Escherichia coli (E coli) Infections Workup

Updated: Feb 11, 2019
  • Author: Tarun Madappa, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Laboratory Studies

All patients with suspected E coli infection should undergo routine CBC count with differential to evaluate for leukocytosis or a left shift.

Gram stain results determine if the organism is gram-negative, but findings do not distinguish among the other aerobic gram-negative bacilli that cause similar infectious diseases.

E coli is a gram-negative bacillus that grows well on commonly used media. It is lactose-fermenting and beta-hemolytic on blood agar. Most E coli strains are nonpigmented. The image below shows Escherichia coli on Gram staining.

Escherichia coli on Gram stain. Gram-negative bac Escherichia coli on Gram stain. Gram-negative bacilli.

In the image below Escherichia coli can be seen growing on MacConkey agar.

Escherichia coli culture on MacConkey agar. Escherichia coli culture on MacConkey agar.

Definitive diagnosis is based on the isolation of the organism in the microbiology laboratory from clinical specimens. Specimens may be blood, urine, sputum, or other fluids such as cerebrospinal, biliary, abscess, and peritoneal.

Recovery of the organism in contaminated sites, such as sputum and wounds, must be analyzed in the context of the patient's clinical state to determine if it represents colonization or infection. Recovery from sterile sites, such as the CSF, should be considered diagnostic of infection.

Lumbar puncture and a CSF culture positive for E coli establish the diagnosis of acute E coli meningitis; however, lumbar puncture is not justified in all babies presenting with sepsis. Indications for lumbar puncture include positive blood culture results, abnormal neurological signs, and detection of bacterial antigens in the urine.

Patients with pneumonia should undergo blood cultures and sputum Gram stain and culture. The results of a Gram stain of the sputum help to differentiate a good specimen (many PMN leukocytes, few squamous epithelial cells) from a bad specimen (few PMN leukocytes, many squamous epithelial cells). In addition, obtain the sputum culture before antibiotic therapy is initiated.

In enteric infections, the causative organism is suggested based on the clinical presentation and the characteristic of the patient's stool. Enterotoxigenic E coli (ETEC), enteropathogenic E coli (EPEC), enteroaggregative E coli (EAggEC), and enteroadherent E coli (EAEC) infections produce watery stools without inflammatory cells. Enteroinvasive E coli (EIEC) infection produces dysentery-type stools, and enterohemorrhagic E coli (EHEC) infection produces hemorrhagic-type stools.

In urinary tract infections, a urine dipstick test may be performed to rapidly determine if the patient has pyuria or bacteriuria based on the detection of leukocyte esterase and nitrites, respectively. Definitive diagnosis is based on urine culture results. Collect the specimen from a midstream clean void or from the catheter in patients with an indwelling Foley catheter. Colonization must be differentiated from infection based on urinalysis results. In cases of infection, pyuria is usually present.


Imaging Studies

In pneumonia, chest radiography or CT scanning is indicated.

In cholecystitis/cholangitis, ultrasonography or CT scanning of the RUQ is indicated.

In intra-abdominal abscess, abdominal and pelvic CT scanning is indicated; abscesses may be missed on sonograms.

In UTI, ultrasonography or CT scanning may be performed to help evaluate the kidneys and to look for any other source of abscess, stones, or obstruction.


Other Tests

E coli strains that cause diarrhea can be differentiated based on results from tests that are not widely used, such as DNA probes and polymerase chain reaction.

EPEC can be identified based on findings from serotyping, assays of adherence, and DNA probes. These tests are difficult to perform and not available widely. Also, results are difficult to interpret.

EIEC can be identified based on results from animal pathogenicity tests such as the Sereny test.

EHEC can be identified by looking for the major serotype involved, 0157:H7.

EHEC strains are cultured in a sorbitol MacConkey agar. Strains that are sorbitol-negative are then serotyped with 0157:H7 antisera.

EAEC and EAggEC are identified based on their adherence pattern on tissue culture cells. Serotyping is not useful.



The following procedures may be indicated:

  • Meningitis -Lumbar puncture with CSF Gram stain/culture

  • Pneumonia - Bronchoscopy, blood and urine cultures

  • Cholecystitis/cholangitis - Decompression of biliary system through endoscopic drainage, sphincterotomy for stone extraction, or endoscopic cholangiography

  • Intra-abdominal abscess - Aspiration and drainage

  • UTI - In cases of ureteral obstruction, placement of stent or stone extraction

  • Prostatic hypertrophy - Transurethral prostatectomy or transurethral resection of the prostate (TURP)

  • Prostatic abscess - Drainage