an epidemiological study of listeriosis in dairy cattle

Investigating Non-specific Bindings : Some non-specific binding was still detectable
when BSA, Marvel or Pig Albumin were used whereas this was not detectable when
FCS was used. FCS was used in the blocking step in all subsequent tests.
Investigating Specificity of ELISA: Absorption of hyper-immune serum to L.
monocytogenes with CP-LLO and culture supernatant abolished the antibody activity.
Table 5. 10. indicates that antigen-antibody binding was specific.
Table 5. 10. Plate plan and Optical densities
SD B Sample serum Standard Goat hyper-immune serum B
1 2 3 4 5 6 7 8 9 10 11 12
2 0.055 0.059 0.078 0.063 0.505 0.800 0.833 0.056 0.289 0.057 0.465 0.048
4 0.055 0.058 0.068 0.060 0.554 0.696 0.736 0.058 0.236 0.055 0.406 0.049
8 0.055 0.058 0.061 0.057 0.214 0.568 0.630 0.057 0.173 0.057 0.306 0.051
16 0.055 0.058 0.061 0.058 0.163 0.523 0.493 0.057 0.115 0.057 0.271 0.052
32 0.055 0.057 0.059 0.058 0.121 0.361 0.344 0.057 0.094 0.057 0.211 0.052
64 0.055 0.058 0.058 0.058 0.090 0.268 0.239 0.055 0.075 0.055 0.150 0.052
128 0.052 0.055 0.055 0.056 0.071 0.055 0.054 0.054 0.065 0.054 0.114 0.052
256 0.054 0.055 0.055 0.054 0.062 0.054 0.054 0.054 0.061 0.056 0.86 0.050
SD Serum dilution, B Blank, Columns 2 and 8 treated with LLO, Columns 3 and 9 treated with culture
supernatant, Columns 4 and 10 heat killed Listeria organisms, Columns 5 and 11 untreated serum
(control).
Immune Status of the animals (a field trial of the ELISA) : Blood samples were taken
only once from 77 animals. The OD values obtained were converted to Log values and
then the relationship between age, bacteriological results and antibody level was
investigated. When animals were grouped by the frequency of being positive for
Listeria there was a negative correlation between the frequency of detection of
Listeria and the concentration of the antibody to Listeria. This was statistically
significant (Pearson’s correlation coefficient = -0.35, 95% CL -0.54-0.13, P=0.03).
Animals that became positive for Listeria on 6 and 7 occasions were excluded because
there were very small numbers in these groups (Table 5. 11).
151