A method for retention testing sterilizing grade filters comprises: a) providing a stock of Acholeplasma laidlawii; b) growing up the stock of A. laidlawii for about 24 hours or less in a single serum-free growth medium that supports cell growth to a high titer and yields a cellular morphology where the cells are small, deaggregated and spherical, thereby producing a bacterial culture; c) challenging a test filter by filtering the bacterial culture through the test filter at a known challenge level, thereby producing a filtrate downstream of the test filter; and d) detecting concentration of A. laidlawii in the filtrate. Serum-free growth media for cultivating or storing A. laidlawii are also described.