Swabs culture

Mrs. Dalia Kamal Eldien

Msc in Microbiology Lecture NO 11

Objectives

Identify the different types of swabs Method of collection the swabLaboratory diagnosis of the different swab Case study of patient suffering from tonsillitisCase study of patient suffering from gas

gangrene

Introduction

A swab can be taken from any area of the body that appears to have an infection. The microbiology laboratory attempt to culture (grow) a wide range of organisms (especially bacteria) from this sample, to determine which ‘germ’ is causing the infection.

Examples to the common swabs send to the microbiology lab Throat swab Nasal swab Eye swab Ear swab High vaginal swab Urethral swab Wound swab Skin swab

Method of collection: A sterile cotton-wool swab, which is like an ear-bud, is

used to collect a small amount of fluid from a wound or surface.

By rolling the swab in affected area. The swab is transported in a special medium that

encourages growth of bacteria. For the throat swab In a good light and using the handle

of a spoon to depress the tongue, examine the inside of the mouth. Look for inflammation, and the presence of any membrane, exudate, or pus, Swab the affected area

Swab

Collection of throat swab

Collection of Wound Swabs Gentle cleansing of a skin wound prior to sample collection

by sterile normal saline is recommended to reduce commensal flora.

Purulent exudates must be expressed onto swabs. Place the swab into the transport media.

Deep wound specimens (aspirates) are optimal as the predictive value of superficial swabs is low.

Investigation of deep wounds for anaerobes

Collection of the wound swab

Collection of Eye Swabs Collect before topical or anesthetics are applied. Swab pus or purulent discharge taken from the lower

inverted lid. Place the swab into the transport media. Collection of Ear Swabs Swab the external ear canal.

Urethral swab: Express exudates from the urethra and collect it on a swab.

If exudate is unavailable, insert an urethrogenital swab about 2 cm into the urethra, gently rotate it and remove.

Place the swab into the transport medium. Vaginal swab Wipe away any excessive amount of secretion. Obtain secretions from the mucosal membrane with a swab. Place the swab into the transport medium.

Collection of Vaginal swab

Laboratory diagnosis

Specimen Processing Microbiologic testing should include both direct microscopic

examination and culture of the specimen. Clinical specimens should be inoculated onto both general

purpose and selective media to maximize bacterial recovery.

Common media used are: 2 blood agar(earobic& anearobic) MacConkey agar

Special media Neomycin blood agar or kanamycin BA when anaerobic

bacteria is suspected Modified Tinsdale medium (MTM)or Tellurite blood agar

(TBA) When Corynediphtheriae suspected Crystal violet blood agar for streptococcus pyogen LJ for M. tuberculosis Modified New York City (MNYC) or Thayer Martin medium

isolating Nisseria gonorrhoeae Cooked meat medium (CMM)for Clostridium perfringens Sabroud dextrose agar(SDA) for fungi

Bacterial Identification The first step in culture evaluation is the visual examination of

plated media. Most bacteria produce visible colonies in 24 hr, although some

require 48–72 hr. Inspection includes examination of colonial morphology, noting both types and numbers of colonies and any hemolytic reactions on blood-based agar.

Further classification is based on the presence or absence of growth on differential or selective media.

After the evaluation of plated media, examination of Gram stains made from each different colony type is performed.

Those reactions, combined with colonial morphology, may allow for the presumptive identification of organisms. If more than one colony type is present, subcultures of each are made.

Several microbial identification systems are commercially available. Systems may be manual or automated.

Antimicrobial sensitivity test: The agar disk diffusion method is more widely used for testing the

common, rapidly growing aerobic to facultative anaerobic pathogenic bacteria.

The Kirby-Bauer method is based on the diffusion of an antimicrobial agent impregnated within a paper disk through an agar medium. Briefly, a suspension of actively growing test organism is standardized to a turbidity equivalent to 0.5 on the McFarland scale. Within 15 min of standardization, a sterile swab is dipped into the bacterial suspension and a dry Müller-Hinton agar plate is inoculated by streaking the swab over the entire surface 3 times. To ensure an even distribution of the inoculum, the plate is rotated ~60° each time. Antimicrobial disks are placed on the plate and are gently pressed down to ensure their close contact with the agar surface. Inverted plates are placed in an incubator at 35°C. Plates are examined after 18 hr of incubation.

Results: Zones of complete inhibition are measured in

millimeters using a ruler. The zone sizes around each drug are compared to those

published by the CLSI (Clinical and Laboratory Standards Institute ) in order to make an interpretation of susceptible, intermediate, or resistant for each agent tested.

CASE STUDY (1)

A 5-year-old boy presented to the hospital suffering from acute malaise, shivering and vague pains in his legs, sore throat and had vomited twice. He was febrile (temperature 40.2°C) with a tachycardia  . His pharynx, tonsils and buccal mucosa were red and inflamed and his tonsils were studded with white areas of exudate. He was diagnosed as having acute bacterial tonsillitis .proceed to identify the causative agent

BLOOD AGAR

Macconkey agar

CASE STUDY (2)

• An 75-year-old diabetic man with swelling, blisters that contain gas bubbles near the area of infection, increased heart rate, and high fever. Skin in the affected area often turns from pale to brownish-red. Symptoms progress at a very rapid rate, the physician collect 2 wound swab and part of the necrotic tissue, proceed to identify the causative agent(s)

DIRECT MICROSCOPICAL EXAM

Blood agar and CMM

Gram stain

Gram stain

?????

Spore stain

Nagler test

Litmus milk reduction test

Litmus milk reduction test