MicrobiologyTier 1Medical Sciences concept

Urinary tract pathogens - nitrite reduction and biochemical identification

Core concept

Urinary nitrite = bacterial nitrate reductase converting dietary nitrate -> nitrite

Nitrite-POSITIVE	Nitrite-NEGATIVE despite bacteriuria
Enterobacterales: E. coli, Klebsiella, Proteus, Enterobacter, Citrobacter, Serratia	Enterococcus faecalis
	Staph saprophyticus
	Pseudomonas aeruginosa (a non-fermenter)
	Streptococci, Candida

Three requirements for a positive nitrite - all must hold
- A nitrate-reducing organism
- Dietary nitrate present
- Urine held in the bladder >=4 hours (first-morning sample is best)
Nitrite is highly SPECIFIC (~95%+) but poorly SENSITIVE (~50%)
- A positive nitrite rules IN bacteriuria; a negative does NOT rule it out
Leukocyte esterase - the converse: more sensitive, less specific; positive in sterile pyuria too

Biochemical identification

Test	Positive	Discriminates
Urease	Proteus, Morganella, Providencia, Ureaplasma, Klebsiella (some), Staph saprophyticus	Alkaline urine, struvite stones
Lactose fermentation (MacConkey)	E. coli, Klebsiella, Enterobacter (pink)	vs Salmonella, Shigella, Proteus, Pseudomonas (non-lactose, pale)
Oxidase	Pseudomonas, Neisseria, Vibrio, Campylobacter, Helicobacter	Enterobacterales are oxidase-NEGATIVE
Indole	E. coli	vs Klebsiella (negative)
Coagulase	Staph aureus	vs coagulase-negative staphylococci
Novobiocin	Staph saprophyticus RESISTANT	vs Staph epidermidis sensitive
Swarming motility on agar	Proteus mirabilis	Characteristic
Green pigment (pyocyanin), grape-like odour	Pseudomonas aeruginosa

Organism by clinical context

Context	Likely organism
Uncomplicated community UTI	E. coli ~75-85%
Young sexually active woman	Staph saprophyticus (2nd commonest)
Alkaline urine, staghorn calculus	Proteus mirabilis
Catheter, recent hospital, recurrent instrumentation	Pseudomonas, Enterococcus, Klebsiella, Candida, ESBL producers
Pregnancy	E. coli; group B streptococcus is significant at any count
Sterile pyuria	TB, chlamydia/gonorrhoea, partially treated UTI, interstitial nephritis, stones, tumour, analgesic nephropathy

Dipstick has value only when negative in a low-risk patient - a negative nitrite AND negative leukocyte esterase makes UTI unlikely in a woman with mild symptoms; in the elderly, dipstick is close to useless because asymptomatic bacteriuria is so common
Do NOT treat asymptomatic bacteriuria - exceptions: pregnancy and before urological procedures breaching the mucosa
- Antibiotics for asymptomatic bacteriuria in an older person with delirium generate resistance and C. difficile without improving outcomes
Australian empirical therapy (Therapeutic Guidelines)
- Uncomplicated cystitis: trimethoprim 300 mg nocte for 3 days, or nitrofurantoin 100 mg 6-hourly for 5 days; nitrofurantoin needs adequate renal function and does not treat pyelonephritis
- Acute pyelonephritis: IV gentamicin plus amoxicillin (or ceftriaxone), then oral by susceptibility
- Enterococcus -> amoxicillin; Pseudomonas -> ciprofloxacin or an antipseudomonal beta-lactam
Culture before antibiotics in pyelonephritis, recurrent UTI, pregnancy, men, catheters and treatment failure

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