A process for fermenting syngas comprising propagating a culture of acetogenic bacteria effective for inoculating a main reactor, the propagating including, i) inoculating a first culture of acetogenic bacteria into a pre-reactor to provide a minimum viable cell density. ii) growing the culture of acetogenic bacteria in the pre-reactor to provide a pre-reactor target cell density, (a) wherein, if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is greater than or equal to a minimum viable cell density, transfer a volume of the pre-reactor to the main reactor in an amount effective for providing a minimum viable cell density in the main reactor, or (b) if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is less than a minimum viable cell density, transfer a volume of the pre-reactor to a subsequent pre-reactor in an amount effective for providing a minimum viable cell density in a subsequent pre-reactor, and iii) repeat step ii until a volume of pre-reactor is transferred to the main reactor, and wherein 25% to 75% of the volume of a pre-reactor is used to inoculate a subsequent pre-reactor or main reactor, whilst the remaining volume is maintained for use for re-inoculation should any subsequent reactor fail. i) inoculating a first culture of acetogenic bacteria into a pre-reactor to provide a minimum viable cell density. ii) growing the culture of acetogenic bacteria in the pre-reactor to provide a pre-reactor target cell density, (a) wherein, if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is greater than or equal to a minimum viable cell density, transfer a volume of the pre-reactor to the main reactor in an amount effective for providing a minimum viable cell density in the main reactor, or (b) if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is less than a minimum viable cell density, transfer a volume of the pre-reactor to a subsequent pre-reactor in an amount effective for providing a minimum viable cell density in a subsequent pre-reactor, and (a) wherein, if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is greater than or equal to a minimum viable cell density, transfer a volume of the pre-reactor to the main reactor in an amount effective for providing a minimum viable cell density in the main reactor, or (b) if (the pre-reactor target cell density multiplied by the pre-reactor volume) ÷ (a volume of the main reactor multiplied by (a volume of the pre-reactor ÷ a volume of the pre-reactor which is transferred)) is less than a minimum viable cell density, transfer a volume of the pre-reactor to a subsequent pre-reactor in an amount effective for providing a minimum viable cell density in a subsequent pre-reactor, and iii) repeat step ii until a volume of pre-reactor is transferred to the main reactor, and wherein 25% to 75% of the volume of a pre-reactor is used to inoculate a subsequent pre-reactor or main reactor, whilst the remaining volume is maintained for use for re-inoculation should any subsequent reactor fail.