chrome Content of Bacteria
A. E F F E C T OF OXYGEN T E N S I O N
T h e cytochromes of yeast grown under anaerobic conditions are qualita
tively different from those of aerobically grown y e a s t .1 1 9 , 1 2 0 Similar experi
ments with bacteria have shown quite different effects with different species.
I n a number of kinds of bacteria, there was no change observed in t h e positions of the cytochrome absorption bands when t h e organisms were grown aerobically as compared with those grown under anaerobic condi
tions; however, there was a decrease in the intensities of t h e absorption bands in the anaerobically grown cells.1 2 1 - 1 2 3 On the other hand, M o s s1 2 4 observed t h a t the cytochrome a2 content of E. coli and of Aerobacter aero
genes125 was greater when the bacteria were grown at lower oxygen tensions.
When Aerobacter aerogenes were grown with measured oxygen tensions,1 2 5 the absorption band of cytochrome a2 was found to be greatest a t an oxygen concentration of 10~"6 M. T h e content of cytochrome b remained relatively
7. CYTOCHROMES IN AEROBIC ELECTRON TRANSPORT 391 constant when the oxygen concentration was varied between 10~6 and 10~3 Μ. Particles prepared from E. coli grown anaerobically were found to be lacking in cytochrome a 2, while the pigment was present on particles pre
pared from cells grown with aeration.3 3 T h e observation of C a s t o r5 0 t h a t cytochrome a2 was present only in cells harvested in the logarithmic growth phase m a y be related to effects of oxygen tension. Lenhoff and K a p l a n1 1 4*1 2 6 found t h a t the content of cytochrome c in Pseudomonas fiuorescens de
creased when t h e bacteria were grown under strong aeration. I n t h e case of Pseudomonas fiuorescens, nitrate was present during growth. Since the bac
teria are nitrate reducers, this is a more complicated case. Verhoeven and T a k e d a6 3 found similar effects of air in other denitrifying organisms, as well as on one t h a t could not reduce nitrate. Pseudomonas denitrificans also fits this p a t t e r n of effect of oxygen tension; but no change in cytochrome content of Micrococcus denitrificans with changes in oxygen tension was observed during growth.5 4 V e r n o n3 4 a found t h a t the relative amounts of t h e two c-type cytochromes of a nitrate-reducing pseudomonad were different when t h e cells were grown in t h e presence or absence of air.
Faced with so m a n y different effects, it is impossible to make any gener
alizations about the relationship between oxygen tension during growth and cytochrome content of t h e bacteria. I t is even more difficult to explain why variations of oxygen tension make a difference in some species and have no effect in others. More careful and extensive studies are indicated.
B . E F F E C T OF GROWTH R A T E
Chaix and P e t i t2 9 ·1 2 7 examined t h e cytochrome absorption bands of B. subtilis a t t h e temperature of liquid air and observed changes in t h e relative intensities of the bands which appear to be influenced b y the growth rate. These observations were made under conditions where oxygen was stated not to be limiting.
C. E F F E C T OF THE IRON CONTENT OF THE M E D I U M
Growth of Aerobacter indologenes in an iron-deficient m e d i u m1 2 8 resulted in a loss of all of the cytochrome bands. Under similar conditions, the cyto
chromes of Aerobacter aerogenes also decreased, with the cytochrome a2 de
creased to a greater extent t h a n t h e o t h e r s .3 4 b Corynebacterium diphtheriae grown within a certain range of iron concentration in the medium, showed on addition of iron an increase of the cytochrome content of the cells. T h e changes in cytochrome content could not always be correlated with changes in respiratory activity. Lenhoff et al.129 found t h a t the iron content of the medium affected the growth rate of Pseudomonas fiuorescens only when the cells were grown anaerobically, since t h e cytochrome content was low when the bacteria were grown in t h e presence of air.
392 LUCILE SMITH D . OTHER EFFECTS
Changes in the cytochrome content of bacteria have been observed under apparently constant conditions of culture. Tissi£res3 4 b described pronounced differences in the cytochrome a2 content of cultures of Aero
hacter aerogenes grown under similar conditions. Keilin1 3 0 found t h a t t h e cytochrome a2 of Acetobacter pasteurianum varied with t h e culture medium as well as the age of the organism. Effects of composition of the growth medium on the cytochrome content have been observed in other instances.1 3 1 ·1 3 2 I n a strain of Azotobacter chroococcum which originally showed a substantial absorption peak of cytochrome a2, this absorption peak disappeared after several transfers, with no measurable change in the respiratory activity.1 7
Although the work on changes in cytochrome content of bacteria has not been helpful so far in delineating the functions of t h e different cyto
chromes, it has pointed to the need for well-regulated conditions in experi
m e n t s with bacterial cytochromes.
X. Summary
As in mammalian and plant cells and yeast, a series of cytochrome pig
ments forms part of the respiratory chain of m a n y bacteria. T h e cytochrome pigments are at the end of the chain where reaction with oxygen takes place, some of t h e cytochromes being able to react rapidly with molecular oxygen. Although the cytochromes appear to perform similar functions in all respiratory chain systems, t h e cytochromes of bacteria often differ from those of plant and animal tissues in absorption spectra and other physical and chemical characteristics and always in specificity. Two in
teresting aspects of the bacterial systems a r e : the variety of cytochromes which m a y be present, and t h e existence of more t h a n one oxidase in some cells. T h e cytochromes of t h e bacterial systems, which are linked to insolu
ble particulate m a t t e r within t h e cells, can have very high turnover rates.
T h e bacterial respiratory chain systems have also been shown t o be able to link the oxidation reactions with the formation of A T P .
I n recent years considerable success has been achieved in isolating and purifying cytochromes from bacteria. This type of progress should soon lead t o an increase in our knowledge of t h e reactions of these pigments.
I n some instances t h e wide variety met among t h e cytochromes of bacteria has been useful in elucidating relationships among t h e different cytochromes. For example, Μ. pyogenes var. albus with cytochrome a but no &z and B. subtilis with a different ratio of cytochrome a to a3 from t h a t in mammalian tissues were valuable material for studying t h e reactions of these two cytochromes.5 1 A s t u d y of t h e combinations of cytochrome components of different bacteria leads t o t h e conclusion t h a t t h e only
re-7. CYTOCHROMES I N AEROBIC ELECTRON TRANSPORT 393
quirement is for a mixture of several cytochromes with appropriately sep
a r a t e d redox potentials.
Finally, it m u s t be noted t h a t some bacteria d o n o t h a v e a n y cytochrome pigments.3 1 ·1 3 3 Examples of these a r e n o t only t h e obligately anaerobic bac
teria with no oxidative metabolism, such as Clostridia, b u t also some fac
u l t a t i v e anaerobes like streptococci or pneumococci. Some bacteria m a y respire b y means of a flavoprotein oxidase (see C h a p t e r 9 ) . All of t h e ob
ligately aerobic bacteria studied h a v e relatively high concentrations of cytochromes.
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