Keep post format class with isotope filter

you did not describe what you edited to add the isotope filtering, but you seem to have removed post_class() from this line to add your isotope class:

<article class="<?php echo $termsString; ?>item"> 

change it to:

<article <?php post_class( $termstring . 'item' ); ?>"> 

http://codex.wordpress.org/Function_Reference/post_class