Traditionally, Salmonella isolates are separated into serotypes based on structural differences on the surface of the cells (O antigens) and thread‑like portions of the flagella (H antigens), using the Kauffman-White classification scheme. In this technique, antibodies are prepared against these specific antigens in a blood serum known as antiserum. Confirmed Salmonella sp. isolates are then tested with this antisera and are observed for agglutination reactions.
Through testing unknown samples against a series of antisera, the specific serotype of an isolate can be discerned. As previously discussed, there are a great number of serological variants of Salmonella and so this process can be very long and labour intensive, requiring highly experienced staff with a vast library of antisera at their disposal. Because of this, the Kauffman-White serotyping method is often only carried out by reference laboratories, with routine microbiology laboratories only stocking a small number of antisera.
As an example, at ALS Rotherham we stock the antisera for our in-house control strain, Salmonella enterica subsp. enterica ser. Nottingham, which enables us to distinguish our strain from others using the antisera O16, Henz15 and Hd. This serotype is recommended by the health protection agency in the UK for use as a control strain, due to being a very rare serotype and thus very unlikely to be isolated as a wild type. Historically, when further analysis was required for one of our samples, the isolates would be subcontracted to a reference laboratory capable of full serological testing. For a plethora of reasons, this type of analysis all too often had a lengthy turnaround time which, while accurate, was often too little too late and unhelpful in making a practical difference to our client, the FBO (food business operator).