Serotyping of Bdellovibrios by Agglutination and Indirect Immunofluorescence
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INTERNATIONAL JOURNAL OF SYSTEMATICBACT~RIOLOGY. Oct. 1983, p. 816-821 Vol. 33, No. 4 0020-7713/83/040816-06$02.00/0 Copyright 0 1983, Internationnl Union of Microbiologicdl Societie\ Serotyping of Bdellovibrios by Agglutination and Indirect Immunofluorescence M. E. SCHELLING’* AND S. F. CONTI’ Department Biology, Texus A &M Unitwsit?, College Station, Texus 77843‘ und University of Mussa ch us et t s , A rn ii erst, Muss N c h iiset t J 0 1 002’ A comparative serological study was undertaken to clarify the uncertain interrelationships among various bdellovibrios. A total of 17 predacious (host- dependent) strains and 12 nonpredacious strains were serotyped. Strains were grouped into serotypes on the basis of positive agglutination and strongly positive fluorescence cross-reactions. The results of division of the strains into serogroups by these two methods were identical. Bdellovihrio starrii strain A3.12 and Bdellovibrio stolpii strain UKi2 were found to be antigenically distinct from each other and from the group of strains comprising Bdellovihrio hacteriovorus; the latter was found to be an antigenically heterogeneous group consisting of at least nine serogroups. All nonpredacious strains were found to be related antigenically to their obligately predacious counterparts. An antigen(s) common to all of the Bdellovibrio strains examined was exhibited as weakly positive fluorescence. Selected strains of Bdellovihrio were studied by Ouchterlony immunodiffusion, which confirmed the serogrouping results and provided a technically simple method of serotyping. Further immunochemical characterization of the strains of Bdellovihrio in conjunction with increased knowledge of other differences among strains will likely result in the formation of additional species of Bdellovihrio. Research interest in Bdcllovihrio is related to As serotyping of bacterial strains has been the many intriguing characteristics of this genus. widely accepted as useful in determining rela- Bdellovihrio strains are gram-negative, bacterio- tionships among groups of bacteria, and few lytic organisms which prey (10, 14) on gram- studies (22-24) have reported data showing rela- negative bacteria (3, 5, 21). The role of Bdello- tionships among bdellovibrios, an immunologi- vihrio in the environment has not been well cal study of the interrelationships among various established, but its primary importance may lie strains of Bdellovihrio was undertaken to pro- in the ecological balance between pathogenic vide additional information which may be useful and nonpathogenic bacteria in soil, sewage, and in the formation of a more unified concept of the water (22). genus Bdellovibrio. The taxonomic position of the genus Bdcdlo- vibrio and the interrelationships among the MATERIALS AND METHODS strains of this genus have remained uncertain. Culture of nonpredacious strains of Bdeffovibrio.The The use of predation as a primary criterion to medium used for culture of nonpredacious strains of allow an unknown strain to be recognized as Bdellovihrio was peptone-yeast extract broth, which Bdellovihrio has resulted in a heterogeneous contained 1% peptone and 0.3% yeast extract (Difco assemblage of organisms. Determination of Laboratories) and was adjusted to pH 7.2. Cultures were transferred by using 10% (vol/vol) inocula and strains of Bdellovihrio on the basis of prey range were grown for 16 h at 30°C in a Psychrotherm shaking and location of isolation has been unsatisfac- incubator (New Brunswick Scientific Co.). tory, and at present other differences among Culture of predacious strains of Bdeffovibrio.Esche- strains are used in determining the interrelation- richia coli ATCC 15144 was used as prey to propagate ships among strains. Known differences among all of the predacious Bdellovihrio strains examined selected strains of Bdellovihrio include differ- except strains 6-5-S and 118. Strain 6-5s was grown ences in guanine-plus-cytosine content (15, 16, on Spiriflurn serpens ATCC 12638, and strain 118 was 18), deoxyribonucleic acid ribosomal binding grown on Serratia marcc’scens ATCC 13880. sites, enzyme migration patterns (16), levels of The prey cells were grown in nutrient broth (Difco) at 30°C for 10 h in a shaking incubator, centrifuged, protease activity (8), and susceptibility to phage washed, and suspended in dilute nutrient broth lysis (1). Our knowledge of the nutritional and (0.008% nutrient broth, 0.005% Casamino Acids, metabolic characteristics of strains of Bdellovih- 0.001% yeast extract [Difco]) supplemented with 2 rio is incomplete and, therefore, not useful taxo- mM CaClz and 3 mM MgC12. Lysates were transferred nomically. to prey cell cultures by using 10% (vol/vol) inocula and 816 VOL. 33, 1983 SEROTYPING OF BDELLOVIBRIOS 817 these cultures were grown for 12 to 48 h at 30°C in a 0.85% (wtlvol) NaCl to densities of approximately 3 x Psychrotherm shaking incubator. lo9 bacteria per ml. Preparation of anti-Bdellovibrio antisera. Nonpreda- Agglutination procedure. A 1 :5 dilution of heat- cious Bdellovibrio antigens were prepared by adding inactivated (56°C- 30 min) antiserum was further dilut- equal volumes of 1% Formalin (40% formaldehyde) in ed through a series of twofold dilutions to 163,840. An 0.85% (wt/vol) NaCl to 16-h cultures grown as de- equal volume of the antigen preparation being tested scribed above. was added to each tube, which doubled the dilution of Antigens of predacious Bdellovihrio strains were the antiserum. prepared from lysates which were filtered through A control series of tubes contained heat-inactivated 0.45-pm membrane filters (Millipore Corp.) to sepa- normal rabbit sera starting at a dilution of 1:5 in 0.85% rate the bdellovibrios from any remaining prey cells. (wt/vol) NaCl and equal volumes of antigen suspen- The filtrates were sedimented at 7,710 x g in a Sorvall sion. model RC-2 preparative centrifuge and suspended in All tubes were incubated at 37°C for 1 h and then 0.5% Formalin in 0.85% (wt/vol) NaCl. Facultative refrigerated overnight at 4°C. strain UKi2 was grown saprophytically in peptone- Indirect immunofluorescence procedure. A smear yeast extract broth and transferred three times at 12-h was prepared from a broth culture or from a Formalin- intervals. A 10% (vol/voL) inoculum was taken from treated Bdellovibrio stock culture and mixed by gentle the third transfer and grown on prey cells in the same heat. The slide was placed in a petri dish containing a manner as other predacious strains. After three cycles disk of moistened filter paper to prevent drying on the of predacious growth, an antigen preparation was slide. The smear was covered with rabbit anti-Bdello- made as described above. Antigen preparations were vibrio antiserum and incubated at room temperature standardized spectrophotometrically. (22°C) or at 30°C for 30 min, washed gently three times The antisera used for the agglutination and indirect by allowing pipettefuls of phosphate buffer (6.4 g of immunofluorescence procedures were raised in New Na2 HP04, 1.3 g of NaH2P04 H20, pH 7.4) to run Zealand white rabbits which were given six intrave- over the area of the smear, and incubated in a second nous injections increasing from 0.2 to 4.0 ml of antigen moist chamber for 30 min at room temperature (22°C) during an 8-week period. or at 30°C with smear covered with FITC-labeled Antisera with titers high enough to be used in caprine anti-rabbit IgG antibody (Microbiological As- precipitation tests were obtained by weekly intramus- sociates). The slide was again washed three times with cular injection of 2 ml of antigen which had been phosphate buffer, air dried, covered with Difco fluo- emulsified with an equal volume of Freund incomplete rescent antibody mountant, and protected with a cover adjuvant (Difco) for 8 months to 1 year. slip. Slides were observed immediately to avoid The sera were obtained by centrifuging coagulated bleaching, which is particularly important when read- whole blood at 7,000 x g for 15 min in an International ing both weak and strong fluorescence, such as in the Equipment Co. refrigerated centrifuge and were stored Bdello vibrio system. at -20°C until they were used. Five controls were run for each group of slides FITC-labeled antisera. The fluorescein isothiocya- tested by the indirect immunofluorescence procedure. nate (F1TC)-labeled caprine anti-rabbit immunoglob- A negative control for nonspecific fluorescence result- ulin G (IgG) antibody with specificity for both heavy ing in false positives consisted of a primary incubation and light immunoglobulin chains and the FITC-labeled with the smear coated with known negative normal caprine anti-rabbit IgM antibody with specificity for rabbit serum, followed by a second incubation with the p chain (Microbiological Associates) contained 10 FITC-labeled anti-rabbit IgG. This control detected mg of protein per ml and 1:10,000 parts of thimerosal nonspecific binding of FITC-labeled anti-rabbit IgG or preservative. inadequate washing. A negative control for the detec- Preparation of antigens for use in indirect immunoflu- tion of any inherent Bdellovibrio fluorescence which orescence and agglutination. Antigens for use in the was not specific for the system consisted of a primary indirect immunofluorescence procedure were pre- incubation only. A pre-injection serum control for the pared from cultures grown as described above. rabbit from which the experimental serum was taken Cultures of nonpredacious organisms up to 24 h old was used to determine that no innate antibodies which were used in this study. Bdellovibrio broth cultures could cross-react with Bdellovibrio antigens were were centrifuged at 4,000 X g for 20 min in a Sorvall present. A known strongly positive fluorescence sys- model RC-2 preparative centrifuge. The cells were tem and a known weakly positive fluorescence system suspended in 1% Formalin (40% formaldehyde) in were run as standards for reading of the fluorescent 0.85% (wt/vol) NaCl and stored at 4°C.