For detail, please see my other website: https://www.microbiologylearning.weebly.com for all the bacteria that cause UTIs and their microscopic and macroscopic growth on agars.
Urinary tract infections are commonly caused by coliform bacteria, particularly of the family Enterobacteriaceae. Urine is normally sterile, however, if coliform bacteria or if the normal microflora found in the genitourinary tract end up traveling into the urethra, it can lead to a urinary tract infection, whether it be cystitis, a bladder infection, a kidney infection, or kidney stones associated with certain bacteria. Some sexually transmitted bacteria, such as Neisseria gonorrhoeae or Chlamydia are responsible for urethritis and STDs.
Coliform bacteria are Gram-negative bacilli. The most common coliform bacteria associated with UTI across the board is Escherichia coli. It is a Gram-negative bacilli, indole +, oxidase -, lactose-fermenter on MacConkey agar, and Beta-hemolytic on blood agar. Some coliform bacteria are mucoid and slimy, like Klebsiella species, some are spreaders like Proteus mirabilis, and some are shiny, metallic, and have a "grape-like" or "tortilla chip" odor like Pseudomonas aeruginosa. Candida albicans and other yeasts can also be isolated in the urine as a cause of UTIs, not just genital yeast infections. Gram-positive, catalase +, latex agglutination (coagulase) + Staphylococcus aureus also causes UTIs, as does S. epidermidis and occasionally other coagulase negative staphylococci. Insertion of urinary catheters is a very common cause of UTI, and is the #1 cause of nosocomial, or hospital-acquired infection.
Since there are so many bacteria that colonize the skin, in order to be labeled as the cause of a UTI, the bacteria are quantitated on a plate using a semi-quantititative streak method, and there must be >5,000 CFU/mL to be considered. Most UTI's have >100,000 CFU/mL of bacteria growing on the agar.
As seen in the images under urinalysis, bacterial urine infections are associated with cloudy/turbid urine, pyuria, blood in the urine (not always), a positive leukocyte (WBC) test, and a positive nitrite test. Sometimes the urine will have a very strong odor as well, such as ammonia. The urine may have a high specific gravity if there is pus, and it tends to be alkaline. This is to show an example of a positive urine culture that was sent to the Microbiology Department for processing.
E. coli bacteria, >100,000 CFU/mL, growing on both the blood agar and MacConkey agar on a urine culture
Most bacteria causing UTIs are Gram-negative bacilli, such as E. coli.
Some coliform bacteria, not all, are indole +, as indicated in the top 2 squares of the test above. Indole positive tests appear pink-red.