Common Bacteria Causing UTI (Urinary Tract Infection)

A urinary tract infection (UTI) is an infection that affects any part of the urinary system, which includes the kidneys, ureters, bladder and urethra. The urinary system is responsible for producing, storing and eliminating urine from the body. Urine is a liquid waste that contains water, salts and other substances filtered from the blood by the kidneys.

UTIs are very common, especially among women and people assigned female at birth (AFAB), who have a shorter urethra than men and people assigned male at birth (AMAB). This makes it easier for bacteria to enter the urinary tract and cause an infection. UTIs can also affect children, but they are less common.

The most common cause of UTIs is bacteria from the gastrointestinal (GI) tract, such as Escherichia coli (E. coli), that enter the urinary tract through the urethra. The urethra is the tube that carries urine from the bladder to the outside of the body. The bacteria can then multiply and spread to the bladder, causing cystitis (bladder infection), or to the kidneys, causing pyelonephritis (kidney infection).

Other factors that can increase the risk of UTIs include:

• Sexual activity, which can introduce bacteria into the urinary tract.
• Pregnancy, which can put pressure on the bladder and ureters and make it harder to empty the bladder completely.
• Menopause, which can reduce estrogen levels and affect the balance of bacteria in the vagina and urethra.
• Certain birth control methods, such as diaphragms or spermicides, which can irritate the urethra or change the vaginal flora.
• Kidney stones or other urinary tract abnormalities, which can block or slow down urine flow and create a breeding ground for bacteria.
• Medical conditions such as diabetes, multiple sclerosis, stroke or spinal cord injury, which can affect bladder function or nerve signals to the urinary tract.
• Weakened immune system, which can make it harder to fight off infections.
• Use of catheters or other medical devices in the urinary tract, which can introduce bacteria or cause irritation or injury.

UTIs can cause various symptoms depending on which part of the urinary tract is affected. Some common symptoms include:

• A strong urge to urinate that doesn`t go away
• A burning feeling when urinating
• Urinating often and passing small amounts of urine
• Urine that looks cloudy, bloody or foul-smelling
• Pelvic pain or pressure
• Lower back pain or flank pain
• Fever, chills, nausea or vomiting

UTIs are usually diagnosed by testing a urine sample for bacteria and other signs of infection. Sometimes, other tests such as urine culture, cystoscopy (examining the inside of the bladder with a camera), CT scan or MRI may be needed to identify the type of bacteria causing the infection or to check for any abnormalities in the urinary tract.

UTIs are usually treated with antibiotics that kill the bacteria causing the infection. The type, dose and duration of antibiotics depend on the severity of the infection and the type of bacteria involved. It is important to complete the prescribed course of antibiotics even if symptoms improve to prevent recurrence or complications.

Some self-care measures that can help prevent or relieve UTI symptoms include:

• Drinking plenty of water to flush out bacteria from the urinary tract
• Wiping from front to back after using the toilet to avoid spreading bacteria from the anus to the urethra
• Avoiding feminine hygiene products such as sprays or douches that can irritate the urethra or alter the vaginal flora
• Cleaning the genital area before and after sexual intercourse and urinating after sex to remove any bacteria that may have entered the urinary tract
• Using heating pads or warm compresses to ease pain in the pelvic or lower back area

UTIs are usually not serious if treated promptly and properly. However, if left untreated or if they recur frequently, they can lead to complications such as:

• Recurrent infections that may damage the kidneys or bladder
• Permanent kidney damage that may impair kidney function or cause kidney failure
• Narrowing of the urethra (urethral stricture), especially in men and AMAB people, that may obstruct urine flow and increase infection risk
• Sepsis, a life-threatening condition that occurs when an infection spreads to the bloodstream and causes inflammation throughout the body

If you have any symptoms of a UTI or if you have any questions or concerns about your urinary health, you should consult your healthcare provider as soon as possible. UTIs are common and treatable, but they can also be prevented with proper hygiene and care.

Urinary tract infections (UTIs) are mainly caused by bacteria that enter the urinary tract through the urethra and spread to the bladder and sometimes the kidneys. The most common bacteria that cause UTIs are Escherichia coli (E. coli), which are normally found in the gastrointestinal (GI) tract and account for 60-90% of all UTIs . However, other bacteria can also cause UTIs, depending on the location and severity of the infection, the host factors, and the exposure to medical devices or antibiotics.

Bacteria that cause UTIs can be classified based on their Gram staining reaction, which reflects their cell wall structure and composition. Gram staining is a technique that uses a violet dye and an iodine solution to stain bacteria. Bacteria that retain the violet color are called Gram-positive, while bacteria that lose the violet color and appear pink are called Gram-negative. Gram staining helps to identify and differentiate bacteria based on their morphology and characteristics.

Some of the common Gram-negative bacteria that cause UTIs are:

• Klebsiella spp., which are rod-shaped, facultatively anaerobic, capsule-forming coliform bacteria that belong to the family Enterobacteriaceae. They are the third most common cause of UTI after E. coli and Staphylococcus spp. They usually migrate from the anorectal region to the urethral opening and ascend to the urinary tract, causing UTIs. The most common species is Klebsiella pneumoniae, which can produce toxins and enzymes that damage the urinary tract tissues.
• Proteus spp., which are rod-shaped, aerobic and facultative anaerobic, motile bacteria that belong to the family Enterobacteriaceae. They are best known for their swarming colonies on agar plates. They are commonly found in the human GI tract and also in the environment. The most common species is Proteus mirabilis, which is associated with urinary catheter-associated UTIs and community-acquired UTIs. They have adhesins and pili for attachment and can produce hemolysins, capsules, and other toxins and enzymes as virulence factors.
• Pseudomonas spp., which are rod-shaped, aerobic, Gammaproteobacteria that belong to the family Pseudomonadaceae. They are mainly associated with hospital-acquired infections, including UTIs. The most common species is Pseudomonas aeruginosa, which is the third most common hospital-acquired uropathogen causing catheter-associated UTI. It can form biofilms on medical devices and has several virulence factors such as exopolysaccharides, extracellular DNA, capsule, protease, elastase, hemolysins, exotoxin A, siderophores, etc.
• Salmonella spp., which are rod-shaped, flagellated, facultative anaerobic Gammaproteobacteria that belong to the family Enterobacteriaceae. They are commonly found in the human GI tract and fecal contaminated environmental sources. Non-typhoidal Salmonella (NTS) is a rare cause of UTI, occupying only about 0.015 – 0.118% of total UTI cases. They are primarily associated with immunosuppression, chronic disease, or abnormalities in the structure of the genitourinary tract.
• Citrobacter spp., which are rod-shaped, non-sporing, coliform bacteria that belong to the family Enterobacteriaceae. Two species of this genus, Citrobacter freundii and Citrobacter koseri are responsible for UTIs, mostly in catheterized, immunosuppressed, and prolonged hospitalized patients. C. freundii is more common than C. koseri.
• Aeromonas spp., which are rod-shaped, facultative anaerobic Gammaproteobacteria that belong to the family Aeromonadaceae. They are not known uropathogens and are rarely responsible for UTIs. Mainly Aeromonas hydrophila and Aeromonas veronii are found to cause UTIs in a few patients.
• Serratia spp., which are rod-shaped, facultative anaerobic Gammaproteobacteria that belong to the family Yersiniaceae. They produce a characteristic red pigment on agar plates. Serratia marcescens is responsible for occasional UTIs in catheterized patients. S. fonticola is rarely reported.
• Neisseria spp., which are Gram-negative, aerobic and facultative anaerobic Betaproteobacteria that belong to the family Neisseriaceae. They are usually found in the mucous membranes of humans and animals. Neisseria meningitidis, Neisseria gonorrhoeae, Neisseria mucosa, Neisseria sicca, Neisseria subflava, and Neisseria cinerea are species in this genus that are associated with UTIs in some cases.
• Providencia spp., which are rod-shaped, motile Gammaproteobacteria that belong to the family Morganellaceae. Providencia rettgeri and Providencia stuartii are common species in this genus that mostly cause UTIs in catheterized elderly people.
• Acinetobacter spp., which are rod-shaped, strict aerobic Gammaproteobacteria that belong to the family Moraxellaceae. Among this genus, Acinetobacter baumanii is responsible for several cases of hospital-acquired UTIs associated with catheterization and prolonged antibiotic therapy.

Some of the common Gram-positive bacteria that cause UTIs are:

• Staphylococcus spp., which are catalase-positive, cocci bacteria that belong to the family Staphylococcaceae. They produce grape-like clusters under a microscope. Three species of Staphylococcus; Staphylococcus saprophyticus, Staphylococcus aureus, and Staphylococcus epidermidis are uropathogens. S. saprophyticus is the second most common cause of UTI after UPEC causing about 5 – 20% of community-associated UTIs, and about 40 – 45% of UTIs in females of 16 -25 years. While in males, they cause UTIs only in immune-compromised conditions. They are normally present in the rectum, urethra, cervix, and skin around the urethral opening. From here, they can enter the urinary tract and ascend upward, causing UTIs. They have adhesion factors for attachment and can even produce toxins and enzymes like urease, and catalase. S. aureus is another species causing UTIs, especially in urinary-catheterized and pregnant individuals. Most of the isolated strains are found to be MRSA (methicillin-resistant S. aureus). S. epidermidis is another rarely found opportunistic species. It is normally present in human skin and gains access to the urinary tract by biofilm formation in indwelling medical devices. S. simulans is another species that is found in very few cases of UTIs.
• Enterococcus spp., which are facultatively anaerobic, lactose fermenting cocci (diplococci) bacteria that belong to the family Enterococcaceae. They tolerate bile salt concentrations up to 40%. They are found as commensals in the human GI tract, hence causing UTIs by passing through the anorectal region. Enterococcus fecalis and Enterococcus faecium are two commonly associated uropathogens. They together are responsible for a minority of community-acquired UTIs but account for 15 – 30% of hospital-acquired catheter-associated UTIs, making them the second most common cause of nosocomial UTIs. E. fecalis is more frequently isolated than E. faecium. They have the capacity to form a biofilm and have adhesion factors and pili for attachment.
• Streptococcus spp., which are anaerobic and facultative anaerobic, catalase-negative, cocci bacteria that belong to the family Streptococcaceae. They arrange in the form of a chain of spheres under a microscope. Group B Streptococcus (GBS), commonly known as Streptococcus agalactiae is responsible for UTIs in nursing elderly people above 70 years and few females especially pregnant and immune-compromised females. GBS are commensals in the GI tract and female genitourinary tract and are found to be associated with UTI in case of underlying conditions like diabetes mellitus, malignancy, chronic kidney disease, recurrent urinary tract infections, obstructive neuropathy, and neurogenic

Gram-negative bacteria are a group of bacteria that have a thin cell wall and stain pink with Gram stain. They are responsible for most cases of UTI, especially in the lower urinary tract (urethra and bladder). Some of the common gram-negative bacteria that cause UTI are:

• Escherichia coli (E. coli): This is the most common cause of both uncomplicated and complicated UTIs, accounting for 60 - 90% of the total UTI cases . E. coli strains that cause UTIs are called uropathogenic E. coli (UPEC). They usually enter the urinary tract from the anorectal region, where they are part of the normal gut flora. They have various factors that help them adhere to the urinary tract lining, invade the bladder cells, produce toxins and enzymes, and acquire iron. They can also multiply rapidly and spread to the kidneys, causing pyelonephritis.
• Klebsiella spp.: This is the third most common cause of UTI after E. coli and Staphylococcus spp. Klebsiella pneumoniae is the main species that causes UTI. It also enters the urinary tract from the anorectal region and has a capsule that protects it from the immune system. It does not produce toxins or enzymes that damage the tissue.
• Proteus spp.: This is a group of motile bacteria that can form swarming colonies on agar plates. They are commonly found in the human gut and can also enter the urinary tract from the anorectal region. Proteus mirabilis is the most common species that causes UTI, especially in catheterized patients and those with urinary stones. It has adhesins and pili for attachment and can produce hemolysins, capsules, and other toxins and enzymes. It can also produce urease, an enzyme that breaks down urea into ammonia and carbon dioxide, making the urine alkaline and favoring stone formation.
• Pseudomonas spp.: This is a group of aerobic bacteria that can cause hospital-acquired UTIs, especially in patients with catheters, indwelling devices, or prolonged antibiotic therapy. Pseudomonas aeruginosa is the most common species that causes UTI. It can form biofilms on medical devices and has several virulence factors such as exopolysaccharides, extracellular DNA, capsule, protease, elastase, hemolysins, exotoxin A, siderophores, etc.
• Salmonella spp.: This is a group of flagellated bacteria that are mainly associated with gastrointestinal infections but can also cause UTIs in rare cases. Non-typhoidal Salmonella (NTS) are responsible for most cases of Salmonella UTIs, which occur mainly in immunocompromised patients or those with urinary tract abnormalities.
• Citrobacter spp.: This is a group of coliform bacteria that are rarely associated with UTIs. Citrobacter freundii and Citrobacter koseri are the main species that cause UTIs, mostly in catheterized, immunosuppressed, and hospitalized patients.
• Aeromonas spp.: This is a group of aquatic bacteria that are not known to be uropathogens but can occasionally cause UTIs. Aeromonas hydrophila and Aeromonas veronii are the main species that cause UTIs.
• Serratia spp.: This is a group of bacteria that produce a characteristic red pigment on agar plates. They are rarely associated with UTIs but can cause infections in catheterized patients. Serratia marcescens is the main species that causes UTIs.
• Neisseria spp.: This is a group of aerobic cocci that are mainly associated with respiratory and genital infections but can also cause UTIs in some cases. Neisseria meningitidis, Neisseria gonorrhoeae, Neisseria mucosa, Neisseria sicca, Neisseria subflava, and Neisseria cinerea are some of the species that have been reported to cause UTIs.
• Providencia spp.: This is a group of motile bacteria that are mostly associated with UTIs in catheterized elderly patients. Providencia rettgeri and Providencia stuartii are the common species that cause UTIs.
• Acinetobacter spp.: This is a group of aerobic bacteria that are mainly associated with hospital-acquired infections, including UTIs. Acinetobacter baumannii is the main species that causes UTIs, especially in patients with catheters and prolonged antibiotic therapy.

These are some of the gram-negative bacteria that cause UTIs. They have different mechanisms of infection and resistance, which make them challenging to treat. Therefore, it is important to identify the causative agent and its susceptibility pattern before initiating antibiotic therapy.

Gram-positive bacteria are bacteria that have a thick cell wall that retains a purple stain when treated with a chemical called Gram stain. They are less common than gram-negative bacteria as causes of UTI, but they can still cause significant infections, especially in certain groups of people. Some of the most frequently isolated gram-positive bacteria in UTI are:

• Staphylococcus spp.: These are spherical bacteria that form clusters under the microscope. They are catalase-positive, meaning they can break down hydrogen peroxide into water and oxygen. They can produce various toxins and enzymes that contribute to their virulence. Three species of Staphylococcus are commonly associated with UTI: S. saprophyticus, S. aureus, and S. epidermidis .
  • S. saprophyticus is the second most common cause of uncomplicated UTI after E. coli, accounting for 5 to 20% of community-acquired UTIs, and up to 45% of UTIs in young women . It is normally found in the rectum, urethra, cervix, and skin around the urethral opening. It can adhere to the urinary tract mucosa and resist flushing by urine flow. It can also produce urease, an enzyme that breaks down urea into ammonia and carbon dioxide, increasing the urine pH and favoring the formation of kidney stones.
  • S. aureus is another species that can cause UTI, especially in catheterized and pregnant patients . It is normally found on the skin and mucous membranes, but it can also enter the bloodstream and spread to different organs, causing serious infections. Many strains of S. aureus are resistant to methicillin, a type of antibiotic, and are called methicillin-resistant S. aureus (MRSA).
  • S. epidermidis is a less common cause of UTI, mainly affecting immunocompromised and catheterized patients . It is also normally present on the skin and mucous membranes, but it can form biofilms on medical devices and resist antibiotics.
• Enterococcus spp.: These are spherical bacteria that form chains or pairs under the microscope. They are facultatively anaerobic, meaning they can grow with or without oxygen. They are also lactose-fermenting, meaning they can break down lactose into lactic acid and gas. They can tolerate high concentrations of bile salts, which are normally toxic to bacteria . Two species of Enterococcus are commonly involved in UTI: E. faecalis and E. faecium .
  • E. faecalis and E. faecium are commensals of the human gastrointestinal tract, but they can also cause UTIs by migrating from the rectum to the urethra . They account for 15 to 30% of hospital-acquired UTIs, making them the second most common cause of nosocomial UTIs after E. coli . They can form biofilms on catheters and other devices, and have adhesion factors and pili that help them attach to the urinary tract mucosa . They are also often resistant to many antibiotics, especially E. faecium .
• Streptococcus spp.: These are spherical bacteria that form chains under the microscope. They are anaerobic or facultatively anaerobic, and catalase-negative, meaning they cannot break down hydrogen peroxide . One species of Streptococcus that is frequently associated with UTI is S. agalactiae .
  • S. agalactiae, also known as group B streptococcus (GBS), is a common cause of UTI in elderly people over 70 years old and in pregnant women . It is normally found in the gastrointestinal tract and the female genital tract, but it can also cause infections in other sites such as the blood, lungs, bones, joints, and central nervous system . It can cause complications such as preterm labor, neonatal sepsis, and meningitis in pregnant women and newborns .
• Corynebacterium spp.: These are rod-shaped bacteria that have a club-like appearance under the microscope. They are aerobic, meaning they require oxygen to grow. They are usually commensals of the human skin and mucous membranes, but they can occasionally cause UTIs. Some species of Corynebacterium that have been reported to cause UTIs are C. coyleae, C. aurimucosum, C. amycolatum, C. renale, and C. pseudodiphtheriticum.
• Aerococcus spp.: These are spherical bacteria that form tetrads or clusters under the microscope. They are aerobic and catalase-negative. They are rare causes of UTIs, mainly affecting elderly men and catheterized patients. Two species of Aerococcus that have been implicated in UTIs are A. urinae and A. sanguinicola.
• Actinobaculum spp.: These are rod-shaped bacteria that are facultatively anaerobic and non-spore-forming. They are emerging causes of UTIs in elderly patients, especially men with urinary tract abnormalities or stones. Two species of Actinobaculum that have been identified as uropathogens are A. schalii and A. massiliense.

These are some of the gram-positive bacteria that can cause UTIs, but there may be others that are less common or underreported. It is important to identify the causative agent of a UTI and its antibiotic susceptibility to choose the appropriate treatment and prevent complications.

• Veillonella spp.
  
  • Veillonella spp. are Gram-negative anaerobic cocci that are part of the normal oral and genital flora .
  • Veillonella parvula is a rare causative agent of UTI, especially in pregnant and immunosuppressed patients .
  • It can cause bacteremia of urinary source and severe complications such as emphysematous pyelonephritis .
  • It can be isolated from urine using cystine-lactose-electrolyte deficient media and identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry or 16S rRNA gene sequencing 
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• Lactobacillus spp.
  
  • Lactobacillus spp. are Gram-positive rod-shaped microaerophilic non-sporing homofermentative bacteria that are normal flora in the genitourinary tract (vagina).
  • They can cause UTIs in case of immunosuppression or disruption of the vaginal microbiota.
  • Generally, L. gasseri and L. debrueckii are species reported as causatives of UTIs, but UTIs by Lactobacillus are very rare.
    
    
• Ureaplasma and Mycoplasma spp.
  
  • Ureaplasma and Mycoplasma spp. are bacteria lacking cell-wall, hence can’t be classified based on Gram staining (but stains pink).
  • They are associated with some urethritis, cystitis, and upper renal tract infections.
  • M. hominis, M. genitalium, U. urealyticum, and U. parvum are reported as causatives of UTIs in a few cases.

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