Wound Infection Laboratory Diagnosis IInd Semester Unit V.

WOUND INFECTIONS: Pus is used as a specimen to investigate wound infection.

Laboratory diagnosis of Wound infections

1. Collection of specimen: Do not apply antiseptic on the wound just before collection of specimen. Collect specimen just after incision of wound by using sterile swab. Transfer the swab into a sterile test tube. Or put a swab into the transport medium. Use specimen within 6 hours.

Normally two swabs are collected. One specimen is for microscopic examination and the other for culture media.

Aspirates from wound are used to identify the presence of anaerobic bacteria.

2. Procedure:

First day: Prepare a pus smear and stain it using Gram stain solution.

• · Examine it for the presence of Gram-positive cocci such as S. aureus, Anaerobic streptococci and enterococci,

• · Gram-negative rods such as E. coli, etc.

• · Ziehl-Neelsen stain is used to examine M. tuberculosis.

• · KOH is used to examine presence of fungus in the specimen.

Culture: Inoculate specimen in two blood agar plates, a MacConkey agar plate, Neomycin blood agar, and cooked meat medium.

One blood agar plate and MacConkey agar plate are incubated aerobically at 37o C for 24 hours. The second blood agar plate, Neomycin blood agar, and cooked meat medium are incubated anaerobically for 48 hours.

Second day: Examine incubated culture media for the presence of Staph. aureus, Strep. pyogens, Pseudomonas aeruginosa, E. coli. Enterococci. Biochemical tests are performed as confirmatory test.

URINARY TRACT INFECTIONS (UTI):

UTI is very common especially in females and during pregnancy. UTI moves from the urethra to the urinary bladder to the ureter to the kidney. E. coli is the most common pathogen to cause UTI.

Laboratory diagnosis of Urinary tract infections:

• 1. Urine collection: The first urine passed in the morning time is concentrated urine. It is most suitable for culture, microscopy, and biochemical tests. Thus it is preferred as a specimen.

Instruct patient to clean urethral opening and its surroundings using clean water. Discard the first portion of passed urine. Collect about 20 ml of midstream urine in a dry sterile wide-mouthed leakproof screw-capped container. Discard the last portion of passed urine.

• a. Prefer to use collected urine immediately

• b. If required, store in refrigerator at 4oC

• c. If required to store for more than one hour then mix 1% w/v boric acid as preservative. This urine specimen does not need to be stored in the refrigerator.

• d. For Tuberculosis examination, collect the entire first urine for three regular days. Preserve it in the refrigerator.

• 2. Urine microscopy: Urine microscopy is performed to detect the presence of pus, RBCs, crystals, fungus, parasites, etc.

• 3. Urine culture: MacConkey’s agar media and Eosin methylene blue agar media are used as isolation media for E.coli in urine.

Use a calibrated wired loop to inoculate urine specimen into culture media by streak plate method. Incubate it at 37o C for 24 hours. Observe colony characteristics. Shape-Circular, Size 1 mm in diameter, Margin-complete, Elevation- slightly raised or convex, Consistency- soft and opacity-opaque.

• 4. Biochemical tests for E. coli in urine: Oxidase negative,  VP negative, Citrate negative, Gelatin -negative, MR- positive, Indole -positive.

• 5. Fermentation of sugar by E. coli.: Sugar fermentation with the release of acid and gas.