Bob S. Roberson

Specificity of slide agglutination test for detecting bacterial fish pathogens

Abstract The usefulness of the slide agglutination assay for a rapid diagnosis of fish diseases was evaluated using a total of 80 pathogenic bacteria and environmental isolates belonging to the genera Vibrio (28), Pasteurella (5), Aeromonas (26), Yersinia (6), Edwardsiella (8), Pseudomonas (6) and Lactobacillus (1). Selected strains from each bacterial group were used as antigens for rabbit immunization. The specificity of the reactions between whole-cell antigens and the different whole-cell antisera varied according to the bacterial group analyzed. In the case of V. anguillarum , it was found that by using two sera against serotypes 1 and 2, it was possible to detect most of the strains causing vibriosis regardless of their origin. Cross-reactions were not detected between either both serotypes or with other pathogenic or environmental vibrios. In general, the whole-cell antisera from P. piscicida, A. salmonicida, Y. ruckeri and E. tarda detected all the strains within each species. However, a more heterogeneous pattern was exhibited by the motile Aeromonas species. The majority of A. hydrophila and A. sobria isolates did not agglutinate with the anti A. caviae serum, indicating that this antiserum is not adequate for identifying all motile Aeromonas strains. The antiserum against A. salmonicida subsp. masoucida displayed weak cross-reactions with some A. salmonicida subsp. salmonicida and A. hydrophila whole-cell antigens. Strong cross-agglutinations occurred also between types I and II of Y. ruckeri as well as between E. tarda and E. ictaluri . These cross-reactions were eliminated by using the respective thermostable somatic “O” antigens, which indicates that common thermolabile antigens are shared by these strains. The slide agglutination test, using anti whole-cell sera and two antigen preparations (whole-cells and “O” antigen) for each strain, is useful for a rapid detection of fish pathogens, with the additional advantage of its applicability for serotyping studies. Furthermore, some cross-reactions using the whole-cell antigens are reliable in identifying a wide range of strains causing similar diseases with a small number of anti whole-cell sera.

Hydrocortisone suppresses the chemiluminescent response of striped bass phagocytes

Phagocytic cells obtained from the pronephros of striped bass (Morone saxatilis) were exposed in vitro to various levels of hydrocortisone acetate. This treatment reduced the normal ability of the cells to generate a chemiluminescent response when exposed to bacteria or phorbol myristate acetate. Although the levels of hydrocortisone were higher than is physiological for fish, suppression was dose dependent and was not attributable to a reduction in cell viability. Whereas phagocytic chemiluminescence has been linked to the respiratory burst and bactericidal activity, the possibility exists that stress-induced elevations in serum corticosteroids lead to increased susceptibility to infection.

Chemiluminescence of phagocytic cells isolated from the pronephros of striped bass

Phagocytosis of bacterial fish pathogens by cells isolated from the pronephros of striped bass (Morone saxatilis) was measured using an assay of chemiluminescence. Results of the assay, which proved to be quite reproducible, indicated that the degree of phagocytosis was related to the number of bacteria employed and to the species of bacteria eliciting the response. Cells from individual fish gave similar phagocytic responses but of different magnitudes.

Animal-passed, virulence-enhancedCampylobacter jejuni causes enteritis in neonatal mice

An animal model resembling the human disease caused byCampylobacter jejuni has been developed. Characteristic illness followed intragastric challenge of neonatal mice with strains ofC. jejuni enhanced for virulence by serial intraperitoneal passage in weanling mice of organisms suspended in either mucin or iron dextran. Such passage lowered the LD50 in weanlings from 2×1011 colony-forming units (CFU) to 2×105 CFU per mouse. Neonatal mice chellenged by intragastric intubation with 2×109 CFU of the virulence-enhanced organisms suspended in mucin or iron dextran showed signs of infection by day 5, including severe diarrhea, increased musus discharge occasionally with blood, and reduced weight gain. Diarrhea contionued for eight days, after which most animals recovered. This mouse infection model provides a means for assessing the determinants of virulence among strains ofC. jejuni.

Phytohemagglutinin-induced enhancement and IgG subclass restriction of the immune response to type III pneumococcal polysaccharide☆☆☆

Abstract The splenic plaque-forming-cell (PFC) response of mice to immunization with pneumococcal capsular polysaccharide (SSS-III), coupled with T-cell activation by phytohemagglutinin (PHA), is characterized by enhanced numbers of IgG-producing cells, largely restricted to the IgG2a and IgG2b subclasses. In contrast, immunization with SSS-III alone results in low numbers of IgG-producing cells, fairly evenly distributed among the subclasses IgG1, IgG2a, IgG2b, and IgG3. The enhanced IgG response and a concomitantly enhanced IgM response are T-cell dependent and occur only if PHA is given 2 days after SSS-III immunization. The absence of immunologic memory to SSS-III in mice previously immunized and treated with PHA implies that enhanced IgG production results from the activation of amplifier T cells and not the helper T cells which are required for memory.