Small universal tissue P systems with symport/antiport rules

In this note, we consider the problem of looking for small universal one-symbol tissue P systems with symport/antiport rules. It is proved that six cells suffice to generate any recursively enumerable set of natural numbers by such a one-symbol tissue P system with symport/antiport rules, under the restriction that only one channel is allowed between two cells or between a cell and the environment. As for the case of allowing two channels between a cell and the environment, it is shown that the computational completeness can be obtained by one-symbol tissue P systems with symport/antiport rules having at most five cells. These results partially answer an open problem formulated by Artiom Alhazov, Rudolf Freund and Marion Oswald