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Polyclonal antibodies can be made to different subspecies of bacteria and these will

recognise surface epitopes on them. Because the subspecies are related then they will

share some surface epitopes and the closer they are related the more they will share.

Antibodies made to one organism will therefore react to a greater or lesser degree to a

related organism depending on how many surface epitopes they share.

This has given rise to a systematic method of identification known asserotyping

which is based on the reaction of microorganisms to antibodies. The system works

well with closely related organisms but is not definitive, as it only assesses surface

markers. The pattern of precipitin bands obtained to reference antibodies is specific

and can be used to assign samples into serotypes. The method was used until very

recently to characteriseSalmonellastrains as their pathogenicity can be assessed

according to their relatedness to known strains.Salmonellas from food and water

samples were tested by this method to establish if they had been the cause of food

poisoning incidents.

Agglutination reactions using sensitised erythrocytes (red blood cells) or latex

particles are carried out in liquid phase usually in small tubes or more recently in

round-bottom microtitre plates. As discussed before, the agglutination reaction only

occurs at the point of equivalence. A positive agglutination test appears cloudy to the

eye as the erythrocytes or latex particles are suspended in solution. A negative result is

characterised by a ‘button’ at the bottom of the reaction vessel which is formed from

non-reacted particles. A negative result may be obtained from excess antigen or

antibody, as the binding reaction favours the production of small aggregates of

antigen/marker particles rather than the agglutination gel.

7.3.1 Enzyme immunosorbent assays

By far, the vast majority of immunoassays carried out fall into the category of enzyme

immunosorbent assays. These are routinely used for the diagnosis of infectious agents

Band occurs

where antigen

and antibody form

immunoprecipitate

Precipitin

band

Antigen

Antibody

Fig. 7.11Ouchterlony double diffusion plate.

285 7.3 Immunoassay formats