Wound infections

ADVANCE ORGANIZER

A wound or a break in the skin can allow the development of serious disease. This includes both tiny scratches and large catastrophes. Bacteria and fungi cross the first line of defense.

WOUNDS
Abscesses are localized collections of pus. Since they lack blood vessels and inflammation surrounds the wound–it may be difficult to get antibiotics to the site (systemically anyway). Additionally since the bacteria may not be growing-antibiotics may not be effective. These abscesses are sources from which infection can spread.
Certain wounds are likely to permit the growth of anaerobic bacteria. Deep wounds such as puncture and those with dead tissue are chief candidates. Since a wide variety of microorganisms can colonize, many different diagnostic methods are required.

BACTERIAL INFECTIONS OF WOUNDS

Staphylococcus aureus (coagulase positive Staphylococcus) is the most common cause of infections in surgical wounds. The development and type of surgical wound infection can be predicted based on the duration of surgery, presence of foreign material, status of host defenses. A major problem is the resistance which is arising to antimicrobial medicines.

Pseudomonas aerginosa is a common cause of burn infections. They make the burns green because they make a green pigment. P. aeruginosa is usually resistant to most antibiotics and they also release a potent toxin.

Clostridial wound infections:
Tetanus is spastic paralysis caused by a toxin produced by Clostridium tetani. , a motile anaerobic Gram-positive rod with a terminal spore. Colonization of wounds by C. tetani can result in tetanus. Immunization and care of wounds prevents this otherwise fatal disease.
Gas gangrene is caused by the anaerobe Clostridium perfringens. It causes necrosis (death) of muscle tissue.

Other anaerobic infections of wounds are caused by Bacteroides, Fusobacterium, and Peptostreptococcus.
Actinomycosis is caused by Actinomyces israelii [an anaerobic branching bacterium] which is part of the normal flora of the mouth and intestinal tract. Infections can originate from dental and intestinal surgeries[which is why they are classed under wound infections. When this is a chronic condition, it is characterized by abscesses that burrow to the surface and drain pus containing “sulfur granules.”

Bite wounds
Pastuerella multocida a facultatively anaerobic, Gram-negative rod infection is a common complication of animal bites.

Cat scratch disease is caused by Bartonella henselar and is characterized by localized skin papules with progression to pus filled lymph nodes.
Rhodococus
Rat bite fever can be caused by Spirillum minus , a spiral Gram-negative rod or Streptobacillus moniliformis. , a facultative anaerobe. These are two distinct diseases both called rat-bite fever. Relapsing fevers and a rash are common.

Human bites often result in multiple synergistic infections involving mouth streptococci and Gram-negative anaerobes.

FUNGAL INFECTIONS

Sporotrichosis is caused by Sporothrix schenckii. It is common in gardeners, berry pickers and wood workers because it is usually introduced by thorns and splinters. It is a chronic fungal disease that causes chains [lines] of draining ulcers.

SKIN INFECTIONS
ADVANCE ORGANIZER
Skin is our boundary between our body and the rest of the world. Properties of the skin cause it to resist microbial colonization, provide a physical barrier to the entry of potential pathogens. We will concentrate on a few of the things that can invade and colonize the body when the barrier is broken. Sometimes the skin is attacked by organisms entering through the respiratory or digestive systems.

ANATOMY AND PHYSIOLOGY
In addition to assisting in the body’s regulation of temperature and in fluid balance, skin repels potential pathogens by being tough, dry, acidic and toxic.

NORMAL BIOTA
Diphtheroids are gram-positive, rod shaped bacteria that come in many sizes. This group includes Propionibacterium acnes whose growth is enhanced by oily secretions from the
sebaceous glands. P. acnes produces fatty acids from the oils. They are usually strict anaerobes but some tolerate air. Staphylococci are gram-positive cocci that help prevent colonization by potential pathogens and maintain the balance among flora of the skin. Staphylococcus epidermidis is universally present on the skin and can be pathogenic at times.
Single celled yeasts of the Malassezia species are found on the skin and may at times cause dandruff as well as pityriasis vericolor and in AIDS patients serious rashes.

SKIN DISEASES CAUSED BY BACTERIA
Staphylococcus aureus is the cause of furuncles and carbuncles. It is coagulase positive and can be a problem because it is often resistant to penicillin and other antibiotics. In
furunculosis (boils) the S. aureus invades the skin via hair follicles. The bacteria generally originate from the nose. Protein A plays a role in pathogenesis. Carbuncles are
dangerous because the infection can spread easily to brain or bone or the heart. Certain strains of Staphylococcus aureus also cause of scalded skin syndrome because
they make an exotoxin that causes exfoliation. It got its name because it resembles a severe burn. Most cases are in newborn infants.
Impetigo, a superficial skin infection can be caused by Staphylococcus aureus and by Streptococcus pyogenes. Pathogenesis is aided by the capsule (group A carbohydrate)
and the M-protein of the cell wall of S. pyogenes (an air tolerant Gram-positive cocci occurring in chains) which interferes with phagocytosis. Fortunately, penicillin or
erythromycin is effective in most S. pyogenes infections. A complication of this and other streptococcal skin infections can be acute glomerulonephritis of the kidneys.
Rocky Mounted Spotted Fever is transmitted to humans through the bite of an infected Dermacentor tick. It is caused by Rickettsia rickettsee, a small Gram- negative rod that
must live intracellularly. Blood vessel damage causes hemorrhages throughout the body. Tetracycline and chloramphenicol are used in the early treatment of Rocky Mountain
spotted fever but diagnosis is difficult. Lyme disease is caused by transmission to humans by lxodes ticks. It is caused by a
spirochete, Borrelia burgdorferi. The bullet shaped rash left by the bite is called an erythema chronicum migrans. In addition the heart, joints, nervous system and other
tissues can be involved. Amoxicillin, erthromycin and doxycycline are useful in treating this disease if diagnosed early. Ehrlichiosis has been referred to as “Rocky Mountain spotless fever”. Ehrlichia chaffeensis causes a human monocyte ehrlichiosis. The ticks which transmit this are Amblyomma americanum [the lone star tick] and Dermacentor variabilis [the Americandog tick]. This is an emerging disease. Most doctors have the tools to diagnose it. Can you figure out what E. chaffee, the small gram-negative, oligately intracellular bacteria isnamed after?

SKIN DISEASES CAUSED BY VIRUSES
Both chicken pox and measles enter the body through the respiratory system and are spread by respiratory droplets. Chicken pox (also known as Varicella) is caused by a member of the herpesvirus family, varicella zoster. Months or even years after chicken pox, reactivation of a varicella virus is possible. The distribution of the virus in the sensory nerves–is apparent from the symptoms. Such reactivations can cause new chicken pox epidemics or may occur in adults as herpes zoster (shingles). Rubeola (measles) can be serious–that is fatal in some cases since it can cause pneumonia and encephalitis and in other cases can lead to serious secondary bacterial infections. This disease is now being controlled by immunizing young children with a two dose regimen. Susceptible adults are also immunized. The Rubeola virus is a member of the Paramyxovirus family. The vaccination is with a live attenuated virus. Rubella (also called three day measles or German measles) if contracted by a woman in the first 8 weeks of pregnancy results in congenital rubella syndrome. Otherwise the symptoms are generally mild. The Rubella virus is a member of the Togavirus family. Again a live virus vaccine is now available.

Other Rash causing viruses
Erythema infectiosum [or Fifth disease] is caused by parvovirus B 19. It resembles a “slapped” check, this symptom comes and goes.
Exanthem subitum [or roseola infantum] is the result of herpesvirus, type 6. The disease occurs in infants. The symptoms are several days of a high fever followed by a transitory
rash [which appears as the temperature returns to normal].
Warts are skin tumors caused by a number of papillomaviruses. Some genital warts are associated with cancer of the cervix. Most others are benign. All can have a long
incubation period but removal of the wart itself may not eliminate the infection.
DISEASES CAUSED BY FUNGI
Dermatophytosis, Athlete’s foot and ringworm are caused by mold-type fungi [of the genera Trichophyton, Microsporum, Epidermophyton and others] which feed on keratin
containing cells. These outer layers of the skin, hair and nails are particularly susceptible if moist.
Invasive skin infections are sometimes caused by a yeast–Candida albicans.