INTRODUCTION:

Skin is the major organ of the human body. It collects sensory information from the environment and functions as a protective barrier to the human body organs. It helps in maintaining the proper body temperature. Bacterial infections on skin are the common ailment for generation of other diseases in the body. Specially, the wound on skin caused by bacteria produces very chronic results, if it is not properly treated on time. The wound on skin can occur in all ages of persons and can causes harm in number of ways. Therefore for correct antimicrobial therapy for the treatment of wound, proper identification of microorganism is necessary so that wound healing activity can be accelerated in less period of time^{1, 2}. Wound is a break in the integrity of the skin and tissues. It may be a superficial cut, scrap or scratch. It may also include puncture, burn or may be the result of surgical or dental procedures. The microorganisms which are likely to infect the skin and tissues depend on the wound extent, its depth, the environment in which it occurs and the types of microorganisms present on the person skin.

The skin consists of three layers: the outer epidermis, the dermis where many hair follicles and sweat glands are found and the fatty subcutaneous layer. Below these layers are membranes that protect connective tissues, muscles and bones. Wound can penetrate any of these layers and can cause skin infections^3-5.Wound and skin infections represent the invasion of tissues by one or more species of microorganisms. This infection disturbs the body immune system and causes inflammation, tissue damage thus resulting in delayed wound healing process. Many infections remain confined to a small area like an infected scratch or hair follicle which usually heals on their own. Others may persist and if kept untreated may increase in severity and may become cause of other chronic diseases. Wound healing itself is a complex process which involves many related systems, chemicals and cells working together so as to clean the wound, heal its edges and to produce new tissues and blood vessels^6-9.

Signs and symptoms:

Common signs and symptoms of wound infection include redness, warmth, swelling, tenderness and pus drainage from the skin. Sometimes the skin may also become harden or tighten in that particular area and red streaks may radiate from the wound. Wound infections may also cause fevers, especially when spread to the blood. Skin infections may also cause pustules, pain, scaling, and itching in that particular wounded area¹⁰.

Types of wound infections and types of microorganisms responsible for wound infection:

Wounds may be typed according to their cause, their extent, the environment in which they occur and whether they are clean or contaminated. The microorganisms that infects skin and wound generally depends on immune system of the person, depth of the wound and the conditions of the surrounding environment.

Bacteria, virus and fungi can cause wound infections on skin (table 1, 2). Bacteria can be classified as per the environment in which they grow: aerobic (grow in air), microaerophilic (grow in reduced oxygen environment) and anaerobic (grow in little to no oxygen). Microaerophilic and anaerobic bacterias are generally found in deeper wounds and abscesses^11-
13.

Wound infections on superficial skin areas:

Superficial infections occur in the outer layer of the skin but may also extend deeper into the subcutaneous layer. They are caused by aerobic microorganisms; deeper wounds may also be infected with anaerobic microbes. Bacterial infections are typically caused by bacteria like Staphylococcus (Staph) and Streptococcus (Strep). They may also be caused by antibiotic resistant bacteria like MRSA (Methicillin Resistant Staphylococcus aureus). Brackish water wound infection occurs due to waterborne Vibrio or Aeromonas species. Hot tub-associated infection occurs by Pseudomonas aeruginosa. When wounds are deeper, the possible pathogens would be anaerobes like Bacteroides and Clostridium species. Necrotizing fasciitis a serious but uncommon type of infection often involves Group A Streptococci, which are sometimes called as “flesh-eating bacteria. Other common skin infections such as ringworm and athlete’s foot are not caused by bacteria but by fungi. Yeast infections occur by Candida species which is generally found in the mouth (thrush) or on other moist areas of the skin.

Wound infections through bites:

A wound infection due to animal bites tends to reflect the microorganisms which are generally present in oral cavity of animal which created the bite wound. They may involve aerobic, anaerobic and microaerophilic microorganisms. The most of the animal bites are from dogs and cats. The common bacteria found in these animals oral cavity is Pasteurella multocida.

Wound infections through trauma:

Trauma is a type of injury which occurs by the physical force applied on the skin. It includes injuries from motor vehicle accidents, cuts from knives or other sharp instruments and gunshot wounds etc. The type of wound infections that trauma victims acquires depend basically on the environment in which the injury occurred, the extent of the injury, the microorganisms present on the affected person skin and the immune status of the infected person. A deep puncture wound occurs mostly by anaerobic bacteria such as Clostridium tetani.

Wound infections through burns:

Burns are generally caused by flammable liquids, fires, chemicals and electricity. First-degree burns involve the epidermis. Second-degree burns involve the dermis. Third-degree burns involve all the layers of the skin and thus damages the tissues below it. Initial infections tend to bacterial. Fungal infections due to Candida, Aspergillus, Fusarium, and other species may arise later since they are not inhibited by antibacterial treatment. Viral infections, such as those caused by the herpes simplex virus, may also occur^14-15.

Wound infections after surgery:

Surgical sites are most commonly infected by exposure to microorganisms in the hospital environment. Hospital-acquired bacteria like MRSA. Deep surgical wounds become infected both superficially with aerobic microorganisms and deep within the body by anaerobes.

Table 1: Microorganisms related with wound infections and their frequency of occurrence on skin

Pathogens	Frequency (%)
Staphylococcus aureus	20
Coagulase-negative staphylococci	14
Enterococci	12
Escherichia coli	8
Pseudomonas aeruginosa	8
Enterobacter species	7
Klebsiella pneumoniae	3
Streptococci	3
Candida albicans	3
Other gram-positive aerobes	2

Table 2: Some potential wound pathogens category responsible for wound infection

Pathogen Category	Name of pathogen
Gram-positive cocci	Beta Haemolytic Streptococci (Streptococcus pyogenes)*
	Enterococci (Enterococcus faecalis)
	Staphylococci (Staphylococcus aureus/ MRSA)*
Gram-negative aerobic rods	Pseudomonas aeruginosa*
Gram-negative facultative rods	Enterobacter species
	Escherichia coli
	Klebsiella species
Anaerobes	Bacteroides
Fungi	Yeasts (Candida)
Fungi	Aspergillus

*Most common causative microorganisms related with wound infections

Laboratory tests to diagnose the microorganisms responsible for wound infections:

A clinical evaluation cannot definitively tell the doctor which microorganism is causing a wound infection or what treatment it is likely to be susceptible to. For that, laboratory testing is required. Laboratory testing is primarily used to diagnose bacterial wound infections, to identify the microorganism responsible, and to determine its likely susceptibility to specific antimicrobial agents. Sometimes testing is also performed to detect and identify fungal infections. Sample collection may involve swabbing the surface of a wound to collect cells or pus with a needle and syringe or biopsy^16-18. For fungal evaluation, scrapings of the skin may be collected. Test includes bacterial wound culture, gram staining, antimicrobial susceptibility test and fungal as well as urine cultures.

Bacterial wound culture:

It is a primary test which is used to diagnose the bacterial infection. The sample is streaked onto nutrient media and is then incubated at body temperature so as to grow and identify bacteria if any present in the sample or not. Growth and identification of bacterial wound cultures is usually available within 24-48 hrs from the time the sample is received in the lab while for slow growing microorganisms, such as fungi it may take several weeks for growth and its identification^19-20.

Gram stains:

After the growth of microorganism in the culture media, staining allows bacteria to be evaluated under the microscope. It involves smearing individual colony types onto glass slides and treating them with a special stain. Under the microscope, the bacteria can be classified into gram-positive and gram-negative organisms, there they are distinguished by their shape such as cocci (spheres) or rods (bacilli) and are easily separated by color into gram positive or gram-negative microorganisms. This test provides preliminary information about the quality and potentiality of microorganisms that may be causing the infection ^21-26.

Antimicrobial susceptibility:

When a pathogen is identified and isolated using the wound culture, this test is used to determine the bacteria likely susceptibility to certain drug treatments. This information helps in selecting appropriate antibiotics for the treatment^27-
31. Other tests may includes

Fungal culture:

It is performed when a fungal infection is suspected³².

Urine culture:

It is performed when a urinary tract infection is suspected.

Treatments:

The risk of wound infection can be minimized with prompt and proper wound cleansing and treatment. Most wound infections that do occur can be successfully resolved. Many superficial bacterial and viral infections will resolve on their own without any kind of treatment. Other bacterial infections may require some type of topical antimicrobial drugs treatment. Deeper infections typically require oral antimicrobial therapy. The choice of which type antimicrobials drug is to be used is based upon the data results of wound culture and antimicrobial susceptibility tests. Patients with antibiotic resistant bacteria or with an infection which is located in difficult area for drug therapy to penetrate (such as bone) may require treatment with intravenous medications. The antimicrobial drugs may act by destroying the bacteria (bactericidal) or by inhibiting the growth of bacteria (bacteriostatic). The mechanism of action of these drugs can be divided into four groups^33-36.

Drugs interfering with cell wall synthesis:

Penicillin, cephalosporin, bacitracin, vancomycin and cycloserine. All b-lactum antibiotics bind to receptors (penicillin binding protein at the cell wall -cell membrane interface). After attachment of the b-lactum drug to the receptor there is interference with the synthesis of peptidoglycan of cell wall. This makes the cell membrane vulnerable to damage by solutes of the environment (plasma). The cell walls of gram negative bacteria being more complex, these drugs cannot penetrate the cell in adequate concentration.

Drugs affecting cytoplasmic membrane:

Polymxcin B and colistin bind selectively with outer membranes of gram negative bacteria that are rich in phoshatidylethanolamine and act as cationic detergents. The inhibition of cell membrane function leads to escape of macro molecules and ions from the cell resulting in cell damage or death. Polyenes are effective against fungus which contains sterol on cell membrane. Bacteria generally do not contain sterol on cell membrane and hence polyene is ineffective in bacterial infection.

Drugs inhibiting protein synthesis and impairment of function of the ribosomes:

Aminoglycosides, tetracyclines, chloramphenicol and macrolide antibiotics lincomycin inhibit protein synthesis in bacterial ribosomes (70 S) without any major effect on mammalian ribosomes (80 S).

Drugs inhibiting synthesis of nucleic acid:

These drugs interfere with transcription of genetic information on the ribosomes. Rifampin binds with DNA- dependent RNA polymerase, actinomycin binds with deoxyguanosine residues and quinolones block the DNA gyrase and thus they inhibit DNA synthesis of bacteria.

Choice of treatment:

The choice of which antimicrobials drugs to be used is generally based upon the results of wound culture and antimicrobial susceptibility tests. Susceptibility testing is used to determine which antibiotic or antibiotic combinations will be most effective in treating the different types of bacteria causing the wound infection ^{37, 38, 39}. The test include following three terms:

Susceptible: Likely, but not guaranteed to inhibit the pathogenic microorganism; may be an appropriate choice for treatment.

Intermediate: May be effective at a higher dosage or more frequent dosage or effective only in specific body sites where the antibiotic penetrates to provide adequate concentrations but drugs may produce higher risk for medication side effects.

Resistant: Not effective at inhibiting the growth of the organism; may not be an appropriate choice for treatment. A combination of antibiotics that work together to inhibit the bacteria when neither one alone will be effective.

CONCLUSION:

Bacterial infections are one of the most prominent causes of chronic diseases and physical disabilities around the world. The most common causative microorganisms related with wound infections include Staphylococcus aureus/MRSA, Streptococcus pyogenes, Enterococci, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. These pathogens can be easily detected and identified by using designed laboratory testing procedures and methods. Thus by the help of wound culture and antimicrobial susceptibility tests, we can easily find out appropriate antimicrobial therapy for the particular pathogen responsible for causing wound infection on human skin.

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Received on 31.03.2013 Accepted on 28.04.2013

Asian J. Res. Pharm. Sci. 2013; Vol. 3: Issue 2, Pg 68-71