Urinary tract infection (UTI) is an infection in any part of the urinary tract, including the kidneys, ureters, urinary bladder, and/or the urethra. It is most commonly caused by bacteria; especially the bacteria from the GI tract in the anal and perianal region. Candida spp. (C. albicans) is common fungi species causing UTIs in female and catheterized patients. The microorganisms that cause UTIs are called uropathogens.
UTI may be classified as lower UTI i.e. infection in the bladder and urethra and upper UTI i.e. infection of the kidneys and ureters. The infection of the bladder is called cystitis. The infection of the urethra is called urethritis, and the infection of the kidney is called pyelonephritis. It is more commonly associated with females due to the comparatively shorter urethra.
List of Bacteria Causing UTI
|Gram –ve Bacteria||Gram +ve Bacteria||Others Bacteria|
|E. coli * |
Klebsiella spp. *
Proteus spp. *
Pseudomonas spp. *
|Staphylococcus spp. *|
Streptococcus spp. *
Enterococcus spp. *
Gram +ve and Gram -ve Bacteria Causing UTI
Escherichia coli (E. coli)
- Lactose fermenting
- Facultatively anaerobic
- Mesophilic coliform bacteria
- Genus Escherichia
- Enterobacteriaceae family
It is the most common cause of both complicated and uncomplicated UTIs; responsible for 60 – 90% of the total UTI cases. It is responsible for about 90% of UTI cases in infants. E. coli strains causing UTIs are called uropathogenic E. coli (UPEC). They mostly enter the urinary tract via migration from the anorectal region. They can adhere to the mucosal lining of the urinary tract, invade the bladder epithelium, release toxins and proteases, and synthesize siderophores. They rapidly multiply and migrate towards the kidney causing complicated UTIs.
- Cocci bacteria
- Family Staphylococcaceae
- Producing grape-like clusters under a microscope
Three species of Staphylococcus; S. saprophyticus, S. aureus, and S. 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.
- Facultatively anaerobic,
- Capsule-forming coliform bacteria
- Family Enterobacteriaceae
It is the third most common cause of UTI after E. coli and Staphylococcus spp. They also migrate mainly through the anorectal region to the urethral opening and ascend upward to the urinary tract causing UTIs.
Mainly K. pneumoniae is responsible for UTIs. Rarely K. oxytoca is also associated with UTI. Their pili aid in attachment and prevent the flushing action and the polysaccharide capsule helps to escape the immune system. They don’t produce toxins and enzymes for tissue degradation.
- Aerobic and Facultative Anaerobic
- Motile bacteria
- Family Enterobacteriaceae
- Best known for their swarming colonies.
They are commonly found in the human GI tract and also in the environment. Hence, they can also enter the environment but mainly migrate from the anorectal region.
P. mirabilis is the most common species of Proteus causing UTIs. It is mostly associated with urinary-catheter-associated UTIs but is also responsible for community-acquired UTIs. They have adhesins and pili for attachment and are capable of producing hemolysins, capsules, and other toxins and enzymes as virulence factors.
P. vulgaris is another common uropathogenic species in this genus after P. mirabilis. P. penneri and P. myxofaciens are also rarely found associated with UTIs.
- Family Pseudomonadaceae
They are mainly associated with hospital-acquired infections, including UTIs.
P. aeruginosa is the most common uropathogen in this genus. It is the third most common hospital-acquired uropathogen causing catheter-associated UTI. It can cause biofilm formation on medical devices and have several virulence factors like exopolysaccharides, extracellular DNA, capsule, protease, elastase, hemolysins, exotoxin A, siderophores, etc.
P. putida is another species of Pseudomonas causing UTI in a few patients.
- Facultatively anaerobic
- Lactose fermenting cocci (diplococci) bacteria
- Family Enterococcaceae
- 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.
E. fecalis and E. 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.
- Anaerobic and facultative anaerobic
- Cocci bacteria
- Family Streptococcaceae
- Arrangement in the form of a chain of spheres
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 bladder.
Besides these seven genera, other bacteria are responsible for a minority of UTI cases. They are not considered uropathogens, but occasionally they cause UTIs. These include;
- facultative anaerobic
- family Enterobacteriaceae
It is commonly found in the human GI tract and fecal contaminated environmental aspects.
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.
- coliform bacteria
- family Enterobacteriaceae.
Two species of this genus, C. freundii and C. koseri are responsible for UTIs, mostly in catheterized, immunosuppressed, and prolonged hospitalized patients. C. freundii is more common than C. koseri.
- facultative anaerobic
- family Aeromonadaceae
They are not known uropathogens and are rarely responsible for UTIs. Mainly A. hydrophila and A. veronii are found to cause UTIs.
- facultative anaerobic
- family Yersiniaceae
- produces a characteristic red pigment
S. marcescens is responsible for occasional UTIs in catheterized patients. S. fonticola is rarely reported.
- aerobic and facultative anaerobic
- family Neisseriaceae
N. meningitides, N. gonorrhoeae, N. mucosa, N. sicca, N. subflava, and N. cinerea are species in this genus that are associated with UTIs in some cases.
- family Morganellaceae
P. rettgeri and P. stuartii are common species in this genus that mostly cause UTIs in catheterized elderly people.
- strict aerobic
- family Moraxellaceae
Among this genus, A. baumanii is responsible for several cases of hospital-acquired UTIs associated with catheterization and prolonged antibiotic therapy.
- family Veillonellaceae
It is a normal flora of the oral and intestinal tract of mammals. V. parvula is described as a rare causative agent of UTI in pregnant and immunosuppressed patients.
- homofermentative bacteria
- family Lactobacillaceae
It is normal flora in the genitourinary tract (vagina) but can cause UTIs in case of immunosuppression. Generally, L. gasseri and L. debrueckii are species reported as causatives of UTIs. But UTIs by Lactobacillus are very rare incidence.
- rod-shaped (club-shaped)
- aerobic bacteria
- family Corynebacteriaceae
They usually are found as commensals to humans and other mammals, but rarely are associated with UTIs. C. coyleae, C.aurimucosum, C. amycolatum, C. reneyi, C. group G1, C. muciefacines, C. propinquum, C. pseudodiphtheriticum, and C. renale are found to cause UTIs.
- aerobic cocci
- family Aerococcaceae
Two species of Aerococcus, namely; A. urinae and A. sanguinicola are rare causative agents of UTIs, especially in elderly males and catheterized males.
- facultatively anaerobic bacteria
- family Actinomycetaceae
A. schalii and A. massiliense are reported as emerging causes of UTIs in elderly patients.
Other Bacteria Causing UTIs
Ureaplasma and Mycoplasma spp.
These 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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