A sensitive and rapid method was developed to measure the mineralization of 14 C-labeled organic compounds at picogram-per-milliliter or lower levels in samples of natural waters and sewage. Mineralization was considered to be equivalent to the loss of radioactivity from solutions. From 93 to 98% of benzoate, benzylamine, aniline, phenol, and 2,4-dichlorophenoxyacetate at one or more concentrations below 300 ng/ml was mineralized in samples of lake waters and sewage, indicating little or no incorporation of carbon into microbial cells. Assimilation of 14 C by the cells mineralizing benzylamine in lake water was not detected. Mineralization in lake waters was linear with time for aniline at 5.7 pg to 500 ng/ml, benzylamine at 310 ng/ml, phenol at 102 fg to 10 mg/ml, 2,4-dichlorophenoxyacetate at 1.5 pg/ml, and di-(2-ethylhexyl) phthalate at 21 pg to 200 ng/ml, but it was exponential at several p -nitrophenol concentrations. The rate of mineralization of 50 and 500 ng of aniline per ml and 200 pg and 2.0 ng of the phthalate per ml increased with time in lake waters. The phthalate and 2,4-dichlorophenoxyacetate were mineralized in samples from a eutrophic but not an oligotrophic lake. Addition to eutrophic lake water of a benzoate-utilizing bacterium did not increase the rate of benzoate mineralization at 34 and 350 pg/ml but did so at 5 and 50 ng/ml. Glucose and phenol reduced the percentage of p -nitrophenol mineralized at p -nitrophenol concentrations of 200 ng/ml but not at 22.6 pg/ml and inhibited the rates of mineralization at both concentrations. These results show that the kinetics of mineralization, the capacity of the aquatic community to assimilate carbon from the substrate or the extent of assimilation, and the sensitivity of the mineralizing populations to organic compounds are different at trace levels than at higher concentrations of organic compounds.

The rates of mineralization of phenol, benzoate, benzylamine, p -nitrophenol, and di(2-ethylhexyl) phthalate added to lake water at concentrations ranging from a few picograms to nanograms per milliliter were directly proportional to chemical concentration. The rates were still linear at levels of <1 pg of phenol or p -nitrophenol per ml, but it was less than the predicted value at 1.53 pg of 2,4-dichlorophenoxyacetate per ml. Mineralization of 2,4-dichlorophenoxyacetate was not detected in samples of lake water containing 200 ng of the chemical per ml. The slope of a plot of the rate of phenol mineralization in samples of three lakes as a function of its initial concentration was lower at levels of 1 to 100 μg/ml than at higher concentrations. In lake water and sewage supplemented with <60 ng of 14 C-labeled benzoate or phenylacetate per ml, 95 to 99% of the radioactivity disappeared from solution, indicating that the microflora assimilated little or none of the carbon. The extent of mineralization of some compounds in samples of two lakes and sewage was least in the water with the lowest nutrient levels. No mineralization of 2,4-dichlorophenoxyacetate and the phthalate ester was observed in samples of an oligotrophic lake. These data suggest that mineralization of some chemicals at concentrations of <1 μg/ml is the result of activities of organisms different from those functioning at higher concentrations or of organisms that metabolize the chemicals at low concentrations but assimilate little or none of the substrate carbon.

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We determined how lactic acid inhibits growth of Salmonella typhimurium in yogurt. This inhibition was demonstrated by microscopic examination not to be due to bacteriolysis. Neither growth nor metabolic activity could be initiated after cells were washed in phosphate buffer and exposed to 1.5% lactic acid for 1 h at 37 degrees C, indicating that lactic acid inhibition is irreversible. The growth rate of S. typhimurium at 37 degrees C, was computed at various combinations of pH and lactic acid concentrations, and the intracellular conditions (pH and lactic acid concentration) at bacteriostasis subsequently were extrapolated. Cellular death resulted when these intracellular bacteriostatic conditions were surpassed. Thus, growing cells could be used indirectly to determine intracellular conditions at the time of death. Intracellular pH (pHi) and inhibition of the growth rate were unrelated. Also, bacteriostasis was observed when hydrochloric acid was used to lower the pHi of Salmonella to 5.5 whereas a bactericidal effect was observed when the pHi was lowered to 5.5 with lactic acid. The lactate anion, rather than the hydrogen ion, exerted the inhibitory effect against S. typhimurium. When the pHi became less than 5.3, inhibition was from the hydrogen ion concentration. Thus, lactic acid inhibition was a complex and variable mechanism in relationship with pHi Lactic acid entered the cell in the undissociated state. Once inside the cell, it dissociated because the pHi was higher than the external pH. The dissociated moiety accumulated because it could not leave the cell in this form consequently lowering the pHi. Thus, inhibition of S. typhimurium in yogurt is from the intracellular dissociated moiety of lactic acid.

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