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E. coli OK O pool for detection of Big Six non-O157 STEC serotypes

SSI Diagnostica offers an E. coli OK O pool that detects the STEC serogroups O26, O45, O103, O111, O121, and O145 by slide agglutination.

In addition to continued testing for E.coli O157:H7, new regulations in the United States now require the meat industry to monitor for six non-O157 Shiga toxin-producing E.coli (STEC) serogroups. The regulation requires testing for the so called “Big Six” STEC (O26, O45, O103, O111, O121, and O145) as these serotypes have been most frequently associated with E. coli outbreaks of food-borne illnesses worldwide. If the products test positive they will be prevented from entering commerce.

The Big Six STEC strains cause outbreaks of hemorrhagic colitis (bloody diarrhea) and hemolytic uremic syndrome (kidney failure).

Product description

The OK O pool Big Six is intended for screening of the serotypes O26, O45, O103, O111, O121, and O145 using slide agglutination. 
The test is supplied in 3 mL bottles – enough to perform 150 tests. All 6 antisera can also be provided as single antisera.
Shelf life is minimum two years from date of shipment.
Storage conditions are 2 – 8 degrees Celsius.

We offer the full range of antisera for E. coli serotyping

High quality

All absorbed free of cross-reactions

We offer experienced technical support

How to use

The test is performed by placing a small drop of antisera on a glass slide. Culture is transferred from two to three colonies to the drop of antiserum and mixed well. Positive result will show as agglutination within 10 seconds. The reaction can be read by the naked eye.
Positive slide results need to be confirmed with O antisera for boiled culture.

Product range

of tests
88010 OK O pool Big Six150 3 mL 


Click the question to get your answer

Antibodies can be frozen, but the process of freezing and thawing is damaging for the antibodies, and the titer will drop (even though not dramatically) due to decomposition.
We have not made further studies on this, and cannot guarantee the stability of the antisera, but we have experienced that the titer mostly stays at the same level if an antiserum is only frozen and thawn once.

SSI Diagnostica cannot guarantee that the titer of our antiserum is at the correct level when it has expired. Generally high-titer (undiluted) products keep the titer better than ready-to-use sera. Some antisera (especially H sera) have a tendency to maintain the titer better than others (such as OK O86). Generally, some antisera can be used after the expiry date and some cannot. Since we cannot say which antisera that maintain the titer and which that do not, we recommend that you buy new ones. The problem in using expired antisera is that you will get false negatives without knowing it.

No, not unless you order large amounts. Please send an inquiry to

Yes, it is possible if you place an order in advance with the expected total volume for the period. In return we guarantee to keep the reserved lot for you.

Please send an inquiry. Depending on the needed volume, we might be able to develop the requested product. It can be anything from a new pool composition to a new serum.

Yes, you can buy all reference strains for the O, H and K antigens.

Antiserum consists primarily of IgG antibodies.

E. coli strains may be what is called rough, and these cannot be serotyped in the normal way. For experienced laboratory technicians it is possible to determine if a colony on an agar plate is rough, just by looking at it. If you suspect you have a rough strain, take a little amount of the colony from an agar plate (it’s important it is a non-selective agar plate) and suspend it in saline water instead of broth. Boil the saline water suspension for at least an hour, and use it for O serotyping, in the same way which you would have used the boiled broth. If the serotyping was unsuccessful it is possible to take a new non-selective agar plate and flood it with ethanol and dry it. Innoculate a rough colony onto the plate and repeat the procedure for suspension and boiling as described above.

SSI Diagnostica recommends that you use the volume stated in the product insert. It is particularly important to note that if the dilution of the antiserum is too high it will not catch the weakest reactions. Further, you will experience a weak agglutination.

OK O serum is based on vaccines using whole bacteria. It is therefore only sensible to produce OK O sera on bacteria which have a very weak or no capsule, as the capsule covers the O antigens. In cases where the rabbits are vaccinated with a strain where the capsule covers the O antigens, the rabbit will produce antibodies against unspecific antigens on the surface of the bacteria, and not against the desired O antigen.

When testing on live bacteria, one can not exclude that some bacteria have a capsule that is so thick that it covers all bacterial O antigens. Therefore, false negative reactions may occur. Due to this OK O antiserum is only for screening.

There may be seen false positive results when using the OK O antisera, although this is rare. This is the result of OK O antisera beeing based on whole cells vaccines. When vaccinated with whole cells, the rabbit make antibodies to all antibodies on the cell surface, and therefore can cause nonspecific agglutinations.
Be aware that you can only read a positive result within the first 10 seconds. After 20 seconds of sliding, agglutinations can cause nonspecific reactions.
Using a very large amount of colony mass for slide agglutination may show lumps in the reaction, which should not be misread as an agglutination.

O antisera are based on a vaccine containing boiled culture. This means that most capsules and surface proteins are removed and it is primarily the O antigens that are left. Taking a live culture and adding a matching antiserum will usually not give an agglutination, since the O antigens are covered by the capsule and surface proteins.
Beside this, O agglutinations normally take several hours to give a visible reaction.

OK O antiserum is used for live bacteria, whereas O antiserum is for boiled cultures.
OK O antiserum is meant for screening. It gives you an easy and quick result (10 seconds), but the result needs to be confirmed by E. coli antisera for boiled culture. OK O antisera is used with live bacteria, and because of this, unspecific antibody reactions sometimes occur.
O antiserum is used for confirmation or for serotyping.

SSI Diagnostica offers a yearly training course. Contact us for more information.

A reference strain is a microorganism defined to at least the genus and species level, catalogued and described according to its characteristics e.g. O, K and H serotype and stating its origin.
A test strain is generally used for quality control and performance testing of culture media or serving as a positive/negative control in a diagnostic kit ensuring that the performance of the test has been successful.
Wild type strains are isolates from patients, animals or other sources, and these are not well defined.

You should send the strain according to your national legislation. The strain should be transported in an ordinary transport medium (Stuarts, Cary Blair, lyophilised or likewise), and the package should be marked with an UN number corresponding to the content of the package.

The main advantages are:
1. Reaction time for OK O antisera is only 10 seconds reducing the risk of false positive reactions leading to a more accurate result compared to other tests having a reaction time of 1 minute.
2. SSI Diagnostica antisera are absorbed and thus very specific.
3. We offer technical expertise and assistance / training.


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