Epitope specificity and application of Salmonella typhimurium O-antigen-specific monoclonal antibodies.

Drs. J. Jaradat and J. Zawistowski have recently reported three monoclonal antibodies (MAbs) against the O antigens of Salmonella typhimurium lipopolysaccharide (LPS) (9). These antibodies were characterized to bind O antigen 5 (or O:5), which corresponds immunochemically to O-acetyl abequose (7). Other laboratories have also developed MAbs against O antigens of S. typhimurium (summarized in Table 1). By using immunochemical and/or genetic approaches, these antibodies were charcterized to recognize O:1, O:4, O:5, O:12, and conformational epitopes in the S. typhimurium LPS molecule. Smooth strains of S. typhimurium (and other group B serotypes) are known to display O antigens 4 and 12 (the latter is also found in serogroups A and D). In addition, some strains also express O antigens 1 and 5, but none of these is exclusive for S. typhimurium (17). These O antigens are present on the surface of the bacteria and are conveniently detected by the slide and/or tube bacterial agglutination test (6). Thus, antibodies (both polyclonal and monoclonal) directed against O antigen 4 are routinely used for the serological identification of serogroup B salmonellae. The fact that the MAbs developed by Drs. Jaradat and Zawistowski bind to live S. typhimurium cells is to be expected if they are directed against O antigen 5. Nevertheless, this does not make them suitable for the isolation and/or identification of S. typhimurium per se, because not all strains of S. typhimurium make O antigen 5 and O antigen 5 is not produced exclusively by S. typhimurium alone (36 of 144 serotypes in serogroup O:4 [B] may produce O antigen 5 [17]). Therefore, to develop an enzyme-linked immunosorbent assay (ELISA) for detection of S. typhimurium using MAbs to O antigen 5 would lead to both false positives and false negatives. The only limited application of MAb to O antigen 5 is for epidemiological differentiation of serogroup B Salmonella strains, especially S. typhimurium because of its prevalence, into those that have and those that do not have O antigen 5 (S. typhimurium copenhagen). To accomplish this differentiation, there is a need to first isolate and identify the strain as S. typhimurium based on conventional culture techniques followed by biochemical and serological testings using specific antisera to identify the somatic O antigen 4 and the flagellar H antigens i and 1,2 (17). The serological identification of S. typhimurium copenhagen is easily, economically, and rapidly accomplished by the simple slide agglutination test which is a routine and standard procedure used by all diagnostic microbiology laboratories. The modern approaches such as capture ELISA (3) or immunomagnetic bead separation (15) for detection of Salmonella contamination are best accomplished with either Salmonella genus-specific (20) or serogroup-specific MAbs (14). Ng et al. have recently reported a capture ELISA using genusspecific anti-core LPS MAb as the capturing reagent and serogroup-specific MAbs for the simultaneous detection and serogroup differentiation of Salmonella spp. in food (16). PCR which targets the Salmonella O-antigen precursor rfb genes had also been demonstrated as a promising approach to detect salmonellae in stool specimens (12, 13). Drs. Jaradat and Zawistowski also reported the analysis of antibody avidity by simple titration of their MAbs against solidphase antigen (bacterial whole cells) using indirect ELISA. ELISA can be applied for measurement of antibody avidity, but the antibodies have to be titrated in a chaotropic agent and the results have to be compared to results of titration of the same antibody in the absence of the chaotropic agent (8).