 Urine Colony Counts Inoculate the agar plate To perform a urine colony count, inoculate the agar plate with one microliter of urine. The sterile loop is calibrated to hold one microliter. Pass this loop down the center of the plate and carefully spread the urine over the entire plate. Incubate the plates overnight at 37 degrees Celsius. Let's take a look. Colony Counts setup requires the agar plate to grow the organisms on a one microliter loop and the urine specimen. You take the one microliter calibrated loop, this is sterile, open carefully, and we'll pick up one loop full, which is calibrated to be one microliter of specimen. It is straight down the center of the plate and then back and forth to spread out the colony so that you can get a colony count reading after it has incubated. Diagram of urine culture inoculation procedure. First, streak down the middle of the plate. Next, streak from side to side to distribute the organisms evenly on the agar plate. Reading the plates. After the plates have incubated, count each colony on the plate. Each colony represents one bacterium, which forms a colony and is called a colony forming unit. Counting the colonies. Count the number of colonies on the plate. This is the number of colonies per one microliter of urine. Use the conversion factor to convert one microliter to one milliliter. Counting the colonies, the calculation. Colonies per plate, 54. Amount plated, one microliter. Here's the calculation. The microliters cancel and you are left with 54,000 colonies per one milliliter. Note, if you counted 50 or 52 or 56 for this sample, no problem. Just use the number you counted in your calculations. Your answer will be different from the answer above, but the exponent will be the same. Also, always remember to put the units of colonies and milliliters in your calculations and in your answer. Counting the colonies. The number of colonies on this plate is greater than 100. The number used for your calculations is greater than 100 colonies per one microliter. Use your conversion factor to convert this value to colonies per milliliter of urine. Counting the colonies, the calculation. Colonies per plate, greater than 100. Amount plated, one microliter. Here's the calculation. The microliters cancel and you are left with greater than 100,000 colonies per one milliliter. Note, remember to put the units of colonies and milliliters in your calculations and in your answer. You must use the greater than sign in your answer. Count the colonies. Count the number of colonies on this plate. This is the number of colonies per one microliter of urine. Use the conversion factor to convert one microliter to one milliliter. Counting the colonies, calculation. Colonies per plate, 75. Amount plated, one microliter. Here's the calculation. The microliters cancel and you are left with 75,000 colonies per one milliliter. Note, if you counted a slightly different number, no problem. Just use the number you counted in your calculations. Your answer will be different from the answer above, but the exponent will be the same. Also, always remember to put the units of colonies and milliliters in your calculations and in your answer. Counting the colonies. Now, count the number of colonies on this plate. This is the number of colonies per one microliter of urine. Use the conversion factor to convert one microliter to one milliliter. Counting the colonies, calculation. Colonies per plate, greater than 100. Amount plated, one microliter. Here's the calculation. The microliters cancel and you are left with greater than 100,000 colonies per one milliliter. In summary, certain criteria are used to evaluate a clean catch midstream urine colony count to determine if it represents an infection. A few bacteria will typically be present on the agar from a clean catch urine colony count because the urine is contaminated with skin organisms as it leaves the urethra. If there is an infection, it is usually due to just one type of organism. Cultures with more than three organisms present on the agar are most likely contaminated. The following results are typical of a urinary tract infection. High colony counts of one organism, especially greater than 100,000, are indicative of an infection. Lower counts can also indicate an infection in some circumstances. White blood cells are seen on the microscopic evaluation of the urine. Very few epithelial cells are seen on the microscopic evaluation. All urine cultures should be taken to the laboratory or stored in the refrigerator as soon as possible after collection to retard replication of organisms in the sample. This concludes this activity. Urine colony counts.