Antibiotic choice in urology is governed by a principle that does not apply elsewhere in medicine: antimicrobials are excreted in the urine at concentrations far higher than in serum. This makes urinary drug levels — not serum levels — the key driver of cure in uncomplicated infection, while serum levels remain critical whenever the kidney, prostate, or bloodstream is involved.
Principles of Antibiotic Therapy
Selecting empirical therapy depends on whether the infection is complicated or uncomplicated, the drug's spectrum against the probable pathogen, hypersensitivity history, side-effect profile, and cost. Duration depends on the extent and duration of tissue invasion, the bacterial and achievable antimicrobial concentration in urine, and host risk factors that impair natural defences.
Urine versus serum concentration:
- Resolution of infection tracks the susceptibility of the organism to the urinary concentration of the drug, but susceptibility testing is based on serum concentrations.
- Some agents never reach adequate serum levels for bacteraemia yet are effective at their achievable urinary concentration — e.g. E. coli may test resistant to amoxicillin while amoxicillin still clears urinary E. coli because of the high urine concentrations achieved.
- Blood concentration is unimportant in uncomplicated UTI but critical in bacteraemia and febrile infection with renal or prostatic parenchymal involvement.
Renal function:
- Dose modification is required for renally cleared agents: ciprofloxacin, nitrofurantoin, trimethoprim/sulfamethoxazole, trimethoprim, amoxicillin, piperacillin/tazobactam, cephalexin, cefuroxime, levofloxacin, clarithromycin, and tetracycline.
- In renal failure the kidney may fail to concentrate the drug in the urine, making eradication difficult. Urinary tract obstruction can likewise reduce urinary drug concentration.
Bacterial resistance — three mechanisms:
- Inherited chromosomal — the species lacks the target the drug acts on (e.g. Proteus and Pseudomonas are always resistant to nitrofurantoin).
- Acquired chromosomal — induced by exposure to antimicrobial agents.
- Extrachromosomal (plasmid-mediated) — acquired and transferable via plasmids ("R-factor" resistance). It arises in the bowel flora and is far more common than selection of pre-existing mutants in the urinary tract. All classes can produce plasmid-mediated resistance, but it is rare with fluoroquinolones and has never been reported for nitrofurantoin.
Because bowel flora is the major reservoir for uropathogens, infections after antibiotic therapy are commonly multidrug-resistant — yet resistant E. coli from bowel flora almost always remain susceptible to nitrofurantoin or the quinolones. Resistance is also influenced by the duration and amount of antibiotic used.
Mechanism of Action
| Drug or class | Mechanism of action | Mechanism of resistance |
|---|---|---|
| β-Lactams (penicillins, cephalosporins, carbapenems, aztreonam) | Inhibit bacterial cell-wall synthesis | β-lactamase production; altered penicillin-binding protein; reduced porin size (decreased penetration) |
| Vancomycin | Inhibits cell-wall synthesis (binds D-Ala-D-Ala) | Enzymatic alteration of peptidoglycan at a point other than the target |
| Fosfomycin | Inhibits cell-wall synthesis (inactivates MurA/enolpyruvyl transferase) | Novel amino-acid substitutions or loss of transporter function |
| Aminoglycosides (gentamicin, tobramycin) | Inhibit ribosomal protein synthesis | Downregulated drug uptake; aminoglycoside-modifying enzymes |
| Clindamycin, macrolides (erythromycin, clarithromycin, azithromycin) | Inhibit ribosomal protein synthesis (50S subunit) | Ribosomal methylation; efflux |
| Quinolones (ciprofloxacin, levofloxacin) | Inhibit bacterial DNA gyrase | Mutation in the DNA-gyrase binding site; reduced porin size; active efflux |
| Trimethoprim-sulfamethoxazole | Competitive inhibition of dihydrofolate reductase | Draws folate from the environment (enterococci) |
| Nitrofurantoin | Inhibits several bacterial enzyme systems | Not fully elucidated; develops slowly with prolonged exposure |
Spectrum of Coverage
| Agent or class | Gram-positive | Gram-negative |
|---|---|---|
| Amoxicillin / ampicillin | Streptococcus, enterococci | Proteus mirabilis |
| Amoxicillin-clavulanate | Streptococcus, enterococci | Proteus, Klebsiella |
| Ampicillin-sulbactam | Staphylococcus (not MRSA), enterococci | Proteus, Klebsiella, H. influenzae |
| Anti-staphylococcal penicillins (methicillin, nafcillin, oxacillin, cloxacillin, dicloxacillin) | Streptococcus, Staphylococcus (not MRSA); not enterococci | None |
| Anti-pseudomonal penicillins (piperacillin, ticarcillin) | Streptococcus, enterococci | Most, including Pseudomonas |
| 1st-gen cephalosporins (cefazolin, cephalexin) | Streptococcus, Staphylococcus (not MRSA) | E. coli, Proteus, Klebsiella |
| 2nd-gen cephalosporins (cefamandole, cefuroxime, cefaclor) | Streptococcus, Staphylococcus (not MRSA) | E. coli, Proteus, Klebsiella, H. influenzae |
| 2nd-gen cephalosporins (cefoxitin, cefotetan) | Streptococcus | E. coli, Proteus (incl. indole-positive), Klebsiella, H. influenzae |
| 3rd-gen cephalosporins (ceftriaxone) | Streptococcus, Staphylococcus (not MRSA) | Most, excluding P. aeruginosa |
| 3rd-gen cephalosporins (ceftazidime) | Streptococcus | Most, including P. aeruginosa |
| Aztreonam | None | Most, including P. aeruginosa |
| Aminoglycosides (gentamicin, tobramycin) | Staphylococcus (urine) | Most, including P. aeruginosa |
| Fluoroquinolones (e.g. ciprofloxacin) | Streptococcus (agent-dependent); not enterococci | Most, including P. aeruginosa |
| Nitrofurantoin | Staphylococcus (not MRSA), enterococci | Many Enterobacteriaceae (not Klebsiella, Proteus); does not cover P. aeruginosa, Providencia, Serratia, Acinetobacter |
| Fosfomycin | Enterococci; variable against S. saprophyticus | Most Enterobacteriaceae (variable against Klebsiella, Enterobacter); does not cover P. aeruginosa |
| Pivmecillinam | None | Most, excluding P. aeruginosa |
| Trimethoprim-sulfamethoxazole | Streptococcus, Staphylococcus; not enterococci | Most Enterobacteriaceae; does not cover P. aeruginosa |
| Vancomycin (option in penicillin allergy) | All, including MRSA | None |
| Clindamycin (option in penicillin allergy) | Streptococcus, Staphylococcus; not enterococci | Anaerobes; not Enterobacteriaceae |
| Carbapenems (ertapenem, imipenem, meropenem) | Broad | Broad — ertapenem has weak Pseudomonas coverage compared with meropenem |
Adverse Reactions and Contraindications
- Aminopenicillins (amoxicillin, ampicillin, ampicillin-sulbactam): hypersensitivity (immediate or delayed), diarrhoea/GI upset, pseudomembranous colitis, a non-allergic maculopapular rash, decreased platelet aggregation. Rash risk rises with concurrent viral illness or allopurinol. Clavulanate adds diarrhoea/GI upset.
- Anti-staphylococcal penicillins: as above, plus acute interstitial nephritis (especially methicillin).
- Anti-pseudomonal penicillins: as above, plus hypernatraemia (sodium-salt load — caution in sodium-sensitive patients) and injection-site reactions.
- Cephalosporins: hypersensitivity (less than penicillins), GI upset, pseudomembranous colitis, positive Coombs test, decreased platelet aggregation (cefotetan, cefamandole, cefoperazone). Avoid with immediate penicillin hypersensitivity; ceftriaxone is contraindicated in neonates.
- Aztreonam: <1% cross-reactivity in penicillin/cephalosporin-allergic patients — usable with caution.
- Aminoglycosides: ototoxicity (vestibular and auditory), nephrotoxicity (non-oliguric azotaemia), neuromuscular blockade at high levels. Avoid in pregnancy except for pyelonephritis; caution in renal impairment, diabetes, hepatic failure, myasthenia gravis, and with other oto-/nephrotoxic drugs.
- Fluoroquinolones: tendon rupture and aortic rupture (each ~20/100,000 — stop at first sign of tendon pain), QT prolongation, photosensitivity, CNS effects (dizziness, tremor, confusion, mood change, hallucinations), and dysglycaemia with anti-diabetic agents.
- Fosfomycin: headache, GI upset, vaginitis.
- Pivmecillinam: rash, GI upset; caution in penicillin hypersensitivity.
- Nitrofurantoin: pulmonary hypersensitivity (acute cough/dyspnoea/fever through to chronic interstitial fibrosis), hepatotoxicity, peripheral neuropathy (worse with renal impairment, anaemia, diabetes, electrolyte imbalance, B-vitamin deficiency), GI upset, and haemolysis in G6PD deficiency. Avoid if renal function is <50 mL/min and avoid concomitant probenecid, magnesium, or quinolones.
- Trimethoprim-sulfamethoxazole: hypersensitivity/rash, GI upset, photosensitivity, haematologic toxicity (higher in AIDS patients and the elderly).
Antibiotics by Class
Aminopenicillins
Resistance now affects 40–60% of common urinary isolates (see Toronto antibiograms), limiting ampicillin/amoxicillin. Their effect on bowel and vaginal flora predisposes to reinfection with resistant strains and to Candida vaginitis. Adding clavulanate improves activity against β-lactamase producers but is costly with frequent GI effects. Extended-spectrum derivatives (pivmecillinam, piperacillin, mezlocillin, azlocillin) keep anti-enterococcal activity and add many ampicillin-resistant gram-negatives. Safe in pregnancy.
Cephalosporins
As a group, activity is high against Enterobacteriaceae and poor against enterococci. First-generation agents favour gram-positives plus E. coli and K. pneumoniae; second-generation adds anaerobe activity; third-generation is the most reliable against community-acquired and nosocomial gram-negatives. Reserve broad-spectrum agents for complicated infections, parenteral need, or likely resistance. Safe in pregnancy; ceftriaxone contraindicated in neonates.
Nitrofurantoin
Effective against common uropathogens but not Pseudomonas or Proteus. Rapidly excreted in urine but does not reach therapeutic tissue levels — so it is not used for upper-tract or complicated infection. It has minimal effect on bowel/vaginal flora (good for prophylaxis), and acquired resistance is exceedingly low.
- Pregnancy (ACOG 2017): in the first trimester, discuss benefits versus the unknown risks; nitrofurantoin or sulfonamides remain appropriate when no suitable alternative exists. In the second and third trimesters they can be first-line. Contraindicated in G6PD deficiency.
- Pregnancy (Campbell's): safe in the first and second trimester in patients without G6PD deficiency; discontinue at 35 weeks because of neonatal haemolytic-anaemia risk.
Trimethoprim-sulfamethoxazole
Effective against most common uropathogens but not Enterococcus or Pseudomonas. TMP alone is as effective as the combination for most uncomplicated infections with fewer side effects, but adding SMX gives synergistic bactericidal activity for upper-tract infection, reaches therapeutic tissue levels, and may slow resistance. It is inexpensive with minimal effect on bowel flora; main downsides are rash and GI upset. TMP blocks tubular secretion of creatinine, so serum creatinine rises while GFR is unchanged. Avoid TMP-SMX in pregnancy (early teratogenicity, late kernicterus); avoid TMP alone in pregnancy (megaloblastic anaemia), though TMP alone is acceptable in neonates.
Fosfomycin
Effective against most uropathogens — including the majority of gram-negatives and vancomycin-resistant Enterococcus — but not Pseudomonas. There is limited cross-resistance with other classes. It is effective as a single-dose empirical treatment for uncomplicated cystitis and is generally well tolerated.
Fluoroquinolones
Broad spectrum, highly effective against Enterobacteriaceae and P. aeruginosa, with good activity against S. aureus and S. saprophyticus but only marginal anti-streptococcal and modest anti-enterococcal coverage. Anaerobes are resistant, so vaginal/bowel flora are spared. Resistance is rising from overuse. Not nephrotoxic, but renal impairment prolongs the half-life — dose-adjust if creatinine clearance <30 mL/min. Contraindicated in children, adolescents, and pregnant/nursing women (cartilage damage). Interactions: may enhance warfarin; antacids (Mg/Al), iron, zinc, and sucralfate markedly reduce oral absorption; enoxacin and ciprofloxacin raise theophylline levels; avoid other QT-prolonging drugs such as amiodarone.
Surgical Prophylaxis
Surgical Wound Classification
- Clean — uninfected, no inflammation, no entry into the genital, urinary, or alimentary tract.
- Clean-contaminated — uninfected, controlled entry into those tracts.
- Contaminated — major break in sterile technique (gross GI spillage or non-purulent inflammation).
- Dirty — pre-existing clinical infection or perforated viscus.
Host Risk Factors for Post-Operative Infection
Advanced age, anatomic anomalies, poor nutritional status, smoking, chronic corticosteroid use, immunodeficiency, chronic indwelling hardware, infected endogenous/exogenous material, distant coexistent infection, and prolonged hospitalisation.
Recommended Regimens
| Procedure | Antibiotic | Alternative | Duration |
|---|---|---|---|
| Cystoscopy with minor manipulation | If risk factors: TMP-SMX or amoxicillin/clavulanate | 1st/2nd-gen cephalosporin; aminoglycoside ± ampicillin; aztreonam ± ampicillin | Single dose |
| Transurethral resection | Cefazolin or TMP-SMX | Amoxicillin/clavulanate; aminoglycoside ± ampicillin; aztreonam ± ampicillin | Single dose |
| Transrectal prostate biopsy | Fluoroquinolone or 1st/2nd-gen cephalosporin ± aminoglycoside or 3rd-gen cephalosporin | Aztreonam (may need ID consult) | Single dose |
| Percutaneous renal surgery | 1st/2nd-gen cephalosporin or aminoglycoside + (clindamycin or metronidazole) or aztreonam + (metronidazole or clindamycin) | Ampicillin/sulbactam | ≤24 hours |
| Ureteroscopy | 1st/2nd-gen cephalosporin or TMP-SMX | Aminoglycoside ± ampicillin; aztreonam ± ampicillin; amoxicillin/clavulanate | Single dose |
| Open/lap/robotic — no urinary-tract entry | Cefazolin | Clindamycin | Single dose |
| Open/lap/robotic — controlled urinary-tract entry | Cefazolin or TMP-SMX | Ampicillin/sulbactam; aminoglycoside or aztreonam + (metronidazole or clindamycin) | Single dose |
| Open/lap/robotic — involving small bowel | Cefazolin | Clindamycin + aminoglycoside; cefuroxime; aminopenicillin + β-lactamase inhibitor ± metronidazole | Single dose |
| Implanted prosthetics (AUS, IPP, sacral neuromodulator) | Aminoglycoside or aztreonam + (1st/2nd-gen cephalosporin or vancomycin) | Aminopenicillin or β-lactamase inhibitor (ampicillin/sulbactam, ticarcillin, tazobactam) | ≤24 hours |
Typical doses: TMP-SMX 800/160 mg PO ×1; cefazolin (1st-gen) 2 g IV ×1; ceftriaxone (3rd-gen) 2 g IV ×1; clindamycin 600 mg IV ×1; levofloxacin 500 mg PO ×1 or ciprofloxacin 500 mg PO ×1; gentamicin 2 mg/kg IV ×1; vancomycin 1 g IV ×1; metronidazole 500 mg IV ×1. Intraoperative redosing: cefazolin 2 g q4h, clindamycin 600 mg q8h, gentamicin 1 mg/kg q8h, metronidazole 500 mg q6h.
Catheter Removal and Endocarditis Prophylaxis
Antibiotics after catheter removal do not significantly reduce UTI risk after radical prostatectomy or TURP. A 2021 systematic review and meta-analysis (Liu et al.) of 8 RCTs of prophylaxis after removal of a temporary (≤14-day) catheter — including two laparoscopic radical prostatectomy trials and one TURP trial — found only 2 of 8 studies showed benefit, and none of the three urological trials did (the two positive trials were in abdominal-surgery patients or women with bacteriuria). Overall prophylaxis reduced UTIs (RR 0.47, 95% CI 0.28–0.72), and subgroups most likely to benefit were age >60, those given TMP-SMX, and indwelling catheters >5 days.
Endocarditis risk: the risk of infective endocarditis (IE) after urologic procedures is low, and Enterococcus faecalis is the organism most likely to cause IE following GU bacteraemia. Prophylaxis solely to prevent IE is not recommended (a change from earlier AHA guidance). For patients with high-risk cardiac conditions (prosthetic valve, prior IE, congenital heart disease, cardiac transplant) and an active GU infection/colonisation who are to undergo GU manipulation — including elective cystoscopy — antibiotics to sterilise the urine may be reasonable (Class IIb): amoxicillin/ampicillin first-line for enterococci, vancomycin for penicillin allergy.
Indwelling orthopaedic hardware: prophylaxis is generally not indicated for urologic patients with joint replacements, pins, plates, or screws. It is advised only for those at higher risk of prosthetic-joint seeding — recently inserted implants (within 2 years) and/or the host risk factors above.
Self-Test
1. Give the mechanism of action of each agent.
- TMP-SMX — competitive inhibition of dihydrofolate reductase.
- Nitrofurantoin — inhibits several bacterial enzyme systems.
- Ciprofloxacin — inhibits bacterial DNA gyrase.
- Ampicillin — inhibits bacterial cell-wall synthesis (β-lactam).
- Fosfomycin — inhibits cell-wall synthesis (inactivates MurA/enolpyruvyl transferase).
- Gentamicin — inhibits ribosomal protein synthesis.
2. Which antibiotics should be avoided in patients on warfarin? Fluoroquinolones, TMP-SMX, metronidazole, and ketoconazole (an antifungal, not strictly an antibiotic) — all can potentiate the anticoagulant effect; monitor coagulation closely.