A comparison of the serum agar grouping and slide agglutination methods for the identification of Neisseria meningitidis strains

Bulletin of the World Health Organization, 55 (6): 659-661 (1977) A comparison of the serum agar grouping and slide agglutination methods for the identification of Neisseria meningitidis strains P. KUZEMENSKA,1 V. BURIAN,' C. E. FRASCH,2 & E. WVANDOVA' The serum agar grouping (SAG) and slide agglutination (SA) tests were comparedfor their ability to identify strains of N. meningitidis. There was no significant difference between the two methods in respect of their relative sensitivity and detection ability. The most suitable concentration of antimeningococcal serum for SAG was found to be 1:10; however, the most suitable concentration will need to be established for each serum used. The ability ofSAG to identify large numbers of N. meningitidis strains was confirmed. The serological identification of Neisseria meningi- tidis strains is normally carried out by slide agglu- tination. Recently, however, the serum agar method of Petrie (1) has been reintroduced. The merit of this method is its simplicity and its capacity to identify easily many strains of N. meningitidis; this would be useful during an outbreak of meningococcal cerebro- spinal meningitis. In this communication, we compare slide agglu- tination (SA) with serum agar grouping (SAG) and determine the optimum conditions for SAG. MATERIALS AND METHODS Strains The strains of N. meningitidis used were: 1. Reference neotype strains: A (M1027), B (M2092), C (M1628), D (M158), X (18/68), Y (19/68), Z (20/68), 29E, WI 35. 2. Group B neotype strains: S3032, M986, M978, M136, M982, M992, B16B6, MIOII, M981, M990, M1080. 3. Field strains isolated from carriers or patients with respiratory infections in Czechoslovakia. 1 Centre of Epidemiology and Microbiology, Institute of Hygiene and Epidemiology, Srobarova 48, 100 42 Prague 10; Czechoslovakia. ' US Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, Bethesda, MD 20014, USA. Sera For SA, rabbit sera prepared by Professor N. A. Vedros, University of California, Berkeley, USA and obtained from WHO were used. Anti- meningococcal sera A, B, C, D, X, Y, Z, 29E, and W135 were used both undiluted and diluted 1: 10 and 1: 20. For SAG, horse sera provided by WHO were used. Antimeningococcal sera A, B, and C were tested at concentrations of 1: 5, 1: 10, 1: 20, and 1: 40; sera Y, 29E, and W135 were tested at a concentration of 1: 10. Slide agglutination The strains, after culture for 18-20 h on Mueller- Hinton medium in a CO2 atmosphere at 37°C, were suspended in a drop of distilled water on a slide. Drops of antimeningococcal serum were placed on another slide. One drop of the suspension was added to each drop of serum by loop. After mixing, the slide was agitated by hand and the results were read within 3 min. Agglutination in saline solution was also evaluated. Scores of +++, ++, +, or were given according to the degree of agglutination. The strains were classified into the following groups: (a) Typable Monoagglutinable-agglutinating with one anti- meningococcal serum. 3652 - 659 - P. KUZEMENSKA ET AL. (b) Nontypable Polyagglutinable-agglutinating with two or more antimeningococcal sera. Autoagglutinable-agglutinating with saline solution. Nonagglutinable-not agglutinating with anti- meningococcal sera or saline solution. Serum agar grouping After the strains had been cultured on Mueller- Hinton medium as described above, they were inocu- lated on to TSB medium containing horse anti- meningococcal serum at concentrations of 1: 5, 1: 10, 1: 20, and 1: 40. The diameter of the ino- culum was 6-8 mm and it was possible to accom- modate 8-12 inocula on one plate. Inoculated plates were incubated at 37°C in a CO2 atmosphere, and after 18-20 h the results were read. The results were scored +±+ , ±+,+, or - according to the strength and diameter of the precipitin halo around the growth. RESULTS Comparison ofSA and SAG The results are shown in Table 1. The two methods were in agreement for 73 (94.8 %) of the 77 strains tested: 44 were positive and 29 negative. Of the four strains that gave different results in the two tests, one was positive with SAG and negative with SA and three were negative with SAG and positive Table 1. Comparison of the results of the serum agar grouping (SAG) and slide agglutination (SA) tests SAG SA Positive Negative Total Positive 44 3 47 Negative 1 29 30 Total 45 32 77 with SA. The 95% confidence interval was 87.2-98.6%; the estimated proportion of disagree- ment was only 5.2%. There was no statistical differ- ence between the two tests in respect of their relative sensitivity and detection capabilities. Concentration of horse sera in TSB medium for SAG Sera A, B, and C were tested at concentrations of 1: 5, 1: 10, 1: 20, and 1: 40. According to the SAG results, the most suitable concentration in TSB medium appeared to be 1: 10, since at this concen- tration the majority of SAG results were ± ++ or ++. Sera Y, 29E, and W135 were tested only at a concentration of 1: 10. The results were + or + + and we can suppose that a concentration of 1: 5 would be more suitable. However, it was impossible to verify this owing to the small quantity of serum available. ACKNOWLEDGEMENTS The technical cooperation of Mrs M. Myskova, Medical Faculty of Hygiene, Charles University, Prague, is gratefully acknowledged. RtSUM1t COMPARAISON DES METHODES DE GROUPAGE SUR AGAR SERIQUE ET D'AGGLUTINATION SUR LAME POUR L IDENTIFICATION DES SOUCHES DE NEISSERIA MENINGITIDIS Bien que l'agglutination sur lame (SA) soit la methode habituellement utilisee pour l'identification des souches de N. meningitidis, la methode de groupage sur agar s6rique (SAG) a ete recemment reintroduite a cet effet. La presente communication relate une experience visant a comparer les deux methodes. Dans l'epreuve SAG, les souches ont ete inoculees sur un milieu TSB contenant du serum antimeningococcique de cheval A differentes concentrations. Les boites de P6tri inoculees ont et6 incubees pendant 18-20 heures a 370 dans une atmosphere C02, et les resultats ont ete notes + + +, + +, + ou - selon l'intensite et le diametre du halo de precipitines autour de la colonie. On n'a pas constate de difference statistiquement significative entre les deux types d'epreuve quant a leur pouvoir de d6tection ou leur sensibilite relative. Sur un total de 77 souches testees, les deux methodes ont donne des resultats similaires pour 73 souches, avec 44 resultats 660 IDENTIFICATION OF N. MENINGITIDIS STRAINS 661 positifs et 29 negatifs. La concentration optimale de serum antimeningococcique s'est rev6l6e 8tre 1: 10 pour 1'epreuve SAG; elle devra neanmoins 8tre determinee pour chaque type de serum utilise. La methode SAG a l'avantage de permettre d'identifier aisement un grand nombre de souches de N. meningitidis, ce qui peut 8tre tres utile en cas d'epidemie de meningite c6r6bro-spinale meningococcique. REFERENCES 1. PETRIE, G. F. A specific precipitin reaction associated with the growth on agar plates of Meningococcus, Pneumo- coccus and B. dysenteriae (Shiga). British journal ofexperimental pathology, 13: 380-394 (1932).