Multiple modes of inhibition of spore germination and outgrowth by reduced pH and sorbate

Germination and outgrowth of three strains of Clostridium botulinum in PYEG medium were measured by phase contrast microscopy. Reduction in pH from 7 to 5.5 completely inhibited germination of strain 12885A, reduced the extent of germination of strain 62A and had no effect on the extent of germination of strain 53B. At pH 5.5, 225 mg/l of undissociated sorbic acid had no effect on the germination of strain 53B, while at pH 6.5, 225 mg/l of undissociated sorbic acid completely inhibited germination of strains 62A and 12885A. Outgrowth of germinated spores of strains 62A and 53B was not inhibited at pH 5.5, but the addition of sorbate (225 mg/l undissociated sorbic acid) completely inhibited outgrowth. Sorbate inhibited germination of Cl. botulinum and Bacillus cereus spores triggered to germinate by amino acids. Inhibition occurred after germinant binding, as measured by commitment to germinate.