Importance of pfkA for rapid growth of Enterobacter cloacae during colonization of crop seeds

Enterobacter cloacae A-11 is a prototrophic, glycolytic mutant of strain 501R3 with a single transposon insertion in pfkA. The populations of strain A-11 on cucumber and radish seeds were smaller than the populations of strain 501R3 in natural soil, but the populations of these two strains on pea, soybean, sunflower, and sweet corn seeds were similar (D. P. Roberts, P. D. Dery, I. Yucel, J. Buyer, M. A, Holtman, and D. Y. Kobayashi, Appl. Environ. Microbiol. 65:2513-2519, 1999). The net effect of the mutation in pfkA in vitro was a shift from rapid growth on certain carbohydrates detected in seed exudates to much slower growth on other carbohydrates, amino acids, and organic acids. The impact of the mutation in pfkA was greatest on the growth rate of E. cloacae on the seeds that released the smallest quantities of fructose, other carbohydrates, and amino acids. Corn, pea, soybean, and sunflower seeds released total amounts of carbohydrates and amino acids at rates that were approximately 10- to 100-fold greater than the rates observed with cucumber and radish seeds for the first 24 h after inhibition began. The growth rate of strain A-11 was significantly less (50% less) than the growth rate of strain 501R3 on radish seeds, and the growth rate of strain A-11 was too low to estimate on cucumber seeds in sterile sand for the first 24 h after inhibition began. The growth rate of strain A-11 was also significantly lower on soybean seeds, but it was only 17% lower than the growth rate of strain 501R3. The growth rates of strains 501R3 and A-11 were similar on pea, sunflower, and corn seeds in sterile sand for the first 30 h after imbibition began. Large reductions in the growth rates of strain A-11 on seeds were correlated with subsequent decreased levels of colonization of seeds compared to the levels of colonization of strain 501R3. The strain A-11 populations were significantly smaller than the strain 501R3 populations only on radish and cucumber seeds. The mutation in pfkA appears to decrease the level of colonization by E. cloacae for seeds that release small quantities of reduced carbon compounds by decreasing the size of the pool of compounds that support rapid growth by this bacterium.