Pyelonephritis and Urinary Tract Infection

Slide 1 · Pyelonephritis and Urinary Tract Infection

Ascending infection • Renal inflammation • Reflux and scarring

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Localize infection within the urinary tract and renal parenchyma.

Explain ascending and hematogenous infection, including bacterial adherence.

Connect urinary stasis, vesicoureteral reflux and intrarenal reflux to renal injury.

Interpret the key morphologic, clinical and urinary findings of pyelonephritis.

Slide 3 · Pyelonephritis is an infection-driven tubulointerstitial disease

The inflammatory process involves renal tubules, interstitium and pelvis.

It belongs to the tubular and interstitial group of renal disorders.

Infection can progress from the lower urinary tract to renal parenchyma.

Recurrent injury in a susceptible kidney may produce scarring.

Slide 4 · Three compartments define the lesion

Tubular lumen and epithelium

Renal interstitium

Renal pelvis and calyces

The pathologic diagnosis is an anatomic one: Infection extends beyond bladder urine into renal tissue.

Slide 5 · The urinary tract: Infection has a direction

Normal urine drainage

Potential ascending infection

Renal parenchyma

Renal parenchyma

Renal pelvis

Renal pelvis

Ureter

Ureter

Urinary bladder

Urinary bladder

Distal urethra

Distal urethra

Downward

Upward when barriers fail

Ascending infection is the dominant route to the kidney.

Slide 6 · Lower and upper urinary tract infection

Lower tract

Urethra and bladder

Urethritis / cystitis

May remain anatomically localized

Irritative voiding symptoms predominate

Upper tract

Collecting system and renal parenchyma

Pyelonephritis

Tubules + interstitium involved

Systemic symptoms and flank pain may occur

Slide 7 · Asymptomatic bacteriuria is not pyelonephritis

Bacteria may be present in urine without symptoms.

Bacterial colonization can remain localized to the lower urinary tract.

An infected bladder does not necessarily imply renal parenchymal infection.

Clinical localization must not rely on bacteriuria alone.

Slide 8 · An uncomplicated ascent is not inevitable

Entry into lower tract

Survival and bladder colonization

Impaired clearance or reflux

Renal involvement

The decisive transition is from lower-tract colonization to kidney exposure.

Slide 9 · The central pathogenesis model

Enteric flora

Periurethral colonization

Ascending bladder infection

Reflux / stasis

Renal inflammation

Microbial factors and host urinary-tract abnormalities act together.

Slide 10 · Enteric gram-negative bacilli dominate

More than 85% of UTIs are attributed to enteric gram-negative bacilli in the supplied Robbins text.

The organisms are commonly part of normal intestinal flora.

Periurethral contamination links the enteric reservoir to urinary infection.

This is an etiologic distribution of UTI, not a diagnostic threshold.

Slide 11 · Major bacterial pathogens

Organism

Pathologic context

Escherichia coli

By far the most common cause

Proteus spp.

Enteric gram-negative bacillus

Klebsiella spp.

Enteric gram-negative bacillus

Enterobacter spp.

Enteric gram-negative bacillus

Enterococci and staphylococci

Additional bacterial causes

Slide 12 · Adherence converts exposure into colonization

P-fimbriae (pili)

Bacterial adhesins

Urothelial receptors

Persistent attachment

Adhesion permits establishment of bacteria along the mucosal surface.

Slide 13 · Host and organism both determine whether infection progresses

An organism must reach the mucosal surface and persist.

Fimbrial adhesins promote contact with epithelial receptors.

Urine flow and voiding oppose colonization.

Abnormal anatomy or function shifts the balance in favor of ascent.

Slide 14 · Unusual renal pathogens require clinical context

Context

Fungal or mycobacterial infection

Granulomatous renal inflammation

Candida may accompany fungal pyelonephritis

Examples

Immunocompromised or transplant patient

Polyomavirus, cytomegalovirus, adenovirus

Also nonenteric bacteria and certain fungi

Slide 15 · Two routes can seed the kidney

Ascending route

Starts at lower urinary tract

Progresses via ureter / collecting system

Most common route in pyelonephritis

Hematogenous route

Originates in bloodstream

Seeding from a distant infection

Less common; Favors selected host settings

Slide 16 · Ascending infection is a multistep process

Colonize distal urethra

Enter the bladder

Multiply if not cleared

Reach ureter / pelvis

Enter renal tissue

The steps explain why bladder infection is commoner than renal infection.

Slide 17 · Initial colonization occurs at the distal urethra

Enteric coliforms can colonize the urethra and female introitus.

Bacterial adhesion influences persistence at these sites.

This step precedes invasion of the bladder in the usual ascending pathway.

Slide 18 · P-fimbriae are a microbial attachment apparatus

Fimbrial tip adhesin

Urothelial receptor binding

Resistance to washout

Colonization

Surface recognition is a plausible mechanistic target—not simply passive contamination.

Slide 19 · Bladder entry can follow urethral instrumentation

Catheterization and instrumentation provide a route into the bladder.

Long-term urinary catheterization markedly changes the exposure context.

The source identifies catheterization as a common predisposing circumstance.

Slide 20 · Why sex influences UTI frequency

Predisposing anatomy / physiology

Shorter female urethra

Shorter route from periurethral organisms to the bladder

Consequence for ascent

Absence of antibacterial prostatic secretions in females

Loss of one local antibacterial influence

Hormonal changes and intercourse-related urethral trauma

May alter adherence or bacterial access

Slide 21 · Voiding is a major anti-infective mechanism

Bacteria enter bladder

Effective voiding

Mechanical washout

Reduced persistence

Retention of residual urine interrupts the clearance mechanism.

Slide 22 · Urinary stasis favors bacterial proliferation

Obstructed outflow

Incomplete emptying

Residual urine

Bacterial multiplication

Obstruction increases risk by altering urinary flow, not by introducing a new organism.

Slide 23 · Anatomic causes of obstruction and stasis

Abnormality

Pathogenetic effect

Benign prostatic enlargement

Bladder outlet resistance / residual urine

Tumor

Mechanical urinary outflow obstruction

Calculus

Luminal obstruction

Urinary-tract structural lesion

Interruption of normal passage and emptying

Slide 24 · Functional stasis: Neurogenic bladder

Diabetes or spinal cord injury

Neurogenic dysfunction / atony

Incomplete emptying

Residual infected urine

A functional emptying defect can mimic the microbial consequences of obstruction.

Slide 25 · Normal vesicoureteral junction versus reflux

Competent ureteric valve

Oblique intravesical ureter is compressed

Raised bladder pressure does not drive urine into ureter

Flow is predominantly one-way

Incompetent ureteric valve

Shortened or incompetent intramural segment

Bladder urine can move retrograde into ureter

Ascending exposure to the renal pelvis

Slide 26 · How vesicoureteral reflux develops

Valve incompetence

Rise in bladder pressure

Retrograde urine flow

Renal pelvic exposure

Reflux delivers bladder contents upward during filling or voiding.

Slide 27 · Primary reflux commonly reflects abnormal ureteric insertion

Congenital deficiency or shortening of the intravesical ureter weakens the anti-reflux mechanism.

During micturition the intramural ureter may fail to compress adequately.

Affected children are especially important because recurrent injury can create polar scars.

Slide 28 · Acquired reflux can accompany bladder disease

Bladder infection and inflammation may affect the vesicoureteral junction.

Altered ureteral contractility is proposed as a mechanism, especially in children.

This mechanism is described as a postulate, not a universal rule.

Slide 29 · Voiding cystourethrography demonstrates reflux

Bladder filled with contrast

Imaging during micturition

Contrast moves into ureters

Reflux becomes visible

The test documents retrograde movement of contrast; It does not by itself prove active renal infection.

Slide 30 · Intrarenal reflux extends beyond the pelvis

Retrograde movement of infected urine

Renal parenchyma

Papillary ducts

Renal pelvis

Refluxing ureter

Bladder urine

Read the mechanism upward: Bladder → ureter → renal parenchyma

Open collecting ducts at the papillary tip can transmit infected urine into kidney tissue.

Slide 31 · Papillary anatomy creates polar vulnerability

Upper and lower pole papillae are often relatively flat or concave.

This shape favors open ductal orifices and intrarenal reflux.

Midzonal convex, pointed papillae are relatively less susceptible.

Slide 32 · Why pyelonephritic scars are often polar

Vesicoureteral reflux

Intrarenal papillary reflux

Upper / lower pole injury

Irregular corticomedullary scar

The distribution of chronic scars reflects the route and site of ascending exposure.

Slide 33 · Stasis and reflux are different but complementary

Urinary stasis

Increases bacterial residence time

Promotes bacterial growth in residual urine

Examples: Obstruction, atony

Vesicoureteral reflux

Permits retrograde flow toward kidney

Moves infected bladder urine to renal pelvis

Often congenital; May be acquired

Slide 34 · Most lower-tract infections do not become pyelonephritis

The source emphasizes that infection often remains in the bladder without reflux.

Repeated bacteriuria may produce cystitis or urethritis alone.

A barrier failure or predisposing condition helps explain renal extension.

Slide 35 · Hematogenous renal infection bypasses the lower tract

Distant focus of infection

Bacteremia / viremia / fungemia

Renal vascular seeding

Renal inflammation

Bloodstream spread is less common than ascending infection.

Slide 36 · Infective endocarditis is an example of hematogenous seeding

Bacteria from a distant focus can enter the bloodstream.

The vascular route can expose both kidneys to circulating microbes.

Debilitated hosts and urinary obstruction increase susceptibility.

Slide 37 · Immunosuppression changes the etiologic spectrum

Renal transplant recipients and other immunocompromised hosts may develop nonenteric infections.

Robbins mentions staphylococci, certain fungi and viruses.

The host setting matters when interpreting infectious renal lesions.

Slide 38 · Ascending and hematogenous infection: Clues

Feature

Ascending

Hematogenous

Primary starting site

Bladder / urethra

Bloodstream

Usual frequency

Predominant

Less frequent

Typical predisposing factor

Reflux or urinary stasis

Distant infection, host debility

Renal exposure

Pelvis → parenchyma

Vascular delivery

Slide 39 · Clinical risks converge on three mechanisms

Greater bacterial access

Reduced urinary clearance

Retrograde / vascular delivery

Kidney infection

Anatomy, instrumentation, comorbidity and microbial virulence contribute at different steps.

Slide 40 · Pregnancy can increase UTI susceptibility

Pregnancy as a clinical setting associated with infection.

Bacteriuria during pregnancy may progress to symptomatic infection.

This teaching section focuses on predisposition; It does not specify screening or drug protocols.

Slide 41 · Diabetes predisposes through more than one pathway

Increased susceptibility

Bladder dysfunction

Urinary stasis

Higher risk of complicated infection

Diabetes also increases concern for severe complications such as papillary necrosis.

Slide 42 · Age changes the predominant risk factors

In young children, congenital urinary abnormalities are important.

After infancy through early adulthood, UTIs are more frequent in females.

In older males, prostatic enlargement and instrumentation become important.

Slide 43 · Pyelonephritis becomes clinically apparent when the kidney is inflamed

Flank or costovertebral-angle pain with fever and malaise supports upper-tract involvement.

Irritative bladder symptoms can coexist.

The clinical syndrome reflects both local tissue injury and systemic inflammation.

Symptoms alone do not document the microbiologic cause.

Slide 44 · Local irritation and systemic inflammation

Lower-tract manifestations

Dysuria

Increased urinary frequency

Urgency

Renal / systemic manifestations

Costovertebral-angle pain

Fever

Malaise

Slide 45 · The classic onset of acute pyelonephritis is abrupt

The patient commonly develops fever and costovertebral-angle pain.

Malaise supports the presence of a systemic inflammatory response.

Cystitis-associated urinary symptoms may be present simultaneously.

Slide 46 · Pyuria demonstrates inflammation—but not its location

Leukocytes in the urine occur in lower and upper tract infection.

Pyuria arises from the inflammatory response.

The finding cannot independently distinguish cystitis from pyelonephritis.

Localization requires additional clinical and laboratory context.

Slide 47 · Leukocyte casts implicate renal tubules

Neutrophils enter tubular lumen

A leukocyte cast forms within a renal tubule

Urinary leukocyte cast is excreted

Casts are formed within renal tubules, providing a localization clue unavailable from isolated pyuria.

Slide 48 · Urine culture establishes the microbiologic diagnosis

The source identifies culture as the method establishing urinary infection.

Interpret organism recovery with the symptoms and site of disease.

Pyuria alone cannot identify the causative microbe.

Slide 49 · Interpret urinary findings by what they prove

Finding

Supported interpretation

Limitation

Bacteriuria

Organisms present in urine

May be asymptomatic

Pyuria

Urinary tract inflammation

Does not localize upper vs lower

Leukocyte casts

Renal tubular inflammatory involvement

Not an organism identification test

Urine culture

Microbiologic evidence of infection

Needs clinical interpretation

Slide 50 · Acute pyelonephritis is a suppurative renal lesion

Neutrophils infiltrate tubules and interstitium, with associated tubular injury.

The process is often patchy.

Infected tubular lumens can provide conduits for extension.

Focal abscesses may form when the interstitial reaction becomes destructive.

Slide 51 · Acute gross lesions are often patchy

Focal bacterial involvement

Neutrophilic inflammation

Cortical abscesses

Wedge-shaped suppurative areas

Schematic progression; Actual gross pathology requires authentic specimen evidence.

Slide 52 · Intratubular neutrophils: The defining microscopic clue

Neutrophils accumulate within tubular lumens during acute renal infection.

Tubular lumen

Neutrophil aggregates

Injured epithelium

Schematic microanatomy — not an H&E photomicrograph.

Slide 53 · Neutrophilic tubulitis indicates epithelial involvement

Neutrophils can cross or permeate injured tubular epithelium.

Tubular lumen

Neutrophil aggregates

Injured epithelium

Tubular wall inflammation links microbial infection to structural damage. Schematic only.

Slide 54 · Interstitium can develop a suppurative abscess

Intraluminal infection

Tubular epithelial injury

Interstitial spread

Localized tissue destruction

An abscess represents more than colonization: It is destructive parenchymal inflammation.

Slide 55 · Glomeruli are initially relatively resistant

Early acute pyelonephritis as predominantly tubular and interstitial.

Glomeruli are often relatively spared in less extensive disease.

Destructive infection can eventually involve glomeruli when severe.

Slide 56 · Papillary necrosis: A severe complication

Predisposition

Morphologic consequence

Diabetes

Ischemic injury affecting renal papillae

Sickle cell disease

Susceptibility of medullary/papillary tissue

Urinary obstruction

Compromised medullary and papillary environment

Pathologic appearance

Coagulative necrosis of papillary tissue

Slide 57 · Pyonephrosis means pus cannot drain

High-grade urinary obstruction

Acute suppurative infection

Pus trapped in collecting system

Pelvis and calyces distend

Obstructed infected urine is a distinct anatomic complication of pyelonephritis.

Slide 58 · Perinephric abscess crosses the capsule

Suppurative renal focus

Capsular extension

Perinephric tissue infection

The site of disease is no longer confined to renal parenchyma.

Slide 59 · Healing can convert acute inflammation into scar

Neutrophilic phase

Macrophages / lymphocytes

Fibrosis and tubular atrophy

Irregular cortical depression

Persistent or recurrent injury can permanently distort the renal architecture.

Slide 60 · Chronic pyelonephritis has a calyceal fingerprint

Chronic tubulointerstitial injury is accompanied by scarring and deformation of the calyces and pelvis.

The renal surface is often irregularly scarred.

The calyx deep to the scar may be blunted or distorted.

This anatomic pairing strongly supports reflux-related disease.

Slide 61 · Scarring pattern helps classify chronic injury

Chronic pyelonephritis

Coarse, uneven, discrete scars

Often polar and asymmetric

Underlying calyceal abnormalities

Chronic glomerulonephritis

More diffusely and symmetrically contracted kidneys

Primary glomerular process

Not defined by focal calyceal clubbing

Slide 62 · Thyroidization represents chronically damaged tubules

Tubular atrophy / dilation

Flattened tubular epithelium

Proteinaceous luminal casts

Thyroid-like histologic pattern

This pattern must be interpreted in an appropriate tubulointerstitial context.

Slide 63 · Chronic scarring can impair tubular function

Loss of urine-concentrating capacity may cause polyuria and nocturia.

Progressive parenchymal loss can reduce renal function.

Hypertension may accompany chronic reflux-associated renal damage.

Slide 64 · Reflux nephropathy can begin in childhood

Abnormal antireflux mechanism

Intrarenal reflux

Recurrent renal injury

Polar scars and later dysfunction

Underlying reflux and scarring can be present even without overt repeated acute episodes.

Slide 65 · Acute and chronic pyelonephritis: Pattern comparison

Feature

Acute

Chronic

Dominant cell response

Neutrophils

Chronic mononuclear infiltrate

Tubules

Tubulitis and luminal pus

Atrophy, dilatation, thyroidization

Gross pattern

Patchy suppurative foci

Irregular corticomedullary scars

Collecting system

Active inflammation

Calyceal deformity and blunting

Slide 66 · Not every tubulointerstitial lesion is infectious

Pattern

Useful discriminating clue

Pyelonephritis

Infectious context; Neutrophilic tubulitis / pelvic involvement

Drug-induced interstitial nephritis

Medication exposure; Variable eosinophils

Other chronic interstitial lesions

May scar tubules without characteristic reflux-pattern calyceal deformity

Analgesic nephropathy

Also may involve papillae and calyces

Slide 67 · Clinical case 1: Where is the infection?

A patient has dysuria and urinary frequency. Urine contains many leukocytes, but there is no reported flank pain or fever.

Does pyuria alone establish acute pyelonephritis?

A. Yes—pyuria proves kidney involvement.

B. No—pyuria does not distinguish upper from lower UTI.

Slide 68 · Case 1: Pyuria is not anatomically specific

No. Pyuria can arise from cystitis or pyelonephritis.

Look for renal symptoms and systemic inflammatory features.

A leukocyte cast, if present, supports intrarenal involvement.

Culture is important for identifying the infectious agent.

Slide 69 · Clinical case 2: Recurrent infection in a child

A child has repeated bacterial UTIs and imaging shows retrograde movement of contrast from the bladder into a ureter during voiding.

What mechanism increases the risk of renal scarring?

A. Vesicoureteral and intrarenal reflux

B. Isolated glomerular immune-complex deposition

Slide 70 · Case 2: Reflux carries infection toward the poles

Valve incompetence permits ascending exposure of the renal pelvis.

Intrarenal reflux transmits infected urine through papillary ducts.

Flattened or concave polar papillae are more susceptible.

Repeated injury can generate upper- and lower-pole scars.

Slide 71 · Clinical case 3: Infected obstructed system

An adult with urinary outflow obstruction develops severe upper urinary tract infection. The renal pelvis and calyces contain purulent material that cannot drain.

Which complication does this describe?

A. Pyonephrosis

B. Isolated asymptomatic bacteriuria

Slide 72 · Case 3: Pyonephrosis is suppuration behind an obstruction

An obstructed collecting system fills with purulent exudate.

The fundamental combination is obstruction plus infection.

The lesion involves the pelvis/calyces, not simply bladder colonization.

It is distinguished from perinephric abscess, which extends beyond the capsule.

Slide 73 · Five principles to retain

Pyelonephritis localizes to tubules, interstitium and renal pelvis.

The dominant pathway is ascending infection by enteric organisms.

Urinary stasis and reflux determine which infections reach the kidney.

Neutrophilic tubulitis and leukocyte casts are strong renal clues.

Reflux-related chronic injury creates irregular scars with calyceal deformation.

Slide 74 · Textbook reference

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th edition. Elsevier, 2026.

Relevant textbook sections: Pyelonephritis and Urinary Tract Infection; Acute Pyelonephritis; Chronic Pyelonephritis and Reflux Nephropathy.