Bacterial Wound Culture

Bacterial wound culture test is performed for nonsurgical and surgical wound for both chronic and acute infections. Some symptoms of an infected wound are like:

Slow healing wound

Redness, heat, and swelling at the wound site

Tenderness

Fever

Pus or fluid drainage at the site

Marchand-Senécal has found that patients who have low quality wound swap samples have a reduced rate of antibiotic treatment for infection in wound. This is due to patients who have low quality samples, their samples are rejected as they can not match the Q-score quality matrix. The reflexive antibiotics treatment rates for low and high-quality groups are not different before the sample rejection, but after the rejection of samples, the rate is 4.5% and 9.4%.

Bacterial wound culture tests and clinical examinations are used to know the infection which is present in wounds.

The negative bacterial wound culture test result is dependent on the methods.

Qualitative wound culture method: There is no growth of normal skin flora or pathogenic bacteria.

Semiquantitative wound culture method: Below 4+ growth of bacteria

Quantitative wound culture method: Below 100000 organisms/g if tissue sample is used, 100000/swab if swab sample is used, or 100000/ml if fluid sample is used.

Quantitative wound culture method gives colony counts to differentiate between infection, colonization, and contaminations.

Contamination: Bacteria is present in reduced quantity which do not lead to active infection.

Colonization: Bacteria are growing in the wound but do not lead to any symptoms of infection.

Infection: Bacteria are present in more quantities, which can lead to symptoms of infection.

A bacteria wound culture test is used with clinical examinations as both can be analyzed simultaneously. To interpret the result of qualitative bacterial wound culture test result needs to determine whether the isolated pathogen us the actual cause of the infection or it is a contamination or a part of the normal flora of skin. Quantitative tests like semiquantitative method and quantitative method are used to determine whether the isolated pathogen can cause the infection by the standard cutoff levels for bacterial wound culture in normal range.

Qualitative culture method is used to identify the pathogenic bacteria.

Pathogens which are part of normal flora of skin and is not pathogenic include:

Corynebacterium species

Staphylococcus epidermidis

Other diphtheroid

Other coagulase-negative staphylococci

Propionibacterium acnes

Brevibacterium species

Alpha or gamma streptococci

Pityrosporum species

Bacillus species which do not include Bacillus anthracis or Bacillus cereus

Growth of the above bacteria shows contamination except negative coagulase staphylococcus species which can show contamination or infection.

The common bacteria which are pathogenic and isolated in chronic and acute wound infections include:

Pseudomonas aeruginosa

staphylococcus aureus

Enterococci

Beta-hemolytic streptococci

Coagulase-negative staphylococci

Coliform bacteria like Enterobacter species, Escherichia coli, and Klebsiella pneumoniae

Nonpigmented gram-negative anaerobes like Prevotella, Bacteroides, and Fusobacterium species

Pigmented gram-negative anaerobes like Porphyromonas species and Prevotella

Clostridium species

Peptostreptococcus species

Quantitative bacteria wound cultures, which are collected by biopsy, are the standard method in all wound culture techniques. Growth greater than 100000 organisms/gm of tissue samples or 10⁵ /ml of fluid aspirate samples are the positive results for infection in wound.

Blood agar plates are marked 3 times on 1 quadrant and 3 times on each of the quadrants by a sterile loop for every quadrant in a semiquantitative culture test. Growth of greater than 30 colonies in quadrant is interpreted as 4+ growth. It is a positive result for infection. Semiquantitative and quantitative culture methods can correlate to detect the sepsis in wound with a 100% positive value and 93.7% negative value.

There are 3 common methods which are used to collect the samples for bacteria wound culture test: Swab, tissue biopsy, and needle aspiration

Needle aspiration:

Sample collection container: Sterile container or tube

Sample volume: 0.5 to 2 ml

Procedure: This procedure includes focal fluid sample collections. The area from which sample is taken are prepared aseptically. Several aspirations are made near the wound by using a 10 ml syringe, which is connected to a 22-gauge needle. The syringe which contains samples are capped or samples are transferred into sterile tube and sent to laboratories in 60 minutes after the collection of samples.

Tissue biopsy:

Sample collection container: Sterile container or tube

Sample volume: 0.005 to 0.2 g, which include tissue or 3 to 4 mm punch biopsy

Procedure: A tissue biopsy is collected aseptically by a punch biopsy or excision with a scalpel. Other techniques which are used cauterization of superficial devitalized tissue in diabetic foot ulcers and less invasive dermabrasion procedure for deep tissue. Samples must be sent to a laboratory in 60 minutes to get the optimum results.

Swab

Sample amount: 1 swab

Procedure: Swabbing is the most commonly method which is used to collect the samples for culture. It is performed on the open wounds. It is necessary to irrigate and clean the surface of wound with saline till it can free from necrotic debris, drainage, purulent material or eschar. This process can avoid contamination of the samples.

The Z-stroke method include rotating a swab in the fingers as swab can move in zigzag pattern on the wound. The edges of wound can be avoided.

The Levine method include the rotating of swab in the area of 1 cm² with proper pressure to release the fluid from he wound. This method is superior rather than Z-stroke to identify the infected wound because it has the ability to release the pus from wound.