Esi on arrear salary

Suppose if an employee is drawing salary of Rs 14,500/- for the month of Apr 2011. ESI has been deducted and its paid to the department. If in case company revised his salary in June 2011 to Rs 16,000/- with RETROSPECTIVE EFFECT and paid arrear salary for apr and may 2011, whether he need to contribute towards ESI and shall company contribute towards ESI for the remaining months in the same contribution period?
 If yes then on which amount - On upper limit of Rs 15,000/- or on actual salary? Whether he need to contribute ESI on the Arrear salary?