Reactivity of phage clones by ELISA with sera of patients and controls. Microtiter plates were coated with anti-phage antibody (1:800) and incubated with 2 × 1010 phages/mL followed by the addition of sera (1:100). The detection of the reaction was carried out with peroxidase conjugated anti-human antibody and OPD as chromogen. Each cell in the table represents an individual. The analysis was performed in duplicate and the hatched cells represent positive samples. The cut-off value was calculated as the average of the optical density plus twice the standard deviation obtained with endemic controls.