Relation between chemotaxis and consumption of amino acids in bacteria

Abstract

Chemotaxis enables bacteria to navigate chemical gradients in their environment, accumulating toward high concentrations of attractants and avoiding high concentrations of repellents. Although finding nutrients is likely to be an important function of bacterial chemotaxis, not all characterized attractants are nutrients. Moreover, even for potential nutrients, the exact relation between the metabolic value of chemicals and their efficiency as chemoattractants has not been systematically explored. Here we compare the chemotactic response of amino acids with their use by bacteria for two well-established models of chemotactic behavior, Escherichia coli and Bacillus subtilis. We demonstrate that in E. coli chemotaxis toward amino acids indeed strongly correlates with their utilization. However, no such correlation is observed for B. subtilis, suggesting that in this case, the amino acids are not followed because of their nutritional value but rather as environmental cues.

Figure 1

FRET‐based analysis of chemotactic responses of E . coli to amino acids. A. Exemplary measurement of FRET response to alanine in the wild‐type cells, using the phosphorylation‐dependent interaction between CheZ‐CFP and CheY‐YFP. Buffer‐adapted cells were stimulated with step‐like addition and subsequent removal of indicated concentrations of amino acid (indicated by arrows). The response to a saturating stimulus, 100 μM α‐methyl‐DL‐aspartate (MeAsp), was used as a positive control. Ratio of YFP to CFP fluorescence is proportional to the amount of FRET complex formed and thus to the kinase activity. B. Dose responses of wild‐type cells to amino acid attractants. Relative kinase activity, derived from the YFP/CFP ratio, was plotted relative to the steady‐state activity in the buffer. Zero activity was obtained by a saturating stimulation with 100 μM MeAsp. Data were fitted using a Hill equation. Error bars here and throughout indicate standard errors. C and D. Correlation between the values of EC ₅₀ in the wild type and in receptorless cells expressing Tar (C) or Tsr (D) as a sole receptor. Amino acid ligands sensed by Tar fall into two groups indicated by dotted lines, suggesting that Tar is primary sensor for group 1 and secondary sensor for group 2 amino acids.

Figure 2

Consumption of amino acids by E . coli. A. Profile of amino acid uptake by E . coli  MG1655 in minimal medium containing equimolar mixture of amino acids as the sole source of carbon and nitrogen. Profiles of amino acids with less than 50% utilization are shown in Fig. S5A. B. Correlation between t_1/2 of amino acid uptake, derived from (A), and the EC ₅₀ of chemotactic response as determined in Fig. 1B. C. Profiles of amino acid uptake by E . coli  NCM3722 under the same growth conditions as in (A). Profiles of amino acids with less than 50% utilization are shown in Fig. S9A. D. Profiles of amino acid uptake by E . coli  NCM3722 in minimum medium under anaerobic conditions. Profiles of amino acids with less than 50% utilization are shown in Fig. S9B.

Figure 3

Repellent responses and growth inhibition by amino acids in E. coli. A. Dose responses to amino acid repellents. Measurements were done and plotted as in Fig. 1A and B. The response was normalized to the prestimulus pathway activity as in Fig. 1B. B. Growth inhibition by amino acids. Cells were grown in the M9 minimal glucose medium containing individual L‐amino acids at the final concentration of 1 mM. Culture density was determined after 8 h, and relative growth was quantified by normalizing OD ₆₀₀ values to the control culture with no addition of amino acids. Red line indicates the relative growth of control culture.

Figure 4

Chemotactic response to amino acids in B . subtilis. A. Exemplary FRET measurement of the chemotactic response in B . subtilis cells, using the phosphorylation‐dependent interaction between CFP‐FliY and CheY‐YFP. Measurements were performed as in Fig. 1A. The response to a saturating stimulus of 100 μM asparagine served as positive control. Arrows indicate addition and removal of the indicated stimuli. B. Dose responses to all amino acids attractants in B . subtilis, derived from FRET measurements performed as in (A).

Figure 5

Consumption of amino acids by B . subtilis. A. Amino acid uptake profiles for B . subtilis cells grown in the minimal medium containing equimolar mixture of 20 l‐amino acids as the sole source of carbon and nitrogen. Profiles of amino acids with less than 50% utilization are shown in Fig. S12A. B. Correlation analysis between t_1/2 of amino acid uptake derived from (A) and the EC ₅₀ of chemotactic responses determined from Fig. 4B. C. Amino acid uptake profiles for B . subtilis  NCIB3610 grown under the same conditions as in (A). Profiles of amino acids with less than 50% utilization are shown in Fig. S12B.