Common Bacteria Causing UTI (Urinary Tract Infection)

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 and Gram -ve Bacteria Causing UTI

Escherichia coli (E. coli)

• Gram-negative
• Rod-shaped
• 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.

Staphylococcus spp.

• Gram-positive
• Catalase-positive
• 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. 

Klebsiella spp.

• Gram-negative
• Rod-shaped
• 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.    

Proteus spp.

• Gram-negative
• Rod-shaped
• 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.      

Pseudomonas spp.

• Gram-negative
• Rod-shaped
• Aerobic
• Gammaproteobacteria
• 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. 

Enterococcus spp.

• Gram-positive
• 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. 

Streptococcus spp.

• Gram-positive
• Anaerobic and facultative anaerobic
• Catalase-negative
• 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;

Salmonella spp.

• Gram-negative
• rod-shaped
• flagellated
• facultative anaerobic
• Gammaproteobacteria
• 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.   

Citrobacter spp.

• Gram-negative
• rod-shaped
• non-sporing
• 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. 

Aeromonas spp.

• Gram-negative
• rod-shaped
• facultative anaerobic
• Gammaproteobacteria
• family Aeromonadaceae

They are not known uropathogens and are rarely responsible for UTIs. Mainly A. hydrophila and A. veronii are found to cause UTIs.

Serratia spp. 

• Gram-negative
• rod-shaped
• facultative anaerobic
• Gammaproteobacteria
• family Yersiniaceae
• produces a characteristic red pigment

S. marcescens is responsible for occasional UTIs in catheterized patients. S. fonticola is rarely reported.  

Neisseria spp. 

• Gram-negative
• aerobic and facultative anaerobic
• Betaproteobacteria
• 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.    

Providencia spp.

• Gram-negative
• rod-shaped
• motile
• Gammaproteobacteria
• family Morganellaceae

P. rettgeri and P. stuartii are common species in this genus that mostly cause UTIs in catheterized elderly people.    

Acinetobacter spp.

• Gram-negative
• rod-shaped
• strict aerobic
• Gammaproteobacteria
• family Moraxellaceae

Among this genus, A. baumanii is responsible for several cases of hospital-acquired UTIs associated with catheterization and prolonged antibiotic therapy.  

Veillonella spp.

• Gram-negative
• anaerobic
• cocci
• 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. 

Lactobacillus spp.

• Gram-positive
• rod-shaped
• microaerophilic
• non-sporing
• 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. 

Corynebacterium spp.

• Gram-positive
• 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.

Aerococcus spp.

• Gram-positive
• 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. 

Actinobaculum spp.

• Gram-positive
• rod-shaped
• 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.

References

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