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21: Skin and Eye Infections - Biology

21: Skin and Eye Infections - Biology


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  • 21.1: Anatomy and Normal Microbiota of the Skin and Eyes
    Human skin consists of two main layers, the epidermis and dermis, which are situated on top of the hypodermis, a layer of connective tissue. The skin is an effective physical barrier against microbial invasion. The skin’s relatively dry environment and normal microbiota discourage colonization by transient microbes. The skin’s normal microbiota varies from one region of the body to another. The conjunctiva of the eye is a frequent site for microbial infection and deeper infections are less commo
  • 21.2: Bacterial Infections of the Skin and Eyes
    Staphylococcus and Streptococcus cause many different types of skin infections, many of which occur when bacteria breach the skin barrier through a cut or wound. S. aureus are frequently associated with purulent skin infections that manifest as folliculitis, furuncles, or carbuncles. aureus is also a leading cause of staphylococcal scalded skin syndrome (SSSS). aureus is generally drug resistant and current MRSA strains are resistant to a wide range of antibiotics.
  • 21.3: Viral Infections of the Skin and Eyes
    Papillomas (warts) are caused by human papillomaviruses. Herpes simplex virus (especially HSV-1) mainly causes oral herpes, but lesions can appear on other areas of the skin and mucous membranes. Roseola and fifth disease are common viral illnesses that cause skin rashes; roseola is caused by HHV-6 and HHV-7 while fifth disease is caused by parvovirus 19. Viral conjunctivitis is often caused by adenoviruses and may be associated with the common cold. Herpes keratitis is caused by herpesviruses.
  • 21.4: Mycoses of the Skin and Eyes
    Mycoses can be cutaneous, subcutaneous, or systemic. Common cutaneous mycoses include tineas caused by dermatophytes of the genera Trichophyton, Epidermophyton, and Microsporum. Tinea corporis is called ringworm. Tineas on other parts of the body have names associated with the affected body part. Aspergillosis is a fungal disease caused by molds of the genus Aspergillus. Primary cutaneous aspergillosis enters through a break in the skin, such as the site of an injury or a surgical wound.
  • 21.5: Protozoan and Helminthic Infections of the Eyes
    The protozoan Acanthamoeba and the helminth Loa loa are two parasites that can breach the skin barrier, causing infections of the skin and eyes. Acanthamoeba keratitis is a parasitic infection of the eye that often results from improper disinfection of contact lenses or swimming while wearing contact lenses. Loiasis, or eye worm, is a disease endemic to Africa that is caused by parasitic worms that infect the subcutaneous tissue of the skin and eyes. It is transmitted by deerfly vectors.
  • 21.E: Skin and Eye Infections (Exercises)
    These are excerise for Chapter 21 "Skin and Eye Infections" in OpenStax's Microbiology Textmap.

Thumbnail: Multiple plantar warts have grown on this toe.


21.1 Anatomy and Normal Microbiota of the Skin and Eyes

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.

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.

Check Your Understanding

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. 1 2 3

Check Your Understanding

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

Check Your Understanding

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.

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).

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.

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

Check Your Understanding

Footnotes

    Belkaid, Y., and J.A. Segre. “Dialogue Between Skin Microbiota and Immunity,” Science 346 (2014) 6212:954–959. 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. 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. 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. 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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    • Authors: Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster
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    Introduction

    Eye infections are a common presenting problem in primary care. ‘Red eye’, 𠆌onjunctivitis’ and 𠆌orneal ulcer/keratitis’ were among the top five problems most commonly referred to two ophthalmology departments in Brisbane. 1

    To ensure a good visual outcome for the patient, the practitioner should make a prompt diagnosis and start appropriate treatment. Conjunctivitis typically does not threaten vision, but infections of the cornea or inside the eye are serious threats and require immediate referral to an ophthalmologist.


    Erysipelas

    Erysipelas ( Figure 3 ) , also known as St. Anthony's fire, usually presents as an intensely erythematous infection with clearly demarcated raised margins, and often with associated lymphatic streaking (Table 1) . Common sites are the legs and face.15 Most cases do not have an inciting wound or skin lesion and are preceded by influenza-like symptoms. The incidence of erysipelas is rising, especially in young children, the elderly, persons with diabetes, alcoholic persons, and patients with compromised immune systems or lymphedema.6


    Protective Features of the Eyes

    The orbit, eyelashes, eyelids, conjunctiva, and lacrimal glands help protect the eyes.

    Structures That Protect the Eye

    The bony structures of the orbit (the bony cavity that contains the eyeball and its muscles, nerves, and blood vessels, as well as the structures that produce and drain tears) protrude beyond the surface of the eye. They protect the eye while allowing it to move freely in a wide arc.

    The eyelashes are short, tough hairs that grow from the edge of the eyelid. The upper lashes are longer than the lower lashes and turn upward. The lower lashes turn downward. Eyelashes keep insects and foreign particles away from the eye by acting as a physical barrier and by causing the person to blink reflexively at the slightest sensation or provocation.

    The upper and lower eyelids are thin flaps of skin and muscle that can cover the eye. They reflexively close quickly (blink) to form a mechanical barrier that protects the eye from foreign objects, wind, dust, insects, and very bright light. This reflex is triggered by the sight of an approaching object, the touch of an object on the surface of the eye, or the eyelashes being exposed to wind or small particles such as dust or insects.

    On the moist back surface of the eyelid, the conjunctiva loops around to cover the front surface of the eyeball, right up to the edge of the cornea. The conjunctiva protects the sensitive tissues underneath it.

    When blinked, the eyelids help spread tears evenly over the surface of the eye. Tears consist of a salty fluid that continuously bathes the surface of the eye to keep it moist and transfers oxygen and nutrients to the cornea, which lacks the blood vessels that supply these substances to other tissues. When closed, the eyelids help trap the moisture against the surface of the eye. Small glands at the edge of the upper and lower eyelids secrete an oily substance that contributes to the tear film and keeps tears from evaporating. Tears keep the surface of the eye moist. Without such moisture, the normally transparent cornea can become dried, injured, infected, and opaque. Tears also trap and sweep away small particles that enter the eye. Moreover, tears are rich in antibodies that help prevent infection. The eyelids and tears protect the eye while allowing clear access to light rays entering the eye.

    Tears contain 3 layers: water, mucous, and oil. The lacrimal (tear) glands produce the water layer. The lacrimal glands, located at the top outer edge of each eye (see figure Where Tears Come From) and within the conjunctiva, produce the watery portion of tears, which flow to the eye surface through the lacrimal excretory ducts. Mucous glands in the conjunctiva produce mucus, and oil (lipid) glands in the eyelid margin produce an oil. The mucus and oil mix with the watery portion of the tears to create a more protective tear film.

    Tears drain from each eye into the nose through one of the two nasolacrimal ducts. Each of these ducts has openings at the edge of the upper and lower eyelids near the nose, called the punctum.


    Layers of the Skin

    The skin has three layers:

    Fat layer (also called the subcutaneous layer)

    Each layer performs specific tasks.

    Getting Under the Skin

    The skin has three layers. Beneath the surface of the skin are nerves, nerve endings, glands, hair follicles, and blood vessels.

    Epidermis

    The epidermis is the relatively thin, tough, outer layer of the skin. Most of the cells in the epidermis are keratinocytes. They originate from cells in the deepest layer of the epidermis called the basal layer. New keratinocytes slowly migrate up toward the surface of the epidermis. Once the keratinocytes reach the skin surface, they are gradually shed and are replaced by newer cells pushed up from below.

    The outermost portion of the epidermis, known as the stratum corneum, is relatively waterproof and, when undamaged, prevents most bacteria, viruses, and other foreign substances from entering the body. The epidermis (along with other layers of the skin) also protects the internal organs, muscles, nerves, and blood vessels from injury. In certain areas of the body that require greater protection, such as the palms of the hands and the soles of the feet, the stratum corneum is much thicker.

    Scattered throughout the basal layer of the epidermis are cells called melanocytes, which produce the pigment melanin, one of the main contributors to skin color. Melanin's primary function, however, is to filter out ultraviolet radiation from sunlight (see Overview of Sunlight and Skin Damage), which damages DNA, resulting in numerous harmful effects, including skin cancer.

    Specialized cells called melanocytes produce the pigment melanin. Melanocytes originate from cells in the deepest layer of the epidermis called the basal layer.

    The epidermis also contains Langerhans cells, which are part of the skin's immune system. Although these cells help detect foreign substances and defend the body against infection, they also play a role in the development of skin allergies.

    Dermis

    The dermis, the skin's next layer, is a thick layer of fibrous and elastic tissue (made mostly of collagen, with a small but important component of elastin) that gives the skin its flexibility and strength. The dermis contains nerve endings, sweat glands and oil glands (sebaceous glands), hair follicles, and blood vessels.

    The nerve endings sense pain, touch, pressure, and temperature. Some areas of the skin contain more nerve endings than others. For example, the fingertips and toes contain many nerves and are extremely sensitive to touch.

    The sweat glands produce sweat in response to heat and stress. Sweat is composed of water, salt, and other chemicals. As sweat evaporates off the skin, it helps cool the body. Specialized sweat glands in the armpits and the genital region (apocrine sweat glands) secrete a thick, oily sweat that produces a characteristic body odor when the sweat is digested by the skin bacteria in those areas.

    The sebaceous glands secrete sebum into hair follicles. Sebum is an oil that keeps the skin moist and soft and acts as a barrier against foreign substances.

    The hair follicles produce the various types of hair found throughout the body. Hair not only contributes to a person's appearance but has a number of important physical roles, including regulating body temperature, providing protection from injury, and enhancing sensation. A portion of the follicle also contains stem cells capable of regrowing damaged epidermis.

    The blood vessels of the dermis provide nutrients to the skin and help regulate body temperature. Heat makes the blood vessels enlarge (dilate), allowing large amounts of blood to circulate near the skin surface, where the heat can be released. Cold makes the blood vessels narrow (constrict), retaining the body's heat.

    Over different parts of the body, the number of nerve endings, sweat glands and sebaceous glands, hair follicles, and blood vessels varies. The top of the head, for example, has many hair follicles, whereas the soles of the feet have none.

    Fat layer

    Below the dermis lies a layer of fat that helps insulate the body from heat and cold, provides protective padding, and serves as an energy storage area. The fat is contained in living cells, called fat cells, held together by fibrous tissue. The fat layer varies in thickness, from a fraction of an inch on the eyelids to several inches on the abdomen and buttocks in some people.


    Diagnosis Diagnosis

    Systemic candidiasis is usually suspected in people who have an increased risk of developing an invasive Candida infection and have symptoms of an infection. Blood cultures (or cultures of other sterile fluids from the body such as cerebrospinal fluid) are then ordered to confirm the diagnosis. [1] [2] [3]

    However, in 40-50% of cases when systemic candidiasis affects another part of the body, the blood culture may be negative. [1] [2] In these cases, further testing will depend on which organ (s) or body system is infected. [1] As of 2016, medical researchers are hoping T2 magnetic resonance testing will be able to more accurately and easily detect all forms of systemic candidiasis. [5]


    Microbiology Ch. 21: Microbial Diseases of the Skin and Eyes

    TREATMENT: Topical antibiotics
    *Mostly affects children 2 to 5 years of age.

    METHOD OF TRANSMISSION: Aerosol
    DIAGNOSIS: Usu. diagnosed by clinical signs and symptoms and may be confirmed by serological tests or PCR

    METHOD OF TRANSMISSION: Direct contact with or aerosols from infected small mammals
    DIAGNOSIS: Usu. diagnosed by clinical signs and symptoms and may be confirmed by serological tests or PCR

    METHOD OF TRANSMISSION: Aerosol
    DIAGNOSIS: Usu. diagnosed by clinical signs and symptoms and may be confirmed by serological tests or PCR

    METHOD OF TRANSMISSION: Recurrence of latent chickenpox infection
    DIAGNOSIS: Usu. diagnosed by clinical signs and symptoms and may be confirmed by serological tests or PCR

    METHOD OF TRANSMISSION: Initial infection by direct contact recurring latent infection
    DIAGNOSIS: Usu. diagnosed by clinical signs and symptoms and may be confirmed by serological tests or PCR

    FUNGAL DISEASES:
    Ringworm (tinea)
    Sporotrichosis

    METHOD OF TRANSMISSION: Direct contact fomites endogenous infection
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: Endogenous infection
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: Direct contact
    DIAGNOSIS: bacterial culture

    TREATMENT: Surgical tissue removal broad spectrum antibiotics

    METHOD OF TRANSMISSION: Endogenous infection
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: swimming water hot tubs
    DIAGNOSIS: bacterial culture

    TREATMENT: usu. self-limiting

    METHOD OF TRANSMISSION: Swimming water
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: Direct contact
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: Contaminated water
    DIAGNOSIS: bacterial culture

    METHOD OF TRANSMISSION: Direct contact
    DIAGNOSIS: usu. diagnosed by clinical signs and symptoms

    PORTAL OF ENTRY: Skin
    SYMPTOMS: Skin lesions of highly varied appearance on scalp may cause local loss of hair

    METHOD OF TRANSMISSION: Direct contact fomites
    DIAGNOSIS: Diagnosis is confirmed by microscopic examination

    METHOD OF TRANSMISSION: Soil
    DIAGNOSIS: Diagnosis is confirmed by microscopic examination


    Virulence Factors

    The success of S. aureus as a pathogen is due to a large extent to its ability to resist antimicrobial agents and circumvent the immune surveillance of the host. MGEs play an essential role in this process and are a means to transfer genetic information within species. Many molecular determinants of resistance and virulence are encoded on MGEs. S. aureus possesses an enormous repertoire of virulence and persistence genes [20•]. Some of the more commonly studied virulence factors include genome-encoded pore-forming α-toxin [87], PSMs, which are a small group of genome-encoded cytolytic peptides that are key determinants in the development of skin, bloodstream, and biofilm-associated infections [23, 88], and leukotoxins, such as Panton-Valentine Leukocidin (PVL), the effect of which on the pathogenesis of MRSA infection is complicated by its epidemic association with CA-MRSA infections, even though animal and human studies have failed to find PVL as a factor in S. aureus virulence The reader is referred to review articles presenting the molecular basis of S. aureus virulence in more detail [23, 89].


    Streptococcus pyogenes

    BSIP/UIG/Universal Images Group/Getty Images

    Streptococcus pyogenes bacteria typically colonize the skin and throat areas of the body. S. pyogenes reside in these areas without causing issues in most cases. However, S. pyogenes can become pathogenic in individuals with compromised immune systems. This species is responsible for a number of diseases that range from mild infections to life-threatening illnesses. Some of these diseases include strep throat, scarlet fever, impetigo, necrotizing fasciitis, toxic shock syndrome, septicemia, and acute rheumatic fever. S. pyogenes produce toxins that destroy body cells, specifically red blood cells and white blood cells. S. pyogenes are more popularly known as "flesh-eating bacteria" because they destroy infected tissue causing what is known as necrotizing fasciitis.


    Watch the video: Conjunctivitis (June 2022).


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