Microbiota of the Human Ear

Introduction

By Kyle Hardacker

The human body can be thought of as a superorganism. Many microbes can be found in or around the human body and microbial cells are in much higher abundance than human cells. The human body serves as a microbial ecosystem with a wide variety of environments ranging from the skin to mucous membranes and the digestive tract. Due to the microbial environmental diversity in and around the human body, the microbial population varies depending on location. This confers a wide range of bacteria and other microbes inhabiting the human body. The human ear serves as a unique environment with its own microbiome due to its distinct anatomy (Belkaid, 2014).

Anatomy & Physiology of the Ear

Anatomical Drawing of the Human Ear. http://enriquem12bio.wikispaces.com/5.+THE+STRUCTURE+AND+FUNCTION+OF+THE+HUMAN+EAR

Three main compartments subdivide the human ear: the outer, middle and inner 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 (Schwaab, 2011; Stransky, 2011). Earwax is produced by a combination of sebaceous and apocrine glands in the outer third portion of the outer ear (Alvord, 1997). 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 (Alberti, 1988). Human cerumen is dimorphic with wet and dry morphotypes (Matsunaga, 1962). 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 (Alberti, 1988; Alvord, 1997).

Middle Ear

The middle ear, or tympanic cavity, is an air-filled cavity contain a set of three ossicles: the malleus, incus and stapes. The ossicles are conjoined sequentially with the malleus anchored to the tympanic membrane and the stapes anchored to the inner ear. The hammer, anvil, and stirrup respectively conduct the oscillations of the tympanic membrane from sound vibrations entering the outer ear to the inner ear via the oval window. The Eustachian tube connects the middle ear to the nasopharynx and functions to equilibrate air pressure between the middle and outer ear to prevent perforation of the ear drum. This is the reason that people can pop their ears by closing their mouth, plugging their nose and exhaling. The increase in pressure in the nasopharynx is transmitted into the middle ear via the Eustachian tube, causing the tympanic membrane to pop (Alvord, 1997; Alberti, 1988).

Inner Ear

The inner ear contains the organs and nerves that are involved in hearing and balance. The cochlea separates the inner and middle ear and is the snail-shaped auditory organ. The oval window of the cochlea vibrates as sound is conducted into the inner ear and the vibrations of the oval window. The perilymph inside the cochlea conducts the sound waves to the vestibular membrane. Inside of the vestibular membrane is endolymph fluid that conducts sound to the basilar membrane. Inside the basilar membrane, specialized hairs detect the sound waves and the action potentials created are sent to the brain via the vestibulocochlear nerves. The vestibule and semicircular canals function to maintain balance. The vestibule and semicircular canals sense the motion of the endolymph with specialized hair cells and assess the bodies position with respect to gravity. The action potentials are sent to the brain via the vestibulocochlear nerve. The endolymph and perilymph differ based on the potassium and sodium concentration. The endolymph contains higher concentration of potassium ions than sodium ions (Konishi, 1978). The difference in ion concentrations between the two presents a different environment to potential bacteria.

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 (Brook, 1981). The outer ear is home to a diverse set of microbes including bacteria, viruses, and fungi (Belkaid, 2014 Segre).

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 (Campos, 2000). 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 (Staley, 1997; Stroman 2009).

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 (Stroman, 2009). 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 (Alvord, 1997). 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 (Alberti, 1988; Schwaab, 2011; Campos, 2000; Lum, 2009). 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 (Lum, 2009). Certain bacteria may or may not be completely wiped out, but cerumen keeps bacteria in the outer ear under control (Stroman, 2009; Schwaab, 2011; , Lum, 2009).

Middle Ear

Although the middle ear is segregated from the external portion of the ear via the tympanic membrane the middle ear is connected to the nasopharynx by the way of the Eustachian tube. In this way the middle ear is somewhat in contact with the external environment. However, bacteria would still need to travel through the nasal cavity and up the Eustachian tube which is no easy task. The mucous and cilia in the nasal cavity function to trap and expel foreign particles including bacteria that may travel up to the middle ear (Dahl, 1998). That being said microbes are present in the middle ear. For instance alpha hemolytic streptococci are present in a healthy middle ear (Roos, 2000).

Inner Ear

Unlike the outer and middle ear, the inner ear is completely secluded from the outside environment. The cochlea, semicircular canals, and vestibule are enclosed by the bony labyrinth. This makes it much more difficult for bacteria to translocate across the oval window. The immune system is more prevalent in the inner ear which also reduces the possibility of microbial entry into the inner ear (Alvord, 1997; Alberti, 1988).