Utilization of the Antibiogram in Clinical Practice


Case Exercises

By simulating real world antibiograms and clinical cases, Case Exercises help reinforce scientific principles explained and demonstrated in Modules and translate antibiogram data into clinical decision-making. Please select the case to review.

Case 1. Persistent E. coli Urinary Tract Infection

A 78-year-old man presents to the outpatient clinic of a hospital with signs and symptoms of a lower urinary tract infection. He recently returned from India (visiting friends and relatives [VFR]) where his symptoms developed. He is completing a 7-day course of ciprofloxacin 250 mg PO BID. However, his symptoms have not improved. A urine culture shows E. coli with the following susceptibility profile.

Bacteria Susceptibility (MIC μg/mL)
Ampicillin Cefazolin Ceftriaxone Piperacillin/Tazobactam Gentamicin Ciprofloxacin Trimethoprim/Sulfamethoxazole Imipenem
E. coli R (>32) R (>64) R (>64) S (8/4) R (>32) R (>16) R (>32) S (0.25)


Based on the susceptibility results and knowing that the patient requires outpatient treatment, appropriate alternative agent(s) to test for potential treatment is/are:

  • Amoxicillin/clavulanate
  • Nitrofurantoin
  • Fosfomycin
  • If possible, all of these
Answer & Rationale

The susceptibility results together with the fact the patient recently visited India suggest that the E. coli phenotype is likely:

  • ESBL
  • AmpC
  • NDM-1
  • Efflux
Answer & Rationale

Potential treatments for this patient may include:

  • Amoxicillin/clavulanate
  • IV ertapenem
  • Nitrofurantoin
  • Any of these
Answer & Rationale

Would you perform stool cultures of the entire family returning home from India?

  • Yes
  • No
  • Uncertain at this time
Answer & Rationale

Adapted from Lagace-Wiens, et al. J Antimicrob Chemother. 2006;57:1262-1263. Link

The correct answer is If possible, all of these.

Nitrofurantoin and fosfomycin are effective in urinary tract infections and often retain activity against ESBL and MDR strains of E. coli.

Despite resistance of this isolate to ampicillin and cephalosporins, amoxicillin/clavulanate may still be an appropriate choice given that this is not a life-threatening infection. Evidence shows that amoxicillin/clavulanate retains in vitro activity against some beta-lactamase producers, even those that are ESBL-producers.1 In E. coli, the CTX-M-15 beta-lactamase is readily inactivated by tazobactam and clavulanate (though less so by sulbactam).2 SHV- and TEM-ESBLs are also inactivated by clavulanate.1 Thus, certain ESBL-producers may be susceptible to amoxicillin/clavulanate at achievable urine concentrations provided that additional beta-lactam resistance mechanisms are not present.

  1. Paterson DL, Bonomo RA. Extended-spectrum β-lactamases: a clinical update. Clin Microbiol Rev. 2005;18:657-686. Link
  2. Walther-Rasmussen J, Hoiby N. Cefotaximases (CTX-M-ases), an expanding family of extended-spectrum β-lactamases. Can J Microbiol. 2004;50:137-165. Link

The correct answer is ESBL.

This isolate is resistant to beta-lactams (ampicillin) and third-generation cephalosporins (i.e., ceftriaxone). However, the isolate remains susceptible to carbapenems (imipenem). The concomitant resistance to gentamicin, ciprofloxacin and trimethoprim/sulfamethoxazole by this isolate suggests an ESBL.

The most likely phenotype is ESBL, probably a CTX-M, as this is a common ESBL phenotype for E. coli.

NDM-1 is a metallo-beta-lactamase-producer that exhibits resistance to carbapenems. AmpC hyperproduction is often observed by the SPACE bacteria (in which E. coli is not included). An efflux mutant would require further phenotypic and/or genotypic testing for definitive identification.

The correct answer is Any of these.

Amoxicillin/clavulanate may be considered as second-line treatment for non-life-threatening infections caused by ESBL-producers.

IV ertapenem can also be effective as the isolate is susceptible to carbapenems. However, the IV formulation may not be preferred if the patient is being considered for outpatient treatment.

Nitrofurantoin can also be used as it is effective in urinary tract infections, though it would be useful to have the nitrofurantoin susceptibility results for this isolate.

The correct answer is Uncertain at this time.

Some authorities suggest to culture the stool of the entire immediate family to ascertain the carriage rate of MDR ESBL E. coli or potentially carbapenem-producing E. coli in the family. It remains uncertain whether it is necessary to eradicate the organisms from the stool of asymptomatic individuals as well as how best to accomplish this.