• Fusobacterium nucleatum;
  • growth;
  • metabolism;
  • continuous culture

Fusobacterium nucleatum ATCC 10953, a type strain of one of the newly proposed subspecies of this group of organisms, was grown anaerobically in continuous culture in a chemically defined medium. Its response to conditions of varying pH, nutritional environment, and imposed growth rate were then examined. The organism failed to grow at pH 7.8 but grew at pH 5.8, although the cell yield was greatly reduced. At pH 6.8 the cell yield was halved and less than 50% of available glucose was consumed. The optimum growth pH was around 7.4 when the culture appeared to be limited for both glucose and the amino acids glutamate, histidine and serine. Some intracellular polyglucose (IP) was produced and acetate, butyrate and ammonia were the major fermentation end-products, as they were under all growth conditions tested. Increasing the available glucose or amino acids did not alter cell numbers but the amount of IP was greatly increased. When glucose was omitted from the medium, the cell yield was halved and the culture then became limited for lysine as well as for glutamate, histidine and serine. Growth rate had little overall effect on the organism's physiology and the maximum growth rate at pH 7.4 was 0.20 h−1, a doubling time of 3.5 h. Glucose was thus channelled into stable IP synthesis only when the growth limitation imposed by lack of fermentable amino acids was relieved. The metabolism of IP and the ability to obtain carbon and energy from a variety of substrates may explain why F. nucleatum is one of the most commonly detected organisms in subgingival dental plaque.