Bacteriuria is the presence of bacteria in urine. Bacteriuria accompanied by symptoms is a urinary tract infection while that without is known as asymptomatic bacteriuria. Diagnosis is by urinalysis or urine culture. Escherichia coli is the most common bacterium found. People without symptoms should generally not be tested for the condition. Differential diagnosis include contamination.

If symptoms are present, treatment is generally with antibiotics. Bacteriuria without symptoms generally does not require treatment. Exceptions may include pregnant women, those who have had a recent kidney transplant, young children with significant vesicoureteral reflux, and those undergoing surgery of the urinary tract.

Bacteriuria without symptoms is present in about 3% of otherwise healthy middle aged women. In nursing homes rates are as high as 50% among women and 40% in men. In those with a long term indwelling urinary catheter rates are 100%. Up to 10% of women have a urinary tract infection in a given year and half of all women have at least one infection at some point in their lives. There is an increased risk of asymptomatic or symptomatic bacteriuria in pregnancy due to physiological changes that occur in a pregnant woman which promotes unwanted pathogen growth in the urinary tract.

Asymptomatic bacteriuria is bacteriuria without accompanying symptoms of a urinary tract infection and is commonly caused by the bacterium Escherichia coli. Other potential pathogens are Klebsiella spp., and group B streptococci. It is more common in women, in the elderly, in residents of long-term care facilities, and in people with diabetes, bladder catheters, and spinal cord injuries. People with a long-term Foley catheter always show bacteriuria. Chronic asymptomatic bacteriuria occurs in as many as 50% of the population in long-term care.

There is an association between asymptomatic bacteriuria in pregnant women with low birth weight, preterm delivery, cystitis, infection of the newborn and fetus death.However, most of these studies were graded as poor quality. Bacteriuria in pregnancy also increases the risk of preeclampsia.

Symptomatic bacteriuria is bacteriuria with the accompanying symptoms of a urinary tract infection (such as frequent urination, painful urination, fever, back pain, abdominal pain and blood in the urine) and includes pyelonephritis or cystitis. The most common cause of urinary tract infections is Escherichia coli.^{[citation needed]}

Testing for bacteriuria is usually performed in people with symptoms of a urinary tract infection. Certain populations that cannot feel or express symptoms of infection are also tested when showing nonspecific symptoms. For example, confusion or other changes in behaviour can be a sign of an infection in the elderly. Screening for asymptomatic bacteriuria in pregnancy is a common routine in many countries, but controversial.^{[citation needed]}

Bacteriuria is assumed if a single bacterial species is isolated in a concentration greater than 100,000 colony forming units per millilitre of urine in clean-catch midstream urine specimens. In urine samples obtained from women, there is a risk for bacterial contamination from the vaginal flora. Therefore, in research, usually a second specimen is analysed to confirm asymptomatic bacteriuria in women. For urine collected via bladder catheterization in men and women, a single urine specimen with greater than 100,000 colony forming units of a single species per millilitre is considered diagnostic. The threshold for women displaying UTI symptoms can be as low as 100 colony forming units of a single species per millilitre. However, bacteria below a threshold of 10000 colony forming units per millilitre are usually reported as "no growth" by clinical laboratories.