Bacterial incubation on petri dish

If the microscopic examination shows signs of infection then a sample of the urine sediment will be placed on a Petri dish and left to incubate over 18 -24 hours. Bacteria can reproduce very quickly given the right conditions, such as warmth, moisture and suitable nutrients.

To ensure the cultures are not contaminated by other microorganisms, the following sterile conditions are needed; the Petri dishes, nutrient agar jelly inside these dishes and other culture media must be sterilised.

The inoculating loops used to transfer microorganisms to the petri dish must be sterilised (usually by passing the metal loop through a Bunsen burner flame) and finally, the lid of the Petri dish is sealed with sticky tape to stop microorganisms from the air getting in and contaminating the culture.

Bacteria will produce colonies differing in appearance as they grow, some colonies may be coloured, some colonies are circular in shape, and others are irregular. Different bacterial strains produce these characteristics. The sample from the petri dish is then “gram stained” to identify the actual strain of bacteria. This means a crystal violet colour stain is applied to the sample and placed under a microscope.

All bacteria are described as either Gram-negative or Gram-positive

Gram-positive bacteria remain purple. Gram-negative bacteria are stained pink. Gram-negative bacterial strains include:

  • Escherichia coli
  • Klebsiella pneumoniae
  • Proteus mirabilis
  • Pseudomonas species
  • Morganella morganii
  • Citrobacter species

Gram-positive strains include:

  • Enterococcus faecalis
  • Enterococcus faecium (E. faecium)
  • Staphylococcus aureus
  • Staph saprophyticus

Under the microscope, the appearance of bacteria is observed. The lab can then finally determine:

  • Are they Gram-positive or negative?
  • What are the physical characteristics?
  • Are the cells individual or are they in chains, pairs etc.?
  • How many are there and how large are the cells?

The final part of the process is to then count the total colony numbers of the single bacteria identified.

These are known as colony-forming units usually abbreviated as CFU. For a positive diagnosis to be made of an infection, the agreed standard is currently greater than >10 5 /ml in a millilitre of urine. Up to 10,000 colonies of bacteria/ml are considered normal. Greater than 100,000 colonies/ml represents a positive urinary tract or kidney infection.

For counts between 10,000 and 100,000, the culture is indeterminate and results will show low growth or mixed growth.

If sufficient bacteria are grown and the single pure strain identified e.g. E-coli, antibiotics will be tested against the bacteria to check their effectiveness in stopping the infection. This is known as susceptibility and resistance and the results will be noted on the lab report to help your GP prescribe the correct antibiotic.

Yeast can also be present in urine. If yeast is found in urine, then the laboratory may recommend tests for a yeast (fungal) infection on vaginal secretions or may culture on a petri dish to identify the yeast and whether it could be causing UTI or other disease symptoms.

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