The present invention relates, e.g., to a minimal set of protein-coding genes which provides the information required for replication of a free-living organism in a rich bacterial culture medium, wherein (1) the gene set does not comprise the 101 genes listed in Table 2; and/or wherein (2) the gene set comprises the 381 protein-coding genes listed in Table 3 and, optionally, one of more of: a set of three genes encoding ABC transporters for phosphate import (genes MG410, MG411 and MG412; or genes MG289, MG290 and MG291); the lipoprotein-encoding gene MG185 or MG260; and/or the glycerophosphoryl diester phosphodiesterase gene MG293 or MG385.