Flux Analyzer

Overview

The flux-analyzer skill transforms raw FBA output into actionable biological knowledge. It operates on FBA result files and the COBRApy model to produce gene essentiality maps, phenotypic phase planes (PPP), flux sampling distributions, pathway-level summaries, and product secretion profiles.

This skill is the metabolic-modelling analogue of event reconstruction and phenomenology summary stage in the ColliderAgent pipeline: it turns numbers into biology.

Workflow

Step 1 — Load Model and FBA Results

import cobra
import cobra.io
import cobra.flux_analysis
import cobra.sampling
import pandas as pd
import numpy as np
import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt

model = cobra.io.load_json_model("my_model.json")
fba_fluxes = pd.read_csv("fba_fluxes.csv", index_col=0)["flux_mmol_gDW_h"]
wt_growth = model.optimize().objective_value
print(f"Wild-type growth: {wt_growth:.4f} h^-1")

Step 2 — Gene Essentiality Analysis

Essential genes are those whose deletion reduces growth to below 5% of wild-type — a widely used lethality criterion.

from cobra.flux_analysis import single_gene_deletion, double_gene_deletion

# --- Single gene essentiality ---
sg_deletion = single_gene_deletion(model)
sg_deletion.columns = ["growth", "status"]
sg_deletion["is_essential"] = sg_deletion["growth"] < 0.05 * wt_growth
sg_deletion["growth_fraction"] = sg_deletion["growth"] / wt_growth

essential_genes = sg_deletion[sg_deletion["is_essential"]]
print(f"Essential genes: {len(essential_genes)} / {len(model.genes)}")
sg_deletion.to_csv("gene_essentiality.csv")

# --- Double gene essentiality (synthetic lethality) ---
# Limit to a focused gene set to reduce compute time
target_genes = list(model.genes)[:50]   # adjust as needed
dg_deletion = double_gene_deletion(model, target_genes, target_genes)
dg_deletion.columns = ["growth", "status"]
dg_deletion["is_synthetic_lethal"] = dg_deletion["growth"] < 0.05 * wt_growth
dg_deletion.to_csv("double_gene_essentiality.csv")

Step 3 — Reaction Essentiality Analysis

from cobra.flux_analysis import single_reaction_deletion

sr_deletion = single_reaction_deletion(model)
sr_deletion.columns = ["growth", "status"]
sr_deletion["is_essential"] = sr_deletion["growth"] < 0.05 * wt_growth
essential_rxns = sr_deletion[sr_deletion["is_essential"]]
print(f"Essential reactions: {len(essential_rxns)} / {len(model.reactions)}")
sr_deletion.to_csv("reaction_essentiality.csv")

Step 4 — Phenotypic Phase Plane (PPP)

The PPP maps growth rate over a 2D grid of two nutrient uptake rates, revealing metabolic phase transitions (aerobic growth, mixed-acid fermentation, etc.).

from cobra.flux_analysis import production_envelope

# Phase plane: glucose uptake vs. oxygen uptake
ppp = production_envelope(
    model,
    ["EX_glc__D_e", "EX_o2_e"],   # x and y axes
    objective=model.reactions.get_by_id("BIOMASS_Ec_iJO1366_core_53p95M"),
    points=20,
)

print(ppp.head())

# Plot heatmap
fig, ax = plt.subplots(figsize=(8, 6))
pivot = ppp.pivot_table(
    index="EX_o2_e", columns="EX_glc__D_e", values="flux_maximum"
)
im = ax.imshow(pivot.values, aspect="auto", origin="lower",
               cmap="viridis",
               extent=[ppp["EX_glc__D_e"].min(), ppp["EX_glc__D_e"].max(),
                       ppp["EX_o2_e"].min(),    ppp["EX_o2_e"].max()])
plt.colorbar(im, ax=ax, label="Growth rate (h$^{-1}$)")
ax.set_xlabel("Glucose uptake (mmol/gDW/h)")
ax.set_ylabel("O$_2$ uptake (mmol/gDW/h)")
ax.set_title("Phenotypic Phase Plane")
fig.tight_layout()
fig.savefig("phenotypic_phase_plane.pdf", dpi=300)
fig.savefig("phenotypic_phase_plane.png", dpi=150)
plt.close(fig)
print("PPP saved to phenotypic_phase_plane.pdf")

Step 5 — Flux Sampling

Flux sampling explores the full space of feasible steady-state flux distributions, revealing which reactions have wide vs. narrow feasible ranges.

# OptGP sampler: Markov-chain Monte Carlo in flux cone
samples = cobra.sampling.sample(model, n=1000, method="optgp",
                                thinning=100, processes=4)
# samples is a DataFrame: rows = samples, columns = reaction IDs
samples.to_csv("flux_samples.csv", index=False)

# Violin plot for key central metabolism reactions
KEY_REACTIONS = ["PFK", "PGI", "PDH", "CS", "AKGDH",
                 "EX_glc__D_e", "EX_ac_e", "EX_co2_e"]
key_data = samples[[r for r in KEY_REACTIONS if r in samples.columns]]

fig, ax = plt.subplots(figsize=(10, 5))
ax.violinplot([key_data[c].values for c in key_data.columns],
              positions=range(len(key_data.columns)),
              showmedians=True)
ax.set_xticks(range(len(key_data.columns)))
ax.set_xticklabels(key_data.columns, rotation=45, ha="right")
ax.set_ylabel("Flux (mmol/gDW/h)")
ax.set_title("Flux Sampling Distribution (n=1000)")
fig.tight_layout()
fig.savefig("flux_sampling_violin.pdf", dpi=300)
plt.close(fig)
print("Flux sampling violin plot saved.")

Step 6 — Pathway-Level Flux Aggregation

Group reactions by metabolic subsystem and compute total absolute flux per pathway — a proxy for pathway activity.

pathway_flux = {}

for rxn in model.reactions:
    subsystem = rxn.subsystem or "Unknown"
    flux_val = abs(fba_fluxes.get(rxn.id, 0.0))
    pathway_flux[subsystem] = pathway_flux.get(subsystem, 0.0) + flux_val

pathway_df = (pd.Series(pathway_flux, name="total_abs_flux")
              .sort_values(ascending=False)
              .reset_index()
              .rename(columns={"index": "subsystem"}))

pathway_df.to_csv("pathway_flux_summary.csv", index=False)

# Bar chart of top 15 pathways
top15 = pathway_df.head(15)
fig, ax = plt.subplots(figsize=(10, 6))
ax.barh(top15["subsystem"][::-1], top15["total_abs_flux"][::-1],
        color="steelblue")
ax.set_xlabel("Sum of |flux| (mmol/gDW/h)")
ax.set_title("Top 15 Pathway Activities (FBA)")
fig.tight_layout()
fig.savefig("pathway_activity.pdf", dpi=300)
plt.close(fig)
print("Pathway activity chart saved.")

Step 7 — Secretion Product Prediction

Identify exchange reactions carrying positive flux (secretion) at optimal growth — these are by-products and potential products of interest.

secretion = {}

for rxn in model.exchanges:
    flux = fba_fluxes.get(rxn.id, 0.0)
    if flux > 1e-6:   # positive = secretion
        met = list(rxn.metabolites)[0]
        secretion[rxn.id] = {
            "metabolite": met.name,
            "formula": met.formula,
            "flux_mmol_gDW_h": flux,
        }

sec_df = pd.DataFrame(secretion).T.sort_values("flux_mmol_gDW_h",
                                                ascending=False)
print("\nSecreted products:")
print(sec_df.to_string())
sec_df.to_csv("secretion_profile.csv")

Key Conventions

Output File Conventions

Common Failure Modes

• PPP returns all zeros: objective reaction ID does not match model; check model.objective.expression for correct reaction ID.
• Flux sampling ACHR crashes: optgp (default) is more stable for large models; for models with >5000 reactions use processes=1 to debug.
• No secretion products: model may be forced to be strictly aerobic with all carbon converted to CO2; verify exchange bounds allow secretion (ub = 1000 on exchange reactions).