Enterobacterales

Escherichia coli

Gram-negative bacilli

Identification & Growth

Identification

Oxidase-negative, indole-positive, lactose-fermenting
TSI A/A with gas; IMViC ++--

Growth Conditions

MacConkey agar: pink colonies (lactose-positive)
EMB agar: green metallic sheen; CLED agar: yellow colonies
35–37 °C, aerobic incubation, 18–24 h

Clinical Significance

Leading cause of both outpatient and hospital-acquired urinary tract infection
Bacteremia of urinary/abdominal source, peritonitis, diarrhea (diarrheagenic pathotypes)

Intrinsic Resistance

Penicillin G, macrolides, clindamycin, glycopeptides, linezolid, daptomycin, fusidic acid (EUCAST Expected Resistant Phenotypes v1.2)

Bench Alerts

ESBL: resistance to cefotaxime/ceftriaxone/ceftazidime — report cephalosporins as R and avoid their use
Carbapenemase (KPC/NDM): a reduced meropenem zone requires confirmatory testing and notification to infection control

Clinical Notes

Clinical presentations

The leading cause of urinary tract infection, both uncomplicated and complicated, in outpatient and hospital settings.
Causes bacteremia frequently originating from a urinary or abdominal focus, including cholangitis and peritonitis.
Diarrheagenic strains (ETEC, EPEC, EHEC/STEC, EIEC, EAEC) cause gastroenteritis, with EHEC associated with hemolytic-uremic syndrome.
A common cause of neonatal meningitis, especially strains carrying the K1 capsular antigen.

Specimens and collection

Midstream urine processed within 2 hours (or refrigerated) is the standard specimen for UTI diagnosis.
Stool for STEC/EHEC investigation should be plated on sorbitol-selective agar (sorbitol-MacConkey) and tested for Shiga toxin.
Blood cultures should be drawn before empiric antibiotic therapy in sepsis of probable urinary or abdominal origin.

Epidemiology and at-risk populations

The main pathogen of community-acquired UTI in women, related to the anatomic proximity between urethra and rectum.
Extended-spectrum beta-lactamase (ESBL)-producing strains are increasingly prevalent in the community, not only in hospital settings.
EHEC outbreaks are associated with undercooked meat, unpasteurized milk, and contaminated vegetables, with elevated risk in children.

Resistance and therapeutic implications

ESBL production confers resistance to penicillins and broad-spectrum cephalosporins, generally requiring carbapenem use.
Carbapenemase-producing isolates (KPC, NDM, OXA-48-like) represent a critical therapeutic threat and require phenotypic/molecular confirmation.
Fluoroquinolone and trimethoprim-sulfamethoxazole resistance is increasing and should inform local empiric choice based on institutional antibiogram.

Bench and reporting notes

Gram-negative rod, oxidase-negative, lactose-fermenting on MacConkey agar, indole-positive, guides rapid presumptive identification.
Phenotypic ESBL screening (combination disk with clavulanic acid) should be performed on isolates with elevated MIC to third-generation cephalosporins.
Reports of fecal isolates suspicious for STEC should explicitly mention hemolytic-uremic syndrome risk and avoid suggesting empiric antibiotic therapy.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Escherichia coli in the lab?

Escherichia coli is identified through oxidase-negative, indole-positive, lactose-fermenting, tsi a/a with gas; imvic ++--.

What are the intrinsic resistances of Escherichia coli?

This organism is naturally resistant to penicillin g, macrolides, clindamycin, glycopeptides, linezolid, daptomycin, fusidic acid (eucast expected resistant phenotypes v1.2). These drugs should not be reported as susceptible.

Where is Escherichia coli commonly found?

It is typically associated with leading cause of both outpatient and hospital-acquired urinary tract infection, bacteremia of urinary/abdominal source, peritonitis, diarrhea (diarrheagenic pathotypes).

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