Bacteriology Sample Collection

Swabs

• Swab the area which is showing active signs of inflammation - avoid areas of pus as the neutrophilic destruction of bacteria which forms pus is less likely to produce a live culture.
• Use aseptic techniques and watch the swab tip until it has been safely replaced in the swab container to prevent accidental contamination prior to culture.
• Transport media containing charcoal is suitable for most samples with some notable exceptions such as swabs requiring Strangles PCR. Check with your submitting lad the first time you collect a swab for a specific diagnostic test to see if there are any special transport medium requirements. CEMO swabs must be placed in a charcoal medium for transportation.
• Label the swab immediately and indelibly with the patient details, site, and date.
• Fill in your external lab request form as completely as possible - the more information you include the better the chances of a diagnostic result. Most cultures are likely to give a mixed growth and if the pathologist can rule in or out organisms based on the information you give them then the pathological process becomes considerably more accurate.
• Refrigerate samples that are awaiting postage later in the day, as this will help to prevent an overgrowth of fast growing organisms at the expense of pathogenic organisms.

In House Culture

Accurate paperwork is essential when culturing in house samples so that changes noted on plates can be recorded over time. Always warm culture plates to room temperature prior to inoculation or cold shock can inhibit a delicate organism from growing on a plate. Different mediums can be a used to select for an organism of interest:

1. Blood Agar (BA) is the standard plate used for most samples - in most cases an additional plate is added to give more information about the identity of any organism that successfully grows.
2. COBA is selective for Streptococci species, which are ubiquitous in some species and frequently cause opportunistic infections. Tracheal washes, joint samples, and cervical swabs and potential Strep equi cases require this medium in addition to BA.
3. CLED with Andrade’s indicator is a useful medium when a swab is likely to grow E coli; the indicator will turn pink and the colonies will have a distinctive look and smell.
4. Sabaroud plates are selective for fungi and are useful for skin scrapes where dermatophytes are suspected - for many fungi special incubation requirements (e.g. temperature settings of 30°C for 48 hours) must be met so check the requirement based on the most likely organism to grow.
5. XLD and Brilliant green are used to encourage and identify Salmonella species; the colonies grow with specific characteristics on these two plates. Place your sample in Selenite broth prior to inoculating this media to reduce the number of contaminant organisms that will grow on the plate.
6. MRSA Chromogenic media will give a rapid (18 hour) diagnosis of MRSA infection – growth of the organism is associated with a distinctive colour change.
7. CEMO medium for Taylorella culture requires two plates of specialised agar which suppress the growth of contaminants; one plate contains added Streptomycin, the other is without Streptomycin as some strains of Taylorella have been found to be Streptomycin sensitive.

In addition to using these plates for your original inoculation, there are media available which can confirm the identity of an organism such as Pseudomonas aeruginosa that has grown on the standard media initially. Always test your growth media by maintaining known organisms and inoculating the media using normal conditions so that you can be sure that your methods are giving diagnostic results. Alternative methods of identifying an organism include biochemical tests such as oxidase or catalase reactions or latex agglutination tests.

Sensitivity testing

Sensitivity testing requires a standardised approach to plate inoculation and disc strengths used - the British Society for Antimicrobial Chemotherapy (BSAC) guidelines publish a methodology to ensure this. The plate requirement, dilution of the culture, strength of antibiotic disc used, interpretation of growth inhibition ring size and the culture conditions are all specified in these guidelines, which ensures in vivo antibiotic sensitivity. The remit is based on organism rather than species from which the sample was taken, so it is a valid approach to ensure accurate sensitivity tests in veterinary practice.