Growth of DHFR-TS deficient T. thermophila compared to wildtype. A: Selection of DHFR-TS knock out cells by growth on thymidine. T. thermophila cells with disrupted DHFR-TS gene (clone 1–3) do not grow without the presence of thymidine (CDM-T), whereas wildtype cells do. Addition of thymidine to the medium (CDM+T) recovers growth. B: Growth kinetics of DHFR-TS knock out cells compared to wildtype cells in media with or without thymidine show, that the knock out strain (pKOI) is growing as fast as wildtype cells on thymidine supplemented media (CDM+T). Knock out cells die without thymidine present (CDM-T). The curves are calculated by mean values of at least three independent experiments.