Serotyping of Serratia marcescens: simplified tube O-agglutination test and comparison with other serological procedures.

A simplified tube O-agglutination test was developed and evaluated for the determination of somatic serogroup (O) antigens of Serratia marcescens. Use was made of Tryptic Soy broth (TSB)-grown O-cells that had been boiled for 1 hour; 0.145 M NaCl proved a satisfactory diluent. Various technical parameters of this test were examined as well. Rabbit anti-O immune sera, that had been elicited with 5 x concentrated, TSB-grown, 1 hour-boiled O-cells of all 15 currently employed O-antigen reference strains of S. marcescens yielded satisfactory O-agglutinin titers. The tube O-agglutination test compared favorably with the indirect hemagglutination technic, although the latter technic yielded significantly higher O-agglutinin titers with merely 7 of the 15 O-antigens of S. marcescens. The tube O-agglutination test permitted detection of higher O-agglutinin titers than a microtiter O-agglutination test utilizing O-cells that had been stained with safranin O. Conversely, titers obtained with TTC-stained O-cells in a microtiter agglutination procedure approximated those yielded by the tube O-agglutination test, but O-cells of the various S. marcescens strains were stained nonuniformly by triphenyltetrazolium chloride (TTC). As before, there existed marked serologic cross-reactivity between O-antigens O6 and O14. A new O6 candidate strain, S. marcescens isolate S 1i, serotype O6:H20, was proposed. Contrary to O-agglutinins of human control serum, the O-agglutinins of rabbit anti-O immune sera proved refractory to treatment with 0.1 M of 2-mercaptoethanol (2-ME) and 0.01 M dithiothreitol (DTT) respectively. Dual absorptions of rabbit anti-O immune sera with killed cells of Staphylococcus aureus Cowan I (protein A), failed to significantly reduce O-agglutinin titers, although human IgG and IgM was bound by protein A. It was tentatively concluded that the 2-ME- and DTT-refractory rabbit anti-S. marcescens O-agglutinins resided in the IgM immunoglobulin class.