Metabolism consists of catabolic and anabolic reactions that are organized into metabolic pathways. Catabolic pathways break down molecules to generate energy in the form of ATP via redox reactions. Anabolic pathways use this energy to synthesize larger molecules. Enzymes catalyze reactions and cells regulate pathways through feedback inhibition. Microbes metabolize carbohydrates, proteins, lipids, and nucleic acids to generate energy and building blocks through both aerobic and anaerobic pathways. Metabolic tests can be used to distinguish between microbial species in the laboratory.

1. An overview of
Microbial metabolism

2. What is Metabolism?
 Catabolism (breakdown)
Provides energy and building
blocks for anabolism and
other cellular functions
 Anabolism (biosynthesis)
Uses energy and building
blocks to build larger
molecules, macromolecules,
and cell structures
 Metabolism is composed of Catabolism
and Anabolism
The sum of controlled chemical reactions that occur within a cell
(microbial cell)

3. The role of ATP as an intermediate between
catabolism and anabolism

4.  Metabolic pathway - sequence of chemical reactions
that occur in a cell
A B C D E
 A is the initial molecule/compound (substrate)
 E is the final molecule/compound (product)
 B, C, and D are intermediates
 Each step in the pathway is mediated or facilitated by
a specific catalyst called enzyme (protein)
Catabolic and anabolic reactions are organized
into series of reactions called pathways

5. Cells use redox reactions to extract energy from nutrient
molecules such as glucose
 Redox reaction (oxidation-reduction reaction)
 Redox reactions involve the transfer of electrons from an
electron donor to an electron acceptor

6.  The energy of the reduced coenzyme NADH (or FADH2) is used to
make ATP in later reactions
Cells use redox reactions to extract energy from nutrient
molecules such as glucose
 Most biological oxidations involve the loss of hydrogen atoms
(one electron plus one proton-H+) - dehydrogenation reactions

7. Enzymes are the catalysts of biological reactions
 Specific for a given substrate/chemical reaction
 The shape of the molecule provides a distinctive site called the
active site or catalytic site of the enzyme
 The substrate specifically fits into the enzyme’s active site

8. Examples of enzymes and their names

9. Enzymes can use cofactors
and coenzymes
 Cofactors include magnesium, manganese, iron, copper, zinc,
calcium, cobalt
 Coenzymes (NAD+, NADP+, and FAD derived from vitamins) act as
electron carriers

10. Enzymes lower the activation energy needed to
“trigger a chemical reaction”
 The substrate interacts with the active site of the enzyme to form
an enzyme-substrate complex
 The substrate is transformed into product(s)
 The product(s) is/are released
 The enzyme is recovered unchanged

11.  Factors that influence the activity of enzymes include
 Temperature
 pH
 Enzyme and substrate concentration
 Presence/absence of inhibitors
 Temperature and pH
denature proteins,
therefore enzymes
Inactive
 Cells control synthesis of enzymes (amount/time of synthesis)
and their activity

12. Enzyme inhibitors
 Competitive
inhibition
 Sulfa drugs are
an example of
competitive
inhibitors
 Sulfanilamide/
PABA
 Inhibition can be reversible or irreversible

13.  Noncompetitive
inhibition
 Poisons such as
fluoride are an
example of
noncompetitive
inhibitors
Enzyme inhibitors
 Inhibition can be reversible or irreversible

14. Feedback Inhibition
 It is reversible
 When the final product
accumulates
 It begins to bind to and
inactivates the enzyme
that catalyzes the first
reaction of the pathway
 The pathway is turned
off
Cells regulate metabolic
pathways

15. Carbohydrate Catabolism
 Most microorganisms oxidize carbohydrates as the primary
source of cellular energy
 Two general processes are used
 Cellular respiration
 Fermentation
 Both cellular respiration and fermentation can share a common
pathway called glycolysis or Embden-Meyerhof pathway

16. Aerobic
cellular
respiration
Glucose
(C6H12O6) is
oxidized to CO2
in presence of
O2
~ 38 molecules
of ATP are
formed

17. Fermentation
 Releases energy from
oxidation of organic
molecules, i.e.;
 Sugars
 Organic acids
 Amino acids
 Purines and pyrimidines
 Does not use the Krebs cycle or
the electron transport chain
 Uses pyruvate as the final
electron acceptor from NADH,
regenerating NAD+ for glycolysis
Does not require oxygen

18. End-Products of Fermentation
 Chemical analyses of the end-products
help identify microbes, including
pathogens in clinical specimens

19. Lactic acid and
alcohol Fermentation
 Lactic acid bacteria
are Streptococcus and
Lactobacillus
 Alcohol
fermentation
carried out by
Saccharomyces
cerevisiae (yeast)

20. How do we use this knowledge to distinguish between
bacteria in the lab?
Fermentation test tubes
containing mannitol
Negative control
Samples
S. epidermidis (-)
S. aureus (+)
E. coli (+) plus gas
 Bubbles in the
Durham tube
indicate gas
formation

21. Microorganisms
catabolize complex
sugars and lipids
 Microbes produce
extracellular enzymes
 Amylases to break down
starch
 Lipases to break down
fats into glycerol and
fatty acids

22. Microorganisms
catabolize proteins
 Microbes produce
proteases
 Break down proteins into
their component amino
acids, transported across
the plasma membrane
 Amino acids are chemically
modified by deamination
reaction

23. How do we use this knowledge to distinguish between
bacteria in the lab?
 Biochemical tests
are designed to detect
the presence of enzymes
 The test tubes contain
 glucose
 pH indicator
 amino acid
Negative control Sample
 pH indicator turns to yellow if acid is produced from glucose
 pH indicator turns to purple if alkaline products are produced from
amino acid (decarboxylation reactions)

24. How do we use this knowledge to distinguish between
bacteria in the lab?
 Production of H2S
 Salmonella (+) can be readily
distinguishable from E. coli (-)
by the production of
hydrogen sulfide
 H2S is detected when bacteria
remove sulfur from certain
amino acids
 Other tests take advantage of different components of the
electron transport chain (oxidase test – cytochrome oxidase)

25. Anabolic pathways and their link to catabolic pathways
 ATP made during catabolic pathways is used for cellular
functions such as
 Active transport across plasma membranes
 Flagellar motion
 Most of the ATP is used to synthesize new cellular components
 Amino acids are needed to make proteins/enzymes
 Carbohydrates needed for polysaccharides/peptidoglycan
 Lipids are important components of cell membranes
 Purines and pyrimidines are the building blocks of DNA and
RNA

26. Anabolic pathways and their link to catabolic pathways
 The biosynthesis of
simple lipids
 Glycolysis and
Krebs cycle provide
intermediates
(precursor
metabolites) for
anabolic pathways

27.  Metabolic
pathways that function
in both anabolism and
catabolism are called
amphibolic pathways
Amphibolic
pathways