Escherichia coli generates about 14 muM hydrogen peroxide (H2O2) per s when
it grows exponentially in glucose medium. The steady-state intracellular c
oncentration of H2O2, depends on the rates at which this H2O2 is dissipated
by scavenging enzymes and by efflux from the cell. The rates of H2O2 degra
dation by the two major scavenging enzymes, alkyl hydroperoxide reductase,
and catalase, were quantified. In order to estimate the rate of efflux the
permeability coefficient of membranes for H2O2 was determined. The coeffici
ent is 1.6 x 10(-3) cm/s, indicating that permeability is substantial but n
ot unlimited. These data allowed internal H2O2 fluxes and concentrations to
be calculated. Under these growth conditions, Ahp scavenges the majority o
f the endogenous H2O2, with a small fraction degraded by catalase and virtu
ally none persisting long enough to penetrate the membrane and exit the cel
l. The robust scavenging activity maintains the H2O2 concentration inside g
lucose-grown cells at < 10(-7) M, substantially below the level (10(-6) M)
at which toxicity is evident. When extracellular H2O2 is present, its flux
into the cell can be rapid, but the internal concentration may still be an
order of magnitude lower than that outside. The presence of such gradients
was confirmed in experiments that revealed different degrees of oxidative s
tress in cocultured scavenger-deficient mutants. The limited permeability o
f membranes to H2O2 rationalizes the compartmentalization of scavenging sys
tems and predicts that bacteria that excrete redox-cycling drugs do not exp
erience the same H2O2 dose that they impose on their competitors.