Blog post listing category multiple ids

I have isotope filter system working on my website, and I have the filter working with the categories by the categories id number. however if a post as more that one it only show the first id number. I need it to add multiple id numbers to the class.

<div class="five-col grid-item <perch:blog id="categories" />">
    <a href=" <perch:blog id="postURL" />" rel="bookmark">
        <img class="repson-img" src="<perch:blog id="image" type="image" />" />
    </a>

    <div class="summary">
        <p class="time"> <perch:blog id="time" type="text" textile="true" /> </p>
        <h3> <perch:blog id="postTitle" type="text" textile="true" /> </h3>
        <p> <perch:blog id="excerpt" type="textarea" textile="true" words="20" append="..." /> </p>
    </div>
</div>