How to manually invalidate fields after AJAX call with jQuery validate

Note: the below code is just for demonstration and I am using jQuery and jQuery validate plugin.

Suppose I have a form with two fields (email and inventory number):

<form action="/something" method="post" id="demoForm">
     Inventory No: <input type="text" class="required" name="inventory_no" />
     Email: <input type="text" class="required" name="email" />
     <input type='submit' value='submit' />
</form>

Binding plugin to form:

jQuery(function(){
    jQuery('#demoForm').validate();
});

//handle user submit

jQuery('#demoForm').bind('submit', function (e) {
    e.preventDefault();

    if (jQuery(e.target).valid()) {
        //form is valid submit request thru ajax

        jQuery.ajax({
            /*more options here*/

            success: function (data) {

                /*
                 Server process request and finds that, email is already in use
                 and inventory number is invalid
                 so it sends data in some format may pe JSon
                 with field names and message indicating errors
                 eg: Email:"Already in use"
                 InventoryNo: "Invalid Inventory No",
                 Now can i invalidate these two fields on form mannualy
                 and display the message received from server for each field
                */
            }
        });
    }
});