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Microbiology

21.1 Anatomy and Normal Microbiota of the Skin and Eyes

Microbiology 21.1 Anatomy and Normal Microbiota of the Skin and Eyes
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  1. Preface
  2. 1 An Invisible World
    1. Introduction
    2. 1.1 What Our Ancestors Knew
    3. 1.2 A Systematic Approach
    4. 1.3 Types of Microorganisms
    5. Summary
    6. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  3. 2 How We See the Invisible World
    1. Introduction
    2. 2.1 The Properties of Light
    3. 2.2 Peering Into the Invisible World
    4. 2.3 Instruments of Microscopy
    5. 2.4 Staining Microscopic Specimens
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  4. 3 The Cell
    1. Introduction
    2. 3.1 Spontaneous Generation
    3. 3.2 Foundations of Modern Cell Theory
    4. 3.3 Unique Characteristics of Prokaryotic Cells
    5. 3.4 Unique Characteristics of Eukaryotic Cells
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  5. 4 Prokaryotic Diversity
    1. Introduction
    2. 4.1 Prokaryote Habitats, Relationships, and Microbiomes
    3. 4.2 Proteobacteria
    4. 4.3 Nonproteobacteria Gram-Negative Bacteria and Phototrophic Bacteria
    5. 4.4 Gram-Positive Bacteria
    6. 4.5 Deeply Branching Bacteria
    7. 4.6 Archaea
    8. Summary
    9. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  6. 5 The Eukaryotes of Microbiology
    1. Introduction
    2. 5.1 Unicellular Eukaryotic Parasites
    3. 5.2 Parasitic Helminths
    4. 5.3 Fungi
    5. 5.4 Algae
    6. 5.5 Lichens
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  7. 6 Acellular Pathogens
    1. Introduction
    2. 6.1 Viruses
    3. 6.2 The Viral Life Cycle
    4. 6.3 Isolation, Culture, and Identification of Viruses
    5. 6.4 Viroids, Virusoids, and Prions
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  8. 7 Microbial Biochemistry
    1. Introduction
    2. 7.1 Organic Molecules
    3. 7.2 Carbohydrates
    4. 7.3 Lipids
    5. 7.4 Proteins
    6. 7.5 Using Biochemistry to Identify Microorganisms
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. True/False
      3. Matching
      4. Fill in the Blank
      5. Short Answer
      6. Critical Thinking
  9. 8 Microbial Metabolism
    1. Introduction
    2. 8.1 Energy, Matter, and Enzymes
    3. 8.2 Catabolism of Carbohydrates
    4. 8.3 Cellular Respiration
    5. 8.4 Fermentation
    6. 8.5 Catabolism of Lipids and Proteins
    7. 8.6 Photosynthesis
    8. 8.7 Biogeochemical Cycles
    9. Summary
    10. Review Questions
      1. Multiple Choice
      2. True/False
      3. Matching
      4. Fill in the Blank
      5. Short Answer
      6. Critical Thinking
  10. 9 Microbial Growth
    1. Introduction
    2. 9.1 How Microbes Grow
    3. 9.2 Oxygen Requirements for Microbial Growth
    4. 9.3 The Effects of pH on Microbial Growth
    5. 9.4 Temperature and Microbial Growth
    6. 9.5 Other Environmental Conditions that Affect Growth
    7. 9.6 Media Used for Bacterial Growth
    8. Summary
    9. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  11. 10 Biochemistry of the Genome
    1. Introduction
    2. 10.1 Using Microbiology to Discover the Secrets of Life
    3. 10.2 Structure and Function of DNA
    4. 10.3 Structure and Function of RNA
    5. 10.4 Structure and Function of Cellular Genomes
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. True/False
      3. Matching
      4. Fill in the Blank
      5. Short Answer
      6. Critical Thinking
  12. 11 Mechanisms of Microbial Genetics
    1. Introduction
    2. 11.1 The Functions of Genetic Material
    3. 11.2 DNA Replication
    4. 11.3 RNA Transcription
    5. 11.4 Protein Synthesis (Translation)
    6. 11.5 Mutations
    7. 11.6 How Asexual Prokaryotes Achieve Genetic Diversity
    8. 11.7 Gene Regulation: Operon Theory
    9. Summary
    10. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  13. 12 Modern Applications of Microbial Genetics
    1. Introduction
    2. 12.1 Microbes and the Tools of Genetic Engineering
    3. 12.2 Visualizing and Characterizing DNA, RNA, and Protein
    4. 12.3 Whole Genome Methods and Pharmaceutical Applications of Genetic Engineering
    5. 12.4 Gene Therapy
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  14. 13 Control of Microbial Growth
    1. Introduction
    2. 13.1 Controlling Microbial Growth
    3. 13.2 Using Physical Methods to Control Microorganisms
    4. 13.3 Using Chemicals to Control Microorganisms
    5. 13.4 Testing the Effectiveness of Antiseptics and Disinfectants
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  15. 14 Antimicrobial Drugs
    1. Introduction
    2. 14.1 History of Chemotherapy and Antimicrobial Discovery
    3. 14.2 Fundamentals of Antimicrobial Chemotherapy
    4. 14.3 Mechanisms of Antibacterial Drugs
    5. 14.4 Mechanisms of Other Antimicrobial Drugs
    6. 14.5 Drug Resistance
    7. 14.6 Testing the Effectiveness of Antimicrobials
    8. 14.7 Current Strategies for Antimicrobial Discovery
    9. Summary
    10. Review Questions
      1. Multiple Choice
      2. True/False
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  16. 15 Microbial Mechanisms of Pathogenicity
    1. Introduction
    2. 15.1 Characteristics of Infectious Disease
    3. 15.2 How Pathogens Cause Disease
    4. 15.3 Virulence Factors of Bacterial and Viral Pathogens
    5. 15.4 Virulence Factors of Eukaryotic Pathogens
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  17. 16 Disease and Epidemiology
    1. Introduction
    2. 16.1 The Language of Epidemiologists
    3. 16.2 Tracking Infectious Diseases
    4. 16.3 Modes of Disease Transmission
    5. 16.4 Global Public Health
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  18. 17 Innate Nonspecific Host Defenses
    1. Introduction
    2. 17.1 Physical Defenses
    3. 17.2 Chemical Defenses
    4. 17.3 Cellular Defenses
    5. 17.4 Pathogen Recognition and Phagocytosis
    6. 17.5 Inflammation and Fever
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  19. 18 Adaptive Specific Host Defenses
    1. Introduction
    2. 18.1 Overview of Specific Adaptive Immunity
    3. 18.2 Major Histocompatibility Complexes and Antigen-Presenting Cells
    4. 18.3 T Lymphocytes and Cellular Immunity
    5. 18.4 B Lymphocytes and Humoral Immunity
    6. 18.5 Vaccines
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  20. 19 Diseases of the Immune System
    1. Introduction
    2. 19.1 Hypersensitivities
    3. 19.2 Autoimmune Disorders
    4. 19.3 Organ Transplantation and Rejection
    5. 19.4 Immunodeficiency
    6. 19.5 Cancer Immunobiology and Immunotherapy
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  21. 20 Laboratory Analysis of the Immune Response
    1. Introduction
    2. 20.1 Polyclonal and Monoclonal Antibody Production
    3. 20.2 Detecting Antigen-Antibody Complexes
    4. 20.3 Agglutination Assays
    5. 20.4 EIAs and ELISAs
    6. 20.5 Fluorescent Antibody Techniques
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  22. 21 Skin and Eye Infections
    1. Introduction
    2. 21.1 Anatomy and Normal Microbiota of the Skin and Eyes
    3. 21.2 Bacterial Infections of the Skin and Eyes
    4. 21.3 Viral Infections of the Skin and Eyes
    5. 21.4 Mycoses of the Skin
    6. 21.5 Protozoan and Helminthic Infections of the Skin and Eyes
    7. Summary
    8. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  23. 22 Respiratory System Infections
    1. Introduction
    2. 22.1 Anatomy and Normal Microbiota of the Respiratory Tract
    3. 22.2 Bacterial Infections of the Respiratory Tract
    4. 22.3 Viral Infections of the Respiratory Tract
    5. 22.4 Respiratory Mycoses
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  24. 23 Urogenital System Infections
    1. Introduction
    2. 23.1 Anatomy and Normal Microbiota of the Urogenital Tract
    3. 23.2 Bacterial Infections of the Urinary System
    4. 23.3 Bacterial Infections of the Reproductive System
    5. 23.4 Viral Infections of the Reproductive System
    6. 23.5 Fungal Infections of the Reproductive System
    7. 23.6 Protozoan Infections of the Urogenital System
    8. Summary
    9. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  25. 24 Digestive System Infections
    1. Introduction
    2. 24.1 Anatomy and Normal Microbiota of the Digestive System
    3. 24.2 Microbial Diseases of the Mouth and Oral Cavity
    4. 24.3 Bacterial Infections of the Gastrointestinal Tract
    5. 24.4 Viral Infections of the Gastrointestinal Tract
    6. 24.5 Protozoan Infections of the Gastrointestinal Tract
    7. 24.6 Helminthic Infections of the Gastrointestinal Tract
    8. Summary
    9. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  26. 25 Circulatory and Lymphatic System Infections
    1. Introduction
    2. 25.1 Anatomy of the Circulatory and Lymphatic Systems
    3. 25.2 Bacterial Infections of the Circulatory and Lymphatic Systems
    4. 25.3 Viral Infections of the Circulatory and Lymphatic Systems
    5. 25.4 Parasitic Infections of the Circulatory and Lymphatic Systems
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Fill in the Blank
      3. Short Answer
      4. Critical Thinking
  27. 26 Nervous System Infections
    1. Introduction
    2. 26.1 Anatomy of the Nervous System
    3. 26.2 Bacterial Diseases of the Nervous System
    4. 26.3 Acellular Diseases of the Nervous System
    5. 26.4 Fungal and Parasitic Diseases of the Nervous System
    6. Summary
    7. Review Questions
      1. Multiple Choice
      2. Matching
      3. Fill in the Blank
      4. Short Answer
      5. Critical Thinking
  28. A | Fundamentals of Physics and Chemistry Important to Microbiology
  29. B | Mathematical Basics
  30. C | Metabolic Pathways
  31. D | Taxonomy of Clinically Relevant Microorganisms
  32. E | Glossary
  33. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
    13. Chapter 13
    14. Chapter 14
    15. Chapter 15
    16. Chapter 16
    17. Chapter 17
    18. Chapter 18
    19. Chapter 19
    20. Chapter 20
    21. Chapter 21
    22. Chapter 22
    23. Chapter 23
    24. Chapter 24
    25. Chapter 25
    26. Chapter 26
  34. Index

Learning Objectives

  • Describe the major anatomical features of the skin and eyes
  • Compare and contrast the microbiomes of various body sites, such as the hands, back, feet, and eyes
  • Explain how microorganisms overcome defenses of skin and eyes in order to cause infection
  • Describe general signs and symptoms of disease associated with infections of the skin and eyes

Clinical Focus

Part 1

Sam, a college freshman with a bad habit of oversleeping, nicked himself shaving in a rush to get to class on time. At the time, he didn’t think twice about it. But two days later, he noticed the cut was surrounded by a reddish area of skin that was warm to the touch. When the wound started oozing pus, he decided he had better stop by the university’s clinic. The doctor took a sample from the lesion and then cleaned the area.

  • What type of microbe could be responsible for Sam’s infection?

Jump to the next Clinical Focus box.

Human skin is an important part of the innate immune system. In addition to serving a wide range of other functions, the skin serves as an important barrier to microbial invasion. Not only is it a physical barrier to penetration of deeper tissues by potential pathogens, but it also provides an inhospitable environment for the growth of many pathogens. In this section, we will provide a brief overview of the anatomy and normal microbiota of the skin and eyes, along with general symptoms associated with skin and eye infections.

Layers of the Skin

Human skin is made up of several layers and sublayers. The two main layers are the epidermis and the dermis. These layers cover a third layer of tissue called the hypodermis, which consists of fibrous and adipose connective tissue (Figure 21.2).

The epidermis is the outermost layer of the skin, and it is relatively thin. The exterior surface of the epidermis, called the stratum corneum, primarily consists of dead skin cells. This layer of dead cells limits direct contact between the outside world and live cells. The stratum corneum is rich in keratin, a tough, fibrous protein that is also found in hair and nails. Keratin helps make the outer surface of the skin relatively tough and waterproof. It also helps to keep the surface of the skin dry, which reduces microbial growth. However, some microbes are still able to live on the surface of the skin, and some of these can be shed with dead skin cells in the process of desquamation, which is the shedding and peeling of skin that occurs as a normal process but that may be accelerated when infection is present.

Beneath the epidermis lies a thicker skin layer called the dermis. The dermis contains connective tissue and embedded structures such as blood vessels, nerves, and muscles. Structures called hair follicles (from which hair grows) are located within the dermis, even though much of their structure consists of epidermal tissue. The dermis also contains the two major types of glands found in human skin: sweat glands (tubular glands that produce sweat) and sebaceous glands (which are associated with hair follicles and produce sebum, a lipid-rich substance containing proteins and minerals).

Perspiration (sweat) provides some moisture to the epidermis, which can increase the potential for microbial growth. For this reason, more microbes are found on the regions of the skin that produce the most sweat, such as the skin of the underarms and groin. However, in addition to water, sweat also contains substances that inhibit microbial growth, such as salts, lysozyme, and antimicrobial peptides. Sebum also serves to protect the skin and reduce water loss. Although some of the lipids and fatty acids in sebum inhibit microbial growth, sebum contains compounds that provide nutrition for certain microbes.

a) A micrograph of a large light pink region labeled dermis, a thinner dark pink region on top of that labeled epidermis, and a thin region of clear cells. The division between the dermis and epidermis is wavy; with areas where one projects into the other. B) A diagram of skin. The top layer is dark and is labeled epidermis. The next layer is lighter and much thicker; this is the dermis. Inside the dermis are vase-shaped hair follicles with hairs projecting out of the skin. Next to the hair follicle is a smaller vase-shape labeled sebaceous gland; this empties into the space of the hair follicle. There are also coiled shapes labeled receptor and a variety of long tubes labeled: nerve, lymph vessel and blood vessels. A coiled blob is labeled sweat gland; this leads to a tube that opens at the surface called a sweat pore. Below the dermis is a yellow bubbly-looking layer labeled fatty tissue; this is the hypodermis.
Figure 21.2 (a) A micrograph of a section through human skin shows the epidermis and dermis. (b) The major layers of human skin are the epidermis, dermis, and hypodermis. (credit b: modification of work by National Cancer Institute)

Check Your Understanding

  • How does desquamation help with preventing infections?

Normal Microbiota of the Skin

The skin is home to a wide variety of normal microbiota, consisting of commensal organisms that derive nutrition from skin cells and secretions such as sweat and sebum. The normal microbiota of skin tends to inhibit transient-microbe colonization by producing antimicrobial substances and outcompeting other microbes that land on the surface of the skin. This helps to protect the skin from pathogenic infection.

The skin’s properties differ from one region of the body to another, as does the composition of the skin’s microbiota. The availability of nutrients and moisture partly dictates which microorganisms will thrive in a particular region of the skin. Relatively moist skin, such as that of the nares (nostrils) and underarms, has a much different microbiota than the dryer skin on the arms, legs, hands, and top of the feet. Some areas of the skin have higher densities of sebaceous glands. These sebum-rich areas, which include the back, the folds at the side of the nose, and the back of the neck, harbor distinct microbial communities that are less diverse than those found on other parts of the body.

Different types of bacteria dominate the dry, moist, and sebum-rich regions of the skin. The most abundant microbes typically found in the dry and sebaceous regions are Betaproteobacteria and Propionibacteria, respectively. In the moist regions, Corynebacterium and Staphylococcus are most commonly found (Figure 21.3). Viruses and fungi are also found on the skin, with Malassezia being the most common type of fungus found as part of the normal microbiota. The role and populations of viruses in the microbiota, known as viromes, are still not well understood, and there are limitations to the techniques used to identify them. However, Circoviridae, Papillomaviridae, and Polyomaviridae appear to be the most common residents in the healthy skin virome.123

A diagram showing different regions of the body. Each region has a pie chart that shows which bacteria are most prevalent. The most common bacterium in each region: Glabella (corynebacterineae), Alar Crease (propionibacterineae), External auditory canal (propionibacterineae), Nare (other actinobacteria), manubrioum (propionibacterineae), Axillary vault (proteobacteria), antecubital fossa (proteobacteria), Volar forearm (proteobacteria), interdigital web space (proteobacteria), hypothenar palm (proteobacteria), inguinal crease (corynebacterineae), umbilicus (corynebacterineae), toe web space (corynebacterineae, , propionibacterineae, and staphylococcaceae), reticular crease (propionibacterineae), occiput (staphylococcaceae, back (propionibacterineae), buttock (proteobacteria), gluteal crease (corynebacterineae), popliteal fossa (staphylococcaceae), plantar heel (staphylococcaceae).  Second part of the image shows that different subjects have different bacterial percentages and that these percentages change over time.
Figure 21.3 The normal microbiota varies on different regions of the skin, especially in dry versus moist areas. The figure shows the major organisms commonly found in different locations of a healthy individual’s skin and external mucosa. Note that there is significant variation among individuals. (credit: modification of work by National Human Genome Research Institute)

Check Your Understanding

  • What are the four most common bacteria that are part of the normal skin microbiota?

Infections of the Skin

While the microbiota of the skin can play a protective role, it can also cause harm in certain cases. Often, an opportunistic pathogen residing in the skin microbiota of one individual may be transmitted to another individual more susceptible to an infection. For example, methicillin-resistant Staphylococcus aureus (MRSA) can often take up residence in the nares of health care workers and hospital patients; though harmless on intact, healthy skin, MRSA can cause infections if introduced into other parts of the body, as might occur during surgery or via a post-surgical incision or wound. This is one reason why clean surgical sites are so important.

Injury or damage to the skin can allow microbes to enter deeper tissues, where nutrients are more abundant and the environment is more conducive to bacterial growth. Wound infections are common after a puncture or laceration that damages the physical barrier of the skin. Microbes may infect structures in the dermis, such as hair follicles and glands, causing a localized infection, or they may reach the bloodstream, which can lead to a systemic infection.

In some cases, infectious microbes can cause a variety of rashes or lesions that differ in their physical characteristics. These rashes can be the result of inflammation reactions or direct responses to toxins produced by the microbes. Table 21.1 lists some of the medical terminology used to describe skin lesions and rashes based on their characteristics; Figure 21.4 and Figure 21.5 illustrate some of the various types of skin lesions. It is important to note that many different diseases can lead to skin conditions of very similar appearance; thus the terms used in the table are generally not exclusive to a particular type of infection or disease.

Some Medical Terms Associated with Skin Lesions and Rashes
Term Definition
abscess localized collection of pus
bulla (pl., bullae) fluid-filled blister no more than 5 mm in diameter
carbuncle deep, pus-filled abscess generally formed from multiple furuncles
crust dried fluids from a lesion on the surface of the skin
cyst encapsulated sac filled with fluid, semi-solid matter, or gas, typically located just below the upper layers of skin
folliculitis a localized rash due to inflammation of hair follicles
furuncle (boil) pus-filled abscess due to infection of a hair follicle
macules smooth spots of discoloration on the skin
papules small raised bumps on the skin
pseudocyst lesion that resembles a cyst but with a less defined boundary
purulent pus-producing; suppurative
pustules fluid- or pus-filled bumps on the skin
pyoderma any suppurative (pus-producing) infection of the skin
suppurative producing pus; purulent
ulcer break in the skin; open sore
vesicle small, fluid-filled lesion
wheal swollen, inflamed skin that itches or burns, such as from an insect bite
Table 21.1
a) Acne (labeled whitehead) on a person’s cheek. B) A drawing of skin with a yellow bubble labeled pus. This is below a raised region on the skin.
Figure 21.4 (a) Acne is a bacterial infection of the skin that manifests as a rash of inflamed hair follicles (folliculitis). The large whitehead near the center of the cheek is an infected hair follicle that has become purulent (or suppurative), leading to the formation of a furuncle. (b) An abscess is a pus-filled lesion. (credit b: modification of work by Bruce Blaus)
A table labeled types of skin lesions. Crust is shown as a raised region on the surface of the skin. Cyst is shown as a large white sphere in the upper layers of the skin. Macule is shown as a dark mark on the surface. Papule is shown as a raised bubble on the surface. Pusture is shown as a large yellow sphere in the upper layers of the skin. Ulcer is a large cavity in the skin. Vesicle is a small blue bubble in the upper regions of the skin. Wheal is a small blue bubble on the surface of the skin.
Figure 21.5 Numerous causes can lead to skin lesions of various types, some of which are very similar in appearance. (credit: modification of work by Bruce Blaus)

Check Your Understanding

  • How can asymptomatic health care workers transmit bacteria such as MRSA to patients?

Anatomy and Microbiota of the Eye

Although the eye and skin have distinct anatomy, they are both in direct contact with the external environment. An important component of the eye is the nasolacrimal drainage system, which serves as a conduit for the fluid of the eye, called tears. Tears flow from the external eye to the nasal cavity by the lacrimal apparatus, which is composed of the structures involved in tear production (Figure 21.6). The lacrimal gland, above the eye, secretes tears to keep the eye moist. There are two small openings, one on the inside edge of the upper eyelid and one on the inside edge of the lower eyelid, near the nose. Each of these openings is called a lacrimal punctum. Together, these lacrimal puncta collect tears from the eye that are then conveyed through lacrimal ducts to a reservoir for tears called the lacrimal sac, also known as the dacrocyst or tear sac.

From the sac, tear fluid flows via a nasolacrimal duct to the inner nose. Each nasolacrimal duct is located underneath the skin and passes through the bones of the face into the nose. Chemicals in tears, such as defensins, lactoferrin, and lysozyme, help to prevent colonization by pathogens. In addition, mucins facilitate removal of microbes from the surface of the eye.

Diagram of an eye. Above the eye is the lacrimal gland. At the point nearest the nose is the punctums and tubes leading to the lacrimal sac and nasolacrimal duct.
Figure 21.6 The lacrimal apparatus includes the structures of the eye associated with tear production and drainage. (credit: modification of work by “Evidence Based Medical Educator Inc.”/YouTube)

The surfaces of the eyeball and inner eyelid are mucous membranes called conjunctiva. The normal conjunctival microbiota has not been well characterized, but does exist. One small study (part of the Ocular Microbiome project) found twelve genera that were consistently present in the conjunctiva.4 These microbes are thought to help defend the membranes against pathogens. However, it is still unclear which microbes may be transient and which may form a stable microbiota.5

Use of contact lenses can cause changes in the normal microbiota of the conjunctiva by introducing another surface into the natural anatomy of the eye. Research is currently underway to better understand how contact lenses may impact the normal microbiota and contribute to eye disease.

The watery material inside of the eyeball is called the vitreous humor. Unlike the conjunctiva, it is protected from contact with the environment and is almost always sterile, with no normal microbiota (Figure 21.7).

A cross section of the eye. The large spherical center is the vitreous humor. The layer surrounding this is the retina. A projection out of the back of the eye is the optic nerve. A region on the retina just above the optic nerve is the fovea. At the front of the eye is the lens. In front of this is a space labeled pupil. The colored region around the pupil is the iris. The cornea is the covering in front of the iris and pupil. The conjunctiva is a mucous membrane on the eye.
Figure 21.7 Some microbes live on the conjunctiva of the human eye, but the vitreous humor is sterile.

Infections of the Eye

The conjunctiva is a frequent site of infection of the eye; like other mucous membranes, it is also a common portal of entry for pathogens. Inflammation of the conjunctiva is called conjunctivitis, although it is commonly known as pinkeye because of the pink appearance in the eye. Infections of deeper structures, beneath the cornea, are less common (Figure 21.8). Conjunctivitis occurs in multiple forms. It may be acute or chronic. Acute purulent conjunctivitis is associated with pus formation, while acute hemorrhagic conjunctivitis is associated with bleeding in the conjunctiva. The term blepharitis refers to an inflammation of the eyelids, while keratitis refers to an inflammation of the cornea (Figure 21.8); keratoconjunctivitis is an inflammation of both the cornea and the conjunctiva, and dacryocystitis is an inflammation of the lacrimal sac that can often occur when a nasolacrimal duct is blocked.

a) photo of an eyelid being pulled back to show a red are. B) A photo of inflamed eyelids. C)A photo of an eye with a cloudy cornea.
Figure 21.8 (a) Conjunctivitis is inflammation of the conjunctiva. (b) Blepharitis is inflammation of the eyelids. (c) Keratitis is inflammation of the cornea. (credit a: modification of work by Lopez-Prats MJ, Sanz Marco E, Hidalgo-Mora JJ, Garcia-Delpech S, Diaz-Llopis M; credit b, c: modification of work by Centers for Disease Control and Prevention)

Infections leading to conjunctivitis, blepharitis, keratoconjunctivitis, or dacryocystitis may be caused by bacteria or viruses, but allergens, pollutants, or chemicals can also irritate the eye and cause inflammation of various structures. Viral infection is a more likely cause of conjunctivitis in cases with symptoms such as fever and watery discharge that occurs with upper respiratory infection and itchy eyes. Table 21.2 summarizes some common forms of conjunctivitis and blepharitis.

Types of Conjunctivities and Blepharitis
Condition Description Causative Agent(s)
Acute purulent conjunctivitis Conjunctivitis with purulent discharge Bacterial (Haemophilus, Staphylococcus)
Acute hemorrhagic conjunctivitis Involves subconjunctival hemorrhages Viral (Picornaviradae)
Acute ulcerative blepharitis Infection involving eyelids; pustules and ulcers may develop Bacterial (Staphylococcal) or viral (herpes simplex, varicella-zoster, etc.)
Follicular conjunctivitis Inflammation of the conjunctiva with nodules (dome-shaped structures that are red at the base and pale on top) Viral (adenovirus and others); environmental irritants
Dacryocystitis Inflammation of the lacrimal sac often associated with a plugged nasolacrimal duct Bacterial (Haemophilus, Staphylococcus, Streptococcus)
Keratitis Inflammation of cornea Bacterial, viral, or protozoal; environmental irritants
Keratoconjunctivitis Inflammation of cornea and conjunctiva Bacterial, viral (adenoviruses), or other causes (including dryness of the eye)
Nonulcerative blepharitis Inflammation, irritation, redness of the eyelids without ulceration Environmental irritants; allergens
Papillary conjunctivitis Inflammation of the conjunctiva; nodules and papillae with red tops develop Environmental irritants; allergens
Table 21.2

Check Your Understanding

  • How does the lacrimal apparatus help to prevent eye infections?

Footnotes

  • 1 Belkaid, Y., and J.A. Segre. “Dialogue Between Skin Microbiota and Immunity,” Science 346 (2014) 6212:954–959.
  • 2 Foulongne, Vincent, et al. “Human Skin Microbiota: High Diversity of DNA Viruses Identified on the Human Skin by High Throughput Sequencing.” PLoS ONE (2012) 7(6): e38499. doi: 10.1371/journal.pone.0038499.
  • 3 Robinson, C.M., and J.K. Pfeiffer. “Viruses and the Microbiota.” Annual Review of Virology (2014) 1:55–59. doi: 10.1146/annurev-virology-031413-085550.
  • 4 Abelson, M.B., Lane, K., and Slocum, C.. “The Secrets of Ocular Microbiomes.” Review of Ophthalmology June 8, 2015. http://www.reviewofophthalmology.com/content/t/ocular_disease/c/55178. Accessed Sept 14, 2016.
  • 5 Shaikh-Lesko, R. “Visualizing the Ocular Microbiome.” The Scientist May 12, 2014. http://www.the-scientist.com/?articles.view/articleNo/39945/title/Visualizing-the-Ocular-Microbiome. Accessed Sept 14, 2016.
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