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Chapter: 11th Microbiology : Chapter 12 : Medical Microbiology

Skin and Wound Infections

The skin, which covers and protects the body, is the body’s first line of defense against pathogens.

Skin and Wound Infections

The skin, which covers and protects the body, is the body’s first line of defense against pathogens. As a physical barrier, it is almost impossible for the pathogens to penetrate it. However, microorganisms can enter through skin breaks that are not readily apparent, and the larval forms of a few parasites can penetrate the intact skin. The skin has up to seven layers (Figure 12.5) of ectodermal tissue and guards the underlying tissues viz; muscles, bones, ligaments and internal organs. Nearly all human skin is covered with hair follicles. Because it interfaces with the environment, skin plays an important role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.


Structure of Skin

Skin is composed of three primary layers: the epidermis, the dermis and the hypodermis.



It forms the water proof, protective wrap over the body’s surface. It also serves as a barrier to infection. It is made up of stratified squamous epithelium with an underlying basal lamina. The outermost layer of the epidermis, the stratum corneum, consists of dead cells that contain a waterproofing protein called keratin. The epidermis contains no blood vessels and cells in the deepest layers are nourished exclusively by diffused oxygen from the surrounding air. The main types of cells present in epidermis are Merkel cells, keratinocytes with melanocytes and Langerhan cells.



The dermis is the layer of skin beneath the epidermis that consists of epithelial tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the Stratum basale of the epidermis.



Subcutaneous tissue (also hypodermis and subcutis) is not part of the skin, and lies below the dermis of the cutis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue, adipose tissue and elastin. The main cells are fibroblasts, macrophages and adipocytes (subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body.

The hair follicles, sweat gland ducts, and oil gland ducts in the dermis provide passageways through which the microorganisms can enter the skin and penetrate deeper tissues. Perspiration provides moisture and some nutrients for microbial growth. However, it contains salt, which inhibits many microorganisms; the enzyme lysozyme, which is capable of breaking down the cell walls of certain bacteria and antimicrobial peptides. Sebum, secreted by oil glands, is a mixture of lipids (unsaturated fatty acids), proteins, and salts that prevents skin and hair from drying out. Although the fatty acids inhibit the growth of certain pathogens, sebum, like perspiration, is also nutritive for many microorganisms


Normal Microbiota of the Skin

The skin’s normal microbiota contains relatively large numbers of Gram positive bacteria, such as Staphylococci and Micrococci. Bacteria in the skin tends to be grouped into small clumps. Vigorous washing can reduce their numbers but will not eliminate them. Microorganisms remaining in hair follicles and sweat glands after washing will soon reestablish the normal populations. Areas of the body with high moisture, such as armpits and between the legs, have higher populations of microorganisms. They metabolize secretions from the sweat glands and are the main contributors to body odour.

Also part of the skin’s normal microbiota are Gram positive pleomorphic rods called diphtheroids. Some diphtheroids, such as Propionibacterium acnes, are typically anaerobic and inhabit hair follicles. These bacteria produce propionic acid, which helps maintain the low pH of skin, generally between 3 and 5


Wound Infection

Wound can be defined as any interruption of continuity of external or internal surfaces caused by violence

Wounds may occur following: surgery, trauma or injections

Wound infections may occur mainly after surgical procedures

Wound sepsis is the result of cross infection from human sources and from other outside sources.


Bacteria associated with wound infections

Many bacteria are associated with wound infection (Figure 12.6). The normal flora may also cause infection. The most common normal flora of the skin are: Staphylococci, and various Streptococci, Sarcina sp, anaerobic Diphtheroids, Gram negative rods and others.


Factors determining the ecology of the skin bacteria

The main factors that determine the ecology of skin bacteria:

·        Climate: Temperature and humidity

·        The effect of free fatty acids

·        Maintenance of the flora by products of skin secretions and other bacterial inhibitors.


Defence against infection

·        Intact skin: Normal uninterrupted skin provides protection against invasion by bacteria.

·        Lysozyme in sweat: The enzyme lysozyme provides protection against Gram positive bacteria by lysing the cell wall.

·        IgA antibodies in the sweat and secretions provide first line of defense against infection.

·        Inhibitors like unsaturated fatty acids provide protection against bacteria.

·        Bacteriocins produced by the normal flora prevent the establishment of other bacteria.


Factors responsible for wound infections


a) Host factors

The following factors help the organisms to survive and produce the infections:

·        Extremes of age: Very old and very young people are susceptible to infection.

·        Diabetes mellitus: Hormonal imbal-ance increases susceptibility.

·        Steroid therapy: Immune responses are affected.

·        Obesity: Increases susceptibility.

·        Malnutrition: General health status affected.

·        Immunocompromisedindividual: Immune system will not function properly.

·        Presence of remote infection at the time of surgery.


b) Exogenous Factors

·        Use of unsterile instruments:

They carry pathogens.

·        Surgeons hands / from health workers: May carry pathogens.

·        Air / Hospital environments: Unclean environment harbur pathogens.


c) Endogenous Factors

·        Wound contamination from the patient source: From the normal flora.

·        Wound penetrating through structures containing normal flora.

·        Surgical procedures involving mucous membranes harbouring normal flora.

·        Patients carrying pathogens in their nose, throat, axilla.


Etiological agents

Etiological agents like Pseudomonas aeruginosa, Staphylococcus aureus, Proteus, member of enterobacteriaceae anaerobic organisms, anaerobic cocci and bacteroides cause infections.


Post operative infections

Gasgangrene organisms like Clostridium perfringens, Staphylococcus aureus and Clostridium tetani may cause post operative infections.


Route of entry

Wounds may occur following surgery, trauma or injections. Wound infections may occur mainly after surgical procedures. Wound sepsis is the result of cross infection from human sources and from other outside sources. Infections of skin are listed in Table 12.2.


Mechanisms of damage

1. Organisms enter through the skin, multiply there and produce the disease in the skin.

For example, impetigo, abscess and cellulitis (Figure 12.7) are caused by Staphylococcus aureus and Streptococcus pyogenes.

As soon as the organisms enter the skin they multiply and produce various toxins that kill the cells and produce cellulitis. Further damage leads to necrosis and ulcer formation (Figure 12.8).

2. Organisms multiply in the skin and produce disease in internal organs. For example some Group A Streptococci multiply in the skin and produce disease known as Acute Glomerulonephritis causing damage to the kidneys. Some times Corynebacterium diphtheriae may multiply in the skin and affect the heart due to the toxin

3. Sometimes organism may multiply in the skin and produce the toxin which affect the Central Nervous System (CNS) and the effects seen. In the case of Clostridium tetani infection, convulsions and paralysis occur due to the production of a powerful toxin.


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11th Microbiology : Chapter 12 : Medical Microbiology : Skin and Wound Infections |

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