. http://enriquem12bio.wikispaces.com/5.+THE+STRUCTURE+AND+FUNCTION+OF+THE+HUMAN+EAR]]

Revision as of 21:38, 5 May 2015


By Kyle Hardacker


Anatomy & Physiology of the Ear



Outer Ear

The outer ear consists of the fleshy outer portion most commonly thought of when picturing the ear. This structure is referred to as the auricle or the pinna and is supported by cartilage. The auricle functions to funnel sound from the environment into the next section of the outer ear, the external auditory meatus. The external auditory meatus is the ear canal that leads to the tympanic window. The external auditory meatus is the passageway through the temporal bone and is coated in cerumen (earwax). The outer ear is exposed to the environment and is covered in skin. Earwax is produced in the outer ear in order to clean and lubricate the skin of the outer ear. Earwax contains a mixture of hydrocarbons, fatty acids, and cholesterol as well as a mix of antimicrobial proteins [24, 26]. Earwax is produced by a combination of sebaceous and apocrine glands in the outer third portion of the outer ear [2]. The skin of the ear canal grows from inside to out and pushes skin cells to the exterior of the ear where it is eventually shed. This process expels the cerumen from the ear canal [1]. Human cerumen is dimorphic with wet and dry morphotypes [18]. The external auditory meatus terminates at the tympanic membrane (tympanic window or eardrum). The tympanic membrane is the thin membrane that separates the outer and middle ear [1,2].

Middle Ear


Inner Ear


Microbiota of the Ear

Outer Ear

The outer ear is exposed to the external environment and much like skin on other parts of the human body is in contact with microbial life. Both the auricle and the external auditory meatus house a variety of microbes under healthy conditions. The outer ear is exposed to the outside oxygen-filled environment, the majority of the bacterial flora on the auricle and in the external auditory canal is made up of aerobic species [4]. The outer ear is home to a diverse set of microbes including bacteria, viruses, and fungi [3].

Bacteria in the Auricle & Skin of External Auditory Meatus

The skin of the external auditory canal and auricle is predominantly occupied by Gram-positive over Gram-negative bacteria. The main Gram-positive bacteria are staphylococci, coryneforms, streptococci and enterococci, micrococci, and bacillus. Of the Gram-positive bacteria, the predominant species are Staphylococcus auricularis, S. capitis (both capitis and ureolyticus), S. epidermidis, S. warnen, Turicella otitidis, Alloiococcus otitis, Micrococcus luteus, and E. coli [5]. Gram-negative species inhabit the auricle and skin of the external auditory meatus to a much lesser extent with Pseudomonas aeruginosa and Moraxella osloensis in relative abundance. Some fungal microbes can be found in the skin of the outer ear, but are less abundant than either Gram-positive or Gram-negative bacteria. Candida parapsilosis, C. albicans, and Penicillium species form the majority of fungal isolates of the ear integument [25,27].

Bacteria in Cerumen

The skin and the cerumen present different microbial populations in the external ear. Gram-positive bacteria still dominate the bacterial flora and the species distribution is relatively similar to that in the skin. However, the Gram-negative bacteria species are less common and almost non-existent in the cerumen with Pseudomonas. Fungal microbes across the board are more common, yet the species distribution remained the same [27]. Earwax functions to prevent desiccation of the ear canal by lubricating the passageway, which prevents the thin layer of skin lining the ear canal from abrasion and subsequent infection [2]. Earwax also serves to protect the external auditory canal from microbial infection. Earwax not only is pushed out of the ear by skin growth, thereby physically removing microbes from the ear, earwax also contains antimicrobial peptides that prevent bacterial infestation of the ear [1,5,16,24]. Proteins in cerumen chemically prevent microbes from taking hold in the outer ear and serve as an important barrier to ear infections. These proteins suppress the growth rate of many bacterial and fungal microorganisms in the outer ear including E. coli, P. aeruginosa, S. aureus and C. albicans [16]. Certain bacteria may or may not be completely wiped out, but cerumen keeps bacteria in the outer ear under control [16,24,27].

Middle Ear


Inner Ear


Infections of the Ear

Infections of the Ear

Outer Ear

The outer ear is protected by the skin and earwax. The skin forms a physical barrier to the entry of pathogenic microbes, and earwax, as mentioned above, prevents tearing of the skin by means of lubrication and prevents microbial growth through the presence of antimicrobial proteins in the earwax [2,16,24]. However, outer ear infections do occur due to a variety of reason.


Cerumen does come in two morphotypes. Studies show that the wet cerumen yields increased growth in E. coli, S. epidermidis, and Corynebacterium [4]. Removal of earwax can greatly increase the incidence of outer ear infections due to abrasions and the removal of the antimicrobial properties that accompany earwax [22]. Common infections of the outer ear include acute diffuse otitis externa and otomycosis. Infections of the outer ear are referred to as otitis externa or external otitis.

Pseudomonas aeruginosa

One of the most common outer ear infections is acute diffuse otitis externa, commonly referred to as swimmer’s ear. Swimmer’s ear is caused by the microbe P. aeruginosa. Swimmer’s ear can be caused by swimming in polluted water or by cleaning one’s ears with Q-tips [22]. P. aeruginosa is present in the outer ear of healthy individuals but does not normally cause infection [27]. However P. aeruginosa is able to infect immune compromised individuals. When water is trapped in the ear or earwax is removed via Q-tips allows P. aeruginosa to grow relatively unchecked because of favorable conditions or antimicrobial earwax deficiency [22]. Polluted water may contain an excess of bacteria or an abrasion in the skin could allow P. aeruginosa entry from the environment. In any case, once P. aeruginosa gains access to the outer ear and is able to reproduce to reach a critical mass, the ear becomes inflamed, itchy, irritated, and painful. The ear becomes crusty obstructed leading to conductive hearing loss. Cleansing of the ear canal as well as antibiotic treatment via ear drops can reduce and ultimately eliminate swimmer’s ear [22].

P. aeruginosa can also infect the auricle via ear piercings. Piercing of the upper ear cartilage leads to an external ear infection which presents with swelling, inflammation, irritation, and drainage of pus and/or blood [12]. Infection via S. aureus can occur in a similar fashion due to ear piercing from untrained individuals.

Aspergillus niger & Candida albicans


Otomycosis is another common outer ear infection. There are multiple causes of otomycosis but the most common causes are A. niger and C. albicans. The fungal microbes cause symptoms of pain, hearing loss, aural fullness, and itching. In extreme cases otomycosis can permanently damage the ear canal and the tympanic membrane leading to hearing loss [10]. C. albicans, like P. aeruginosa, is present in normal outer ear microbiota[27]. C. albicans is also an opportunistic pathogen, attacking immune compromised individuals. Most of the C. ablicans in the human body resides in the human gut, but C. albicans is also found in the exterior of the human ear [27]. Otomycosis is treatable with the usage of antifungal drops into the ear [10].

Middle Ear


Alloiococcus otitis & Turicella otitidis

Some causes of otitis media are caused by bacteria found in the outer ear. Both A otitis and T. otitidis can be found in the normal microbial flora of the outer ear yet do not cause [27,29]. There is controversy over whether these bacteria cause acute otitis media. Some say they play no role, some say they cause middle ear infection, and some say that these two bacteria enter the middle ear through the tympanic membrane when it is inflamed by an already pre-existing ear infection with the presence of the two microbes only serve to prolong the middle ear infection [30]. This highlights the complex interplay between the microbiome and the human body. A. otitis and T. otitidis are commensal in the external auditory meatus yet possibly pathogenic in the middle ear.

Streptococcus pneumoniae & Haemophilus influenzae & Moraxella catarrhalis

These microbes are the most common cause for acute otitis media [15]. S. pneumoniae, H. influenzae, and M. catarrhalis are all associated with the upper respiratory tract of humans [19]. They access the middle ear via the Eustachian tube connecting to the nasopharynx.

Both S. pneumoniae and H. influenzae are facultative anaerobes which makes them well-suited for the tympanic cavity environment[19]. Once the microbes have taken hold in the middle ear patients present with ear pain, drainage of fluid and diminished hearing [15]. Acute otitis media is common is children and presents with additional symptoms such as increased irritability, fever of 100 °F or higher, and difficulty sleeping [8]. In most cases acute otitis media will resolve itself without treatment; However, in the severe cases amoxicillin is administered as a first line of defense. This is only done in severe cases to avoid adverse side effects of the antibiotics on young children [15].

Moraxella catarrhalis is another upper respiratory bacterium that is known to migrate to the inner ear causing acute otitis media [19].

Inner Ear


Vestibular neuritis



Beneficial Ear Microbiota

Good Microbes and the Microbiome

More and more scientists are beginning to realize that the interplay between the human body and naturally occurring biological flora can be beneficial for both the host and the microbe [3]. In order to restore proper gut health fecal transplants were performed in order to replace the beneficial bacteria [11]. This logic has been extended to the treatment of ear infections. Alpha hemolytic streptococci are present in the middle ear naturally and evidence notion that these bacteria effectively interfere with the activity of S. pneumonia, H. influenza, and M. catarrhalis. In this way alpha hemolytic streptococci and other probiotics can be utilized to combat pesky ear infecitons by essentially replacing the naturally occurring bacterial flora in the middle ear in this case [7,23,29].


[17] Marill, K.A. (2011). Vestibular Neuronitis : Pathology. eMedicine. Medscape Reference

[1] Hodgkin, J. and Partridge, F.A. "Caenorhabditis elegans meets microsporidia: the nematode killers from Paris." 2008. PLoS Biology 6:2634-2637.
