Skip directly to search Skip directly to A to Z list Skip directly to page options Skip directly to site content


This online archive of the CDC Prevention Guidelines Database is being maintained for historical purposes, and has had no new entries since October 1998. To find more recent guidelines, please visit the following:

E. coli O157:H7: Procedure for Isolation and Identification from Stool Specimens

Foodborne and Diarrheal Diseases Branch, Division of Baterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention MS C-09 1600 Clifton Rd Atlanta, GA 30333 To order call: (703)487-4650

Publication date: 08/01/1994

Table of Contents









In Vitro Diagnostic Products and Control Strains
Strains of Escherichia coli O157:H7

Procedure for Isolation and Identification of E.coli O157:H7


The diagnosis of E. coli O157:H7 infection needs to be considered for all patients who present with diarrhea, especially bloody diarrhea or hemolytic uremic syndrome (HUS) (1). Stool specimens (whole stools, swabs prepared from whole stools or rectal swabs with visible fecal staining) should be collected. Ideally, specimens should be collected as close to the time of onset of diarrhea as possible; however, specimens taken even weeks after the onset of symptoms are sometimes positive (2,3). Antibiotic treatment decreases the chance of recovery of E. coli O157:H7; therefore, when follow-up specimens are being obtained, the patient should have received no antibiotic for a minimum of 48 hours before culture. (Figure 1)


Ideally, stool specimens should be examined as soon as they are received in the laboratory. If whole stool specimens will not be processed immediate- ly, they should be either refrigerated or frozen at -70 C as soon as possible after collection. Refrigerated specimens should be examined within 1-2 hours. If stools cannot be examined within this time, they should be placed in transport medium. All rectal swabs should be placed immediately into transport medium. If specimens in transport medium will be examined within 2-3 days, they should be refrigerated. If specimens will not be examined within 3 days, they should be frozen immediately, preferably at -70 C. Specimens should not be refrigerated for days and then frozen, or placed in transport medium and left at room temperature.

If a transport medium will be used, any of the commercially available transport media (e.g., Cary-Blair, Stuart's, Amie's, buffered glycerol saline) are satisfactory. A swab should be completely covered by the transport medium. If the medium does not cover the swab, the swab will not be kept sufficiently moist and recovery of E. coli O157:H7 and other organisms may be compromised.


E. coli O157:H7 rapidly ferments lactose and is indistinguishable from most other E. coli on traditional lactose-containing media. However, unlike approximately 80% of other E. coli, nearly all isolates of E. coli O157:H7 ferment D-sorbitol slowly, or not at all. Sorbitol-MacConkey (SMAC) agar was developed to take advantage of this characteristic by substituting the carbohydrate sorbitol for lactose in MacConkey agar and is the medium of choice for isolation of E. coli O157:H7 (4).

Inoculate stool specimens onto SMAC and incubate 18-24 hours at 35-37C. Sorbitol-negative colonies will appear colorless on SMAC. Test sorbitol- negative colonies selected from SMAC with E. coli O157 antiserum or latex reagents (O157 antibody-coated latex and control latex) according to the procedures recommended by the manufacturer (5). If using O157 latex reagents, it is important to test isolates in the control latex to detect nonspecific agglutination of organisms with latex. Manufacturers of O 157 latex reagents recommend heating strains that agglutinate in the latex control reagent and then retesting them in both the O157 antibody-coated and control latex reagents. However, E. coli O157:H7 strains have not been shown to agglutinate in both the antibody-coated and control latex reagents (6). For this reason, some laboratories report isolates that agglutinate in the latex control as negative for O157 without heating and retesting the isolate.

Colonies may be tested with antisera directly from the plate, or subcultured to another nonselective medium (blood agar, for example) and tested the next day. If colonies are tested directly from the plate, O157- positive colonies should also be transferred to another medium for subsequent testing. Although it is more labor-intensive and delays results by a day, subculturmg to another medium and testing the next day offers the advantage of providing more bacterial growth on which to perform the O157 agglutination assay. The extra growth makes it easier to observe agglutination and allows repeat testing of the isolate. if necessary. Once one colony from a plate as been identified as O157-positive, no further colonies from the same plate need to be tested.

Isolates agglutinating in O157 antiserum or O157 latex reagent should be identified biochemically as E. coli, since strains of several species cross-react with O157 antiserum (7,8,9). However, because biochemical confirmation may take 24 hours or longer, an oral report of presumptive E. coli O 157 may be given before biochemical identification is completed.

Specimens from which sorbitol-negative colonies have been isolated that agglutinate in O157 antiserum or O157 latex reagent, and are biochemically E. coli, may be reported as presumptively positive for E. coli O157:H7. A preliminary written report should be issued to the clinician and to public health authorities. It may be useful to note on the laboratory report that E. coli 0157:H7 is an enteric pathogen and can cause nonbloody diarrhea, bloody diarrhea, and HUS.


Confirmation of E. coli O157:H7 requires identification of the H7 flagellar antigen. This is usually performed by reference laboratories, although some clinical laboratories do H7 testing. E. coli O157 strams that appear to be H7 negative in the clinical laboratory should be sent to a reference laboratory. H7 serology may be difficult since isolates often require multiple passages before the flagellar antigen is detected.

Testing for the H7 antigen as well as for the production of the Shiga-like toxins, which are associated with pathogenic strains, is available through reference laboratories. Isolates that are nonmotile or that are negative for the H7 antigen should be tested for production of the Shiga-like toxins to identify pathogenic strains. Toxin testing of E. coli O157 strains that have the H7 antigen is not necessary, because virtually all of these strains produce the Shiga-like toxins. Although they are uncommon, some strains of E. coli O157 have other H types and do not produce Shiga-like toxin; these are not recognized pathogens. A final report can be issued after H type results have been obtained. (Table 2)


Some laboratories also test E. coli O157 strains for the enzyme B-glucuronidase using broth or agar medium containing the substrate 4-methylumbelliferyl-B-D-glucuronide (MUG) (10). When MUG is cleaved by this enzyme, a fluorescent product is produced that is detectable with long-wave ultraviolet light. Unlike approximately 92% or E. coli, E. coli O157:H7 and nonmotile E. coli O157 strains that produce Shiga-like toxins lack the enzyme and are MUG negative. For this reason the MUG assay used in conjunction with testing for sorbitol fermentation and agglutination in E. coli O 157 antiserum is a useful screening test for toxigenic strains of O 157. (Table 1)


  1. Griffin PM, Tauxe RV. The epidemiology of infections caused by Escherichia coli O 157:H7, other enterohemorrhagic E. coli, and the associated hemolytic uremic syndrome. Epidemiol Rev 1991: 13:60-98.
  2. Belongia EA, Osterholm MT, Soler JT, Ammend DA, Braun JE, MacDonald KL. Transmission of Escherichia coli 0157:H7 infection in Minnesota child day-care facilities. JAMA 1993; 269:883-888.
  3. Pai CH, Ahmed N, Lior H, Johnson WM, Sims HV, Woods DE. Epidemiology of sporadic diarrhea due to verocytotoxin-producing Escherichia coli: a two-year prospective study. J Infect Dis 1988; 157: 1054-1057.
  4. March SB, Ratnam S. Sorbitol-MacConkey medium for detection of Escherichia coli O157:H7 associated with hemorrhagic colitis. J Clin Microbiol 1986;23:869-872.
  5. March SB, Ratnam S. Latex agglutination test for detection of Escherichia coli serotype O157. J Clin Microbiol 1989;27: 1675-1677.
  6. Borczyk AA, Harnett N, Lombos M, Lior H. False-positive identification of Escherichia coli O157 by commercial latex agglutination tests. Lancet 1990;336:946-947.
  7. Bettelheim KA, Evangelidis H, Pearce JL, Sowers E, Strockbine NA. Isolation of a Citrobacter freundii strain which carries the Escherichia coli O 157 antigen. J Clin Microbiol 1993;31:760-761.
  8. Corbel MJ. Recent advances in the study of brucella antigens and their serological cross-reactions. Vet Bull 1985;55:927-942.
  9. Lior H, Borczyk AA. False positive identifications of Escherichia coli O157. Lancet 1987:i:333. 10.Thompson JS, Hodge DS, Borczyk AA. Rapid biochemical test to identify verocytotoxin-positive strains of Escherichia coli serotype O 157. J Clin Microbiol 1990;28:2 165-2 168.


  • American Type Culture Collection
    10801 University Blvd.
    Manassas, VA 20110-2209 (USA)
  • Becton-Dickinson / BBL
    PO Box 243
    Cockeysville, MD 21030
    410 - 771-0100
  • Boehringer Mannheim Corp.
    9115 Hague Road
    P. O. Box 50414
    Indianapolis, IN 46250-0414
    800 -262-1640
  • Difco Laboratories
    PO Box 331058
    Detroit, MI 48232-7058
    800 - 521-0851
  • DiMed 2956 Yorkton Blvd.
    St. Paul, MN 55117
    612 - 490-5350
  • Gene-Trak Systems
    31 New York Avenue
    Framingham, MA 01701
    508 - 872-3113
  • Pro-Lab Inc.
    2111 Sam Bass Road
    Round Rock, TX 78681
    800 - 522-7740
  • Remel
    12076 Santa Fe Dr.
    Lenexa, KS 66215
    800 - 255-6730
  • Research Organic
    4353 East 49th St.
    Cleveland, OH 44125
    800 - 334-0144
  • Sigma Chemical Company
    PO Box 14508
    St. Louis, MO 63178
    800 - 325-3010
  • Unipath / Oxoid
    PO Box 691
    Ogdensburg, NY 13669
    800 - 567-8378


To request a copy of this document or for questions concerning this document, please contact the person or office listed below. If requesting a document, please specify the complete name of the document as well as the address to which you would like it mailed. DIVISION OF BACTERIAL & MYCOTIC DISEASES
NTIS U.S. Dept. Commerce
5285 Port Royal
Springfield, VA 22161

Table 1

Table 1. In Vitro Diagnostic Products and Control Strains for the Detection
         and Identification of Escherichia coli O157:H7

Item                     Source
Sorbitol-MacConkey Agar:

  Dehydrated medium      Difco
                         Cat. No. 0079-17-7
                         500 gm

                         Cat. No. CM813
                         500 gm

  Prepared Medium(1)     Becton Dickinson/BBL
                         Cat. No. 97953
                         10 plates/pkg

                         Cat. No. 4260-22-1(2)
                         5 plates/pkg

                         Cat. No. 50-1430
                         10 plates/pkg

                         Cat. No. 01-554
                         10 plates/pkg

                         Cat. No. 09-548(3)
                         20 pour tubes (deeps)/pkg

MUG (4-methylumbelliferyl-B-D-glucuronide) products:

  Reagent                Boehringer Mannheim Corp.
                         Cat. No. 270954
                         100 mg

                         Cat. No. BR71
                         Box of 10 vials (50 mg/vial)

                         Research Organic
                         Cat. No. 01 84 M
                         10 mg
                         50 mg
                         100 mg

                         Sigma Chemical Co.
                         Cat. No. M 9130
                         10 mg
                         100 mg

  Media with MUG         Becton Dickinson/BBL
                         Mac Conkey agar with MUG
                         Cat. No. 43-21938
                         20 plates/pkg

                         MacConkey agar with MUG
                         Cat. No. 99057
                         500 gm

                         Nutrient agar with MUG
                         Cat. No. 0023-17-4
                         500 gm

                         Gene-Trak Systems
                         Flourocult(R) E. coli O157:H7
                         Cat. No. 4036
                         500 gm

                         MacConkey agar + MUG
                         Cat. No. 01-554
                         10 plates/pkg

                         MacConkey-sorbitol with MUG
                         Cat. No. 01-563
                         10 plates/pkg

  Disks with MUG              Cat. No. 21-135
                         25 disks/vial

O157 antiserum (rabbit):

                         Cat. No. 2970-47-7
                         3 ml (tube test)

O157 latex reagents(rabbit antiserum conjugated to latex beads):

                         Cat. No. DR 620
                         100 tests (slide test)

                         Cat. No. PL070
                         50 test (slide test)
                         Cat. No. PL071
                         100 tests (slide test)

                         Cat. No. 24-250
                         50 tests (slide test)

H7 antiserum (rabbit)    Difco
                         Cat. No. 2159-47-0
                         3 ml (tube test)

H7 latex reagent (rabbit antiserum conjugated to latex beads):

                         Cat. No. 24-250
                         50 tests (slide test)

1  Except where noted, the shelf life of prepared sorbitol-MacConkey agar
   plates stored at 2-8C ranges from 4 to 12 weeks.
2  Extended shelf life medium; the shelf life of this product stored at
   room temperature is from 6 to 12 months.
3  Plates are prepared from pour tubes (deeps) by melting the agar and
   pouring plates as needed. The shelf life of this product stored at 2-8C
   is 4 months.

Table 2

Table 2. Strains of Escherichia coli O157:H7 available from the
          American Type Culture Collection
ATCC No.      CDC Nos.      Toxin(s)       State       Origin       Source
--------      ---------     ---------      ------      -------      -------
43890           C984         SLT I          WA         Human        Stool
43889          B1409-C1      SLT II         NC         Human        Stool
35150          EDL 931       SLT I & II     OR         Human        Stool
43894          EDL 932       SLT I & II     MI         Human        Stool
43895          EDL 933       SLT I & II     MI         Food         Meat
43888          B6914-MS1     SLT neg        WA         Human        Stool
*SLT I = Shiga-like toxin I (Verocytotoxin 1); SLT= Shiga-like toxin II
(Verocytotoxin 2)

   The above list of commercially available products for the detection and
identification of Escherichia coli O157:H7 is not intended to be a complete
listing of all such products and is not an endorsement of the named products
by the Centers for Disease Control and Prevention.

Figure 1

Procedure for Isolation and Identification of E.coli O157:H7

Prevention Guidelines Image

This page last reviewed: Wednesday, January 27, 2016
This information is provided as technical reference material. Please contact us at to request a simple text version of this document.