Incomplete term slugs output from a foreach loop with get_the_terms

I’m using this PHP to get all the product category slugs on a single product page:

global $post;
$terms = get_the_terms( $post->ID, 'product_cat' );
if(is_array($terms)){
foreach ($terms as $term) {
    $product_cat_slug = $term->slug;
    $product_cat_slugs = ' product_cat-' . $product_cat_slug;
    echo $product_cat_slugs;
}
}

The line echo $product_cat_slugs; outputs product_cat-category1 product_cat-category2, which I will use for div classes.

The problem is that when I delete the echo $product_cat_slugs; from the function above and use <?php echo $product_cat_slugs; ?> elsewhere on the page, all I get for output is the last category slug product_cat-category2, and not product_cat-category1 product_cat-category2.

What’s wrong?