EFFECT OF SHORT-CHAIN FATTY-ACIDS ON GROWTH OF THE RUMINAL BACTERIUM STREPTOCOCCUS-BOVIS

The effects of the sodium salts of acetic, propionic, butyric (volatil e fatty acids, VFAs) and lactic acids on the growth of Streptococcus b ovis JB1 were studied. Increasing concentrations (0.05-0.3 M) of VFAs or lactic acid produced progressive drops in the S. bovis growth rate. With concentrations of 0.1 M and higher, the inhibition was greater a t pH 5.5 than at pH 6.5. Minimum relative growth rates of 0.65 and 0.5 6 were produced by 0.3 M of butyric (at pH 6.5) and lactic (at pH 5.5) acids, respectively. The inhibitory potency at pH 6.5 decreased in th e order butyrate > propionate > acetate = lactate, while at pH 5.5 the order was lactate = butyrate > propionate > acetate. The inhibition e licited by acetate and lactate at pH 6.5 was similar to the inhibition caused by NaCl, but at pH 5.5, all the acids were more inhibitory tha n NaCl. Ethyleneglycol did not affect growth in any case. The effects of VFAs in mixtures seemed to differ from their effects when tested se parately. It is suggested that the inhibitions observed resulted from both the increased ionic strength in the medium and the ability of the acids to solubilize in the bacterial membrane and affect intracellula r enzymes.